Reversibly Tunable Upconversion Luminescence by Host–Guest Chemistry

Tuning upconversion (UPC) luminescence using external stimuli and fields, as well as chemical reactions, is expected to lead to novel and efficient optical sensors. Herein, highly tunable UPC luminescence was achieved through a host–guest chemistry approach. Specifically, interlayer ion exchange reactions reversibly tuned the emission intensity and green-red color of Er/Yb-codoped A₂La₂Ti₃O₁₀ layered perovskite, where A corresponds to proton and alkali metal ions, enabling the visualization of host–guest interactions and reactions

All-Graphene Oxide Flexible Solid-State Supercapacitors with Enhanced Electrochemical Performance

The rapid development of flexible and wearable electronics has led to an increase in the demand for flexible supercapacitors with enhanced electrochemical performance. Graphene oxide (GO) and reduced GO (rGO) exhibit several key properties required for supercapacitor components. Although solid-state rGO/GO/rGO supercapacitors with unique structures are promising, their moderate capacitance is inadequate for practical applications. Herein, we report a flexible solid-state rGO/GO/rGO supercapacitor comprising H₂SO₄-intercalated GO electrolyte/separator and pseudocapacitive rGO electrodes, which demonstrate excellent electrochemical performance. The resulting supercapacitor delivered an areal capacitance of 14.5 mF cm^–2, which is among the highest values achieved for any rGO/GO/rGO supercapacitor. High ionic concentration and fast ion conduction in the H₂SO₄-intercalated GO electrolyte/separator and abundant CH defects, which serve as pseudocapacitive sites on the rGO electrode, were responsible for the high capacitance of this device. The rGO electrode, well separated by the H₂SO₄ molecular spacer, supplied highly efficient ion transport channels, leading to excellent rate capability. The highly packed rGO electrode and high specific capacitance resulted in a high volumetric energy density (1.24 mWh cm^–3) observed in this supercapacitor. The structure, without a clear interface between GO and rGO, provides extremely low resistance and flexibility for devices. Our device operated in air (25 °C 40%) without the use of external electrolytes, conductive additives, and binders. Furthermore, we demonstrate a simple and versatile technique for supercapacitor fabrication by combining photoreduction and electrochemical treatment. These advantages are attractive for developing novel carbon-based energy devices with high device performance and low fabrication costs

Solid Electrolyte Gas Sensor Based on a Proton-Conducting Graphene Oxide Membrane

Graphene oxide (GO) is an ultrathin carbon nanosheet with various oxygen-containing functional groups. The utilization of GO has attracted tremendous attention in a number of areas, such as electronics, optics, optoelectronics, catalysis, and bioengineering. Here, we report the development of GO-based solid electrolyte gas sensors that can continuously detect combustible gases at low concentrations. GO membranes were fabricated by filtration using a colloidal solution containing GO nanosheets synthesized by a modified Hummers’ method. The GO membrane exposed to humid air showed good proton-conducting properties at room temperature, as confirmed by hydrogen concentration cell measurements and complex impedance analyses. Gas sensor devices were fabricated using the GO membrane fitted with a Pt/C sensing electrode. The gas-sensing properties were examined by potentiometric and amperometric techniques. The GO sensor showed high, stable, and reproducible responses to hydrogen at parts per million concentrations in humid air at room temperature. The sensing mechanism is explained in terms of the mixed-potential theory. Our results suggest the promising capability of GO for the electrochemical detection of combustible gases

Coal Oxide as a Thermally Robust Carbon-Based Proton Conductor

Inexpensive solid proton conducting materials with high proton conductivity and thermal stability are necessary for practical solid state electrochemical devices. Here we report that coal oxide (CO) is a promising carbon-based proton conductor with remarkable thermal robustness. The CO produced by simple liquid-phase oxidation of coal demonstrates excellent dispersibility in water owing to the surface carboxyl groups. The proton conductivity of CO, 3.9 × 10^–3 S cm^–1 at 90% relative humidity, is as high as that of graphene oxide (GO). Remarkably, CO exhibits much higher thermal stability than GO, with CO retaining the excellent proton conductivity as well as the capacitance performance even after thermal annealing at 200 °C. Our study demonstrates that the chemical modification of the abundant coal provides proton conductors that can be used in practical applications for a wide range of energy devices

Tunable Graphene Oxide Proton/Electron Mixed Conductor that Functions at Room Temperature

Graphene oxide (GO) and reduced graphene oxide exhibit proton and electron (or hole) conduction, respectively. Owing to this, the conductivity of GO can be controlled via reduction because its electron conductivity increases and its proton conductivity depends on the concentration of epoxide groups. Herein, we report the successful control of the proton and electron conductivities of GO using the photoirradiation and thermal reduction processes. The proton conductivity decreases when the epoxide content and layer distance decreases, whereas the electron conductivity drastically increases with decreasing oxygen content. Both the electron and proton conduction mechanisms for GO are discussed based on the concentrations of various functional groups and defects, changes in the interlayer distance, and the activation energy associated with proton conduction. Finally, we determined the most suitable degree of reduction for obtaining a good mixed conductor useful as an electrode material and a hydrogen separation membrane that functions at room temperature

Intense Photoluminescence from Ceria-Based Nanoscale Lamellar Hybrid

Nanosheets, which are ultrathin inorganic crystals, have the potential to exhibit unique surface states and quantum effects. These nanosheets can be further manipulated to form lamellar structures for the fabrication of advanced hybrid nanomaterials. Here we report that conventionally nonluminescent ceria yields intense UV photoluminescence with an internal quantum yield (QY) of 59% when self-organized into a nanosheet lamellar architecture with dodecyl sulfate (DS) bilayers. The origin of luminescence exist at the organic/inorganic interfaces, where surface Ce³⁺ ions of ceria nanosheet layers graft with DS anions to activate radiative 5d→4f transition

Water Vapor Electrolysis with Proton-Conducting Graphene Oxide Nanosheets

Hydrogen production by membrane water electrolysis has attracted tremendous attention because of its benefits, which include easy separation of hydrogen and oxygen, no carbon emissions, and the possibility to store hydrogen fuel as an electricity source. Here, we study water vapor electrolysis using a proton-conducting membrane comprising graphene oxide (GO) nanosheets. The GO membrane shows good through-plane proton conductivity, as confirmed by concentration-cell measurements, complex impedance spectroscopy, and hydrogen pumping experiments. The results also confirm that most carriers in the GO membrane are protons. The GO membrane fitted with Pt/C and IrO₂–Al₂O₃ as the cathode and the anode, respectively, efficiently electrolyzes humidified air to produce hydrogen and oxygen at room temperature, which indicates bright prospects for this carbon-based electrochemical device

Correlated Optical and Magnetic Properties in Photoreduced Graphene Oxide

Optical and magnetic properties of graphene oxide (GO) have been intensively investigated because of the promising applications of GO-related materials in various technical fields. So far, the optical and magnetic properties of GO have been discussed independently. However, localized electronic states in reduced GO may simultaneously add optical transitions and spin moments in sp² nanodomains in GO nanosheets. In the present study, the structural, optical, and magnetic properties of graphene oxide (GO) photoreduced in an aqueous solution are correlated on the basis of experimental and theoretical investigations. Experimental observations show that photoreduction leads to enhancement of visible absorption, quenching of photoluminescence, and emergence of magnetism. Detailed spectroscopic and microscopic characterizations indicate the presence of photoreduction-produced basal plane CH bonding and carbon vacancies. Ab initio calculations suggest that the presence of these defects in sp² nanodomains results in singly occupied molecular orbital levels in the π–π* gap to afford enhanced visible to near-infrared (NIR) absorption and emergence of magnetism, which is consistent with the experimentally observed change in the optical and magnetic properties of GO by photoreduction. Enhancement of NIR emissions observed in shortly photoreduced GO and their extinction found in longer photoreduced GO are explained with integrating the theoretical calculations and time-resolved fluorescence measurements. The correlation among structural, optical, and magnetic properties, highlighted for the first time, could help accelerate the development of open-shell nanographene devices with concurrently tunable electrical, optical, magnetic, and electrochemical properties

Graphene Oxide Nanosheet with High Proton Conductivity

We measured the proton conductivity of bulk graphite oxide (GO′), a graphene oxide/proton hybrid (GO-H), and a graphene oxide (GO) nanosheet for the first time. GO is a well-known electronic insulator, but for proton conduction we observed the reverse trend, as it exhibited superionic conductivity. The hydrophilic sites present in GO as −O–, −OH, and −COOH functional groups attract the protons, which propagate through hydrogen-bonding networks along the adsorbed water film. The proton conductivities of GO′ and GO-H at 100% humidity were ∼10^–4 and ∼10^–5 S cm^–1, respectively, whereas that for GO was amazingly high, nearly 10^–2 S cm^–1. This finding indicates the possibility of GO-based perfect two-dimensional proton-conductive materials for applications in fuel cells, sensors, and so on

Photochemical Engineering of Graphene Oxide Nanosheets

Many unique properties of graphene oxide (GO) strongly depend on the oxygenated functional groups and morphologies. Here, the photoreaction process is demonstrated to be very useful to control these factors. We report the fast, simple production of nanopores in porous GO via photoreaction in O₂ under UV irradiation at room temperature. Quantitative analysis using X-ray photoelectron spectroscopy showed that nanopores were produced in areas of oxygenated groups (sp³ carbon bonds), creating porous reduced graphene oxide (rGO). The photoreaction mechanism was proposed on the basis of changes in the number of oxygenated groups. Proton conduction occurred at the basal plane of epoxide groups in virgin GO, even at low humidity, and at carboxyl groups for porous rGO at high humidity. Thus, GO and rGO samples with various morphologies, oxygenated functional groups, and conduction types can be easily fabricated by controlling the photoreaction conditions