Showing papers in "Chemistry of Materials in 2011"

π-Conjugated Polymers for Organic Electronics and Photovoltaic Cell Applications†

Abstract: 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 π-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.

Disclosing the Complex Structure of UiO-66 Metal Organic Framework: A Synergic Combination of Experiment and Theory

Abstract: Through a combined use of experimental and theoretical approaches such as XRPD, EXAFS, IR, and UV−vis spectroscopies and ab initio periodic DFT calculations, we report a detailed characterization of structural, vibrational, and electronic properties of UiO-66 (Zr-BDC MOF) in its hydroxylated and dehydroxylated forms. The stability of the materials with respect to the most common solvents, acids, and bases is determined by combining XRPD and TGA/MS techniques. The structures of the two forms of UiO-66 are refined through an interactive XRPD/EXAFS approach and validated by ab initio calculations. Experimental and calculated IR spectra are reported and compared to enlighten the nature of vibrational modes upon dehydroxylation and to show the complete reversibility of the dehydration/hydration phenomenon. Experimental and calculated band gaps are also reported and compared. In this work, we show the necessity to combine, in a synergic way, different experimental techniques and periodic ab initio approaches to...

Toward N-Doped Graphene via Solvothermal Synthesis

Abstract: Theoretical studies predicted that doping graphene with nitrogen can tailor its electronic properties and chemical reactivity. However, experimental investigations are still limited because of the lack of synthesis techniques that can deliver a reasonable quantity. We develop here a novel method for one-pot direct synthesis of N-doped graphene via the reaction of tetrachloromethane with lithium nitride under mild conditions, which renders fabrication in gram scale. The distinct electronic structure perturbation induced by the incorporation of nitrogen in the graphene network is observed for the first time by scanning tunnelling microscopy. The nitrogen content varies in the range of 4.5−16.4%, which allows further modulation of the properties. The enhanced catalytic activity is demonstrated in a fuel cell cathode oxygen reduction reaction with respect to pure graphene and commercial carbon black XC-72. The resulting N-doped materials are expected to broaden the already widely explored potential applicatio...

Small Molecule Solution-Processed Bulk Heterojunction Solar Cells†

Abstract: Although most research in the field of organic bulk heterojunction solar cells has focused on combinations of a p-type conducting polymer as a donor and a fullerene-based acceptor, recent work has demonstrated the viability of solution-processed heterojunctions composed entirely of molecular solids. Molecular solids offer potential advantages over conjugated polymer systems in terms of easier purification, amenability to mass-scale production and better batch-to-batch reproducibility. This article reviews the major classes of molecular donors that have been reported in the literature in the past several years and highlights some of key considerations in molecular heterojunction design compared to polymer-based bulk heterojunctions.

Hierarchical Dendrite-Like Magnetic Materials of Fe3O4, γ-Fe2O3, and Fe with High Performance of Microwave Absorption

Abstract: Iron-based microstructured or nanostructured materials, including Fe, γ-Fe2O3, and Fe3O4, are highly desirable for magnetic applications because of their high magnetization and a wide range of magnetic anisotropy. An important application of these materials is use as an electromagnetic wave absorber to absorb radar waves in the centimeter wave (2−18 GHz). Dendrite-like microstructures were achieved with the phase transformation from dendritic α-Fe2O3 to Fe3O4, Fe by partial and full reduction, and γ-Fe2O3 by a reduction−oxidation process, while still preserving the dendritic morphology. The investigation of the magnetic properties and microwave absorbability reveals that the three hierarchical microstructures are typical ferromagnets and exhibit excellent microwave absorbability. In addition, this also confirms that the microwave absorption properties are ascribed to the dielectric loss for Fe and the combination of dielectric loss and magnetic loss for Fe3O4 and γ-Fe2O3.

Processable Low-Bandgap Polymers for Photovoltaic Applications†

Abstract: Over the last five years, organic photovoltaic devices have emerged as a new competitor to silicon-based solar cells. In particular, the bulk heterojunction architecture (BHJ), in which the photoactive layer consists of a bicontinuous blend of an electron donor and an electron acceptor, has allowed power conversion efficiencies around 8%. We will present in this review the latest conjugated polymers used in such BHJ solar cells. We will mainly focus on electron-donating (p-type) polymers based on thiophenes, 1,3,2-benzodiathiazoles, pyrrolo[3,4-c]pyrrole-1,4-diones, benzo[1,2-b;3,4-b]dithiophenes, and few other materials with more exotic structures. This review should be helpful to evaluate which are the most promising materials and where this research field is going in the years to come.

Na2Ti3O7: Lowest voltage ever reported oxide insertion electrode for sodium ion batteries

Abstract: Na2Ti3O7 is found to reversibly uptake 2 Na ions per formula unit (200 mA h/g) at an average potential of 0.3 V and is hence a very promising negative electrode material for building sodium ion bat...

Controlling Zeolitic Imidazolate Framework Nano- and Microcrystal Formation: Insight into Crystal Growth by Time-Resolved In Situ Static Light Scattering

Abstract: We report on a simple and straightforward method that enables the rapid room-temperature production of nanocrystals (finely tuned in size between ∼10 and 65 nm) and microcrystals (∼1 μm) of the prototypical microporous zeolitic imidazolate framework (ZIF) material ZIF-8. Control of crystal size is achieved in a novel approach by employing an excess of the bridging bidentate ligand and various simple auxiliary monodentate ligands with different chemical functionalities (carboxylate, N-heterocycle, alkylamine). The function of the monodentate ligands can be understood as a modulation of complex formation and deprotonation equilibria during crystal nucleation and growth. Using time-resolved static light scattering, the functioning of modulating ligands is monitored for the first time by in situ experiments, which offered significant insight into the crystal growth processes. Formation of nanocrystals is characterized by continuous, comparatively slow nucleation and fast crystal growth occurring on a time sca...

Anatase TiO2 with Dominant High-Energy {001} Facets: Synthesis, Properties, and Applications

Abstract: Control of the surface structure of inorganic materials, in particular titania (TiO2), by chemical processes under nonequilibrium conditions is of growing interest from scientific and utilitarian viewpoints. Titania is one of the most important materials because of its unique surface, electronic, and photocatalytic properties, which make this material applicable in many areas of science and technology ranging from adsorption, catalysis and photocatalysis to biomedicine, environmental monitoring and cleanup, energy conversion and storage, etc. Here we review the existing strategies for the synthesis of anatase TiO2 micro- and nanosheets with exposed high-energy {001} facets and for the assembly of these nanosheets into various hierarchical structures. The {001} facets are stabilized by specific capping agents (typically, fluoride), which are used to control the growth of titania crystals. The presence of high-energy facets in titania improves significantly its adsorption, electronic, and photocatalytic pro...

Emerging Applications of Carbon Nanotubes

Abstract: On the basis of their unique electrical and mechanical properties, carbon nanotubes (CNTs) have attracted great attention in recent years. A diverse array of methods has been developed to modify CNTs and to assemble them into devices. On the basis of these innovations, many applications that include the use of CNTs have been demonstrated. Transparent electrodes for organic light-emitting diodes (OLEDs), lithium-ion batteries, supercapacitors, and CNT-based electronic components such as field-effect transistors (FETs) have been demonstrated. Furthermore, CNTs have been employed in catalysis and sensing as well as filters and mechanical and biomedical applications. This review highlights illustrative examples from these areas to give an overview of applications of CNTs.

Covalent Organic Frameworks with High Charge Carrier Mobility

Abstract: Two types (imine and boronate) of covalent organic frameworks (COFs) having a porphyrin unit have been synthesized. The two highly crystalline porphyrin COFs (COF-366 and COF-66) display excellent chemical and thermal stability and are permanently porous. Two-dimensional extended layered structures of the two COFs demonstrate very high charge carrier mobility values (8.1 cm2 V−1 s−1).

One-Dimensional Nanostructures of π-Conjugated Molecular Systems: Assembly, Properties, and Applications from Photovoltaics, Sensors, and Nanophotonics to Nanoelectronics†

Abstract: This paper presents a comprehensive review of the literature on one-dimensional (1D) nanostructures (nanowires, nanoribbons, nanotubes, nanobelts, and nanofibers) of π-conjugated small molecules, oligomers, and polymers. The diverse methods used in assembling the molecular building blocks into 1D functional nanostructures and nanodevices are discussed, including hard and soft template-assisted synthesis, electrospinning, nanolithography, self-assembly in solution and at interfaces, physical vapor transport, and other strategies. Optical, charge transport, electronic, and photoconductive properties of nanowires and nanotubes of selected classes of π-conjugated molecular systems are discussed next, highlighting unique features of the 1D nanostructures compared to 2D thin films. Overview of applications of these 1D organic nanostructures ranging from nanoscale light-emitting diodes, field-emission devices, organic photovoltaics, sensors/biosensors, spin-electronics, and nanophotonics to nanoelectronics is th...

Electron Transfer Dynamics in Dye-Sensitized Solar Cells

Abstract: In this review, we address the materials design parameters that control the processes of charge separation, and thereby device efficiency, in dye-sensitized photoelectrochemical solar cells. The review starts with an overview of the structure, energetics and kinetics of dye-sensitized solar cells. It then goes on to consider in more detail the parameters determining the efficiency of the two primary charge separation steps in these devices: electron injection from the dye excited state into the metal oxide electrode, and regeneration of the dye ground state by the redox electrolyte. We consider the kinetic competition between these desired charge separation steps and the undesired loss pathways of excited state decay to ground and electron recombination with dye cations. The review avoids detailed mathematical and spectroscopic discussion, but rather tries to summarize the key conclusions relevant to materials design. A recurring theme of the review is the energy cost of achieving charge separation, and h...

Effect of a Cobalt-Based Oxygen Evolution Catalyst on the Stability and the Selectivity of Photo-Oxidation Reactions of a WO3 Photoanode

Abstract: A bare WO3 electrode and a WO3 electrode coupled with a layer of Co−Pi oxygen evolution catalyst (OEC) were prepared to investigate the effect of Co−Pi OEC on the selectivity of photo-oxidation reactions and photostabilities of WO3 photoanodes. WO3 photoanodes have been reported to produce peroxo species as well as O2 during photo-oxidation reactions, and the accumulation of peroxo species on the surface is known to cause a gradual loss of photoactivity of WO3. The photocurrent to O2 conversion efficiencies of the WO3 and WO3/Co−Pi OEC electrodes were obtained by simultaneously measuring the photocurrent and O2 gas generated during illumination at 0.8 V vs Ag/AgCl. The result shows that the presence of OEC increases the photocurrent to O2 conversion efficiency from approximately 61% to approximately 100%. The complete suppression of peroxo formation provided the WO3/Co−Pi OEC photoelectrode with long-term photostability. The photocurrent−potential characteristics show that the presence of OEC effectively ...

Graphene Nanosheet/Ni2+/Al3+ Layered Double-Hydroxide Composite as a Novel Electrode for a Supercapacitor

Abstract: A hybrid chemically converted graphene nanosheet/Ni2+/Al3+ layered double-hydroxide (GNS/LDH) composite for supercapacitor material has been fabricated by a hydrothermal method. Scanning electron microscopy and transmission electron microscopy results reveal that Ni2+/Al3+ LDH platelets homogeneously grew onto the surfaces of the GNSs as spacers to keep the neighboring sheets separate. Electrochemical properties were characterized by cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy. The composite exhibits a maximum specific capacitance of 781.5 F/g and excellent cycle life with an increase of the specific capacitance of 38.07% after 50 cycle tests. Even after 200 cycle tests, the increase of the capacitance is 22.56% compared with the initial capacitance.

Polymer and Organic Nonvolatile Memory Devices

Abstract: Organic molecules and semiconductors have been proposed as active part of a large variety of nonvolatile memory devices, including resistors, diodes and transistors. In this review, we focus on electrically reprogrammable nonvolatile memories. We classify several possible devices according to their operation principle and critically review the role of the π-conjugated materials in the device operation. We propose specifications for applications for organic nonvolatile memory and review the state of the art with respect to these target specifications. Conclusions are drawn regarding further work on materials and device architectures.

Multifunctional Materials in High-Performance OLEDs: Challenges for Solid-State Lighting†

Abstract: Recent advances in material chemistry have enabled white organic light-emitting device (OLED) efficacy beyond fluorescent tube efficacy up to 100 lm W−1. In this short review, we explore recent developments of small molecule-based multifunctional materials in high-performance OLEDs, especially blue phosphorescent emitters, host materials, and electron-transporting materials.

Synthesis and characterization of an amino functionalized MIL-101(Al): Separation and catalytic properties

Abstract: A new MIL-101 material based on aluminum and containing amine functional groups has been synthesized. The pure phase NH2-MIL-101(Al) can only be formed in very specific synthesis conditions, where both the metal source and the solvent used play a key role. The resulting porous solid shows a high thermal and chemical stability, decomposing at temperatures above 650 K in air. The NH2-MIL-101(Al) framework offers an excellent trade off for separation of CO2: the combination of high stability, acceptable capacity at low adsorbate partial pressures, high selectivity, and fast regenerability makes this new material a very attractive candidate for applications like natural gas or biogas upgrading. CO2 capacities up to 62 wt % are obtained at room temperature and 3 MPa. In addition to an excellent separation performance, the NH2-MIL-101(Al) shows a high activity in the basic catalyzed Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate at 313 K even in an as apolar a solvent as toluene (turn over fre...

Halogenated Materials as Organic Semiconductors

Abstract: Organic semiconductors have great potential as the active material in low-cost, large area plastic electronics, whether as light-emitting diodes (LEDs), field-effect transistors (FETs) or solar cells. Organic semiconducting materials retain the processability associated with polymers while maintaining good optoelectronic properties, for example, high absorption coefficients for photons in the visible, and field-effect mobilities comparable with that of amorphous silicon. The elucidation of important structure−property relationships is vital for the design of functional, high-performance organic semiconductors. In this short review, we summarize such relationships stemming from the halogenation of organic semiconductors. While it has been known in the past decade that fluorination lowers the energy levels in carbon based systems, induces stability and electron transport, less is known about the effect of the other halogens. Chlorination has recently been shown to be a viable route to n-type materials. The ...

Oriented Zeolitic Imidazolate Framework-8 Membrane with Sharp H2/C3H8 Molecular Sieve Separation

Abstract: A highly oriented zeolitic imidazolate framework 8 (ZIF-8) composite membrane was prepared by seeding and secondary growth. By dip-coating, preformed ZIF-8 nanocrystals were attached to the surface of a porous α-alumina support using polyethyleneimine as the coupling agent. After solvothermal treatment, a continuous and well-intergrown ZIF-8 layer was obtained. X-ray diffraction analysis of the membrane showed preferred orientation of the {100} plane parallel to the support. Further time-dependent investigations by scanning and transmission electron microscopy as well as X-ray diffraction indicated that the preferred orientation develops during an evolutionary growth process. In gas mixture permeation experiments, the membrane showed good performance in H2/hydrocarbon separation. A sharp molecular sieve separation is observed for an equimolar H2/C3H8 mixture with a separation factor above 300.

Chemical Insights into the Instability of Cu(2)ZnSnS(4) Films during Annealing

Abstract: Cu(2)ZnSnS(4) (CZTS) shows great potential for cheap, efficient photovoltaic devices. However, one problem during synthesis of CZTS films is the loss of Sn as a result of decomposition and evaporat ...

Graphene-Based Non-Noble-Metal Catalysts for Oxygen Reduction Reaction in Acid

Abstract: Non-noble-metal catalysts based on Fe–N–C moieties have shown promising oxygen reduction reaction (ORR) activity in proton exchange membrane fuel cells (PEMFCs). In this study, we report a facile method to prepare a Fe–N–C catalyst based on modified graphene (Fe–N–rGO) from heat treatment of a mixture of Fe salt, graphitic carbon nitride (g-C3N4), and chemically reduced graphene (rGO). The Fe–N–rGO catalyst was found to have pyridinic N-dominant heterocyclic N (40% atomic concentration among all N components) on the surface and have an average Fe coordination of ∼3 N (Fe–N3,average) in bulk. Rotating disk electrode measurements revealed that Fe–N–rGO had high mass activity in acid and exhibited high stability at 0.5 V at 80 °C in acid over 70 h, which was correlated to low H2O2 production shown from rotating ring disk electrode measurements.

Atomic Structure of a Lithium-Rich Layered Oxide Material for Lithium-Ion Batteries: Evidence of a Solid Solution

Abstract: Li[Li0.2Ni0.2Mn0.6]O2, which is a cathode material for Li-ion batteries with enhanced capacity, has been examined, for the first time, with a combination of aberration-corrected scanning transmission electron microscopy (STEM), STEM computer simulations, and diffraction scanning transmission electron microscopy (D-STEM). These techniques, in combination with X-ray diffraction (XRD) and conventional electron diffraction (ED), indicate that this material is composed of a solid solution with C2/m monoclinic symmetry and multiple planar defects. In addition, we show that XRD and ED alone can give misleading information and cannot resolve the structure of these materials without the additional use of the aforementioned techniques.

Polymorphous Transformations of Nanometric Iron(III) Oxide: A Review

Abstract: There is great interest in iron oxides, especially in nanosized form, for both fundamental and practical reasons. Because of its polymorphism, iron(III) oxide (ferric oxide, Fe2O3) is one of the most interesting and potentially useful phases of the iron oxides. Each of the four different known crystalline Fe2O3 polymorphs (alpha-, beta-, gamma-, and epsilon-Fe2O3) has unique biochemical, magnetic, catalytic, and other properties that make it suitable for specific technical and biomedical applications. High temperature treatment is a key step in most syntheses of iron(III) oxides but often triggers polymorphous transformations that result in the formation of undesired mixtures of Fe2O3 polymorphs. It is therefore important to control the parameters that induce polymorphous transformations when seeking to prepare a given Fe2O3 polymorph as a single phase; identifying and understanding these parameters is a major challenge in the study of the polymorphism of solid compounds. This review discusses the depende...

Materials and Devices toward Fully Solution Processable Organic Light-Emitting Diodes†

Abstract: Organic light-emitting diodes (OLEDs) have been successfully developed and have now entered the commercial marketplace. Besides the impressive performance as displays, one of the key advantages of OLEDs is that they can potentially be made entirely by solution process and thus are more suitable for low cost, large area flexible displays and white lighting panels. To realize this, many efforts have been devoted on the development of solution processable light-emitting materials and charge transporting materials as well as electrode materials, which have resulted in the successful demonstration of high performance fully solution processed OLEDs and have opened a way to achieve all printable roll-to-roll organic optoelectronic devices.

Navigating the Color Palette of Solution-Processable Electrochromic Polymers†

Abstract: Solution-processable electrochromic (EC) polymers that can be switched from one distinct color state to a highly transmissive and near colorless state are required for applications in both EC windows and displays. Using a tour around the color wheel, we describe the various EC polymer (ECP) compositions that now make a full palette of colors available demonstrating a set of structure−property relationships. Electrochemical and electrochromic characterization methodologies are described and their application to ECPs demonstrated. Processing and patterning methods including spray casting, screen-, flexo-, and ink jet printing, along with photo- and soft lithography are described. Absorptive/transmissive (window type) and absorptive/reflective (display type) devices are described as platforms for practical applications.

Phosphates as Lithium-Ion Battery Cathodes: An Evaluation Based on High-Throughput ab Initio Calculations

Abstract: Phosphate materials are being extensively studied as lithium-ion battery electrodes. In this work, we present a high-throughput ab initio analysis of phosphates as cathode materials. Capacity, voltage, specific energy, energy density, and thermal stability are evaluated computationally on thousands of compounds. The limits in terms of gravimetric and volumetric capacity inherent to the phosphate chemistry are determined. Voltage ranges for all redox couples in phosphates are provided, and the structural factors influencing the voltages are analyzed. We reinvestigate whether phosphate materials are inherently safe and find that, for the same oxidation state, oxygen release happens thermodynamically at lower temperature for phosphates than for oxides. These findings are used to recommend specific chemistries within the phosphate class and to show the intrinsic limits of certain materials of current interest (e.g., LiCoPO4 and LiNiPO4).

Small-Molecule, Nonfullerene Acceptors for Polymer Bulk Heterojunction Organic Photovoltaics†

Abstract: In the field of polymer bulk-heterojunction organic photovoltaics, fullerenes and fullerene derivatives clearly play the dominant role as acceptor materials. Recently, a number of research efforts have focused on the development of new small-molecule acceptors for this device configuration. Although few materials prepared to-date have demonstrated power conversion efficiencies close to those achieved with fullerenes, numerous design rules and some interesting new materials classes have been explored. This short review will highlight the progress toward higher efficiency in nonfullerene small-molecule acceptors for organic solar cells.

Size-Tunable, Ultrasmall NaGdF4 Nanoparticles: Insights into Their T1 MRI Contrast Enhancement

Abstract: We report on the size-tunable synthesis of thermodynamically stable (β) NaGdF4 nanoparticles (NPs) below 10 nm. Paramagnetic β-NaGdF4 NPs of four different sizes (2.5–8.0 nm with a narrow size distribution) were synthesized by simple modifications of the reaction conditions affecting nanoparticle growth dynamics. The synthesized NPs were transferred to water by exchanging the oleate ligands with biocompatible polyvinylpyrrolidone, and analyzed for their ability to affect magnetic resonance (MR) T1 longitudinal relaxivity at 1.5 T. The ionic relaxivity (unit Gd3+ concentration) values increased from 3.0 mM–1 s–1 to 7.2 mM–1 s–1 with decreasing particle size, and the relaxivity of the 2.5-nm particle is almost twice that of clinically used Gd-DTPA (Magnevist) relaxivity. The relaxivity per contrast agent (i.e., per nanoparticle) for these NPs is 200–3000 times larger than the clinical agent, showing great potential as local contrast enhancement probes. The rate of increase in ionic relaxivity with decreasin...

Template-Free Synthesis of a Highly Porous Benzimidazole-Linked Polymer for CO2 Capture and H2 Storage

Abstract: A highly porous benzimidazole-linked polymer (SABET 1172 m2/g) exhibits very high gas selectivity CO2/N2 (70) and CO2/CH4 (10) and can store CO2 (19 wt %, 273 K, 1 bar) and H2 (1.9 wt %, 77 K, 1 bar) with Qst values of 26.7 and 7.9 kJ/mol, respectively.