Siva Krishna Karuturi

A new nanoarchitecture photoelectrode design comprising CdS quantum-dot-sensitized, optically and electrically active TiO(2) inverse opals is developed for photoelectrochemical water splitting. The photoelectrochemical performance shows high photocurrent density (4.84 mA cm(-2) at 0 V vs. Ag/AgCl) under simulated solar-light illumination.

A perovskite/CIGS tandem configuration is an attractive and viable approach to achieve an ultra-high efficiency and cost-effective all-thin-film solar cell.

Global energy and environmental crises are among the most pressing challenges facing humankind. To overcome these challenges, recent years have seen an upsurge of interest in the development and production of renewable chemical fuels as alternatives to the nonrenewable and high-polluting fossil fuels. Photocatalysis, photoelectrocatalysis, and electrocatalysis provide promising avenues for sustainable energy conversion. Single- and dual-component catalytic systems based on nanomaterials have been intensively studied for decades, but their intrinsic weaknesses hamper their practical applications. Multicomponent nanomaterial-based systems, consisting of three or more components with at least one component in the nanoscale, have recently emerged. The multiple components are integrated together to create synergistic effects and hence overcome the limitation for outperformance. Such higher-efficiency systems based on nanomaterials will potentially bring an additional benefit in balance-of-system costs if they exclude the use of noble metals, considering the expense and sustainability. It is therefore timely to review the research in this field, providing guidance in the development of noble-metal-free multicomponent nanointegration for sustainable energy conversion. In this work, we first recall the fundamentals of catalysis by nanomaterials, multicomponent nanointegration, and reactor configuration for water splitting, CO2 reduction, and N2 reduction. We then systematically review and discuss recent advances in multicomponent-based photocatalytic, photoelectrochemical, and electrochemical systems based on nanomaterials. On the basis of these systems, we further laterally evaluate different multicomponent integration strategies and highlight their impacts on catalytic activity, performance stability, and product selectivity. Finally, we provide conclusions and future prospects for multicomponent nanointegration. This work offers comprehensive insights into the development of cost-competitive multicomponent nanomaterial-based systems for sustainable energy-conversion technologies and assists researchers working toward addressing the global challenges in energy and the environment.

Abstract Electrochemical water splitting has attracted considerable attention for the production of hydrogen fuel by using renewable energy resources. However, the sluggish reaction kinetics make it essential to explore precious‐metal‐free electrocatalysts with superior activity and long‐term stability. Tremendous efforts have been made in exploring electrocatalysts to reduce the energy barriers and improve catalytic efficiency. This review summarizes different categories of precious‐metal‐free electrocatalysts developed in the past 5 years for alkaline water splitting. The design strategies for optimizing the electronic and geometric structures of electrocatalysts with enhanced catalytic performance are discussed, including composition modulation, defect engineering, and structural engineering. Particularly, the advancement of operando/in situ characterization techniques toward the understanding of structural evolution, reaction intermediates, and active sites during the water splitting process are summarized. Finally, current challenges and future perspectives toward achieving efficient catalyst systems for industrial applications are proposed. This review will provide insights and strategies to the design of precious‐metal‐free electrocatalysts and inspire future research in alkaline water splitting.

Direct synthesis of Ni 3 N/Ni catalyst enriched with N-vacancies using one-step reactive magnetron sputtering with enhanced performance for the hydrogen evolution reaction in photoelectrochemical cells and electrolysers.

Crystalline silicon (c-Si) solar cells have been dominating the photovoltaic (PV) market for decades, and c-Si based photoelectrochemical (PEC) cells are regarded as one of the most promising routes for water splitting and renewable production of hydrogen.In this work, we demonstrate a nanoscale tantalum oxide (TaO x , ∼6 nm) as an electron-selective heterocontact, simultaneously providing high-quality passivation to the silicon surface and effective transport of electrons to either an external circuit or a water-splitting catalyst.The PV application of TaO x is demonstrated by a proof-of-concept device having a conversion efficiency of 19.1%.In addition, the PEC application is demonstrated by a photon-to-current efficiency (with additional applied bias) of 7.7%.These results represent a 2% and 3.8% absolute enhancement over control devices without a TaO x interlayer, respectively.The methods presented in this Letter are not limited to c-Si based devices and can be viewed as a more general approach to the interface engineering of optoelectronic and photoelectrochemical applications.

A hetero-nanostructured photoanode with enhanced near-infrared light harvesting is developed for photo-electrochemical cells. By spatially coating upconversion nanoparticles and quantum dot photosensitizers onto TiO2 inverse opal, this architecture allows direct irradiation of upconversion nanoparticles to emit visible light that excites quantum dots for charge separation. Electrons are injected into TiO2 with minimal carrier losses due to continuous electron conducting interface.

TiO2 inverse opals (TIO) fabricated by the atomic layer deposition (ALD) technique showed a superior infiltration result when compared to those fabricated by the conventional nanoparticles-infiltration method reported in previous studies. The ALD can achieve high filling fractions of more than ca. 96% of the maximum possible infiltration by conformal filling of 288, 390 and 510 nm opals, giving rise to high quality TIO. The photoelectrochemical performances of the ALD-fabricated TIO photoanodes of different sizes are investigated systematically for the first time in dye-sensitized solar cells (DSCs). When the TIO with a size of 288 nm was used as photoanode and indoline dye as a sensitizer in DSCs, the power conversion efficiency of the cell could attain 2.22% (Air Mass 1.5). It is found that the efficiency increases with decreasing lattice size of TIO electrode due to the larger surface area for dye loading. Owing to the selective reflectivity of the inverse opal, IPCE spectra of TIO electrodes revealed a strong wavelength dependence. Strategies relating to the characteristics of selective reflection and the design of composite photoanodes to enhance the efficiency of DSCs are discussed.

Advanced MaterialsVolume 24, Issue 30 p. 4157-4162 Communication A Novel Photoanode with Three-Dimensionally, Hierarchically Ordered Nanobushes for Highly Efficient Photoelectrochemical Cells Siva Krishna Karuturi, Siva Krishna Karuturi School of Materials Science and Engineering, Nanyang Technological University, 639798 SingaporeSearch for more papers by this authorJingshan Luo, Jingshan Luo Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 SingaporeSearch for more papers by this authorChuanwei Cheng, Chuanwei Cheng Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 SingaporeSearch for more papers by this authorLijun Liu, Lijun Liu School of Materials Science and Engineering, Nanyang Technological University, 639798 SingaporeSearch for more papers by this authorLiap Tat Su, Liap Tat Su School of Materials Science and Engineering, Nanyang Technological University, 639798 SingaporeSearch for more papers by this authorAlfred Iing Yoong Tok, Corresponding Author Alfred Iing Yoong Tok MIYTok@ntu.edu.sg School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore Alfred Iing Yoong Tok, School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore Hong Jin Fan, Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore.Search for more papers by this authorHong Jin Fan, Corresponding Author Hong Jin Fan fanhj@ntu.edu.sg Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore Alfred Iing Yoong Tok, School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore Hong Jin Fan, Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore.Search for more papers by this author Siva Krishna Karuturi, Siva Krishna Karuturi School of Materials Science and Engineering, Nanyang Technological University, 639798 SingaporeSearch for more papers by this authorJingshan Luo, Jingshan Luo Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 SingaporeSearch for more papers by this authorChuanwei Cheng, Chuanwei Cheng Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 SingaporeSearch for more papers by this authorLijun Liu, Lijun Liu School of Materials Science and Engineering, Nanyang Technological University, 639798 SingaporeSearch for more papers by this authorLiap Tat Su, Liap Tat Su School of Materials Science and Engineering, Nanyang Technological University, 639798 SingaporeSearch for more papers by this authorAlfred Iing Yoong Tok, Corresponding Author Alfred Iing Yoong Tok MIYTok@ntu.edu.sg School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore Alfred Iing Yoong Tok, School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore Hong Jin Fan, Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore.Search for more papers by this authorHong Jin Fan, Corresponding Author Hong Jin Fan fanhj@ntu.edu.sg Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore Alfred Iing Yoong Tok, School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore Hong Jin Fan, Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore.Search for more papers by this author First published: 29 May 2012 https://doi.org/10.1002/adma.201104428Citations: 89Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract A 3D hierarchically ordered nanobush structure is fabricated for use as a photoanode in photoelectrochemical cells. The photoanode structure has several favorable intrinsic characteristics, including high interface area, direct electron transport pathways, and engineered light scattering centers. Sensitization with CdS quantum dots is demonstrated, and this nanobush photoanode is expected to be advantageous also in solar cells. Citing Literature Supporting Information Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Filename Description adma_201104428_sm_suppl.pdf5.4 MB suppl Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article. Volume24, Issue30Special Issue: Materials Research at Nanyang Technological University, SingaporeAugust 8, 2012Pages 4157-4162 RelatedInformation

Atomic layer deposition (ALD) provides a unique tool for the growth of thin films with excellent conformity and thickness control down to atomic levels. The application of ALD in energy research has received increasing attention in recent years. In this review, the versatility of ALD in solar cells will be discussed. This is specifically focused on the fabrication of nanostructured photoelectrodes, surface passivation, surface sensitization, and band-structure engineering of solar cell materials. Challenges and future directions of ALD in the applications of solar cells are also discussed.

J–V hysteresis in perovskite solar cells is known to be strongly dependent on many factors ranging from the cell structure to the preparation methods. Here we uncover one likely reason for such sensitivity by linking the stoichiometry in pure CH3NH3PbI3 (MAPbI3) perovskite cells with the character of their hysteresis behavior through the influence of internal band offsets. We present evidence indicating that in some cells the ion accumulation occurring at large forward biases causes a temporary and localized increase in recombination at the MAPbI3/TiO2 interface, leading to inverted hysteresis at fast scan rates. Numerical semiconductor models including ion accumulation are used to propose and analyze two possible origins for these localized recombination losses: one based on band bending and the other on an accumulation of ionic charge in the perovskite bulk.

Abstract Realizing solar‐to‐hydrogen (STH) efficiencies close to 20% using low‐cost semiconductors remains a major step toward accomplishing the practical viability of photoelectrochemical (PEC) hydrogen generation technologies. Dual‐absorber tandem cells combining inexpensive semiconductors are a promising strategy to achieve high STH efficiencies at a reasonable cost. Here, a perovskite photovoltaic biased silicon (Si) photoelectrode is demonstrated for highly efficient stand‐alone solar water splitting. A p + nn + ‐Si/Ti/Pt photocathode is shown to present a remarkable photon‐to‐current efficiency of 14.1% under biased condition and stability over three days under continuous illumination. Upon pairing with a semitransparent mixed perovskite solar cell of an appropriate bandgap with state‐of‐the‐art performance, an unprecedented 17.6% STH efficiency is achieved for self‐driven solar water splitting. Modeling and analysis of the dual‐absorber PEC system reveal that further work into replacing the noble‐metal catalyst materials with earth‐abundant elements and improvement of perovskite fill factor will pave the way for the realization of a low‐cost high‐efficiency PEC system.

Following recent developments in photoelectrochemical and photovoltaic–electrosynthetic systems, we present the benefits of III–V semiconductors for solar water splitting. In addition to their interesting light absorption and carrier transport properties, III–V alloys and multijunction structures enable the highest solar-to-hydrogen conversion efficiencies. However, many obstacles still stand in the way of practical realization of III–V solar water-splitting systems. Various surface protection strategies are being developed to address the instability of III–V semiconductors in an electrolyte. Meanwhile, multiple cost-reduction approaches are being implemented, including the use of solar concentration, epitaxial lift-off or spalling for substrate reuse, and monolithic or heterogeneous integration on silicon substrates. All these advances make III–V photoabsorbers a promising route toward decarbonated hydrogen production and pave the way to long-term deployment in real-world applications.

Photoelectrochemical (PEC) reduction of CO2 with H2O is a promising approach to convert solar energy and greenhouse gas into value-added chemicals or fuels. However, the exact role of structures and interfaces of photoelectrodes in governing the photoelectrocatalytic processes in terms of both activity and selectivity remains elusive. Herein, by systematically investigating the InP photocathodes with Au–TiO2 interfaces, we discover that nanostructuring of InP can not only enhance the photoresponse owing to increased light absorption and prolonged minority carrier lifetime, but also improve selectivity toward CO production by providing more abundant interfacial contact points between Au and TiO2 than planar photocathodes. In addition, theoretical studies on the Au–TiO2 interface demonstrate that the charge transfer between Au and TiO2, which is locally confined to the interface, strengthens the binding of the CO* intermediate on positively charged Au interfacial sites, thus improving CO2 photoelectroreduction to form CO. An optimal Au–TiO2/InP nanopillar-array photocathode exhibits an onset potential of +0.3 V vs reversible hydrogen electrode (RHE) and a Faradaic efficiency of 84.2% for CO production at −0.11 V vs RHE under simulated AM 1.5G illumination at 1 sun. The present findings of the synergistic effects of the structure and interface on the photoresponse and selectivity of a photoelectrode provide insights into the development of III–V semiconductor-based PEC systems for solar fuel generation.

The construction of Z-scheme photocatalyst materials mimicking the natural photosynthesis system provides many advantages, including increased light harvesting, spatially separated reductive and oxidative active sites and strong redox ability. Here, a novel Bi2 S3 nanorod@In2 S3 nanoparticle heterojunction photocatalyst synthesized through one-pot hydrothermal method for Cr6+ reduction is reported. A systematic investigation of the microstructural and compositional characteristics of the heterojunction catalyst confirms an intimate facet coupling between (440) crystal facet of In2 S3 and (060) crystal facet of Bi2 S3 , which provides a robust heterojunction interface for charge transfer. When tested under visible-light irradiation, the Bi2 S3 -In2 S3 heterojunction photocatalyst with 15% Bi2 S3 loading content achieves the highest Cr6+ photoreduction efficiency of nearly 100% with excellent stability, which is among the best-reported performances for Cr6+ removal. Further examination using optical, photoelectrochemical, impedance spectroscopy, and electron spin resonance spectroscopy characterizations reveal greatly improved photogenerated charge separation and transfer efficiency, and confirm Z-scheme electronic structure of the photocatalyst. The Z-scheme Bi2 S3 -In2 S3 photocatalyst demonstrated here presents promise for the removal of highly toxic Cr6+ , and could also be of interest in photocatalytic energy conversion.

TiO₂ nanostructures-based photoelectrochemical (PEC) cells are under worldwide attentions as the method to generate clean energy. For these devices, narrow-bandgap semiconductor photosensitizers such as CdS and CdSe are commonly used to couple with TiO₂ in order to harvest the visible sunlight and to enhance the conversion efficiency. Conventional methods for depositing the photosensitizers on TiO₂ such as dip coating, electrochemical deposition and chemical-vapor-deposition suffer from poor control in thickness and uniformity, and correspond to low photocurrent levels. Here we demonstrate a new method based on atomic layer deposition and ion exchange reaction (ALDIER) to achieve a highly controllable and homogeneous coating of sensitizer particles on arbitrary TiO₂ substrates. PEC tests made to CdSe-sensitized TiO₂ inverse opal photoanodes result in a drastically improved photocurrent level, up to ~15.7 mA/cm² at zero bias (vs Ag/AgCl), more than double that by conventional techniques such as successive ionic layer adsorption and reaction.

Atomic layer deposition (ALD) provides a tool for conformal coating on high aspect-ratio nanostructures with excellent uniformity. It has become a technique for both template-directed nanofabrications and engineering of surface properties. This Feature Article highlights the application of ALD in selected fields including photonics, SERS and energy materials. Specifically, the topics include fabrication of plasmonic nanostructures for the SERS applications, fabrication of 3-D nanoarchitectured photoanodes for solar energy conversions (dye-sensitized solar cells and photoelectrochemical cells), and coating of electrodes to enhance the cyclic stability and thus device life span of batteries. Dielectric coating for tailoring optical properties of semiconductor nanostructures is also discussed as exemplified by ZnO nanowires. Future direction of ALD in these applications is discussed at the end.

In order to achieve a high performance-to-cost ratio to photovoltaic devices, the development of crystalline silicon (c-Si) solar cells with thinner substrates and simpler fabrication routes is an important step. Thin-film heterojunction solar cells (HSCs) with dopant-free and carrier-selective configurations look like ideal candidates in this respect. Here, we investigated the application of n-type silicon/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HSCs on periodic nanopyramid textured, ultrathin c-Si (∼25 μm) substrates. A fluorine-doped titanium oxide film was used as an electron-selective passivating layer showing excellent interfacial passivation (surface recombination velocity ∼10 cm/s) and contact property (contact resistivity ∼20 mΩ/cm2). A high efficiency of 15.10% was finally realized by optimizing the interfacial recombination and series resistance at both the front and rear sides, showing a promising strategy to fabricate high-performance ultrathin c-Si HSCs with a simple and low-temperature procedure.

Ferroelectric semiconductors like BiFeO3 are increasingly being investigated for applications in solar energy conversion and storage due to their intrinsic ability to induce ferroelectric polarization-driven separation of the photogenerated charge carriers resulting in above-bandgap photovoltages. Nevertheless, the BiFeO3 has been commonly prepared using complex and expensive fabrication techniques, e.g., epitaxial growth, radio frequency sputtering and pulsed laser deposition, which are not economically viable for large-scale production. Herein, we report a facile and scalable method for the fabrication of porous perovskite BiFeO3 photoanodes, as well as sequential interfacial engineering methods to enhance their photoelectrochemical performance for water splitting. Upon atomic layer deposition of a TiO2 overlayer and photo-assisted electrodeposition of a cobalt oxide/oxyhydroxide co-catalyst, the photocurrent density of the engineered photoanode for oxygen evolution reaction (1 M NaOH) significantly increased from negligible photocurrent of the pristine BiFeO3 to 0.16 mA cm−2 at 1.23 V vs. reversible hydrogen electrode (RHE) under simulated 1 sun irradiation (100 mW cm−2, AM1.5G spectrum). Furthermore, such functionalization of the BiFeO3 photoanodes shifts the photoelectrochemical oxidation onset potential by 0.7 V down to 0.6 V vs. RHE. The significantly enhanced photoelectro-oxidation activity is facilitated by the improved charge transfer and electrochemical kinetics.

Abstract III–V semiconductor nanowires offer potential new device applications because of the unique properties associated with their 1D geometry and the ability to create quantum wells and other heterostructures with a radial and an axial geometry. Here, an overview of challenges in the bottom‐up approaches for nanowire synthesis using catalyst and catalyst‐free methods and the growth of axial and radial heterostructures is given. The work on nanowire devices such as lasers, light emitting nanowires, and solar cells and an overview of the top‐down approaches for water splitting technologies is reviewed. The authors conclude with an analysis of the research field and the future research directions.

Abstract While direct solar‐driven water splitting has been investigated as an important technology for low‐cost hydrogen production, the systems demonstrated so far either required expensive materials or presented low solar‐to‐hydrogen (STH) conversion efficiencies, both of which increase the levelized cost of hydrogen (LCOH). Here, a low‐cost material system is demonstrated, consisting of perovskite/Si tandem semiconductors and Ni‐based earth‐abundant catalysts for direct solar hydrogen generation. NiMo‐based hydrogen evolution reaction catalyst is reported, which has innovative “flower‐stem” morphology with enhanced reaction sites and presents very low reaction overpotential of 6 mV at 10 mA cm −2 . A perovskite solar cell with an unprecedented high open circuit voltage ( V oc ) of 1.271 V is developed, which is enabled by an optimized perovskite composition and an improved surface passivation. When the NiMo hydrogen evolution catalyst is wire‐connected with an optimally designed NiFe‐based oxygen evolution catalyst and a high‐performance perovskite‐Si tandem cell, the resulting integrated water splitting cell achieves a record 20% STH efficiency. Detailed analysis of the integrated system reveals that STH efficiencies of 25% can be achieved with realistic improvements in the perovskite cell and an LCOH below ≈ $3 kg −1 is feasible.

Abstract The design and development of materials at the nanoscale has enabled efficient, cutting‐edge renewable energy storage, and conversion devices such as solar cells, water splitting, fuel cells, batteries, and supercapacitors. In addition to creating new materials, the ability to refine the structure and interface properties holds the key to achieving superior performance and durability of these devices. Atomic layer deposition (ALD) has become an important tool for nanofabrication as it allows the deposition of pin‐hole‐free films with atomic‐level thickness and composition control over high aspect ratio surfaces. ALD is successfully used to fabricate devices for renewable energy storage and conversion, for example, to deposit absorber materials, passivation layers, selective contacts, catalyst films, protection barriers, etc. In this review article, recent advances enabled by ALD in designing materials for high‐performance solar cells, catalytic energy conversion systems, batteries, and fuel cells, are summarized. The critical issues impeding the performance and durability of these devices are introduced and then the role of ALD in addressing them is discussed. Finally, the challenges in the implementation of ALD technique for nanofabrication on industrial scale are highlighted and a perspective on potential solutions is provided.

Electrochemical water splitting provides a promising approach to store renewable electricity in the form of hydrogen on a grand scale. However, the current techniques for large‐scale electrochemical hydrogen production rely on the use of expensive and scarce noble‐metal catalysts making it uncompetitive to traditional methods using fossil fuels. Thus, replacing noble‐metal electrocatalysts with cheap materials made of abundant elements holds the key to achieve the cost‐effectiveness. Recently, amorphous electrocatalysts emerged as promising candidates due to their unique physical and chemical properties compared with their crystalline counterparts leading to superior catalytic performance. Given the rapid advances made in the design, synthesis, and development of amorphous catalysts, namely monometallic and multimetallic borides, sulfides, phosphides, oxides, and hydroxides based on transition metals, the recent progress on compositional designs, microstructure, morphology, electronic properties, and interaction with host materials are reviewed critically. Special attention is paid to uncover the main strategies adopted in each material category and the underlying structure–property relationship that lead to improved hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performances. As a result, guidance for the design and synthesis of novel amorphous electrocatalysts with high performance for large‐scale electrochemical water splitting application is provided.

Abstract Ferrihydrite (Fh) has been demonstrated as an effective interfacial layer for photoanodes to achieve outstanding photoelectrochemical (PEC) performance for water oxidation reaction owing to its unique hole‐storage function. However, it is unknown whether such a hole‐storage layer can be used to construct highly efficient photocathodes for hydrogen evolution reaction (HER). In this work, we report Fh interfacial engineering of amorphous silicon photocathode (with nickel as HER cocatalyst) achieving a photocurrent density of 15.6 mA cm −2 at 0 V vs. the reversible hydrogen electrode and a half‐cell energy conversion efficiency of 4.08 % in alkaline solution, outperforming most of reported a‐Si based photocathodes including multi‐junction configurations integrated with noble metal cocatalysts in acid solution. Besides, the photocurrent density is maintained above 14 mA cm −2 for 175 min with 100 % Faradaic efficiency for HER in alkaline solution. Our results demonstrate a feasible approach to construct efficient photocathodes via the application of a hole‐storage layer.

A machine-learning methodology was applied to unveil the structure–property relationships of the fabricated ternary Ni, Fe, and Co amorphous oxygen evolution catalyst, showcasing remarkable performance and stability via corrosion engineering.

Semiconductor nanowires-inverse opal hybrid architectures are designed and fabricated as a new type of photoelectrochemical photoanode. ZnO nanowire arrays are directly grown atop TiO2 inverse opals to form a bilayer structure with both electrical and optical coupling. In addition to the individual photocurrent contributions from the TiO2 and ZnO, the inverse opal plays an additional role in enhancing the light harvesting when the photonic stop bands are tailored at appropriate positions by tuning the opal size. Given the fact that materials for both the nanowires and inverse opal are widely available, our strategy offers a new approach to the fabrication of various nanowires–photonic crystal coupled structures for various solar energy applications.

The scalable synthesis of highly transparent and robust sub‐monolayers of Co 3 O 4 nano‐islands, which efficiently catalyze water oxidation, is reported. Rapid aerosol deposition of Co 3 O 4 nanoparticles and thermally induced self‐organization lead to an ultra‐fine nano‐island morphology with more than 94% light transmission at a wavelength of 500 nm. These transparent sub‐monolayers demonstrate a remarkable mass‐weighted water oxidation activity of 2070–2350 A g Co3O4 −1 and per‐metal turnover frequency of 0.38–0.62 s −1 at an overpotential of 400 mV in 1 m NaOH aqueous solution. This mixed valent cobalt oxide structure exhibits excellent long‐term electrochemical and mechanical stability preserving the initial catalytic activity over more than 12 h of constant current electrolysis and 1000 consecutive voltammetric cycles. The potential of the Co 3 O 4 nano‐islands for photoelectrochemical water splitting has been demonstrated by incorporation of co‐catalysts in GaN nanowire photoanodes. The Co 3 O 4 ‐GaN photoanodes reveal significantly reduced onset overpotentials, improved photoresponse and photostability compared to the bare GaN ones. These findings provide a highly performing catalyst structure and a scalable synthesis method for the engineering of efficient photoanodes for integrated solar water‐splitting cells.

Photoelectrolysis of water using solar energy into storable and environment-friendly chemical fuel in the form of hydrogen provides a potential solution to address the environmental concerns and fulfill future energy requirements in a sustainable manner. Achieving efficient and spontaneous hydrogen evolution in water using solar light as the only energy input is a highly desirable but a difficult target. In this work, we report perovskite solar cell integrated CdS-based photoanode for unbiased photoelectrochemical hydrogen evolution. An integrated tandem device consisting of mesoporous CdS/TiO2 photoanode paired with a triple-cation perovskite (Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3) solar cell is developed via a facile fabrication route. The proposed photovoltaic integrated photoanode presents an efficient tandem configuration with high optical transparency to long-wavelength photons and strong photoelectrochemical conversions from short-wavelength photons. On the basis of this integrated tandem device, an unbiased photocurrent density of 7.8 mA/cm2 is demonstrated under AM1.5G illumination.

Heterojunction solar cells with transition-metal-oxide-based carrier-selective contacts have been gaining considerable research interest owing to their amenability to low-cost fabrication methods and elimination of parasitic absorption and complex semiconductor doping process. In this work, we propose tantalum oxide (Ta2O5) as a novel electron-selective contact layer for photo-generated carrier separation in InP solar cells. We confirm the electron-selective properties of Ta2O5 by investigating band energetics at the InP-Ta2O5 interface using X-ray photoelectron spectroscopy. Time-resolved photoluminescence and power dependent photoluminescence reveal that the Ta2O5 inter-layer also mitigates parasitic recombination at the InP/transparent conducting oxide interface. With an 8 nm Ta2O5 layer deposited using an atomic layer deposition (ALD) system, we demonstrate a planar InP solar cell with an open circuit voltage, Voc, of 822 mV, a short circuit current density, Jsc, of 30.1 mA cm-2, and a fill factor of 0.77, resulting in an overall device efficiency of 19.1%. The Voc is the highest reported value to date for an InP heterojunction solar cells with carrier-selective contacts. The proposed Ta2O5 material may be of interest not only for other solar cell architectures including perovskite cells and organic solar cells, but also across a wide range of optoelectronics applications including solid state emitting devices, photonic crystals, planar light wave circuits etc.

In this work, we report on defects generation in TiO2 inverse opal (IO) nanostructures by electrochemical reduction in order to increase photocatalytic activity and improve photoelectrochemical (PEC) water splitting performance. Macroporous structures, such as inverse opals, have attracted a lot of attention for energy-related applications because of their large surface area, interconnected pores, and ability to enhance light-matter interaction. Photocurrent density of electrochemically reduced TiO2-IO increased by almost 4 times, compared to pristine TiO2-IO photoelectrodes. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses confirm the presence of oxygen vacancies in electrochemically reduced TiO2-IO photoelectrodes. Oxygen vacancies extend the absorption of TiO2 from the UV to visible region. The incident photon-to-current efficiency (IPCE) increased by almost 3 times in the absorption (UV) region of TiO2 and slightly in the visible region. Impedance studies show improved electrical conductivity, longer photogenerated electron lifetime, and a negative shift of the flatband potential, which are attributed to oxygen vacancies acting as electron donors. The Fermi level shifts to be closer to the conduction band edge of TiO2-IO.

Abstract This research introduces a pioneering approach to solar water splitting technology, utilizing an innovative, highly efficient immersed system. The system incorporates a flexible array of electrochemical and photoelectrochemical cells, powered by high‐performance III‐V triple‐junction cells. Remarkably, this method significantly boosts the solar‐to‐hydrogen (STH) conversion efficiency, reaching a record 20.7% under 1 sun illumination, employing earth‐abundant catalysts operating at ambient temperature. These findings highlight extensive scope for further optimization, including minimizing optical transmission losses, mitigating shading effects, and reducing the overpotential of the electrochemical cells, thereby augmenting the STH efficiency to an estimated 28%. Through a comprehensive techno‐economic analysis, a levelized cost of hydrogen (LCOH) of 8.3 USD kg −1 is estimated, forecasting the potential for a reduction to a competitive 1.8 USD kg −1 with improved efficiency, increased capacity factors, and decreased production costs. A sensitivity analysis emphasizes the significant influence of factors such as III‐V cell cost, electrolyzer membrane cost and capacity factor on the LCOH. Overall, this study signifies crucial progress toward a highly efficient and economically viable solar water splitting solution, promising a sustainable route for hydrogen production.

We present one-pot hydrothemal syntheis of SnS2-In2S3 p-n heterostructures for photocatalytic reduction of Cr6+ to Cr3+. The conduction bands of In2S3 and SnS2 serve as the photogenerated electron donors and acceptors, respectively, forming a p-n junction band alignment improving the photogenerated charge separation efficiency. Through a detailed investigation of photocurrent spectra, photoluminescence spectra and electrochemical impedance, the heterojunction formation is shown to enhance the photogenerated charge separation and transport properties. In addition, the analysis of 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) spin trapping electron spin resonance (ESR) spectra and band structure further testify the p-n junction of the composite catalyst that could accelerate the electron transport and change the pathway of photocatalytic process toward Cr6+ reduction. Furthermore, SnS2-In2S3 heterojunction photocatalyst achieved 3 times and 67 times higher efficiency than pure In2S3 and SnS2 photocatalysts under visible light toward Cr6+ reduction. Moreover, the SnS2-In2S3 p-n heterojunction photocatalyst demonstrates Cr6+ removal in a weak alkaline solution, breaking through the challenge of Cr6+ reduction in an alkaline environment.

III–V semiconductors are among the highest performing materials for solar energy conversion devices. Exposing III–V semiconductors to a hydrogen plasma can improve optoelectronic properties and is a critical step in fabricating efficient InP solar cells. However, there is a limited understanding of the changes induced by hydrogen plasma exposure to the surface and in the bulk of III–V semiconductors. Herein, it is demonstrated that a 19.3% efficient p‐InP solar cell with a TiO 2 electron selective contact layer can be achieved by exposing the InP substrate to hydrogen plasma. Detailed investigations employing ultraviolet photoelectron spectroscopy and capacitance–voltage measurement unveil that the hydrogen plasma exposure on p‐InP leads to charge carrier polarity inversion in the near‐surface region (charge inversion layer) while simultaneously reducing the carrier concentration (charge‐depleted layer) in the bulk. The study provides important insights into the impact of hydrogen plasma exposures on InP which may lead to more efficient optoelectronic devices such as solar cells, photodetectors, light‐emitting diodes, and photoelectrochemical cells.

Atomic layer deposition (ALD) is shown as a unique method to produce high aspect ratio (AR) nanostructures through conformal filling and replication of high AR templates. The stop-flow process is often used as an alternative to the conventional continuous flow process to obtain high step coverage. However, there is a need for understanding the deposition kinetics and optimizing the deposition process to fabricate defect-free nanostructures. In this Article, TiO2 ALD in high AR self-assembled opal photonic crystal templates was performed in stop-flow fill−hold−purge process in comparison with continuous flow pulse−purge process. Photonic band gap properties of opal templates were characterized and compared with simulated band diagrams for quantitative investigation of filling kinetics and the effect of shrinking pore size on filling uniformity. Γ−L bands in the transmittance spectra of ALD-infiltrated opals accurately represented the depth profile of the depositions without the need for expensive sample preparation techniques and characterization tools. It was found that the stop-flow process attains higher Knudsen flow rates of precursor gases, thereby achieving homogeneous and complete filling at considerably lower cycle time.

An electrochromic photonic crystal (EPC) display device that combines chemical (electrochromic) and physical (photonic) coloring mechanisms is reported for the first time. This EPC exhibits superior and versatile color tunability. The TiO2 inverse opals fabricated by atomic layer deposition are adopted as EPC material. Results show that the photonic band gaps selectively modified the optical properties of the EPC and enabled facile tuning of electrochromic colors. In addition, the reversible photonic and photonic modified electrochromic coloring states with insertion/extraction of lithium ions enable novel and promising approaches for future display applications.

In this work, we report on the photoelectrochemical (PEC) investigation of n-GaN nanopillar (NP) photoanodes fabricated using metal organic chemical vapour deposition and the top-down approach. Substantial improvement in photocurrents is observed for GaN NP photoanodes compared to their planar counterparts. The role of carrier concentration and NP dimensions on the PEC performance of NP photoanodes is further elucidated. Photocurrent density is almost doubled for doped NP photoanodes whereas no improvement is noticed for undoped NP photoanodes. While the diameter of GaN NP is found to influence the onset potential, carrier concentration is found to affect both the onset and overpotential of the electrodes. Optical and electrochemical impedance spectroscopy characterisations are utilised to further explain the PEC results of NP photoanodes. Finally, improvement in the photostability of NP photoanodes with the addition of NiO as a co-catalyst is investigated.

Design of efficient photoelectrodes for water oxidation requires careful optimization of the morphology and structure of a photoactive material to maximize electrical conductivity and balance carrier diffusion length with light penetration depth. Hematite-based photoanodes can theoretically oxidize water at very high rates, as provided by the optimal band-gap, but their performance is limited by the poor charge transport and low charge separation efficiency. Herein, we have developed physically- and chemically-induced morphological and structural tuning procedures, viz. capillary-force-induced self-assembly and corrosion followed by regrowth, which enable significant improvements in the performance of the hematite photoanodes. Specifically, a 24-fold enhancement in the photocurrent density for water oxidation (1 M NaOH) at 1.23 V vs. reversible hydrogen electrode under simulated 1 sun (100 mW cm–2, AM1.5G solar spectrum) irradiation has been achieved. The capillary-force-induced self-assembly improves the crystallinity, promotes preferential orientation of the hematite along the [110] direction, and thereby enhances the electrical conductivity of the material. Subsequent dissolution and regrowth of the hematite nanostructures provide higher light absorption, improve photo-generated charge separation and facilitate photoelectrocatalytic kinetics resulting in the significantly higher photoelectrocatalytic activity. These broadly applicable insights provide a robust set of guidelines for the engineering of efficient photoelectrodes initially made of disordered structures for conversion of solar energy into renewable fuels.

Abstract Crystalline silicon (c‐Si) solar cells featuring carrier‐selective passivating contacts have become a prominent path to develop highly efficient photovoltaic devices. Development of electron‐selective materials that can provide excellent surface passivation and low contact resistivity to c‐Si substrates while presenting good environmental stability is crucial for practical implementation. Here, an easy approach is demonstrated to achieve low resistivity Ohmic contacts between slightly doped n‐type c‐Si and aluminum electrodes via simple spin‐coating of metal acetylacetone (MAcac) film on a c‐Si surface. Contact resistivity of 1.3 m Ω cm 2 (18.2 m Ω cm 2 with an a‐Si:H(i) passivating layer) is realized when a thin calcium acetylacetone (CaAcac) interlayer is introduced between c‐Si and Al. An n‐Type c‐Si solar cell with a full area rear a‐Si:H(i)/CaAcac/Al electron‐selective contact is demonstrated with a power conversion efficiency of 21.6%. This work not only demonstrates an approach to develop highly efficient n‐type c‐Si solar cells with effective electron‐selective passivating contacts, but also contributes toward accomplishing a simplified fabrication process for photovoltaic devices, from vacuum to solution processing.

While photoelectrochemical (PEC) water splitting is a very promising route toward zero-carbon energy, conversion efficiency remains limited. Semiconductors with narrower band gaps can absorb a much greater portion of the solar spectrum, thereby increasing efficiency. However, narrow band gap (∼1 eV) III–V semiconductor photoelectrodes have not yet been thoroughly investigated. In this study, the narrow band gap quaternary III–V alloy InGaAsP is demonstrated for the first time to have great potential for PEC water splitting, with the long-term goal of developing high-efficiency tandem PEC devices. TiO2-coated InGaAsP photocathodes generate a photocurrent density of over 30 mA/cm2 with an onset potential of 0.45 V versus reversible hydrogen electrode, yielding an applied bias efficiency of over 7%. This is an excellent performance, given that nearly all power losses can be attributed to reflection losses. X-ray photoelectron spectroscopy and photoluminescence spectroscopy show that InGaAsP and TiO2 form a type-II band alignment, greatly enhancing carrier separation and reducing recombination losses. Beyond water splitting, the tunable band gap of InGaAsP could be of further interest in other areas of photocatalysis, including CO2 reduction.

Abstract An ideal catalytic interface for photoelectrodes that enables high efficiency and long‐term stability remains one of the keys to unlocking high‐performance solar water splitting. Here, fully decoupled catalytic interfaces realized using surface‐structured cocatalyst foils are demonstrated, allowing optimized photoabsorbers to be combined with high‐performance earth‐abundant cocatalysts. Since many earth‐abundant cocatalysts are deposited via solution‐based methods, deposition on chemical‐sensitive photoabsorbers is a significant challenge. By synthesizing cocatalyst foils prior to device fabrication, photoabsorbers are completely isolated from corrosive chemical environments and are provided with outstanding protection during operation. Si and GaAs photoelectrodes prepared using Ni‐based cocatalyst foils achieve excellent half‐cell efficiencies and generate stable photocurrents for over 5 days. Furthermore, a GaAs artificial leaf achieves a solar‐to‐hydrogen efficiency of 13.6% and maintains an efficiency of over 10% for longer than nine days, an accomplishment that has not been previously reported for an immersed solar water splitting system. These results, together with theoretical calculations of other photoelectrode systems, demonstrate that cocatalyst foils offer a very attractive method for fabricating high‐performance solar water splitting systems.

Cadmium sulfide (CdS) is a unique semiconducting material for solar hydrogen generation applications with a tunable, narrow bandgap that straddles water redox potentials. However, its potential towards efficient solar hydrogen generation has not yet been realized due to low photon-to-current conversions, high charge carrier recombination and the lack of controlled preparation methods. In this work, we demonstrate a highly efficient CdS/TiO2 heterostructured photoelectrode using atomic layer deposition and solution ion transfer reactions. Enabled by the well-controlled deposition of CdS nanocrystals on TiO2 inverse opal (TiIO) nanostructures using the proposed method, a saturation photocurrent density of 9.1 mA cm−2 is realized which is the highest ever reported for CdS-based photoelectrodes. We further demonstrate that the passivation of a CdS surface with an ultrathin amorphous layer (∼1.5 nm) of TiO2 improves the charge collection efficiency at low applied potentials paving the way for unassisted solar hydrogen generation.

With a band gap close to the Shockley–Quiesser limit and excellent conduction band alignment with the water reduction potential, InP is an ideal photocathode material for photoelectrochemical (PEC) water reduction. Here, we develop facile self-assembled Au nanodots based on dewetting phenomena as a masking technique to fabricate wafer-scale InP nanowires (NWs) via a top-down approach. In addition, we report dual-function wet treatment using sulfur-dissolved oleylamine (S-OA) to remove a plasma-damaged surface in a controlled manner and stabilize InP NWs against surface corrosion in harsh electrolyte solutions. The resulting InP NW photocathodes exhibit an excellent photocurrent density of 33 mA/cm2 under 1 sun illumination in 1 M HCl with a highly stabilized performance without needing additional protection layers. Our approach combining large-area NW fabrication and surface engineering synergistically enhances light harvesting and PEC performance and stability, thereby providing a pathway for the development of efficient and durable InP photoelectrodes in a scalable manner.

Nano-crystalline GexC1 − x is a potential third generation solar cell absorber material due to its favourable opto-electronic properties and relatively high abundance of elements. The ability to grow nano-crystalline GexC1 − x in large areas by an industry-friendly process can enhance its scope as a photovoltaic absorber. In this work nano-crystalline GexC1 − x thin films have been grown on Si (100) substrate using radio frequency magnetron sputtering. The crystallinity, composition, structure and optical properties of the films were determined by, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, atomic force microscopy, transmission electron microscopy (TEM) and ultra-violet visible infrared spectroscopy. From TEM results it was found that GexC1 − x crystals were scattered in the film with d-spacing of 3.4 nm between the fringes (calculated a = 5.53 Å), but that a small number of nanoparticles of GeC were present. The Raman signature of the local Ge–C mode is identified near 530 cm− 1 in GexC1 − x film grown at 350 °C. The band gap energy value was estimated to be 0.90 eV from optical reflectance spectra. Maximum 15.5% of GexC1 − x is found in the film deposited at 350 °C using XPS fitting.

Crystalline silicon (c‐Si) solar cells with carrier‐selective passivating contacts have been prosperously developed over the past few years, showing fundamental advantages, e.g., simpler configurations and higher potential efficiencies, compared with conventional c‐Si solar cells using highly doped emitters. Herein, solution‐processed cesium halides (CsX, X represents F, Cl, Br, I) are investigated as electron‐selective contacts for c‐Si solar cells, enabling lowest contact resistivity down to about 1 mΩ cm 2 for slightly doped n‐type c‐Si/CsF/Al contact. After inserting a thin intrinsic amorphous silicon (a‐Si:H(i)) passivating layer, the contact resistivity can still be kept in a low value, about 10 mΩ cm 2 . With full area rear‐side a‐Si:H(i)/CsF/Al electron‐selective passivating contacts, record power conversion efficiencies of about 21.8% are finally demonstrated for n‐type c‐Si solar cells, showing a simple approach to realize high‐efficiency c‐Si solar cells.

Synthesis of ternary NiFeCo hydroxide as an efficient OER electrocatalyst using a versatile, two-step solution corrosion approach.

Abstract This study presents an innovative, statistically‐guided magnetron sputtering technique for creating nanoarchitectonics of high‐performing, NiFeMoN electrocatalysts for oxygen evolution reaction (OER) in water splitting. Using a central composite face‐centered (CCF) design, 13 experimental conditions are identified that enable precise optimization of synthesis parameters through response surface methodology (RSM), confirmed by analysis of variance (ANOVA). The statistical analysis highlighted a interaction between Mo% and N% in the nanostructured NiFeMoN and found optimizing values at 31.35% Mo and 47.12% N. The NiFeMoN catalyst demonstrated superior performance with a low overpotential of 216 mV at 10 mA cm −2 and remarkable stability over seven days, attributed to the modifications in electronic structure and the creation of new active sites through Mo and N additions. Furthermore, the NiFeMoN coating, when used as a protective layer for a Si photoanode in 1 m KOH, achieved an applied‐bias photon‐to‐current efficiency (ABPE) of 5.2%, maintaining stability for 76 h. These advancements underscore the profound potential of employing statistical design for optimizing synthesis parameters of intricate catalyst materials via magnetron sputtering, paving the way for accelerated advancements in water splitting technologies and also in other energy conversion systems, such as nitrogen reduction and CO 2 conversion.

New sustainable energy technologies are an important current research field, which aim to address the global environmental challenge caused by our excessive reliance on fossil fuels. Underpinning this field is...

In pursuit of efficient water-splitting technologies, the development of high-performing electrocatalysts is crucial, particularly for the hydrogen evolution reaction (HER). In addition, it is paramount to adopt cost-effective approaches that leverage earth-abundant metals, together with scalable synthesis methods. In this study, we introduce a synthesis approach that combines a facile solution corrosion technique with plasma modification, thereby enhancing the hydrogen evolution activity of NiMo alloys. The inclusion of NH3 plasma modification plays a dual role by concurrently reducing and nitriding as-synthesized NiMo hydroxide. The treatment results in a significant reduction in the HER overpotential to 95 mV at 10 mA/cm2 compared to its initial overpotential. This improvement is attributed to enhanced kinetics due to substantial reductions in charge transfer resistance and an increased double-layer capacitance. Furthermore, the catalyst demonstrates excellent stability of close to 120 h, thereby highlighting the potential of this synthesis method for large-area synthesis of HER electrocatalysts.

Abstract The increasing adoption of solar energy as a renewable power source marks a significant shift toward clean, sustainable alternatives to conventional energy forms. A notable development in this field is the advancement of thin monocrystalline silicon (c‐Si) solar cells. Characterized by their lightweight, flexible nature, these solar cells promise to transform the renewable energy landscape with enhanced durability, adaptability, and portability. Amidst the growing demand for sustainable energy solutions, refining and evolving thin c‐Si solar cell technologies is crucial. This review comprehensively examines the latest progress in thin c‐Si solar energy conversion device technologies, offering an extensive overview of current methodologies for producing thin c‐Si films, advanced light trapping techniques, surface passivation strategies, and methods for managing thin wafers. It also explores the wide‐ranging applications of thin c‐Si solar cells. Furthermore, this article sheds light on the techno‐economic aspects, highlighting the intertwined challenges of commercializing these innovative cells. Through an in‐depth analysis of the latest advancements, this review serves as a valuable resource for researchers and industry experts, keeping them abreast of current trends and catalyzing further advancements in thin c‐Si solar cell technology.

A method of spatially controlled template filling and replication is reported herein using Knudsen diffusion-limited atomic layer deposition (ALD). Experimental and theoretical investigations based on the porous framework of self-assembled polystyrene opals further confirm the well controlled z-directional TiO2 spatial gradients paving the way for the fabrication of gradient index inverse opal photonic crystals for the first time.

The research interest in photoelectrochemical (PEC) water splitting is ever growing due to its potential to contribute towards clean and portable energy. However, the lack of low energy band gap materials with high photocorrosion resistance is the primary setback inhibiting this technology from commercialisation. The ternary alloy InGaN shows promise to meet the photoelectrode material requirements due to its high chemical stability and band gap tunability. The band gap of InGaN can be modulated from the UV to IR regions by adjusting the In concentration so as to absorb the maximum portion of the solar spectrum. This paper reports on the influence of In concentration on the PEC properties of planar and nanopillar (NP) InGaN/GaN multi-quantum well (MQW) photoanodes, where NPs were fabricated using a top-down approach. Results show that changing the In concentration, while having a minor effect on the PEC performance of planar MQWs, has an enormous impact on the PEC performance of NP MQWs, with large variations in the photocurrent density observed. Planar photoanodes containing MQWs generate marginally lower photocurrents compared to photoanodes without MQWs when illuminated with sunlight. NP MQWs with 30% In generated the highest photocurrent density of 1.6 mA cm-2, 4 times greater than that of its planar counterpart and 1.8 times greater than that of the NP photoanode with no MQWs. The InGaN/GaN MQWs also slightly influenced the onset potential of both the planar and NP photoanodes. Micro-photoluminescence, diffuse reflectance spectroscopy and IPCE measurements are used to explain these results.

InP is currently being used in various (opto)electronic and energy device applications. However, the high cost of InP substrates and associated epitaxial growth techniques has been huge impediments for its widespread use. Here, large-area monocrystalline InP thin films are demonstrated via a convenient cracking method, and the InP thin films show material properties identical to their bulk counterparts. Furthermore, the same substrate can be reused for the production of additional InP thin films. This cracking technique is also shown to be a versatile tool to form an ultrasmooth surface or a microscale periodic triangular grating structure on the surface, depending on the orientation of the donor substrate used. Strain-induced band gap energy shift is also observed in localized regions of the thin film with a grating structure. The simplicity of this technique, which does not require any sophisticated equipment and complex fabrication process, is promising to reduce the cost of InP thin-film devices.

The production of flexible monocrystalline semiconductor thin films less than a few tens of micrometers in thickness is currently receiving huge interest in various emerging applications such as mobile health care (mHealth), wearable devices, smart cities, and Internet of things (IoT). However, conventional techniques fail to produce wafer-scale monocrystalline thin films without the use of sophisticated equipment. Recently, the controlled cracking method has shown promise as a facile and scalable method to produce monocrystalline inorganic semiconductor thin films such as Si, Ge, III–V, and III–N materials. In this method, a crystalline semiconductor thin film can be exfoliated from its thick donor substrate via subsurface crack propagation. The cracking based layer transfer approach does not require expensive processing equipment and enables the production of multiple thin films from the same donor substrate. In this review, we present the working principles, recent progress, and future prospects of this emerging crack-assisted layer transfer technology. The unique advantages of this technology for state-of-the-art flexible (opto)electronics are also highlighted. This review offers insights for the fabrication of large-scale flexible monocrystalline semiconductors, which is crucial for the development of next-generation (opto)electronics.

In this work, surface-plasmon mediated enhanced photoluminescence emission has been investigated on Ni, Ti, and NiTi coated ZnO nanowires (NWs). ZnO NWs have been synthesized using a facile hydrothermal process, where NWs are coated with three different metals (Ni, Ti, and NiTi) using sputter deposition technique. It has been found that there is a significant improvement in near band edge emission (NBE) and passivation in deep level emission (DLE) in such metal embedded ZnO NWs and these emission properties can be tuned as we change the metal. Notably, we have achieved the highest enhancement of ∼6 times in NBE and best suppression of ∼15 times) in DLE by alloying of such metals (Ni and Ti). Such a remarkable DLE suppression is attributed to the presence of defect centers in ZnO NWs. The defect transition energy of ZnO is in resonance with the surface plasmon energy of metal nanoparticles, which leads to the conversion of DLE into NBE. The enhancement of NBE and suppression of DLE are possible due to the surface plasmon resonance coupling between metal nanoparticles (NPs) and ZnO NWs. Therefore, we conclude that earth abundant metals, such as Ni and Ti show significant SPR coupling on ZnO NWs and the alloying (NiTi) of such metals presents further improved SPR compared to the respective individual metals.

Amorphous silicon (a-Si) has been extensively used to fabricate solar cells for efficient light-to-electricity conversion due to its outstanding light-harvesting properties and its facile and low-cost preparation method. However, photoelectrodes based on single-junction-structured a-Si have not been demonstrated for overall water splitting due to their insufficient photovoltage. Herein, we report the fabrication of single-junction a-Si-based photocathodes and photoanodes and further construct dual-photoelectrode devices for unassisted photoelectrochemical (PEC) water splitting. The p/i/n and n/i/p junction a-Si are used as photoabsorbers and the sputtered Pt nanoparticles and Co3O4 film as cocatalysts for photocathodes and photoanodes, respectively. The photocathode yields a photocurrent density up to 12.03 mA cm–2 at 0 V versus reversible hydrogen electrode (RHE), which outperforms all previous results of a-Si-based photocathodes for PEC hydrogen evolution reactions. Additionally, the Co3O4/nip photoanode generated a photocurrent density of 7.3 mA cm–2 at 1.23 V vs RHE. The maximum applied bias photo-to-current efficiencies are 3.3% for the photocathode and 0.93% for the photoanode in alkaline solution. The as-fabricated biphotoelectrode system is able to yield a solar-to-hydrogen efficiency of 0.61%, which presents an example enabling single-junction-structured a-Si for unassisted overall water splitting.

Abstract Catalyst‐free InGaAs nanowires are promising building blocks for optoelectronic devices operating at telecommunication wavelengths. Despite progress, the applications of InGaAs nanowires remain limited due to their high density of surface states that degrade their optical properties. Here, InGaAs nanowires with superior optical properties are achieved by effectively suppressing their surface states with an InP passivation shell. Optimal InP shell growth conditions and thickness to maximize the minority carrier lifetime are identified. The photoluminescence intensity of these passivated InGaAs nanowires is up to three orders of magnitude higher than that of their bare counterparts. Moreover, a long minority carrier lifetime of up to ≈13 ns is measured with these passivated nanowires at room temperature. Optimal passivation of InGaAs nanowires with an emission wavelength of 1530 nm results in an ultra‐low surface recombination velocity of ≈280 cm s −1 . In addition to the shell, the crystal structure of these nanowires plays an important role in the luminescence intensity. Combined cathodoluminescence mapping and high‐resolution transmission electron microscopy along the nanowires reveal significantly lower emission intensities in wurtzite predominant sections of the nanowires than zinc blende predominant ones.These insights on the optimal passivation of InGaAs provide directions for engineering high‐performance nanoscale‐devices in the telecommunication wavelength.

To realize low-cost and sustainable hydrogen production, it is imperative to develop facile approaches to fabricate water splitting (photo)electrodes based on earth-abundant catalysts. In addition, cost benefits can be unlocked if the expended catalysts can be regenerated multiple times on the same substrate. Here, we demonstrate a substrate-agnostic method of depositing NiFe layered double hydroxide (LDH) catalyst via solution corrosion on diverse substrates as water splitting (photo)anodes. Across various substrates, the catalyst deposited electrodes exhibit consistent and sustained water splitting performance as well as possessing regenerative capabilities. Using this method, we also demonstrate a record performance for NiFe LDH/GaAs photoanode, whereby an applied bias photon-to-current efficiency of 11.7% is achieved with excellent photocurrent stability up to 100 h. This study also shows that NiFe LDH deposited by using this technique can sustain high current density operations in alkaline electrolyzer cells for the benefit of industrial water splitting applications. Using the method developed here in preparing low-cost (photo)electrodes on diverse substrate materials, we foresee excellent prospects for delivering high performance and stable water splitting activity for large-scale application.

Reduced quantum dot (QD) absorption due to state filling effects and enhanced electron transport in doped QDs are demonstrated to play a key role in solar energy conversion. Reduced QD state absorption with increased n-doping is observed in the self-assembled In0.5Ga0.5As/GaAs QDs from high resolution below-bandgap external quantum efficiency (EQE) measurement, which is a direct consequence of the Pauli exclusion principle. We also show that besides partial filling of the quantum states, electron-doping produces negatively charged QDs that exert a repulsive Coulomb force on the mobile electrons, thus altering the electron trajectory and reducing the probability of electron capture, leading to an improved collection efficiency of photo-generated carriers, as indicated by an absolute above-bandgap EQE measurement. The resulting redistribution of the mobile electron in the planar direction is further validated by the observed photoluminescence intensity dependence on doping.

HfO2 has many favorable characteristics for use in energy conversion devices including high thermodynamic stability, good chemical stability in corrosive electrolytes, high refractive index, and wide bandgap. Here, we report surface passivation of a c-Si photocathode by ultrathin HfO2 prepared using atomic layer deposition as an effective approach for enhancing its photoelectrochemical (PEC) performance. The effect of the thickness of HfO2, deposition temperature, and annealing in forming gas on the passivation performance are systematically investigated. We demonstrate that the Si photocathode with a p+/n/n+ structure decorated with a Ni3N/Ni cocatalyst and an HfO2 interlayer follows a metal–insulator–semiconductor mechanism with thicker HfO2 films proving detrimental to the PEC performance. The Si photocathode passivated with a 1 nm HfO2 layer exhibits an enhancement in the onset potential by 100 mV, an applied-bias photon-to-current efficiency of 9%, and improved operational stability. This work provides insights into the application of HfO2 as a passivating layer for Si photoelectrodes for solar hydrogen production.

Abstract Transparent electronics are rapidly evolving with the development of transparent conducting oxides (TCOs). This work investigates both the electrical and optical properties of n‐type tin oxide (SnO x ) film, deposited at various levels of oxygen concentration using magnetron sputtering. The band alignment at InP–SnO x interface is further studied and SnO x deposited at various conditions is used to demonstrate InP nanowire (NW) light emitting diodes (LEDs) to understand the trade‐off between absorption and resistance. It is found that the device with lower resistance but higher absorption outperforms in terms of output power. Emission from the devices consists of two peaks at room temperature due to conduction band‐to‐heavy hole band transition and conduction band‐to‐light hole band transition. Decreasing the operating temperature brings about quite a complex transition in all the devices, which consists of two additional peaks due to Zn acceptor level and zincblende/wurtzite (ZB/WZ) recombination at NW/substrate interface. At temperatures below 208 K, the emission peak from conduction band‐to‐light hole band transition quenches, however the emission peaks from Zn acceptor level and ZB/WZ recombination become prominent. This work lays the foundation for realizing a new generation of efficient transparent electrodes in conjunction with NWs, eliminating the complex epitaxial growth process optimization of NW shell growth, heterostructure and doping.

Solar energy conversion devices with charge-selective contacts are attracting significant research interest as a cost-effective alternative to homojunction counterparts. This study presents a novel approach for fabricating high-performance solar cells based on InP heterojunctions using a solution-processed ferri-hydrite (Fh) electron-selective contact (ESC). The champion cell efficiency of 16.6% is achieved, which is a significant improvement over those from previous studies using other solution-processed ESC materials. X-ray photoelectron spectroscopy measurements showed that the low conduction band offset at the Fh–InP interface facilitated selective transport of photogenerated electrons from InP. Moreover, the Fh electron-selective contact layer provided an excellent photoelectrochemical half-cell water reduction efficiency of 8.4%. The Fh layer not only selectively extracts photogenerated electrons from InP but also simultaneously serves as a surface protection layer, improving the cell’s long-term stability. These results demonstrate the potential of Fh as a low-cost and easily fabricated material for use in high-efficiency photovoltaic and photoelectrochemical devices. Our findings pave the way for further improvements in the efficiency of InP heterojunction solar cells by addressing the losses incurred in the cells.

Recognized for its high calorific value, clean and non-polluting properties, hydrogen is considered to be one of the most attractive solutions to the impending energy crisis. Photocatalytic water splitting has been widely investigated for its promise to produce hydrogen in a sustainable, environmentally friendly and low-cost approach. In pursuit of high photocatalytic efficiency, multiple strategies haven been explored such as doping, co-catalyst modification and heterojunction construction realizing promising improvements. We synthesized CdS doped with Zn ions and modified with MoS2 as co-catalyst to form ZnxCd1−xS solid solution and MoS2-Zn0.25Cd0.75S heterojunction using a simple one-step hydrothermal method. The synthesized catalyst was used water splitting under visible light irradiation. The structural and morphological characteristics of these materials, along with their photocatalytic mechanisms, were investigated using multiple techniques such as SEM, TEM, XPS, PL, etc. The photocatalyst consisting of 0.9 wt.% MoS2-Zn0.25Cd0.75S achieved a production rate of 6276 μmol h−1 g−1, which is 17.4 times higher than that of pure CdS and 1.6 times higher than that of Zn0.25Cd0.75S. The composite also demonstrated a high apparent quantum yield of 11.1 % at 420 nm. The mechanism of the water splitting process, revealed through various techniques, introduces novel pathways for crafting high-efficiency photocatalysts with an internal electron-hole separation heterostructure.

Abstract Crucial advancements in versatile catalyst systems capable of achieving high current densities under industrial conditions, bridging the gap between fundamental understanding and practical applications, are pivotal to propel the hydrogen economy forward. In this study, vertically oriented hierarchically multiscale nanoflakes of NiFeCo electrocatalysts are presented, developed by surface modification of a porous substrate with nano‐structured nickel. The resulting electrodes achieve remarkably low overpotentials of 139 mV at 10 mAcm −2 and 248 mV at 500 mAcm −2 . Further, scaled‐up electrodes are implemented in a water‐splitting electrolyser device exhibiting a stable voltage of 1.82 V to deliver a constant current density of 500 mA cm −2 for over 17 days. Moreover, the role of the unique structures on electrochemical activity is systematically investigated by fractal analysis, involving computation of structure factors such as Minkowski connectivity, fractal dimension, and porosity using scanning electron microscope images. It is found that such structures offer higher surface area than typical layered double hydroxide structures due to morphological coherence that results in a superhydrophilic surface, while the base Ni layer boosts the charge transfer. This study demonstrates a Ni/NiFeCo(OH) x heterostructure with highly porous morphology, a key to unlocking extremely efficient oxygen evolution reaction activity with exceptional stability. Moreover, fractal analysis is presented as a valuable tool to evaluate the electrochemical performance of catalysts for their structured morphology.

Silicon (Si) has been widely investigated as a feasible material for photoelectrochemical (PEC) water splitting. Compared to thick wafer-based Si, thin Si (<50 μm thickness) could concurrently minimize the material usage allowing the development of cost-effective and flexible photoelectrodes for integrable PEC cells. This work presents the design and fabrication of thin Si using crack-assisted layer exfoliation method through detailed optical simulations and a systematic investigation of the exfoliation method. Thin free-standing Si photoanodes with sub-50 μm thickness are demonstrated by incorporating a nickel oxide (NiOx) thin film as oxygen evolution catalyst, light-trapping surface structure, and a rear-pn+ junction, to generate a photo-current density of 23.43 mA/cm2 with an onset potential of 1.2 V (vs. RHE). Our work offers a general approach for the development of efficient and cost-effective photoelectrodes with Si films with important implications for flexible and wearable Si-based photovoltaics and (opto)electronic devices.

Crystalline silicon (c-Si) solar cells using interdigitated back contact (IBC) configurations are one of the most promising candidates to reach the practical efficiency limits of c-Si solar cells. However, the complexity of the process flow hinders the mass production of the IBC cells with conventional doped regions. One of the simple fabrication methods is to introduce the dopant-free carrier-selective contacts, which utilizes the fabrication processes with low temperature, e.g., the thermal evaporation or the spin coating. In this paper, we investigated efficiency close to 20% silicon IBC solar cells with dopant-free asymmetric hetero-contacts. In this solar cell configuration, the high work function material MoOx was chosen as the hole transporting layer, while the low work function material LiF was chosen as the electron transporting layer, respectively. The simulation results indicate that the perspective efficiency exceeding 22% for this type of cells is achievable with the optimized pitch width and improved passivation quality of the contacts, which has a great potential for the industrialization of IBC solar cells with simple fabrication processes.

Si photocathode with industrially relevant charge selective passivation and physically deposited earth-abundant catalyst is developed with an efficiency above 10%. Solar-to-hydrogen efficiency of 17% is achieved by combining perovskite PV in tandem.

Thin semiconductors attract huge interest due to their cost-effective, flexible, lightweight, and semi-transparent properties. Here, we present a protocol on the preparation of thin semiconductor via controlled crack-assisted layer exfoliation technique. The protocol details the fabrication procedure for producing thin monocrystalline semiconductors with thicknesses in the range of a few tens of micrometers from thick donor substrates. In addition, we describe proof-of-concept application of the thin semiconductors for photoelectrochemical water-splitting to produce hydrogen fuel. For complete details on the use and execution of this protocol, please refer to Lee et al. (2021).

Atomic layer deposition (ALD) technique shows superior application in the fabrication of TiO 2 inverse opals (IO), compared with conventional infiltration methods. In the present report, TiO 2 IO structures were infiltrated by ALD method in a continuous-flow and internally developed stop-flow process, respectively. The corresponding optical and optoelectrical properties of TiO 2 IO structures were investigated. The prepared uniform IO structure of 288 nm was used as a photoanode for dye-sensitized solar cells. An efficiency of 2.22% was achieved, which was much higher than that of prepared by conventional solution-infiltration method. It is indicated that ALD method is an effective approach for fabricating TiO 2 IO photoanode.

A nanoarchitectured photoelectrode based on TiO2 inverse opals and CdS quantum dot sensitization is used for solar hydrogen generation. The 3D percolated periodical pore structure of TiO2 inverse opal provides a high surface area for sensitizer loading plus a good electrical transport path and intimate contact with the electrolyte, all of which contribute to the photoelectrochemical performance. The inverse opals provide a unique platform for further hybridization with other photoactive nanostructures and enhancement of light manipulation.

Nanomaterials that are enclosed by high index, reactive facets exhibit a significantly enhanced chemical reactivity, leading to superior performances in electronics, photonics, energy conversion and storage, as well as interfacing with living cells for various biological purposes. This article investigates the formation of single crystalline rare earth hydroxide nanobelts with selective control of high energy surfaces. Combining experimental and theoretical efforts, a carboxylic group and carbon–carbon double bond of oleic acid molecules are found to reduce the (0001) surface energy by 60%, leading to the growth of nanobelts and the stabilization of their high energy surfaces. These findings shed light on the growth of one-dimensional nanostructures that exhibit a simultaneous control over the crystallographic facets, shapes, and sizes. Our results engender a versatile synthesis method for nanomaterials that exhibit enhanced physical and chemical properties by overcoming their energy barrier to grow with high energy surfaces.

In article number 2000772, Siva Krishna Karuturi, Heping Shen and co-workers report a perovskite/Si dual absorber tandem cell for stand-alone solar water splitting. An unprecedented over 17% solar-to hydrogen conversion efficiency is achieved when a Si photocathode is paired in tandem with a high bandgap (≈1.75 eV) semitransparent perovskite solar cell.

Three-dimensional (3D) ordered macroporous (such as inverse opal) heterostructure materials are attractive for photocatalysis because of their interconnected pores, high surface area, light-harvesting properties, and favorable charge-transfer properties. In this work, we report the preparation of TiO2–TaOxNy heterostructure inverse opals using atomic layer deposition and investigate their photoelectrochemical performance. Through ultraviolet photoelectron spectroscopy analyses of the band alignment of TiO2 and TaOxNy, we confirm that a type II heterojunction is formed. The deposition temperature of TaOxNy is found to play an important role in achieving homogeneous infiltration, leading to a uniform TiO2/TaOxNy heterostructure. The TiO2–TaOxNy photoanode achieved a twofold increase in photocurrent density, a lower onset potential, and improved stability in an alkaline electrolyte; overcoming the drawbacks of standalone TiO2 and TaOxNy. These improvements can be attributed to the appropriate type II band alignment, which generates a large built-in electric field, thus promoting charge carrier separation, reducing the accumulation of photogenerated carriers at the semiconductor/electrolyte interface, and improving minority carrier transport.

Nanostructured materials are crucial to the light harvesting and power conversion efficiency of solar energy conversion systems. The template-assisted fabrication method offers a versatile route to produce nanostructured materials of controlled morphology and optoelectronic properties. Toward this, self-assembled opal colloidal crystals and nanoporous anodic aluminum oxide have been used as templates to produce nanostructures of a wide range of materials with different dimensionalities and enhanced photocatalytic properties. This chapter introduces various approaches developed for producing the templates and methods of infiltrating them with the desired photoactive materials. This chapter also critically assesses the advantages and limitations of the proposed methods, and summarizes the beneficial outcomes realized using nanostructures developed through the template-assisted route for photocatalysis.

Nanostructured III-V semiconductors are attractive for solar energy conversion applications owing to their excellent light harvesting and optoelectronic properties. Here, we present a protocol for scalable fabrication of III-V semiconductor nanopillars using a simple and cost-effective top-down approach, combining self-assembled random mask and plasma etching techniques. We describe the deposition of Au/SiO2 layers to prepare random etch mask. We then detail the fabrication of nanopillars and photocathodes. Finally, we demonstrate III-V semiconductor nanopillars as a photoelectrode for photoelectrochemical water splitting. For complete details on the use and execution of this protocol, please refer to Narangari et al. (2021).1.

Abstract Ternary metal (hydro)oxide amorphous catalysts are attractive oxygen evolution reaction (OER) catalysts due to their high performance and cost-effectiveness, but a fundamental understanding of their structure-property relationships remains elusive. Herein, we fabricate a highly active ternary metal (hydro)oxide (NiFeCo) OER catalyst, showing an overpotential of 146 mV at 10 mAcm-2 and ~300 hours durability in 1M KOH. Inspired by this finding, a dataset with first-principles adsorption energies of reaction intermediates at over 300 active-site structures for both oxides and hydroxides is computed and used to train a machine-learning model for screening the dominating factors and unveiling their intrinsic contributions. The computational work confirms that adding Fe and Co makes the original Ni (hydro)oxide reach ultra-low overpotentials below 200 mV through the modulation from hydroxide towards oxide and the formation of active-sites of ternary metallic components. A series of physical properties of the Fe, Co and Ni atoms in the active-sites are further included in the analysis, and it is found that the magnetic moment (mag) plays an important role in the OER activity. This work demonstrates the application of machine-learning methods in atomic-scale active-site engineering to understand the activity mechanism of ternary metal (hydro)oxide amorphous catalysts for water oxidation, and it has the potential to be extended to wider applications.

Large-area epitaxial growth of III-V nanowires and thin films on van der Waals substrates is key to developing flexible optoelectronic devices. In our study, large-area InAs nanowires and planar structures are grown on hexagonal boron nitride templates using metal organic chemical vapor deposition method without any catalyst or pre-treatments. The effect of basic growth parameters on nanowire yield and thin film morphology is investigated. Under optimised growth conditions, a high nanowire density of 2.1×109cm-2is achieved. A novel growth strategy to achieve uniform InAs thin film on h-BN/SiO2/Si substrate is introduced. The approach involves controlling the growth process to suppress the nucleation and growth of InAs nanowires, while promoting the radial growth of nano-islands formed on the h-BN surface. A uniform polycrystalline InAs thin film is thus obtained over a large area with a dominant zinc-blende phase. The film exhibits near-band-edge emission at room temperature and a relatively high Hall mobility of 399 cm-2/(Vs). This work suggests a promising path for the direct growth of large-area, low-temperature III-V thin films on van der Waals substrates.

Mimicking natural photosynthesis, semiconductor photoelectrocatalysis (PEC) is a promising approach to directly convert and store renewable solar energy in the form of chemical bonds. Solar water splitting to produce hydrogen fuel is one of the most-studied examples for PEC. This chapter covers the fundamentals of three key steps of PEC: light absorption, charge separation and chemical catalysis, with an emphasis on the selection criteria of favorable material properties for semiconductor light-absorbers and electrocatalysts. Judicious integration of semiconductor and catalyst with desired energetics is key to optimizing both energy efficiency and operational stability obtained at solid/liquid interface. Prospects of integrated PEC devices for practical applications are also discussed based on recent developments.

Carrier‐selective contacts offer promising opportunities for solar cells. By alleviating the need for p–n junctions and acting as passivation layers, they significantly simplify the device design and fabrication. Herein, this strategy is applied to a narrow‐bandgap (≈0.91 eV) InGaAsP solar cell. Such a solar cell, lattice‐matched to InP, possesses a bandgap ideal for the bottom subcell of a tandem cell. It is shown that TiO 2 forms an electron‐selective contact to InGaAsP. The TiO 2 /InGaAsP solar cell exhibits a short‐circuit current density of 35.2 mA cm −2 , an open‐circuit voltage of 0.49 V, and an efficiency of 8.9%. The cell J – V characteristics and quantum efficiency highlight the beneficial aspect of TiO 2 as a passivating layer for InGaAsP. The reduced open‐circuit voltage and lower response at longer wavelengths, on the other hand, indicate that the quaternary alloy material quality could be further improved to increase the carrier diffusion length. Nevertheless, the performance of this simplified electron‐selective contact solar cell structure is comparable to conventional p–n junction 1 eV InGaAsP solar cells reported in the literature, highlighting the promise toward lower‐cost photovoltaic tandem cells.

When searching for renewable sources of energy that could remediate the energy and environmental crises faced by the society, photoelectrochemical systems are emerging as promising candidates. Using solar radiation to produce hydrogen and other value-added fuels, these systems have demonstrated encouraging laboratory-scale results but are still far from practical applications. In this view, 2D materials present interesting morphological, optical, and electrical properties that could help lower the balance of the costs of photoelectrochemical systems. These can play multiple roles such as cocatalysts, sensitizers, or protective layers. Moreover, the heterojunctions formed with other (dimensional) materials can lead to significant enhancement of the charge separation, transport, transfer, and collection processes. These synergistic effects enable an increased overall photoelectrochemical performance and stability. In this chapter, we review these advantages in detail, providing examples for each role and type of heterostructure.

Abstract Ferrihydrite (Fh) has been demonstrated as an effective interfacial layer for photoanodes to achieve outstanding photoelectrochemical (PEC) performance for water oxidation reaction owing to its unique hole‐storage function. However, it is unknown whether such a hole‐storage layer can be used to construct highly efficient photocathodes for hydrogen evolution reaction (HER). In this work, we report Fh interfacial engineering of amorphous silicon photocathode (with nickel as HER cocatalyst) achieving a photocurrent density of 15.6 mA cm −2 at 0 V vs. the reversible hydrogen electrode and a half‐cell energy conversion efficiency of 4.08 % in alkaline solution, outperforming most of reported a‐Si based photocathodes including multi‐junction configurations integrated with noble metal cocatalysts in acid solution. Besides, the photocurrent density is maintained above 14 mA cm −2 for 175 min with 100 % Faradaic efficiency for HER in alkaline solution. Our results demonstrate a feasible approach to construct efficient photocathodes via the application of a hole‐storage layer.

Two-dimensional (2D) materials have received a considerable interest in photocatalysis owing to their unique electrochemical and opto-electronic properties. These materials have been used in various catalytic energy conversion reactions such as water splitting, oxygen reduction reaction, nitrogen reduction reaction, and CO 2 reduction while exhibiting unique catalytic properties. Moreover, 2D materials can be integrated with other 2D materials or 0D, 1D, and 3D materials to form heterostructure to synergistically enhance their electrochemical and optoelectronic properties. Here, we highlight the important characteristics of 2D materials heterostructures and discuss their fabrication methods and applications in photocatalytic redox reactions. We also provide our perspective on future research directions of these heterostructures and the challenges in the implementation of practical applications.

A hetero-nanostructured photoanode with enhanced near-infrared light harvesting is developed for photoelectrochemical cells by Alfred Iing Yoong Tok, Xiaogang Liu and co-workers on page 1603. The photoanode comprises upconversion nanoparticles on the surface of titanium oxide inverse opal. A nanoshell of titanium oxide is deposited on the nanoparticles, followed by CdSe quantum dots. Upon near infrared excitation, upconversion nanoparticles emit visible light that excites quantum dots for charge separation. Electrons are injected into titanium oxide with minimal carrier losses. This heteronanostructure is envisioned to be applicable to thin film, liquid-junction, and organic solar cells where near-infrared light can be harvested for improved photon-to-electricity conversion efficiency.

Atomic layer deposition (ALD) technique shows superior application in the fabrication of TiO2 inverse opals (IO), compared with conventional infiltration methods. In the present report, TiO2 IO structures were infiltrated by ALD method in a continuous-flow and internally developed stop-flow process, respectively. The corresponding optical and optoelectrical properties of TiO2 IO structures were investigated. The prepared uniform IO structure of 288 nm was used as a photoanode for dye-sensitized solar cells. An efficiency of 2.22% was achieved, which was much higher than that of prepared by conventional solution-infiltration method. It is indicated that ALD method is an effective approach for fabricating TiO2 IO photoanode.

Dielectric nanoantennas have emerged in recent years as a promising platform for nanoscale second-harmonic generation (SHG) light sources and as building blocks for SHG metasurfaces. The group of III-V semiconductor materials with zincblende (ZB) crystal structure has played a key role in this development since it contains materials that feature high refractive indices and low losses in the near infrared (NIR), and strong second-order nonlinearities owing to the broken inversion symmetry in these crystals. However, one drawback of these materials is the peculiar nature of the second-order nonlinear susceptibility χ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ijk</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(2)</sup> where i, j and k relate to the major crystalline axes [100], [010] and [001]. Its components are only nonzero for i ≠ j ≠ k ≠ i. This commonly leads to "doughnut-shaped" radiation patterns with zero power radiated along the optical axis for SHG nanocylinders fabricated from (100) wafers, where the crystal axes align with the laboratory frame defined by the nanocylinder orientation [1,2]. In order to attain higher directivity along the optical axis and hence improving collection efficiency, the system's symmetry has to be reduced [2,3].

Research into hydrogen generation via photoelectrochemical (PEC) water splitting is ever growing owing to its potential to generate clean and portable form of energy. In PEC water splitting, a semiconductor absorbs sunlight and splits water into hydrogen and oxygen to produce hydrogen fuel. 1, 2 However, the lack of a single robust semiconductor material with a narrow bandgap that straddles both water redox potentials is the main barrier to develop practical PEC water splitting systems. III-V semiconductors can make ideal materials for PEC water splitting due to their outstanding optoelectronic properties including tunable band gaps to cover the entire solar spectrum, band edges straddling water redox potentials, high absorption coefficients and high crystalline quality. In addition, III-V photoelectrodes based on one-dimensional nanostructures are shown to greatly enhance the PEC performance from improved light absorption, increased semiconductor/electrolyte interface area and reduced carrier diffusion length over their planar counterparts. In this talk, I will introduce cost-effective and scalable fabrication of random semiconductor nanopillars (NPs) with controlled dimensions developed in our group using ICP etching and self-assembled random mask techniques for PEC water splitting (as illustrated in Figure 1). I will also present our results on the investigation of GaN, InGaN quantum wells and InP nanopillars (NPs), fabricated using top-down method, as photoelectrodes for PEC water splitting application. The band gap tunability with varying In content and high chemical stability makes In x Ga 1-x N a superior candidate for photoelectrodes. The PEC performance of random GaN NPs photoanodes was tested in 1 M NaOH under simulated one sun illumination. NPs generated the highest ever reported photocurrent density for GaN photoanodes due to exceptional absorption by the NPs, and increased depletion layer area and semiconductor/electrolyte interface area. Moreover, the PEC performance of NPs was found to be strongly influenced by carrier concentration and NP dimensions. 3 We further engineered the band gap of GaN NPs by incorporating InGaN/GaN multiple quantum wells (MQWs) to further improve the PEC performance of GaN NPs. Photoluminescence and diffuse reflectance measurements confirmed that the introduction of InGaN/GaN MQWs into GaN extended the optical absorption by the NPs into the visible part of the solar spectrum, thereby contributing to the substantial improvement in photocurrent density for InGaN/GaN MQW NPs. 4 Furthermore, GaN NP photoanodes were found to exhibit improved photostability after being decorated with co-catalysts such as Co 3 O 4 . The accumulation of photogenerated charge carriers at semiconductor surface trigger the self-oxidation of GaN photoanode during water oxidation reaction and results in photocorossion of GaN photoanodes. Deposition of co-catalyst significantly reduced the self-oxidation of GaN photoanode by rapidly extracting the photogenerated holes out of semiconductor that participate in water oxidation reaction. 5 We also developed InP NPs using the random mask technique via top-down approach, removed plasma surface damage using wet treatment in sulphur dissolved oleylamine (S-OA) solution and investigated the PEC performance of InP NP photocathodes. We achieved stable and excellent PEC performance for NP photocathodes without any oxide protection layer after wet treatment in S-OA. The NPs exhibited saturation photocurrent density of ~34 mA/cm 2 , which is close to theoretical limit and power-saved cathodic efficiency of over 5%. The saturation photocurrent density and power-saved cathodic efficiency of NPs improved by 60% and 33%, respectively, compared to their counterpart planar photocathodes. We also carried out the time-resolved photoluminescence, optical and impedance spectroscopy characterisations to elucidate the PEC performance of InP photocathodes. References Fujishima, K. Honda, “ Electrochemical Photolysis of Water at a Semiconductor Electrode ”, Nature 1972, 238, 37–38. Walter, E. Warren, J. McKone, S. Boettcher, Q. Mi, E. Santori and N. S. Lewis, “ Solar Water Splitting Cells ”, Chemical Society Reviews 2010, 110, 6446–6473. Parvathala Reddy Narangari, Siva Krishna Karuturi, Mykhaylo Lysevych, Hark Hoe Tan, and Chennupati Jagadish, “Improved Photoelectrochemical Performance of GaN Nanopillar Photoanodes” Nanotechnology 2017, 28, 154001. Joshua Butson, Parvathala Reddy Narangari, Siva Krishna Karuturi, Rowena Yew, Mykhaylo Lysevych, Hark Hoe Tan, and Chennupati Jagadish, “ Photoelectrochemical Studies of InGaN/GaN MQW Photoanodes ”, Nanotechnology 2018, 29 045403. Guanyu Liu and Siva Krishna Karuturi, Alexandr N. Simonov, Monika Fekete, Hongjun Chen, Noushin Nasiri, Nhien H. Le, N Parvathala Reddy, Mykhaylo Lysevych, Thomas R. Gengenbach, Adrian Lowe, Hark Hoe Tan, Chennupati Jagadish, Leone Spicciac and Antonio Tricoli, “ Robust Sub-Monolayers of Co 3 O 4 Nano-Islands: a Highly Transparent Morphology for Efficient Water Oxidation Catalysis ”, Advanced Energy Materials 2016, 6, 1600697. Figure 1

InP is currently being used in various (opto)­electronic and energy device applications. However, the high cost of InP substrates and associated epitaxial growth techniques has been huge impediments for its widespread use. Here, large-area monocrystalline InP thin films are demonstrated via a convenient cracking method, and the InP thin films show material properties identical to their bulk counterparts. Furthermore, the same substrate can be reused for the production of additional InP thin films. This cracking technique is also shown to be a versatile tool to form an ultrasmooth surface or a microscale periodic triangular grating structure on the surface, depending on the orientation of the donor substrate used. Strain-induced band gap energy shift is also observed in localized regions of the thin film with a grating structure. The simplicity of this technique, which does not require any sophisticated equipment and complex fabrication process, is promising to reduce the cost of InP thin-film devices.