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Lead halide perovskite solar cells (PSCs) have been rapidly developed in the past decade. Owing to its excellent power conversion efficiency with robust and low-cost fabrication, perovskite quickly becomes one of the most promising candidates for the next-generation photovoltaic technology. With the development of PSCs, the interface engineering has witnessed its increasingly critical role in maximizing the device performance as well as the long-term stability, because the interfaces in PSCs are closely correlated with the defect management, carrier dynamics and surface passivation. This review focuses on interfacial modification between the perovskite active layer and the charge transport layer, as well as the recent advances on high-efficiency and stable PSCs driven by interface engineering strategies. The contributing roles of interface engineering in terms of defect passivation, inhibiting ion migration, optimization of energy band alignment and morphological control are discussed. Finally, based on the latest progress and advances, strategies and opportunities for the future research on interface engineering for PSCs are proposed to promote the development of perovskite photovoltaic technology.

Atomic catalysts have demonstrated great potential for the rational design of new heterogeneous catalysts with excellent activity, stability, and selectivity for various industrial reactions. However, knowing how to synthesize atomic catalysts in a low‐cost, high‐yield, and environment‐friendly manner remains highly challenging. Molten‐salt‐assisted material synthesis as an old chemical preparation technology that has recently been deliberately modified to meet the requirements of atomic catalyst design. Here we provide a critical review of the recent progress of the molten‐salt‐assisted synthesis of atomic catalysts. The property and working mechanisms of the molten‐salt‐assisted method are firstly introduced, followed by some typical examples to demonstrate the precise atomic catalyst control. In the end, perspectives in this field are briefly discussed. This review is expected to raise attention for applying classic molten salt synthesis in the design of new generations of advanced materials.

Lead (Pb) halide perovskites have witnessed highly promising achievements for high-efficiency solar cells, light-emitting diodes (LEDs), and photo/radiation detectors due to their exceptional optoelectronic properties. However, compound stability and Pb toxicity are still two main obstacles towards the commercialization of halide perovskite-based devices. Therefore, it is of substantial interest to search for non-toxic candidates with comparable photophysical characteristics. Metal-halide double perovskites (MHDPs), A2BBʹX6, are recently booming as promising alternatives for Pb-based halide-perovskites for their non-toxicity and significantly enhanced chemical and thermodynamic stability. Moreover, this family exhibits rich combinatorial chemistry with tuneable optoelectronic properties and thus a great potential for a broad range of optoelectronic/electronic applications. Herein, we present a comprehensive review of the MHDPs synthesized so far, and classified by their optical and electronic properties. We systematically generalize their electronic structure by both theoretical and experimental efforts to prospect the relevant optoelectronic properties required by different applications. The progress of the materials in various applications is explicated in view of the material structure-function relationship. Finally, a perspective outlook to improve the physical and optoelectronic properties of the materials is proposed aiming at fostering their future development and applications.

Atomic co-catalysts offer high potential to improve the photocatalytic performance, of which the preparation with earth-abundant elements is challenging. Here, a new molten salt method (MSM) is designed to prepare atomic Ni co-catalyst on widely studied TiO2 nanoparticles. The liquid environment and space confinement effect of the molten salt leads to atomic dispersion of Ni ions on TiO2 , while the strong polarizing force provided by the molten salt promotes formation of strong Ni-O bonds. Interestingly, Ni atoms are found to facilitate the formation of oxygen vacancies (OV) on TiO2 during the MSM process, which benefits the charge transfer and hydrogen evolution reaction. The synergy of atomic Ni co-catalyst and OV results in 4-time increase in H2 evolution rate compared to that of the Ni co-catalyst on TiO2 prepared by an impregnation method. This work provides a new strategy of controlling atomic co-catalyst together with defects for efficient photocatalytic water splitting.

Rational design of photocatalysts is essential to achieve efficient solar energy conversion. For narrow bandgap Ta3N5 photocatalyst, various charge recombination occurring in the bulk, interface and on the surface significantly impairs its activity for solar hydrogen (H2) generation. Herein, a synergistic engineering approach is designed to solve this critical recombination challenge. First, hollow spherical structure of Ta3N5 with Mg doping is prepared to not only reduce the charge migration distance and increase the surface area, but also increase the electron mobility for facilitated charge transfer. Second, an MgO nano-layer covers the surface of hollow Ta3N5 structure to passivate surface defects, thus promoting the interfacial charge transfer between Ta3N5 and co-catalysts. Finally, dual co-catalysts (Pt/CoOx) for redox reactions are loaded onto the hollow Ta3N5 structure to reduce the surface recombination and overcome the sluggish surface reaction. Remarkably, the combination of hollow structure, Mg2+ doping, MgO interfacial layer, and dual co-catalysts effectively improves the charge separation and transfer in Ta3N5 photocatalyst. This newly designed photocatalyst exhibits a considerably improved H2 generation performance of 56.3 μmol h−1 under simulated sunlight, compared to that of reference Pt/Ta3N5 hollow spheres.

To fulfil the potential of Li-S batteries (LSBs) with high energy density and low cost, multiple challenges need to be addressed simultaneously. Most research in LSBs have been focusing on sulfur cathode design, while the performance are also known to be sensitive to other parameters such as binder, current collector, separator, lithium anode, and electrolyte. Here, an integrated LSB system based on the understanding of the different roles of binder, current collector, and separator is developed. By using the cross-linked carboxymethyl cellulose-citric acid (CMC-CA) binder, Toray carbon paper current collector, and reduced graphene oxide (rGO) coated separator, LSBs achieve a high capacity of 960 mAh g-1 after 200 cycles (2.5 mg cm-2) and 930 mAh g-1 after 50 cycles (5 mg cm-2) at 0.1C. Moreover, the failure mechanism at a high sulfur loading with character of fast capacity decay and infinite charging are discussed. This work highlights the synergistic effect of different components and the challenges towards more reliable LSBs of high sulfur loading.

Photocatalysis, which utilizes solar energy to trigger chemical reactions, is one of the most desirable solar-energy-conversion approaches. Graphitic carbon nitride (g-C3N4), as an attractive metal-free photocatalyst, has drawn worldwide research interest in the area of solar energy conversion due to its easy synthesis, earth-abundant nature, physicochemical stability and visible-light-responsive properties. Over the past ten years, g-C3N4 based photocatalysts have experienced intensive exploration, and great progress has been achieved. However, the solar conversion efficiency is still far from industrial applications due to the wide bandgap, severe charge recombination, and lack of surface active sites. Many strategies have been proposed to enhance the light absorption, reduce the recombination of charge carriers and accelerate the surface kinetics. This work makes a crucial review about the main contributions of various strategies to the light harvesting, charge separation and surface kinetics of g-C3N4 photocatalyst. Furthermore, the evaluation measurements for the enhanced light harvesting, reduced charge recombination and accelerated surface kinetics will be discussed. In addition, this review proposes future trends to enhance the photocatalytic performance of g-C3N4 photocatalyst for the solar energy conversion. (C) 2018 Published by Elsevier B.V. and Science Press.

A noble-metal-free heterostructure composed of Ta3N5 nanosheets and MoS2 co-catalyst was developed as an efficient photocatalyst for hydrogen production. In special, the MoS2/Ta3N5 heterostructure with 5.2 wt% of MoS2 exhibits a H2 generation rate of 119.4 µmol h−1 g−1, which is comparable to that of Ta3N5 nanosheets with Pt as co-catalyst. The efficient H2 generation for MoS2/Ta3N5 heterostructure is resulted from the short diffusion distance of photo-excited charge carriers in Ta3N5 nanosheets, efficient transfer of electrons from Ta3N5 nanosheets to MoS2 surface, and reduced overpotential for water reduction. Furthermore, the stability of the MoS2/Ta3N5 heterostructured photocatalysts are evaluated for the photocatalytic H2 generation. Keywords: MoS2, Ta3N5 nanosheets, Heterostructure, Photocatalysis, Hydrogen evolution

Two-dimensional (2D) layered metal dichalcogenides (LMDs) with semiconducting character have been attracting increasing attention in both fundamental studies and various applications. In the case of electrode material design for energy storage, integrating metal dichalcogenides with other conductive phases such as nanocarbons has been widely recognised as an efficient way to simultaneously achieve good electrochemical activity and conductivity. However, controllable growth of metal dichalcogenides on nanocarbons with well-defined structure and efficient interfacial contact is still highly challenging. In this work, we report a new class of SnS2 nanosheets with distinct growth orientations on the surfaces of reduced graphene oxide (RGO) and carbon nanotubes (CNTs). We further demonstrate a spatial confinement strategy to in situ grow SnS2 nanoparticles, which are homogeneously confined within RGO or CNT based porous carbon matrices. Consequently, these resultant 3D architectures demonstrate outstanding rate capability and cycling stability due to their synergistic effect of electrochemically active SnS2 particles and highly conductive carbon matrixes. In particular, the free-standing CNT sponge based composite delivers specific capacities of 741 and 462 mA h g−1 at 3200 mA g−1 for Li+ and Na+ storage, respectively, among the best values reported for both lithium ion battery (LIB) and sodium ion battery (NIB) systems. This work not only provides better understanding of the growth mechanisms of metal dichalcogenides on nanocarbons but also opens a new way to construct unique 2D heterostructures for various applications.

Photocatalytic water splitting, which directly converts solar energy into hydrogen, is one of the most desirable solar-energy-conversion approaches. The ultimate target of photocatalysis is to explore efficient and stable photocatalysts for solar water splitting. Tantalum (oxy)nitride-based materials are a class of the most promising photocatalysts for solar water splitting because of their narrow bandgaps and sufficient band energy potentials for water splitting. Tantalum (oxy)nitride-based photocatalysts have experienced intensive exploration, and encouraging progress has been achieved over the past years. However, the solar-to-hydrogen (STH) conversion efficiency is still very far from its theoretical value. The question of how to better design these materials in order to further improve their water-splitting capability is of interest and importance. This review summarizes the development of tantalum (oxy)nitride-based photocatalysts for solar water spitting. Special interest is paid to important strategies for improving photocatalytic water-splitting efficiency. This paper also proposes future trends to explore in the research area of tantalum-based narrow bandgap photocatalysts for solar water splitting.

Photocatalysis has attracted much attention in recent years due to its potential in solving energy and environmental issues. Even though numerous achievements have been made, the photocatalytic systems developed to date are still far from practical applications due to the low efficiency and poor durability. Efficient light absorption and charge separation are two of the key factors for the exploration of high performance photocatalytic systems, which is generally difficult to be obtained in a single photocatalyst. The combination of various materials to form heterojunctions provides an effective way to better harvest solar energy and to facilitate charge separation and transfer, thus enhancing the photocatalytic activity and stability. This review concisely summarizes the recent development of visible light responsive heterojunctions, including the preparation and performances of semiconductor/semiconductor junctions, semiconductor/cocatalyst junctions, semiconductor/metal junctions, semiconductor/non-metal junctions, and surface heterojunctions, and their mechanism for enhanced light harvesting and charge separation/transfer.

Lead-free double perovskite Cs2AgBiBr6 has attracted increasing research interest in addressing the toxicity and stability challenges confronted by lead halide perovskites. While most of the studies on this Cs2AgBiBr6 material have been focusing on photovoltaic performance and potential applications, its long-term stability and degradation mechanism are well under-explored. Herein, high-quality Cs2AgBiBr6 thin-films are developed for lead-free double perovskite solar cells with a decent efficiency of 1.91%. By exploring the ambient stability of these photovoltaic devices, it is found that the Cs2AgBiBr6 exhibits a unique dual-ion-migration phenomenon, where Ag and Br ions gradually diffuse through the hole-transporting layer in the long-term operation. This phenomenon leads to the degradation of the Cs2AgBiBr6 perovskite and subsequent device failure. Theoretical calculations indicate that low formation energies of the Ag and Br vacancies, and low diffusive energy barriers contribute to the dual-ion-migration effect. A possible mechanism involving a vacancy-mediated ion-migration is proposed to explain this phenomenon. These key findings are essential for halide double perovskites not only in providing a new knowledge base for further addressing the challenge of double perovskite stability, but also in extending their optoelectronic/electronic applications where mixed electronic, ionic and photonic properties may be desired.

Two-dimensional (2D) semiconductor materials have attracted much attention in solar energy utilization due to their unique structure and excellent photoelectrical properties. However, the band bending in the 2D materials-based photoelectrodes is limited when the thickness of 2D flakes are too thin, which normally leads to weak driving force for charge separation and transfer. Constructing heterojunction is an effective way to enhance the band bending by creating additional space charge layer in semiconductors. Herein we intentionally introduce a 2D SnS2 layer on SnO2 nanoflakes to design a new class of 2D heterojunction of SnS2/SnO2. The low lattice mismatch of two materials leads to intimate heterojunction contact with well-matched energy band alignment, which efficiently promote the charge carrier separation of SnS2 photoelectrodes. Moreover, the conductive SnO2 provides a good pathway for electron collection. Consequently, the photocurrent density of the SnS2/SnO2 heterojunction photoanode is largely improved by 2.8 times to 0.7 mA cm−2 at 1.23 V vs RHE compared to the pristine SnS2, which also shows significantly improved photocurrent density than others for wet-chemically prepared SnS2 based photoanodes. Considerable near infrared (NIR) light photoresponse is also observed in the SnS2/SnO2 heterojunction which is mainly induced by the defects.

Heterojunction design is an effective approach to provide built-in electric field for enhancing the charge separation and transfer in semiconductor photocatalysts, however the interface of the junction structures often plays a role of charge trap. Herein, a new class of twin junction photocatalysts with MnxCd1-xS solid solution interface is in-situ synthesized by the principle of the solid solubility limit. The appropriate band alignment and graded solid solution interface between Cd-rich and Mn-rich sulfides provide a gradient built-in electric field and a facilitated channel for the efficient charge separation and transfer. As a result, the obtained twin junction photocatalyst exhibits an excellent photocatalytic activity for hydrogen production in the absence of any cocatalyst. The hydrogen evolution rate of the photocatalyst is as high as 82.4 μmol h−1 with an apparent quantum yield of over 10% under visible-light irradiation, which is about 48 times higher than that of the pristine CdS. The design of twin junction photocatalyst may shed light on the rational construction of novel photocatalyst for efficient solar energy conversion.

Perovskite solar cells (PSCs) have demonstrated high power conversion efficiencies (PCEs) but poor stability against ultraviolet (UV) irradiation. Here, we report a one-pot synthesized luminescent europium-doped titania (Eu-TiO2) via chemical-bath deposition at low-temperature (70 °C) for planar PSCs, which enables simultaneous efficiency and UV-stability enhancement. We show that the Eu-TiO2 could effectively convert damaging UV photons into useful visible luminescence for additional light harvesting. A more optimal energy band alignment at the Eu-TiO2/perovskite interface leads to facilitated charge extraction and suppressed non-radiative recombination. The use of Eu-TiO2 in PSCs results in increased photocurrent and open-circuit voltage, yielding an enhanced PCE of 21.40% with relative to pristine TiO2 device (19.22%). More importantly, the Eu-TiO2 devices exhibit remarkably improved UV stability, retaining 75% of the initial PCE after exposing to UV illumination for 500 h, while the devices with pristine TiO2 lost the majority of their original PCEs in 150 h. As a proof-of-concept, we further demonstrate the scalability of our method by fabricating a large-area Eu-TiO2 film of 64 cm2 showing excellent uniformity.

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

Photocatalysis for solar-driven reactions promises a bright future in addressing energy and environmental challenges. The performance of photocatalysis is highly dependent on the design of photocatalysts, which can be rationally tailored to achieve efficient light harvesting, promoted charge separation and transport, and accelerated surface reactions. Due to its unique feature, semiconductors with hollow structure offer many advantages in photocatalyst design including improved light scattering and harvesting, reduced distance for charge migration and directed charge separation, and abundant surface reactive sites of the shells. Herein, the relationship between hollow nanostructures and their photocatalytic performance are discussed. The advantages of hollow nanostructures are summarized as: 1) enhancement in the light harvesting through light scattering and slow photon effects; 2) suppression of charge recombination by reducing charge transfer distance and directing separation of charge carriers; and 3) acceleration of the surface reactions by increasing accessible surface areas for separating the redox reactions spatially. Toward the end of the review, some insights into the key challenges and perspectives of hollow structured photocatalysts are also discussed, with a good hope to shed light on further promoting the rapid progress of this dynamic research field.

A new type of boron-doped graphitic carbon nitride (B-g-C3N4) nanosheets was prepared by a benign one-pot thermal polycondensation process. Systematic studies revealed that a B-doping amount of 1 at% into g-C3N4 (1at%B-g-C3N4) showed the best photocatalytic H2 evolution activity of 1880 μmol h−1 g−1 under visible light irradiation (>400 nm), which is more than 12 times that of the pristine g-C3N4 bulk. Detailed characterizations revealed that the high photocatalytic performance could be attributed to the combination of band structure engineering and morphological control. B-doping not only reduces the band gap to absorb more visible light but also exhibits a higher surface area of B-g-C3N4 (49.47 m2 g−1) as compared to that of g-C3N4 bulk (8.24 m2 g−1), which subsequently improve the photocatalytic performance drastically. This work demonstrates a synergistic strategy to prepare efficient metal-free B-g-C3N4 nanosheets as a promising photocatalyst for H2 evolution under visible light with good stability.

Tuning the bandgap of a semiconductor to achieve strong band-to-band visible light absorption is highly desirable but challenging for photocatalysis. This work presents a facile molten-salt-assisted route to prepare red-colored polymerized carbon nitride (RPCN) nanosheets with a remarkable redshifted absorption and narrowed bandgap of 1.9 eV. Both experimental findings and theoretical calculations reveal that alkali heteroatoms are effective to tune the surface and electronic structures of carbon nitride, resulting in significantly reduced bandgap and excellent solubility. The RPCN-sensitized TiO nanorod-based photoanode generates an impressive photocurrent density of ≈2.33 mA cm at 1.23 V versus reversible hydrogen electrode under Air Mass 1.5 G illumination without any cocatalyst, which is 2.6 folds higher than that of the bare TiO photoanode. The new findings in this work could inspire the electronic structure engineering of semiconductor photocatalysts to greatly enhance the visible light absorption and provide a generic strategy to enhance the photoelectrochemical performance of wide-bandgap semiconductor photoelectrodes.

Owing to the sluggish kinetics for water oxidation, severe surface charge recombination is a major energy loss that hinders efficient photoelectrochemical (PEC) water splitting. Herein, a simple process is developed for preparing a new type of low‐cost iron‐cobalt oxide (FeCoOx) as an efficient co‐catalyst to suppress the surface charge recombination on bismuth vanadate (BiVO4) photoanodes. The new FeCoOx/BiVO4 photoanode exhibits a high photocurrent density of 4.82 mA cm−2 at 1.23 V versus the reversible hydrogen electrode under AM 1.5 G illumination, which corresponds to >100% increase compared to that of the pristine BiVO4 photoanode. The photoanode also demonstrates a high charge separation efficiency of ≈90% with excellent stability of over 10 h, indicating the excellent catalytic performance of FeCoOx in the PEC process. Density functional theory calculations and experimental studies reveal that the incorporation of Fe into CoOx generates abundant oxygen vacancies and forms a p‐n heterojunction with BiVO4, which effectively promotes the hole transport/trapping from the BiVO4 photocatalyst and reduces the overpotential for oxygen evolution reaction (OER), resulting in remarkably increased photocurrent densities and durability. This work demonstrates a feasible process for depositing cheap FeCoOx as an excellent OER cocatalyst on photoanodes for PEC water splitting.

Tantalum nitride (Ta3N5) with a suitable bandgap (≈2 eV) is regarded as one of the most promising photocatalysts for efficient solar energy harvesting and conversion. However, Ta3N5 suffers from low hydrogen production activity due to the low carrier mobility and fast carrier recombination. Thus, the design of Ta3N5 nanostructures to facilitate charge carrier transport and improve photocatalytic performance remains a challenge. This study reports a new type of ultrathin (≈2 nm) Ta3N5 nanomesh with high specific surface area (284.6 m2 g−1) and excellent crystallinity by an innovative bottom-up graphene oxide templated strategy. The resulting Ta3N5 nanomeshes demonstrate drastically improved electron transport ability and prolonged lifetime of charge carriers, due to the nature of high surface area and excellent crystallinity. As a result, when used as photocatalysts, the Ta3N5 nanomeshes exhibit a greater than tenfold improvement in solar hydrogen production compared to bulk counterparts. This work provides an effective and generic strategy for designing 2D ultrathin nanomesh structures for nonlayered materials with improved catalytic activity.

Two-dimensional (2D) metal oxide nanosheets have demonstrated their great potential in a broad range of applications. The existing synthesis strategies are mainly preparing 2D nanosheets from layered and specific transition metal oxides. How to prepare the other types of metal oxides as ultrathin 2D nanosheets remains unsolved, especially for metal oxides containing alkali, alkaline earth metal, and multiple metal elements. Herein, we developed a half-successive ion layer adsorption and reaction (SILAR) method, which could synthesize those types of metal oxides as ultrathin 2D nanosheets. The synthesized 2D metal oxides nanosheets are within 1 nm level thickness and 500 m · g level surface area. This method allows us to develop many new types of ultrathin 2D metal oxides nanosheets that have never been prepared before.