Birnessite is a layered MnO 2 mineral capable of intercalating nanometric water films in its bulk. With its variable distributions of Mn oxidation states (Mn IV , Mn III , and Mn II ), cationic vacancies, and interlayer cationic populations, birnessite plays key roles in catalysis, energy storage solutions, and environmental (geo)chemistry. We here report the molecular controls driving the nanoscale intercalation of water in potassium-exchanged birnessite nanoparticles. From microgravimetry, vibrational spectroscopy, and X-ray diffraction, we find that birnessite intercalates no more than one monolayer of water per interlayer when exposed to water vapor at 25 °C, even near the dew point. Molecular dynamics showed that a single monolayer is an energetically favorable hydration state that consists of 1.33 water molecules per unit cell. This monolayer is stabilized by concerted potassium–water and direct water–birnessite interactions, and involves negligible water–water interactions. Using our composite adsorption–condensation–intercalation model, we predicted humidity-dependent water loadings in terms of water intercalated in the internal and adsorbed at external basal faces, the proportions of which vary with particle size. The model also accounts for additional populations condensed on and between particles. By describing the nanoscale hydration of birnessite, our work secures a path for understanding the water-driven catalytic chemistry that this important layered manganese oxide mineral can host in natural and technological settings.

Electronic energy transfer across a molecular interface between two dissimilar conjugated polymers in a blend is a critical process for many promising optoelectronic device strategies. [1][2][3][4][5][6][7][8][9] Owing to the short range of electronic energy transfer, this process should in principle be strongly modulated by molecular-scale morphology in the interfacial regions of polymer blends. The most common approach for characterizing energy transfer in polymer blends, namely, ensemble fluorescence measurements, is obscured by morphological and kinetic heterogeneity of these materials and thus has done little to unravel the role of morphology in the energytransfer process. Even nearfield scanning optical microscopy, which has been used to study polymer blends at the sub-100-nm scale, [10,11] lacks the spatial resolution in most cases to study individual morphological regions that can be as small as a few nanometers.Herein, we take an alternative approach by using singlemolecule spectroscopy (SMS) to study energy transfer across a molecular interface in isolated, single diblock polymer DMOS-co-MEH chains (DMOS = 2-dimethyloctylsilyl-1,4-phenylenevinylene; MEH = 2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene) comprised of an energy-donor block (DMOS, l em max = 490 nm) and an energy-acceptor block (MEH, l abs max = 500 nm). The sizes of these molecules ( % 10 nm) make them excellent models for developing a detailed understanding at the molecular level of energy transfer between a molecular interface of two dissimilar polymers. SMS has been used effectively to study intramolecular energy transfer in biand multichromophoric dendrimers, [12][13][14][15] end-capped polymers, [16,17] and conjugated homopolymers. [18,19] The SMS approach reported here provides detailed information at the molecular level on how energy transfer is influenced by polymer morphology within a single diblock polymer chain. Figure 1 shows ensemble fluorescence spectra (solid lines) for DMOS (a) and MEH (b) homopolymers at T = 298 K dissolved at high dilution in a polymethyl methacrylate (PMMA) polymer host. These spectra were obtained by summing single-molecule spectra for isolated polymer chains. For comparison we also show absorption spectra (dotted lines) for the homopolymers which were acquired in dilute liquid solutions (solvent) rather than in polymer thin films because of the weak absorption of the latter. These data indicate that excitation wavelengths of 415 nm and 488 nm selectively excite DMOS and MEH, respectively, and also demonstrate a good spectral overlap of the emission bands of DMOS and the absorption bands of MEH which is important for promoting energy transfer by the Förster mechanism (the dominant mechanism for conjugated polymers). [20,21] Analogous ensemble spectra for the DMOS-co-MEH diblock polymer are shown in Figure 1 c. The emission spectra of DMOS-co-MEH (single molecules and ensembles) are well-modeled by a sum of MEH-and DMOS-like spectral components and do not show evidence of excimer emission between t...

Hydrophilic nanosized minerals exposed to air moisture host thin water films that are key drivers of reactions of interest in nature and technology. Water films can trigger irreversible mineralogical transformations,...

Thin water films that form by the adhesion and condensation of air moisture on minerals can initiate phase transformation reactions with broad implications in nature and technology. We here show important effects of water film coverages on reaction rates and products during the transformation of periclase (MgO) nanocubes to brucite [Mg(OH) 2 ] nanosheets. Using vibrational spectroscopy, we found that the first minutes to hours of Mg(OH) 2 growth followed first-order kinetics, with rates scaling with water loadings. Growth was tightly linked to periclase surface hydration and to the formation of a brucite precursor solid, akin to poorly stacked/ dislocated nanosheets. These nanosheets were the predominant forms of Mg(OH) 2 growth in the 2D-like hydration environments of sub-monolayer water films, which formed below ∼50% relative humidity (RH). From molecular simulations, we infer that reactions may have been facilitated near surface defects where sub-monolayer films preferentially accumulated. In contrast, the 3D-like hydration environment of multilayered water films promoted brucite nanoparticle formation by enhancing Mg(OH) 2 nanosheet growth and stacking rates and yields. From the structural similarity of periclase and brucite to other metal (hydr)oxide minerals, this concept of contrasting nanosheet growth should even be applicable for explaining water film-driven mineralogical transformations on other related nanominerals.

Electronic energy transfer across a molecular interface between two dissimilar conjugated polymers in a blend is a critical process for many promising optoelectronic device strategies. [1][2][3][4][5][6][7][8][9] Owing to the short range of electronic energy transfer, this process should in principle be strongly modulated by molecular-scale morphology in the interfacial regions of polymer blends. The most common approach for characterizing energy transfer in polymer blends, namely, ensemble fluorescence measurements, is obscured by morphological and kinetic heterogeneity of these materials and thus has done little to unravel the role of morphology in the energytransfer process. Even nearfield scanning optical microscopy, which has been used to study polymer blends at the sub-100-nm scale, [10,11] lacks the spatial resolution in most cases to study individual morphological regions that can be as small as a few nanometers.Herein, we take an alternative approach by using singlemolecule spectroscopy (SMS) to study energy transfer across a molecular interface in isolated, single diblock polymer DMOS-co-MEH chains (DMOS = 2-dimethyloctylsilyl-1,4-phenylenevinylene; MEH = 2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene) comprised of an energy-donor block (DMOS, l em max = 490 nm) and an energy-acceptor block (MEH, l abs max = 500 nm). The sizes of these molecules ( % 10 nm) make them excellent models for developing a detailed understanding at the molecular level of energy transfer between a molecular interface of two dissimilar polymers. SMS has been used effectively to study intramolecular energy transfer in biand multichromophoric dendrimers, [12][13][14][15] end-capped polymers, [16,17] and conjugated homopolymers. [18,19] The SMS approach reported here provides detailed information at the molecular level on how energy transfer is influenced by polymer morphology within a single diblock polymer chain. Figure 1 shows ensemble fluorescence spectra (solid lines) for DMOS (a) and MEH (b) homopolymers at T = 298 K dissolved at high dilution in a polymethyl methacrylate (PMMA) polymer host. These spectra were obtained by summing single-molecule spectra for isolated polymer chains. For comparison we also show absorption spectra (dotted lines) for the homopolymers which were acquired in dilute liquid solutions (solvent) rather than in polymer thin films because of the weak absorption of the latter. These data indicate that excitation wavelengths of 415 nm and 488 nm selectively excite DMOS and MEH, respectively, and also demonstrate a good spectral overlap of the emission bands of DMOS and the absorption bands of MEH which is important for promoting energy transfer by the Förster mechanism (the dominant mechanism for conjugated polymers). [20,21] Analogous ensemble spectra for the DMOS-co-MEH diblock polymer are shown in Figure 1 c. The emission spectra of DMOS-co-MEH (single molecules and ensembles) are well-modeled by a sum of MEH-and DMOS-like spectral components and do not show evidence of excimer emission between t...