Goldschmidt tolerance factor (t) is an empirical index for predicting stable crystal structures of perovskite materials. A t value between 0.8 and 1.0 is favorable for cubic perovskite structure, and larger (>1) or smaller (<0.8) values of tolerance factor usually result in nonperovskite structures. CH(NH2)(2)PbI3 (FAPbI(3)) can exist in the perovskite a-phase (black phase) with good photovoltaic properties. However, it has a large tolerance factor and is more stable in the hexagonal delta(H)-phase (yellow phase), with delta(H)-to-alpha phase-transition temperature higher than room temperature. On the other hand, CsPbI3 is stabilized to an orthorhombic structure (delta(o)-phase) at room temperature due to its small tolerance factor. We find that, by alloying FAPbI(3) with CsPbI3, the effective tolerance factor can be tuned, and the stability of the photo active a-phase of the mixed solid-state perovskite alloys FA(1-x)C(sx)bI(3) is enhanced, which is in agreement with our first-principles calculations. Thin films of the FA(0.85)Cs(0.15)PbI(3) perovskite alloy demonstrate much improved stability in a high-humidity environment; this contrasts significantly with the pure FAPbI(3) film for which the alpha-to-delta(H) phase transition (associated with yellowing appearance) is accelerated by humidity environment. Due to phase stabilization, the FA(0.86)Cs(0.15)PbI(3) solid-state alloy showed better solar cell performance and device stability than its FAPbI(3) counterparts. Our studies suggest that tuning the tolerance factor through solidstate alloying can be a general strategy to stabilize the desired perovskite structure for solar cell applications.
Solar cells composed of methylammonium lead iodide perovskite (MAPI) are notorious for their sensitivity to moisture. We show that (1) hydrated crystal phases are formed when MAPI is exposed to water vapor at room temperature and (ii) these phase changes are fully reversed when the material is subsequently dried. The reversible formation of CH3NH3PbI3 center dot H2O followed by (CH3NH3)(4)PbI6 center dot 2H(2)O (upon long exposure times) was observed using time-resolved XRD and ellipsometry of thin films prepared using "solvent engineering", single crystals, and state-of-the-art solar cells. In contrast to water vapor, the presence of liquid water results in the irreversible decomposition of MAPI to form PbI2. MAPI changes from dark brown to transparent on hydration; the precise optical constants of CH3NH3PbI3 center dot H2O formed on single crystal's were determined, with bandgap at 3.1 eV. Using the single-crystal optical constants and thin-film ellipsometry measurements, the time-dependent changes to MAPI films exposed to moisture were modeled. The results suggest that the monohydrate phase forms independent of the depth in the film, suggesting rapid transport of water molecules along grain boundaries. Vapor-phase hydration of an unencapsulated solar cell (initially J(sc) approximate to 19 mA cm(-2) and V-oc approximate to 1.05 V at 1 sun) resulted in more than a 90% drop in short-circuit photocurrent and similar to 200 mV loss in open-circuit potential; however, these losses were fully reversed after the device was exposed to dry nitrogen for 6 h. Hysteresis in the current-voltage characteristics was significantly increased after this dehydration, which may be related to changes in the defect density and morphology of MAPI following recrystallization from the hydrate. Based on our observations, we suggest that irreversible decomposition of MAPI in the presence of water vapor only occurs significantly once a grain has been fully converted to the monohydrate phase.
The hybrid two-dimensional (2D) halide perovskites have recently drawn significant interest because they can serve as excellent photoabsorbers in perovskite solar cells. Here we present the large scale synthesis, crystal structure, and optical characterization of the 2D (CH3(CH2)(3)NH3)(2)(CH3NR3)(n-1)PbnI3n+1 (n = 1, 2, 3, 4, infinity) perovskites, a family of layered compounds with tunable semiconductor characteristics. These materials consist of well-defined inorganic perovskite layers intercalated with bulky butylammonium cations that act as spacers between these fragments, adopting the crystal structure of the Ruddlesden-Popper type. We find that the perovskite thickness (n) can be synthetically controlled by adjusting the ratio between the spacer cation and the small organic cation, thus allowing the isolation of compounds in pure form and large scale. The orthorhombic crystal structures of (CH3(CH2)(3)NH3)(2)(CH3NH3)-Pb2I7 (n = 2, Cc2m; a = 8.9470(4), b = 39.347(2) angstrom, c = 8.8589(6)), (CH3(CH2)(3)NH3)(2)(CH3NH3)(2)Pb3I10 (n = 3, C2cb; a = 8.9275(6), b = 51.959(4) angstrom, c = 8.8777(6)), and (CH3(CH2)(3)NH3)(2)(CH3NH3)(3)Pb4I13 (n = 4, Cc2m; a = 8.9274(4), b = 64.383(4) angstrom, c = 8.8816(4)) have been solved by single-crystal X-ray diffraction and are reported here for the first time. The compounds are noncentrosymmetric, as supported by measurements of the nonlinear optical properties of the compounds and density functional theory (DFT) calculations. The band gaps of the series change progressively between 2.43 eV for the n = 1 member to 1.50 eV for the n = infinity adopting intermediate values of 2.17 eV (n = 2), 2.03 eV (n = 3), and 1.91 eV (n = 4) for those between the two compositional extrema. DFT calculations confirm this experimental trend and predict a direct band gap for all the members of the Ruddlesden Popper series. The estimated effective masses have values of m(h) = 0.14 m(0) and m(e) = 0.08 m(0) for holes and electrons, respectively, and are found to be nearly composition independent. The band gaps of higher n members indicate that these compounds can be used as efficient light absorbers in solar cells, which offer better solution processability and good environmental stability. The compounds exhibit intense room-temperature photoluminescence with emission wavelengths consistent with their energy gaps, 2.35 eV (n = 1), 2.12 eV (n = 2), 2.01 eV (n = 3), and 1.90 eV (n = 4) and point to their potential use in light-emitting diodes. In addition, owing to the low dimensionality and the difference in dielectric properties between the organic spacers and the inorganic perovskite layers, these compounds are naturally occurring multiple quantum well structures, which give rise to stable excitons at room temperature.
Poor oxygen evolution reaction (OER) catalysis limits the efficiency of H-2 production from water electrolysis and photoelectrolysis routes to large-scale energy storage. Despite nearly a century of research, the factors governing the activity of OER catalysts are not well understood. In this Perspective, we discuss recent advances in understanding the OER in alkaline media for earth-abundant, first-row, transition-metal oxides and (oxy)hydroxides. We argue that the most-relevant structures for study are thermodynamically stable (oxy)hydroxides and not crystalline oxides. We discuss thin-film electrochemical microbalance techniques to accurately quantify intrinsic activity and in situ conductivity measurements to identify materials limited by electronic transport. We highlight the dramatic effect that Fe cations added either intentionally or unintentionally from ubiquitous electrolyte impurities-have on the activity of common OER catalysts. We find new activity trends across the first-row transition metals, opposite of the established ones, and propose a new view of OER on mixed-metal (oxy)hydroxides that illustrates possible design principles and applications.
Side chains in conjugated polymers have been primarily utilized as solubilizing groups. However, these side chains have roles that are far beyond. We advocate using side chain engineering to tune a polymer's physical properties, including absorption, emission, energy level, molecular packing, and charge transport. To date, numerous flexible substituents suitable for constructing side chains have been reported. In this Perspective article, we advocate that the side chain engineering approach can advance better designs for next-generation conjugated polymers.
The design of active, robust, and nonprecious electrocatalysts with both H-2 and O-2 evolution reaction (HER and OER) activities for overall water splitting is highly desirable but remains a grand challenge. Herein, we report a facile two-step method to synthesize porous Co-P/NC nano-polyhedrons composed of CoPx (a mixture of CoP and Co2P) nanoparticles embedded in N-doped carbon matrices as electrocatalysts for overall water splitting. The Co-P/NC catalysts were prepared by direct carbonization of Co-based zeolitic imidazolate framework (ZIF-67) followed by phosphidation. Benefiting from the large specific surface area, controllable pore texture, and high nitrogen content of ZIF (a subclass of metal-organic frameworks), the optimal Co-P/NC showed high specific surface area of 183 m(2) g(-1) and large mesopores, and exhibited remarkable catalytic performance for both HER and OER in 1.0, M KOH, affording a current density of 10 mA cm(-2) at low overpotentials of -154 mV for HER and 319 mV for OER, respectively. Furthermore, a Co-P/NC-based alkaline electrolyzer approached 165 mA cm(-2) at 2.0 V, superior to that of Pt/IrO2 couple, along with strong stability. Various characterization techniques including X-ray absorption spectroscopy (XAS) revealed that the superior activity and strong stability of Co-P/NC originated from its 3D interconnected mesoporosity with high specific surface area, high conductivity, and synergistic effect of CoPx encapsulated within N-doped carbon matrices.
Hybrid halide perovskites represent one of the most promising solutions toward the fabrication of all solid nanostructured solar cells, with improved efficiency and long-term stability. This article aims at investigating the structural properties of iodide/ chloride mixed-halide perovskites and correlating them with their photovoltaic performances. We found out that, independent of the components ratio in the precursor solution, Cl incorporation in an iodide-based structure, is possible only at relatively low concentration levels (below 3-4%). However, even if the material band gap remains substantially unchanged, the Cl doping dramatically improves the charge transport within the perovskite layer, explaining the outstanding performances of meso-superstructured solar cells based on this material.
The key attribute of the thiol-Michael addition reaction that makes it a prized tool in materials science is its modular "click" nature, which allows for the implementation of this highly efficient, "green" reaction in applications that vary from small molecule synthesis to in situ polymer modifications in biological systems to the surface functionalization of material coatings. Over the past few decades, interest in the thiol-Michael addition reaction has increased dramatically, as is evidenced by the number of studies that have been dedicated to elucidating different aspects of the reaction that range from an in-depth analysis aimed at understanding the mechanistic pathways of the reaction to synthetic studies that have examined modifying molecular structures with the aim of yielding highly efficient thiol-Michael reaction monomers. This review examines the reaction mechanisms, the substrates and catalysts used in the reaction, and the subsequent implementation of the thiol-Michael reaction in materials science over the years, with particular emphasis on the recent developments in the arena over the past decade.
The double perovskites Cs2AgBiBr6 and Cs2AgBiCl6 have been synthesized from both solid state and solution routes. X-ray diffraction measurements show that both compounds adopt the cubic double perovskite structure, space group Fm (3) over barm, with lattice parameters of 11.2711(1) angstrom (X = Br) and 10.7774(2) angstrom (X = Cl). Diffuse reflectance measurements reveal band gaps of 2.19 eV (X = Br) and 2.77 eV (X = C1) that are slightly smaller than the band gaps of the analogous lead halide perovskites, 2.26 eV for CH3NH3PbBr3 and 3.00 eV for CH3NH3PbCl3. Band structure calculations indicate that the interaction between the Ag 4d-orbitals and the 3p/4p-orbitals of the halide ion modifies the valence band leading to an indirect band gap. Both compounds are stable when exposed to air, but Cs2AgBiBr6 degrades over a period of weeks when exposed to both ambient air and light. These results show that halide double perovskite semiconductors are potentially an environmentally friendly alternative to the lead halide perovskite semiconductors.
Two types of carbon dots (C dots) exhibiting respective excitation-independent blue emission and excitation-dependent full-color emissions have been synthesized via a mild one-pot process from chloroform and diethylamine. This new bottom-up synthetic strategy leads to highly stable crystalline C dots with tunable surface functionalities in high reproducibility. By detailed characterization and comparison of the two types of C dots, it is proved concretely that the surface functional groups, such as C=O and C=N, can efficiently introduce new energy levels for electron transitions and result in the continuously adjustable full-color emissions. A simplified energy level and electron transition diagram has been proposed to help understand how surface functional groups affect the emission properties. By taking advantage of the unique excitation-dependent full-color emissions, various new applications can be anticipated. Here, as an example, a ratiometric pH sensor using two emission wavelengths of the C dots as independent references has been constructed to improve the reliability and accuracy, and the pH sensor is applied to the measurement of intracellular pH values and cancer diagnosis.
Nanosized mesoporous silica particles with high colloidal stability attract growing attention as drug delivery systems for targeted cancer treatment and as bioimaging devices. This Perspective describes recent breakthroughs in mesoporous silica nanoparticle design to demonstrate their high potential as multifunctional drug delivery nanocarriers. These types of nanoparticles can feature a well-defined and tunable porosity at the nanometer scale, high loading capacity, and multiple functionality for targeting and entering different types of cells. We focus on the requirements for an efficient stimuli-responsive and thus controllable release of cargo into cancer cells and discuss design principles for smart and autonomous nanocarrier systems. Mesoporous silica nanoparticles are viewed as a promising and flexible platform for numerous biomedical applications.
Through decades of sustained effort, researchers have made substantial progress on developing technologies for solar-driven water splitting. Nevertheless, more basic research is needed before prototype devices with a chance for commercial success can be demonstrated. In this Perspective, we summarize the major design constraints that motivate continued research in the field of solar-driven water splitting. Additionally, we discuss key device components that are now available for use in demonstration systems and prototypes. Finally, we highlight research areas where breakthroughs will be critical for continued progress toward commercial viability for solar-driven water-splitting devices.
Wet chemistry in organic solvents has proven highly efficient for the preparation of several types of metallic, metal-oxide, and semiconductor nanostructures. This Short Review focuses on the use of oleylamine (OAm) as a versatile reagent for the synthesis of various nanoparticle systems. We describe the ability of OAm to act as a surfactant, solvent, and reducing agent, as a function of other synthesis parameters. We also discuss the specific role of OAm either alone or in combination with other reactants, to form nanostructures using a variety of organic or inorganic compounds as precursors. In certain cases OAm can form complex compounds with the metal ions of the corresponding precursor, leading to metastable compounds that can act as secondary precursors and thus be decomposed in a controlled way to yield nanoparticles. We also point out that OAm-stabilized particles can often be dispersed in different organic solvents yielding solutions with enhanced colloidal stability over long times and the potential to find applications in a number of different fields.
Development of high conductivity solid-state electrolytes for lithium ion batteries has proceeded rapidly in recent years, but incorporating these new materials into high-performing batteries has proven difficult. Interfacial resistance is now the limiting factor in many systems, but the exact mechanisms of this resistance have not been fully explained in part because experimental evaluation of the interface can be very difficult. In this work, we develop a computational methodology to examine the thermodynamics of formation of resistive interfacial phases. The predicted interfacial phase formation is well correlated with experimental interfacial observations and battery performance. We calculate that thiophosphate electrolytes have especially high reactivity with high voltage cathodes and a narrow electrochemical stability window. We also find that a number of known electrolytes are not inherently stable but react in situ with the electrode to form passivating but ionically conducting barrier layers. As a reference for experimentalists, we tabulate the stability and expected decomposition products for a wide range of electrolyte, coating, and electrode materials including a number of high-performing combinations that have not yet been attempted experimentally.
Naturally derived cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) are emerging nanomaterials that display high strength, high surface area, and tunable surface chemistry, allowing for controlled interactions with polymers, nanoparticles, small molecules, and biological materials. Industrial production of nano-celluloses, is increasing rapidly with several companies already producing on the tons-per-day scale, intensifying the quest for viable products across many sectors. While the hydrophilicity of the nanocellulose interface has posed a challenge to the use of CNCs and. CNFs as reinforcing agents in conventional plastics, it is a significant benefit for creating reinforced or structured hydrogel composites (or, when dried, aerogels) exhibiting both mechanical reinforcement and a host of other desirable properties. In this context, this Review describes the quickly growing field of hydrogels and aerogels incorporating nanocelluloses; over 200 references are summarized in comprehensive tables covering the chemistry, preparation, properties, and applications of "nanocellulose-only" and "nanocellulose-containing" gels. Physical and chemical cross-linking strategies, postmodification steps, and routes to control gel structure are discussed, along with key, developments and ongoing challenges in the field. Nanocellulose hydrogels and aerogels show great promise in a wide range of biomedical, energy storage, construction, separations, cosmetic, and food applications.
Since the electrocatalytic activity of layered molybdenum disulfide (MoS2) for hydrogen evolution reaction (HER) closely depends on its exposed edges, the morphology and size of the material are critically important. Herein, we introduce a novel solvent-evaporation-assisted intercalation method to fabricate the hybrid of alternating MoS2 sheets and reduced graphene oxide layers, in which the nanosize of the MoS2 nanosheets can be effectively controlled by leveraging the confinement effect within the two-dimensional graphene layers. Significantly, the resulting MoS2/reduced graphene oxide (RGO) composite shows excellent catalytic activity for HER characterized by higher current densities and lower onset potentials than the conventional pre-exfoliated RGO supported MoS2 nanosheets. Further experiments on the effect of oxidation degree of graphene, the crystallinity of MoS2, and the exposed active site density on the HER performance of the MoS2/RGO composites show that there is an optimum condition for the catalytic activity of HER due to a balance between the numbers of exposed active sites of MoS2 and the internal conductive channels provided by graphene.
The optoelectronic properties of polymeric semiconductor materials can be utilized for the fabrication of organic electronic and photonic devices. When key structural requirements are met, these materials exhibit unique properties such as solution processability, large charge transporting capabilities, and/or broad optical absorption. In this review recent developments in the area of pi-conjugated polymeric semiconductors for organic thin-film (or field-effect) transistors (OTFTs or OFETs) and bulk-heterojunction photovoltaic (or solar) cell (BHJ-OPV or OSC) applications are summarized and analyzed.