Hiroaki Sai

Thermally Induced Structural Evolution and Performance of Mesoporous Block Copolymer-Directed Alumina Perovskite Solar Cells

Structure control in solution-processed hybrid perovskites is crucial to design and fabricate highly efficient solar cells. Here, we utilize in situ grazing incidence wide-angle X-ray scattering and scanning electron microscopy to investigate the structural evolution and film morphologies of methylammonium lead tri-iodide/chloride (CH3NH3PbI3–xClx) in mesoporous block copolymer derived alumina superstructures during thermal annealing. We show the CH3NH3PbI3–xClx material evolution to be characterized by three distinct structures: a crystalline precursor structure not described previously, a 3D perovskite structure, and a mixture of compounds resulting from degradation. Finally, we demonstrate how understanding the processing parameters provides the foundation needed for optimal perovskite film morphology and coverage, leading to enhanced block copolymer-directed perovskite solar cell performance.

Crystallization Kinetics of Organic–Inorganic Trihalide Perovskites and the Role of the Lead Anion in Crystal Growth

Methylammonium lead halide perovskite solar cells continue to excite the research community due to their rapidly increasing performance which, in large part, is due to improvements in film morphology. The next step in this progression is control of the crystal morphology which requires a better fundamental understanding of the crystal growth. In this study we use in situ X-ray scattering data to study isothermal transformations of perovskite films derived from chloride, iodide, nitrate, and acetate lead salts. Using established models we determine the activation energy for crystallization and find that it changes as a function of the lead salt. Further analysis enabled determination of the precursor composition and showed that the primary step in perovskite formation is removal of excess organic salt from the precursor. This understanding suggests that careful choice of the lead salt will aid in controlling crystal growth, leading to superior films and better performing solar cells.

Hierarchical Porous Polymer Scaffolds from Block Copolymers

A Complicated Scaffold, Simply Materials with tailored pore structures can be useful as catalysis supports and for lightweight materials. When preparing medical scaffolds, restrictive preparation conditions have to be met, which can prohibit multistep preparation procedures. Sai et al. (p. 530) describe a method for making porous polymers containing both relatively large (several microns) interconnecting pores and a second population of ∼ tens of nanometer pores. The process exploits spinodal decomposition of a block copolymer blended with small-molecule additives and requires a simple washing step with water, methanol, or ethanol. Spinodal decomposition of block copolymers and oligomeric additives produces three-dimensional hierarchical porous polymers. Hierarchical porous polymer materials are of increasing importance because of their potential application in catalysis, separation technology, or bioengineering. Examples for their synthesis exist, but there is a need for a facile yet versatile conceptual approach to such hierarchical scaffolds and quantitative characterization of their nonperiodic pore systems. Here, we introduce a synthesis method combining well-established concepts of macroscale spinodal decomposition and nanoscale block copolymer self-assembly with porosity formation on both length scales via rinsing with protic solvents. We used scanning electron microscopy, small-angle x-ray scattering, transmission electron tomography, and nanoscale x-ray computed tomography for quantitative pore-structure characterization. The method was demonstrated for AB- and ABC-type block copolymers, and resulting materials were used as scaffolds for calcite crystal growth.

One-pot synthesis of platinum-based nanoparticles incorporated into mesoporous niobium oxide-carbon composites for fuel cell electrodes.

Catalyst-electrode design is crucial for the commercialization and widespread use of polymer electrolyte membrane fuel cells. There are considerable challenges in making less expensive, more durable, and more active catalysts. Herein, we report the one-pot synthesis of Pt and Pt-Pb nanoparticles incorporated into the pores of mesoporous niobium oxide-carbon composites. The self-assembly of block copolymers with niobium oxide and metal precursors results in an ordered mesostructured hybrid. Appropriate heat treatment of this hybrid produces highly crystalline, well-ordered mesoporous niobium oxide-carbon composites with Pt (or Pt-Pb) nanoparticles incorporated into the mesopores. The in situ-generated graphitic-like carbon material prevents the collapse of the mesostructure, while the metal oxide crystallizes at high temperatures and enhances the electrical conductivity of the final material. Formic acid electrooxidation with this novel material shows 4 times higher mass activities (3.3 mA/microg) and somewhat lower onset potentials (-0.24 V vs Ag/AgCl) than the best previously reported values employing Pt-Pb intermetallic nanoparticles supported on conducting carbon (0.85 mA/microg and -0.18 V, respectively).

Highly Aminated Mesoporous Silica Nanoparticles with Cubic Pore Structure

Mesoporous silica with cubic symmetry has attracted interest from researchers for some time. Here, we present the room temperature synthesis of mesoporous silica nanoparticles possessing cubic Pm3n symmetry with very high molar ratios (>50%) of 3-aminopropyl triethoxysilane. The synthesis is robust allowing, for example, co-condensation of organic dyes without loss of structure. By means of pore expander molecules, the pore size can be enlarged from 2.7 to 5 nm, while particle size decreases. Adding pore expander and co-condensing fluorescent dyes in the same synthesis reduces average particle size further down to 100 nm. After PEGylation, such fluorescent aminated mesoporous silica nanoparticles are spontaneously taken up by cells as demonstrated by fluorescence microscopy.

Multicompartment Mesoporous Silica Nanoparticles with Branched Shapes: An Epitaxial Growth Mechanism

Tuning Mesopores Porous materials are of interest for catalysis and filtration because the open channels lend themselves to separating materials or function. Suteewong et al. (p. 337) report on a method to make branched mesoporous silica nanoparticles that contain cubic (core) and hexagonally structured (branch) parts within one particle. Controlling the extent of the branched structure is achieved by tuning the concentration of additives in a simple, one-pot reaction system. A one-pot synthesis method furnishes mesoporous silica nanoparticles with both cubic and hexagonally structured compartments. Mesoporous nanomaterials have attracted widespread interest because of their structural versatility for applications including catalysis, separation, and nanomedicine. We report a one-pot synthesis method for a class of mesoporous silica nanoparticles (MSNs) containing both cubic and hexagonally structured compartments within one particle. These multicompartment MSNs (mc-MSNs) consist of a core with cage-like cubic mesoporous morphology and up to four branches with hexagonally packed cylindrical mesopores epitaxially growing out of the cubic core vertices. The extent of cylindrical mesostructure growth can be controlled via a single additive in the synthesis. Results suggest a path toward high levels of architectural complexity in locally amorphous, mesostructured nanoparticles, which could enable tuning of different pore environments of the same particle for specific chemistries in catalysis or drug delivery.

A silica sol–gel design strategy for nanostructured metallic materials

Batteries, fuel cells and solar cells, among many other high-current-density devices, could benefit from the precise meso- to macroscopic structure control afforded by the silica sol-gel process. The porous materials made by silica sol-gel chemistry are typically insulators, however, which has restricted their application. Here we present a simple, yet highly versatile silica sol-gel process built around a multifunctional sol-gel precursor that is derived from the following: amino acids, hydroxy acids or peptides; a silicon alkoxide; and a metal acetate. This approach allows a wide range of biological functionalities and metals--including noble metals--to be combined into a library of sol-gel materials with a high degree of control over composition and structure. We demonstrate that the sol-gel process based on these precursors is compatible with block-copolymer self-assembly, colloidal crystal templating and the Stöber process. As a result of the exceptionally high metal content, these materials can be thermally processed to make porous nanocomposites with metallic percolation networks that have an electrical conductivity of over 1,000 S cm(-1). This improves the electrical conductivity of porous silica sol-gel nanocomposites by three orders of magnitude over existing approaches, opening applications to high-current-density devices.

Ordered mesoporous silica nanoparticles with and without embedded iron oxide nanoparticles: structure evolution during synthesis

This work reports on the structural evolution during room temperature synthesis of hexagonally ordered mesoporous silica nanoparticles with and without embedded iron oxide particles. Oleic acid-capped iron oxide nanoparticles are synthesized and transferred to an aqueous phase using the cationic surfactant, hexadecyltrimethylammonium bromide (CTAB). MCM-41 type silica and composite nanoparticles are fabricated via sol–gel synthesis. Aliquots are taken from the solution during synthesis to capture the particle formation process. Transmission Electron Microscopy (TEM) and Small Angle X-ray Scattering (SAXS) reveal a transition from a disordered to an ordered structure in both synthesis systems. Along with the evolution of structure, iron oxide nanoparticles acting as seeds at the early stages are relocated from the particle centers to the edges. Nitrogen sorption measurements for iron oxide-embedded mesoporous nanoparticles indicate surface areas as high as for the mesoporous silica nanoparticles without iron oxide.

Networked and chiral nanocomposites from ABC triblock terpolymer coassembly with transition metal oxide nanoparticles

Multicomponent materials with ordered nanoscale networks are critical for applications ranging from microelectronics to energy conversion and storage devices which require charge transport along 3-dimensional (3D) continuous pathways. The network symmetry can facilitate additional properties such as macroscopic polarization for piezoelectric, pyroelectric, and second-order nonlinear optical properties in non-centrosymmetric morphologies. Although pure block copolymers are able to form multiple network morphologies, network tunability remains a challenge for coassembled systems. Here we report the coassembly of niobia nanoparticles with a poly(isoprene-b-styrene-b-ethylene oxide) (ISO) which resulted in multiple network morphologies, one of which was chiral and non-centrosymmetric. Detailed small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) measurements were most consistent with the alternating gyroid (GA) morphology at low nanoparticle loadings and a transition to a centrosymmetric network morphology at higher loadings. This is the first report of multiple network morphologies from coassembly with a single polymer over a ∼10 vol% composition range. The nanoparticle spatial distribution was tomographically reconstructed. Nanocomposite calcination resulted in mesoporous networks. This general approach was further demonstrated with amorphous and anatase titania.

Impact of the organic halide salt on final perovskite composition for photovoltaic applications

The methylammonium lead halide perovskites have shown significant promise as a low-cost, second generation, photovoltaic material. Despite recent advances, however, there are still a number of fundamental aspects of their formation as well as their physical and electronic behavior that are not well understood. In this letter we explore the mechanism by which these materials crystallize by testing the outcome of each of the reagent halide salts. We find that components of both salts, lead halide and methylammonium halide, are relatively mobile and can be readily exchanged during the crystallization process when the reaction is carried out in solution or in the solid state. We exploit this fact by showing that the perovskite structure is formed even when the lead salts anion is a non-halide, leading to lower annealing temperature and time requirements for film formation. Studies into these behaviors may ultimately lead to improved processing conditions for photovoltaic films.