The charge carrier dynamics, efficiency and stability of two-dimensional material-based perovskite solar cells.

Perovskites have been firmly established as one of the most promising materials for third-generation solar cells. There remain several great and lingering challenges to be addressed regarding device efficiency and stability. The photovoltaic efficiency of perovskite solar cells (PSCs) depends drastically on the charge-carrier dynamics. This complex process includes charge-carrier generation, extraction, transport and collection, each of which needs to be modulated in a favorable manner to achieve high performance. Two-dimensional materials (TDMs) including graphene and its derivatives, transition metal dichalcogenides (e.g., MoS2, WS2), black phosphorus (BP), metal nanosheets and two-dimensional (2D) perovskite active layers have attracted much attention for application in perovskite solar cells due to their high carrier mobility and tunable work function properties which greatly impact the charge carrier dynamics of PSCs. To date, significant advances have been achieved in the field of TDM-based PSCs. In this review, the recent progress in the development and application of TDMs (i.e., graphene, graphdiyne, transition metal dichalcogenides, BP, and others) as electrodes, hole transporting layers, electron transporting layers and buffer layers in PSCs is detailed. 2D perovskites as active absorber materials in PSCs are also summarized. The effect of TDMs and 2D perovskites on the charge carrier dynamics of PSCs is discussed to provide a comprehensive understanding of their optoelectronic processes. The challenges facing the PSC devices are emphasized with corresponding solutions to these problems provided with the overall goal of improving the efficiency and stability of photovoltaic devices.

Hybrid Organic-Inorganic Thermoelectric Materials and Devices.

Hybrid organic-inorganic materials have been considered as a new candidate in the field of thermoelectric materials since the last decade owing to their great potential to enhance the thermoelectric performance by utilizing the low thermal conductivity of organic materials and the high Seebeck coefficient, and high electrical conductivity of inorganic materials. Herein, we provide an overview of interfacial engineering in the synthesis of various organic-inorganic thermoelectric hybrid materials, along with the dimensional design for tuning their thermoelectric properties. Interfacial effects are examined in terms of nanostructures, physical properties, and chemical doping between the inorganic and organic components. Several key factors which dictate the thermoelectric efficiency and performance of various electronic devices are also discussed, such as the thermal conductivity, electric transportation, electronic band structures, and band convergence of the hybrid materials.

Crafting Mussel-Inspired Metal Nanoparticle-Decorated Ultrathin Graphitic Carbon Nitride for the Degradation of Chemical Pollutants and Production of Chemical Resources.

The development of efficient photocatalysts for the degradation of organic pollutants and production of hydrogen peroxide (H2 O2 ) is an attractive two-in-one strategy to address environmental remediation concerns and chemical resource demands. Graphitic carbon nitride (g-C3 N4 ) possesses unique electronic and optical properties. However, bulk g-C3 N4 suffers from inefficient sunlight absorption and low carrier mobility. Once exfoliated, ultrathin nanosheets of g-C3 N4 attain much intriguing photocatalytic activity. Herein, a mussel-inspired strategy is developed to yield silver-decorated ultrathin g-C3 N4 nanosheets (Ag@U-g-C3 N4 -NS). The optimum Ag@U-g-C3 N4 -NS photocatalyst exhibits enhanced electrochemical properties and excellent performance for the degradation of organic pollutants. Due to the photoformed valence band holes and selective two-electron reduction of O2 by the conduction band electrons, it also renders an efficient, economic, and green route to light-driven H2 O2 production with an initial rate of 0.75 × 10-6 m min-1 . The improved photocatalytic performance is primarily attributed to the large specific surface area of the U-g-C3 N4 -NS layer, the surface plasmon resonance effect induced by Ag nanoparticles, and the cooperative electronic capture properties between Ag and U-g-C3 N4 -NS. Consequently, this unique photocatalyst possesses the extended absorption region, which effectively suppresses the recombination of electron-hole pairs and facilitates the transfer of electrons to participate in photocatalytic reactions.

Barium titanate at the nanoscale: controlled synthesis and dielectric and ferroelectric properties.

The current trend in the miniaturization of electronic devices has driven the investigation into many nanostructured materials. The ferroelectric material barium titanate (BaTiO3) has garnered considerable attention over the past decade owing to its excellent dielectric and ferroelectric properties. This has led to significant progress in synthetic techniques that yield high quality BaTiO3 nanocrystals (NCs) with well-defined morphologies (e.g., nanoparticles, nanorods, nanocubes and nanowires) and controlled crystal phases (e.g., cubic, tetragonal and multi-phase). The ability to produce nanoscale BaTiO3 with controlled properties enables theoretical and experimental studies on the intriguing yet complex dielectric properties of individual BaTiO3 NCs as well as BaTiO3/polymer nanocomposites. Compared with polymer-free individual BaTiO3 NCs, BaTiO3/polymer nanocomposites possess several advantages. The polymeric component enables simple solution processibility, high breakdown strength and light weight for device scalability. The BaTiO3 component enables a high dielectric constant. In this review, we highlight recent advances in the synthesis of high-quality BaTiO3 NCs via a variety of chemical approaches including organometallic, solvothermal/hydrothermal, templating, molten salt, and sol-gel methods. We also summarize the dielectric and ferroelectric properties of individual BaTiO3 NCs and devices based on BaTiO3 NCs via theoretical modeling and experimental piezoresponse force microscopy (PFM) studies. In addition, viable synthetic strategies for novel BaTiO3/polymer nanocomposites and their structure-composition-performance relationship are discussed. Lastly, a perspective on the future direction of nanostructured BaTiO3-based materials is presented.

Achieving Efficient Incorporation of π-Electrons into Graphitic Carbon Nitride for Markedly Improved Hydrogen Generation.

A rapid and highly efficient strategy for introducing C into g-C3 N4 involves copolymerizing π-electron-rich barbituric acid with melamine via a facile microwave-assisted heating, thereby eliminating the issues in conventional electric furnace heating, such as the severe volatilization, owing to the mismatch of the sublimation temperatures of barbituric acid and melamine. The g-C3 N4 catalyst after optimizing the C-doping content actively generates increased amounts of H2 under visible light exposure with the highest H2 generation rate of 25.0 µmol h-1 , which is nearly 20 times above that using g-C3 N4 produced by conventional electric furnace heating of two identical monomers (1.3 µmol h-1 ). As such, the microwave-assisted heating strategy may stand out as an extremely simple route to incorporating π-electrons into g-C3 N4 with markedly improved photocatalytic performance.

Scrutinizing Defects and Defect Density of Selenium-Doped Graphene for High-Efficiency Triiodide Reduction in Dye-Sensitized Solar Cells.

Understanding the impact of the defects/defect density of electrocatalysts on the activity in the triiodide (I3- ) reduction reaction of dye-sensitized solar cells (DSSCs) is indispensable for the design and construction of high-efficiency counter electrodes (CEs). Active-site-enriched selenium-doped graphene (SeG) was crafted by ball-milling followed by high-temperature annealing to yield abundant edge sites and fully activated basal planes. The density of defects within SeG can be tuned by adjusting the annealing temperature. The sample synthesized at an annealing temperature of 900 °C exhibited a superior response to the I3- reduction with a high conversion efficiency of 8.42 %, outperforming the Pt reference (7.88 %). Improved stability is also observed. DFT calculations showed the high catalytic activity of SeG over pure graphene is a result of the reduced ionization energy owing to incorporation of Se species, facilitating electron transfer at the electrode-electrolyte interface.

From Precision Synthesis of Block Copolymers to Properties and Applications of Nanoparticles.

Inorganic nanoparticles have become a research focus in numerous fields because of their unique properties that distinguish them from their bulk counterparts. Controlling the size and shape of nanoparticles is an essential aspect of nanoparticle synthesis. Preparing inorganic nanoparticles by using block copolymer templates is one of the most reliable routes for tuning the size and shape of nanoparticles with a high degree of precision. In this Review, we discuss recent progress in the design of block copolymer templates for crafting spherical inorganic nanoparticles including compact, hollow, and core-shell varieties. The templates are divided into two categories: micelles self-assembled from linear block copolymers and unimolecular star-shaped block copolymers. The precise control over the size and morphology of nanoparticles is highlighted as well as the useful properties and applications of such inorganic nanoparticles.

Immobilization of Pt Nanoparticles via Rapid and Reusable Electropolymerization of Dopamine on TiO2 Nanotube Arrays for Reversible SERS Substrates and Nonenzymatic Glucose Sensors.

Inspired by mussel-adhesion phenomena in nature, polydopamine (PDA) coatings are a promising route to multifunctional platforms for decorating various materials. The typical self-polymerization process of dopamine is time-consuming and the coatings of PDA are not reusable. Herein, a reusable and time-saving strategy for the electrochemical polymerization of dopamine (EPD) is reported. The PDA layer is deposited on vertically aligned TiO2 nanotube arrays (NTAs). Owing to the abundant catechol and amine groups in the PDA layer, uniform Pt nanoparticles (NPs) are deposited onto the TiO2 NTAs and can effectively prevent the recombination of electron-hole pairs generated from photo-electrocatalysis and transfer the captured electrons to participate in the photo-electrocatalytic reaction process. Compared with pristine TiO2 NTAs, the as-prepared Pt@TiO2 NTA composites exhibit surface-enhanced Raman scattering sensitivity for detecting rhodamine 6G and display excellent UV-assisted self-cleaning ability, and also show promise as a nonenzymatic glucose biosensor. Furthermore, the mussel-inspired electropolymerization strategy and the fast EPD-reduced nanoparticle decorating process presented herein can be readily extended to various functional substrates, such as conductive glass, metallic oxides, and semiconductors. It is the adaptation of the established PDA system for a selective, robust, and generalizable sensing system that is the emphasis of this work.

Plasmon-Mediated Solar Energy Conversion via Photocatalysis in Noble Metal/Semiconductor Composites.

Plasmonics has remained a prominent and growing field over the past several decades. The coupling of various chemical and photo phenomenon has sparked considerable interest in plasmon-mediated photocatalysis. Given plasmonic photocatalysis has only been developed for a relatively short period, considerable progress has been made in improving the absorption across the full solar spectrum and the efficiency of photo-generated charge carrier separation. With recent advances in fundamental (i.e., mechanisms) and experimental studies (i.e., the influence of size, geometry, surrounding dielectric field, etc.) on plasmon-mediated photocatalysis, the rational design and synthesis of metal/semiconductor hybrid nanostructure photocatalysts has been realized. This review seeks to highlight the recent impressive developments in plasmon-mediated photocatalytic mechanisms (i.e., Schottky junction, direct electron transfer, enhanced local electric field, plasmon resonant energy transfer, and scattering and heating effects), summarize a set of factors (i.e., size, geometry, dielectric environment, loading amount and composition of plasmonic metal, and nanostructure and properties of semiconductors) that largely affect plasmonic photocatalysis, and finally conclude with a perspective on future directions within this rich field of research.

Solution-Stable Colloidal Gold Nanoparticles via Surfactant-Free, Hyperbranched Polyglycerol-b-polystyrene Unimolecular Templates.

Hyperbranched polyglycerol-block-polystyrene copolymers, denoted HPG-b-PS, are synthesized and employed as a new and effective unimolecular template for synthesizing colloidal gold (Au) nanoparticles. The coordination of noble metal precursors with polyether within the inner HPG core and subsequent in situ reduction enables the formation of well-dispersed and stable PS-capped Au nanoparticles. The inner HPG core is produced via ring opening multibranching polymerization (ROMBP) and subsequently converted into atom transfer radical polymerization (ATRP) macroinitiators for the controlled growth of polystyrene (PS) arms possessing low polydispersity (PDI < 1.31). An initial investigation into the templating parameters of HPG-b-PS was undertaken by producing templates with different arm numbers (98 and 117) and different PS chain lengths (i.e., molecular weight = 3500-13400 g/mol). It was found that the PS chain length and solvent conditions affect the quality of the resulting PS-capped colloidal Au nanoparticles. This work demonstrates, for the first time, a simple, lower-cost approach for templating nonpolar solvent-soluble PS-capped Au nanoparticles on the order of 10-30 nm in diameter.

Germanium-Based Nanomaterials for Rechargeable Batteries.

Germanium-based nanomaterials have emerged as important candidates for next-generation energy-storage devices owing to their unique chemical and physical properties. In this Review, we provide a review of the current state-of-the-art in germanium-based materials design, synthesis, processing, and application in battery technology. The most recent advances in the area of Ge-based nanocomposite electrode materials and electrolytes for solid-state batteries are summarized. The limitations of Ge-based materials for energy-storage applications are discussed, and potential research directions are also presented with an emphasis on commercial products and theoretical investigations.

Block copolymer/ferroelectric nanoparticle nanocomposites.

Nanocomposites composed of diblock copolymer/ferroelectric nanoparticles were formed by selectively constraining ferroelectric nanoparticles (NPs) within diblock copolymer nanodomains via judicious surface modification of ferroelectric NPs. Ferroelectric barium titanate (BaTiO3) NPs with different sizes that are permanently capped with polystyrene chains (i.e., PS-functionalized BaTiO3NPs) were first synthesized by exploiting amphiphilic unimolecular star-like poly(acrylic acid)-block-polystyrene (PAA-b-PS) diblock copolymers as nanoreactors. Subsequently, PS-functionalized BaTiO3 NPs were preferentially sequestered within PS nanocylinders in the linear cylinder-forming polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer upon mixing the BaTiO3 NPs with PS-b-PMMA. The use of PS-b-PMMA diblock copolymers, rather than traditional homopolymers, offers the opportunity for controlling the spatial organization of PS-functionalized BaTiO3 NPs in the PS-b-PMMA/BaTiO3 NP nanocomposites. Selective solvent vapor annealing was utilized to control the nanodomain orientation in the nanocomposites. Vertically oriented PS nanocylinders containing PS-functionalized BaTiO3 NPs were yielded after exposing the PS-b-PMMA/BaTiO3 NP nanocomposite thin film to acetone vapor, which is a selective solvent for PMMA block. The dielectric properties of nanocomposites in the microwave frequency range were investigated. The molecular weight of PS-b-PMMA and the size of BaTiO3 NPs were found to exert an apparent influence on the dielectric properties of the resulting nanocomposites.