Boosting Photocatalytic Hydrogen Production by MOF-derived Metal Oxide Heterojunctions with a 10.0% Apparent Quantum Yield.

Photocatalytic hydrogen production offers an alternative pathway to establish a sustainable energy economy. While numerous photoactive materials exhibit potential for generating hydrogen from water, the synergy achieved by combining two different materials with complementary properties in the form of heterojunctions can significantly their photocatalytic activity. Our study describes the design and generation of the metal-organic framework-derived (MOF) metal oxide heterojunction composed of RuO2/N,S-TiO2. The RuO2/N,S-TiO2 is generated through the pyrolysis of MOFs, Ru- HKUST-1, and the amino-functionalized MIL-125-NH2. Among the various RuO2/N,S- TiO2 materials tested, the material characterized by the lowest RuO2 content, exhibited the highest hydrogen evolution rate, producing 10,761 µmol·hr-1·g-1 of hydrogen with an apparent quantum-yield of 10.0% in pure water. In addition to RuO2/N,S-TiO2, we generated two other MOF-derived metal-oxide heterojunctions, ZnO/N,S-TiO2 and In2O3/N,S-TiO2, leading to apparent quantum yields of 0.7% and 0.3%, respectively. The remarkable photocatalytic activity observed in RuO2/N,S-TiO2 is thought to be attributed to the synergistic effects arising from the combination of metallic properties inherent in the metal oxides, their band alignment, porosity, and surface properties inherited from the parent MOFs. The photocatalytic efficiency of RuO2/N,S-TiO2 was further demonstrated in actual water samples, producing hydrogen with a rate of 8,190 µmol·hr-1·g-1 in tap water.

The Role of Alkalis in Orchestrating Uranyl-Peroxide Reactivity Leading to Direct Air Capture of Carbon Dioxide.

Spectator ions have known and emerging roles in aqueous metal-cation chemistry, respectively directing solubility, speciation, and reactivity. Here, we isolate and structurally characterize the last two metastable members of the alkali uranyl triperoxide series, the Rb+ and Cs+ salts (Cs-U1 and Rb-U1). We document their rapid solution polymerization via small-angle X-ray scattering, which is compared to the more stable Li+, Na+ and K+ analogues. To understand the role of the alkalis, we also quantify alkali-hydroxide promoted peroxide deprotonation and decomposition, which generally exhibits increasing reactivity with increasing alkali size. Cs-U1, the most unstable of the uranyl triperoxide monomers, undergoes ambient direct air capture of CO2 in the solid-state, converting to Cs4[UVIO2(CO3)3], evidenced by single-crystal X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. We have attempted to benchmark the evolution of Cs-U1 to uranyl tricarbonate, which involves a transient, unstable hygroscopic solid that contains predominantly pentavalent uranium, quantified by X-ray photoelectron spectroscopy. Powder X-ray diffraction suggests this intermediate state contains a hydrous derivative of CsUVO3, where the parent phase has been computationally predicted, but not yet synthesized.

Reversible Cl/Cl- redox in a spinel Mn3O4 electrode.

A unique prospect of using halides as charge carriers is the possibility of the halides undergoing anodic redox behaviors when serving as charge carriers for the charge-neutrality compensation of electrodes. However, the anodic conversion of halides to neutral halogen species has often been irreversible at room temperature due to the emergence of diatomic halogen gaseous products. Here, we report that chloride ions can be reversibly converted to near-neutral atomic chlorine species in the Mn3O4 electrode at room temperature in a highly concentrated chloride-based aqueous electrolyte. Notably, the Zn2+ cations inserted in the first discharge and trapped in the Mn3O4 structure create an environment to stabilize the converted chlorine atoms within the structure. Characterization results suggest that the Cl/Cl- redox is responsible for the observed large capacity, as the oxidation state of Mn barely changes upon charging. Computation results corroborate that the converted chlorine species exist as polychloride monoanions, e.g., [Cl3]- and [Cl5]-, inside the Zn2+-trapped Mn3O4, and the presence of polychloride species is confirmed experimentally. Our results point to the halogen plating inside electrode lattices as a new charge-storage mechanism.

Li2 MnO3 : A Catalyst for a Liquid Cl2 Electrode in Low-Temperature Aqueous Batteries.

Li2 MnO3 has been contemplated as a high-capacity cathode candidate for Li-ion batteries; however, it evolves oxygen during battery charging under ambient conditions, which hinders a reversible reaction. However, it is unclear if this irreversible process still holds under subambient conditions. Here, the low-temperature electrochemical properties of Li2 MnO3 in an aqueous LiCl electrolyte are evaluated and a reversible discharge capacity of 302 mAh g-1 at a potential of 1.0 V versus Ag/AgCl at -78 °C with good rate capability and stable cycling performance, in sharp contrast to the findings in a typical Li2 MnO3 cell cycled at room temperature, is observed. However, the results reveal that the capacity does not originate from the reversible oxygen oxidation in Li2 MnO3 but the reversible Cl2 (l)/Cl- (aq.) redox from the electrolyte. The results demonstrate the good catalytic properties of Li2 MnO3 to promote the Cl2 /Cl- redox at low temperatures.

Hypsochromically-shifted Emission of Metal-organic Frameworks Generated through Post-synthetic Ligand Reduction.

Luminescent materials with tunable emission are becoming increasingly desirable as we move towards needing efficient Light Emitting Diodes (LEDs) for displays. Key to developing better displays is the advancement of strategies for rationally designing emissive materials that are tunable and efficient. We report a series of emissive metal-organic frameworks (MOFs) generated using BUT-10 (BUT: Beijing University of Technology) that emits green light with λmax at 525 nm. Post-synthetic reduction of the ketone on the fluorenone ligand in BUT-10 generates new materials, BUT-10-M and BUT-10-R. The emission for BUT-10-R is hypsochromically-shifted by 113 nm. Multivariate BUT-10-M structures demonstrate emission with two maxima corresponding to the emission of both fluorenol and fluorenone moieties present in their structures. Our study represents a novel post-synthetic ligand reduction strategy for producing emissive MOFs with tunable emission ranging from green, white-blue to deep blue.

Bismuth-Polyoxocation Coordination Networks: Controlling Nuclearity and Dimension-Dependent Photocatalysis.

Bismuth-oxocluster nodes for metal-organic frameworks (MOFs) and coordination networks/polymers are less prolific than other families featuring zinc, zirconium, titanium, lanthanides, etc. However, Bi3+ is non-toxic, it readily forms polyoxocations, and its oxides are exploited in photocatalysis. This family of compounds provides opportunity in medicinal and energy applications. Here, we show that Bi node nuclearity depends on solvent polarity, leading to a family of Bix-sulfonate/carboxylate coordination networks with x = 1-38. Larger nuclearity-node networks were obtained from polar and strongly coordinating solvents, and we attribute the solvent's ability to stabilize larger species in solution. The strong role of the solvent and the lesser role of the linker in defining node topologies differ from other MOF syntheses, and this is due to the Bi3+ intrinsic lone pair that leads to weak node-linker interactions. We describe this family by single-crystal X-ray diffraction (eleven structures), obtained in pure forms and high yields. Ditopic linkers include NDS (1,5-naphthalenedisulfonate), DDBS (2,2'-[biphenyl-4,4'-diylchethane-2,1-diyl] dibenzenesulphonate), and NH2-benzendicarboxylate (BDC). While the BDC and NDS linkers yield more open-framework topologies that resemble those obtained by carboxylate linkers, topologies with DDBS linkers appear to be in part driven by association between DDBS molecules. An in situ small-angle X-ray scattering study of Bi38-DDBS reveals stepwise formation, including Bi38-assembly, pre-organization in solution, followed by crystallization, confirming the less important role of the linker. We demonstrate photocatalytic hydrogen (H2) generation with select members of the synthesized materials without the benefit of a co-catalyst. Band gap determination from X-ray photoelectron spectroscopy (XPS) and UV-vis data suggest the DDBS linker effectively absorbs in the visible range with ligand-to-Bi-node charge transfer. In addition, materials containing more Bi (larger Bi38-nodes or Bi6 inorganic chains) exhibit strong UV absorption, also contributing to effective photocatalysis by a different mechanism. All tested materials became black with extensive UV-vis exposure, and XPS, transmission electron microscopy, and X-ray scattering of the black Bi38-framework suggest that Bi0 is formed in situ, without phase segregation. This evolution leads to enhanced photocatalytic performance, perhaps due to increased light absorption.

Reversible Copper Cathode for Nonaqueous Dual-Ion Batteries.

Most reported cathodes of nonaqueous dual-ion batteries (DIBs) host anions via insertion reactions. It is necessary to explore new cathode chemistry to increase the battery energy density. To date, transition metals have yet to be investigated for nonaqueous DIBs, albeit they may offer high capacity in anodic conversion reactions. Here, we report that bulk copper powder exhibits a high reversible capacity of 762 mAh g-1 at 3.2 V vs. Li+ /Li and relatively stable cycling in common organic electrolytes. The operation of the copper electrode is coupled with the transfer of anion charge carriers. An anion exchange membrane separator is employed to prevent Cu2+ from crossing from the catholyte to the anode side. We designed an unbalanced electrolyte with a more concentrated anolyte than a catholyte. This addresses the concentration overpotential ensued during charge and facilitates the high specific capacity and enhanced reversibility. This finding provides a promising direction for high-energy DIBs.

Copper metal electrode reversibly hosts fluoride in a 16 m KF aqueous electrolyte.

Fluoride is a promising charge carrier for batteries due to its high charge/mass ratio and small radius. Here, we report commercial copper powder exhibits a reversible capacity of up to 222 mA h g-1 in a saturated electrolyte of 16 m KF. This electrolyte suppresses dissolution of CuF2, the charged product. Furthermore, the KF solid comprised in the Cu electrode facilitates a high initial capacity. Our results showcase the potential of aqueous fluoride batteries using copper as an electrode.

From Copper to Basic Copper Carbonate: A Reversible Conversion Cathode in Aqueous Anion Batteries.

Dual-ion batteries that use anions and cations as charge carriers represent a promising energy-storage technology. However, an uncharted area is to explore transition metals as electrodes to host carbonate in conversion reactions. Here we report the reversible conversion reaction from copper to Cu2 CO3 (OH)2 , where the copper electrode comprising K2 CO3 and KOH solid is self-sufficient with anion-charge carriers. This electrode dissociates and associates K+ ions during battery charge and discharge. The copper active mass and the anion-bearing cathode exhibit a reversible capacity of 664 mAh g-1 and 299 mAh g-1 , respectively, and relatively stable cycling in a saturated mixture electrolyte of K2 CO3 and KOH. The results open an avenue to use carbonate as a charge carrier for batteries to serve for the consumption and storage of CO2 .

Reversible intercalation of methyl viologen as a dicationic charge carrier in aqueous batteries.

The interactions between charge carriers and electrode structures represent one of the most important considerations in the search for new energy storage devices. Currently, ionic bonding dominates the battery chemistry. Here we report the reversible insertion of a large molecular dication, methyl viologen, into the crystal structure of an aromatic solid electrode, 3,4,9,10-perylenetetracarboxylic dianhydride. This is the largest insertion charge carrier when non-solvated ever reported for batteries; surprisingly, the kinetic properties of the (de)insertion of methyl viologen are excellent with 60% of capacity retained when the current rate is increased from 100 mA g-1 to 2000 mA g-1. Characterization reveals that the insertion of methyl viologen causes phase transformation of the organic host, and embodies guest-host chemical bonding. First-principles density functional theory calculations suggest strong guest-host interaction beyond the pure ionic bonding, where a large extent of covalency may exist. This study extends the boundary of battery chemistry to large molecular ions as charge carriers and also highlights the electrochemical assembly of a supramolecular system.

An ethyl methyl sulfone co-solvent eliminates macroscopic morphological instabilities of lithium metal anode.

Lithium metal anodes suffer from a short cycle life, and the parasitic reactions of lithium with electrolytes are widely observed. Common sense is to avoid such reactions. Herein, we head in the opposite direction by using an oxidizing co-solvent, ethyl methyl sulfone, in the electrolyte, which addresses the 'dendrite' issue entirely, resulting in a dense and macroscopically smooth surface morphology of the plated lithium. However, a dendrite-free lithium metal anode does not necessarily exhibit a high coulombic efficiency.

An Aqueous Dual-Ion Battery Cathode of Mn3 O4 via Reversible Insertion of Nitrate.

We report reversible electrochemical insertion of NO3 - into manganese(II, III) oxide (Mn3 O4 ) as a cathode for aqueous dual-ion batteries. Characterization by TGA, FTIR, EDX, XANES, EXAFS, and EQCM collectively provides unequivocal evidence that reversible oxidative NO3 - insertion takes place inside Mn3 O4 . Ex situ HRTEM and corresponding EDX mapping results suggest that NO3 - insertion de-crystallizes the structure of Mn3 O4 . Kinetic studies reveal fast migration of NO3 - in the Mn3 O4 structure. This finding may open a new direction for novel low-cost aqueous dual-ion batteries.

Glucose Sensing Using Functionalized Amorphous In-Ga-Zn-O Field-Effect Transistors.

Recent advances in glucose sensing have focused on the integration of sensors into contact lenses to allow noninvasive continuous glucose monitoring. Current technologies focus primarily on enzyme-based electrochemical sensing which requires multiple nontransparent electrodes to be integrated. Herein, we leverage amorphous indium gallium zinc oxide (IGZO) field-effect transistors (FETs), which have found use in a wide range of display applications and can be made fully transparent. Bottom-gated IGZO-FETs can have significant changes in electrical characteristics when the back-channel is exposed to different environments. We have functionalized the back-channel of IGZO-FETs with aminosilane groups that are cross-linked to glucose oxidase and have demonstrated that these devices have high sensitivity to changes in glucose concentrations. Glucose sensing occurs through the decrease in pH during glucose oxidation, which modulates the positive charge of the aminosilane groups attached to the IGZO surface. The change in charge affects the number of acceptor-like surface states which can deplete electron density in the n-type IGZO semiconductor. Increasing glucose concentrations leads to an increase in acceptor states and a decrease in drain-source conductance due to a positive shift in the turn-on voltage. The functionalized IGZO-FET devices are effective in minimizing detection of interfering compounds including acetaminophen and ascorbic acid. These studies suggest that IGZO FETs can be effective for monitoring glucose concentrations in a variety of environments, including those where fully transparent sensing elements may be of interest.

Direct fabrication of nanoporous graphene from graphene oxide by adding a gasification agent to a magnesiothermic reaction.

CaCO3 acts as a gasification agent during magnesiothermic reduction of graphene oxide, thus preventing the newly formed graphene from restacking. The surface area of the obtained graphene increases from 66 m(2) g(-1) to 603 m(2) g(-1) by adding CaCO3 with a high yield of ∼70% based on the carbon content in graphene oxide.

Superior cathode of sodium-ion batteries: orthorhombic V2O5 nanoparticles generated in nanoporous carbon by ambient hydrolysis deposition.

For the first time, we demonstrate that orthorhombic V2O5 can exhibit superior electrochemical performance in sodium ion batteries when uniformly coated inside nanoporous carbon. The encapsulated V2O5 shows a specific capacity as high as 276 mAh/g, while the whole nanocomposite exhibits a capacity of 170 mAh/g. The V2O5/C composite was fabricated by a novel ambient hydrolysis deposition that features sequential water vapor adsorption in nanoporous carbon, followed by a hydrolysis reaction, exclusively inside the nanopores. The unique structure of the nanocomposite significantly enhances the capacity as well as the rate performance of orthorhombic V2O5 where the composite retains a capacity of over 90 mAh/g at a current rate of 640 mA/g. Furthermore, by calculating, we also revealed that a large portion of the sodium-ion storage, particularly at high current rates, is due to the V2O5 pseudocapacitance.

Pyrolysis of cellulose under ammonia leads to nitrogen-doped nanoporous carbon generated through methane formation.

Here, we present a simple one-step fabrication methodology for nitrogen-doped (N-doped) nanoporous carbon membranes via annealing cellulose filter paper under NH3. We found that nitrogen doping (up to 10.3 at %) occurs during cellulose pyrolysis under NH3 at as low as 550 °C. At 700 °C or above, N-doped carbon further reacts with NH3, resulting in a large surface area (up to 1973.3 m(2)/g). We discovered that the doped nitrogen, in fact, plays an important role in the reaction, leading to carbon gasification. CH4 was experimentally detected by mass spectrometry as a product in the reaction between N-doped carbon and NH3. When compared to conventional activated carbon (1533.6 m(2)/g), the N-doped nanoporous carbon (1326.5 m(2)/g) exhibits more than double the unit area capacitance (90 vs 41 mF/m(2)).

Chemical and structural investigation of high-resolution patterning with HafSO(x).

High-resolution transmission electron microscopy (TEM) imaging and energy-dispersive X-ray spectroscopy (EDS) chemical mapping have been used to examine key processing steps that enable sub-20-nm lithographic patterning of the material Hf(OH)4-2x-2y(O2)x(SO4)y·qH2O (HafSOx). Results reveal that blanket films are smooth and chemically homogeneous. Upon exposure with an electron beam, the films become insoluble in aqueous tetramethylammonium hydroxide [TMAH(aq)]. The mobility of sulfate in the exposed films, however, remains high, because it is readily exchanged with hydroxide from the TMAH(aq) solution. Annealing the films after soaking in TMAH(aq) results in the formation of a dense hafnium hydroxide oxide material that can be converted to crystalline HfO2 with a high electron-beam dose. A series of 9 nm lines is written with variable spacing to investigate the cross-sectional shape of the patterned lines and the residual material found between them.

Production of graphene by reduction using a magnesiothermic reaction.

We have, for the first time, employed a magnesiothermic reaction to convert microwave-irradiated graphite oxide to pure graphene. The magnesiothermic reaction increases the carbon to oxygen atomic ratio from 22.2 to 165.7 and maintains a high surface area. The new strategy demonstrates an efficient method for obtaining highly pure graphene materials.