Te Fu Yeh

Nitrogen-Doped Graphene Oxide Quantum Dots as Photocatalysts for Overall Water-Splitting under Visible Light Illumination

Nitrogen-doped graphene oxide quantum dots exhibit both p- and n-type conductivities and catalyze overall water-splitting under visible-light irradiation. The quantum dots contain p-n type photochemical diodes, in which the carbon sp(2) clusters serve as the interfacial junction. The active sites for H2 and O2 evolution are the p- and n-domains, respectively, and the reaction mimics biological photosynthesis.

Graphene oxide-based nanomaterials for efficient photoenergy conversion

The tunable electronic properties of graphene oxide (GO) nanomaterials make them interesting candidates for photoenergy conversion applications, including photoluminescence, photovoltaics, and photocatalysis for water splitting. GO nanomaterials can be categorized as GO sheets or GO quantum dots. Various techniques have been developed for tuning the electronic structures of GO nanomaterials to promote the quantum efficiency of photoenergy conversion. This review highlights the mechanisms governing the photoenergy conversion of GO nanomaterials and provides design strategies for the enhancement of quantum efficiency through the tuning of surface chemistry in accordance with the requirements of specific applications.

Synthesis of graphene oxide dots for excitation-wavelength independent photoluminescence at high quantum yields

This study reports on the synthesis of graphene oxide dots (GODs, 2.5 ± 0.5 nm) exhibiting excitation-wavelength independent photoluminescence (PL) at 530 nm. The GODs, which are of high uniformity and crystallinity, are produced by mildly oxidizing thermally-reduced GO sheets under sonication. The GOD aqueous suspension yields a maximal PL quantum yield (QY) of 16% under excitation at 470 nm. This PL can be ascribed to the irradiative excitation of electrons from the non-bonding oxygen (n) states to the graphene anti-bonding π orbital with subsequent relaxation of the electrons to the n ground states. Nitrogen-doping reduces vacancy defects and donates electrons to compensate for the unbalanced charge in p-type GODs, thereby increasing the PL QY to 22% for the nitrogen-doped GODs (NGODs). Treating the unadorned GODs and NGODs with submerged liquid plasma in tetrahydrofuran suppresses charge leakage from the carbonyl groups on the graphene periphery and increases the QYs to 42% and 50%, respectively. The GODs could be used as phosphors for the generation of white light by combining green emissions (530 nm) with violet light used for excitation. The present study demonstrates facile synthesis of high-quality green-emitting GODs and an effective method for the repair of vacancy defects and the stabilization of oxygen functionalities to enhance PL emission from GODs.

Formation of internal p–n junctions in Ta3N5 photoanodes for water splitting

Tantalum nitride photoanodes for water splitting are fabricated by anodizing tantalum foils, with subsequent nitridation of the foils in NH3. The as-synthesized Ta3N5 film has n-type conductivity. Loading Co ions during and after the anodization process forms a Ta3N5:Co film consisting of p- and n-type Ta3N5 domains. Both the Ta3N5 and Ta3N5:Co electrodes have a band gap of 2.0 eV. The p–n junctions in the Ta3N5:Co electrode create an internal electrical field favorable for hole transfer from the n-type domains to the p-type domains. When the photoanodes are immersed in a 0.5 M KOH aqueous solution for water splitting with one-sun illumination, the Ta3N5:Co photoanode exhibits photocurrents an order of magnitude higher than those of the bare Ta3N5 photoanode. AC impedance spectroscopy analysis reveals that the p–n junctions formed by Co-doping reduce the interfacial charge transfer resistance by an order of magnitude. The diffuse reflectance spectra of the electrodes indicate that Co incorporation minimizes the defect states in the bulk Ta3N5. Intensity-modulated photocurrent spectroscopy analysis reveals that the high electron transit rate of the Ta3N5:Co electrode can be attributed to its fewer defect states. A photoelectrochemical reaction using the Ta3N5:Co photoanode produces H2 and O2 at a ratio close to 2 : 1, and N2 evolution from the reaction is negligible. The present study demonstrates the establishment of a p–n junction configuration that considerably enhances the performance of nitride anodes in photocatalyzed water splitting.

Graphene oxide-based micropatterns via high-throughput multiphoton-induced reduction and ablation

In this study, a developed temporal focusing-based femtosecond laser system provides high-throughput multiphoton-induced reduction and ablation of graphene oxide (GO) films. Integrated with a digital micromirror device to locally control the laser pulse numbers, GO-based micropatterns can be quickly achieved instantly. Furthermore, the degree of reduction and ablation can be precisely adjusted via controlling the laser wavelength, power, and pulse number. Compared to point-by-point scanning laser direct writing, this approach offers a high-throughput and multiple-function approach to accomplish a large area of micro-scale patterns on GO films. The high-throughput micropatterning of GO via the temporal focusing-based femtosecond laser system fulfills the requirement of mass production for GO-based applications in microelectronic devices.

Multiphoton fabrication of freeform polymer microstructures containing graphene oxide and reduced graphene oxide nanosheets

Three-dimensional freeform polymer microstructures containing graphene oxide (GO) nanosheets were fabricated via two-photon polymerization using Rose Bengal (RB) as the photoinitiator. To prevent photothermal damage in fabricated polymer structures and impede the reduction level in the GO nanosheets, the femtosecond laser dose was controlled as low as possible at the laser wavelength of 720 nm appropriate for superior two-photon absorption of RB. Furthermore, the GO nanosheets in the fabricated microstructure can be converted into reduced GO (rGO) by precisely increasing the laser power and controlling the desired reduction positions. In this manner, GO and rGO nanosheets contained in the designated areas of the fabricated microstructures can be achieved.

Minimization of Ion–Solvent Clusters in Gel Electrolytes Containing Graphene Oxide Quantum Dots for Lithium-Ion Batteries

This study uses graphene oxide quantum dots (GOQDs) to enhance the Li+ -ion mobility of a gel polymer electrolyte (GPE) for lithium-ion batteries (LIBs). The GPE comprises a framework of poly(acrylonitrile-co-vinylacetate) blended with poly(methyl methacrylate) and a salt LiPF6 solvated in carbonate solvents. The GOQDs, which function as acceptors, are small (3-11 nm) and well dispersed in the polymer framework. The GOQDs suppress the formation of ion-solvent clusters and immobilize PF6- anions, affording the GPE a high ionic conductivity and a high Li+ -ion transference number (0.77). When assembled into Li|electrolyte|LiFePO4 batteries, the GPEs containing GOQDs preserve the battery capacity at high rates (up to 20 C) and exhibit 100% capacity retention after 500 charge-discharge cycles. Smaller GOQDs are more effective in GPE performance enhancement because of the higher dispersion of QDs. The minimization of both the ion-solvent clusters and degree of Li+ -ion solvation in the GPEs with GOQDs results in even plating and stripping of the Li-metal anode; therefore, Li dendrite formation is suppressed during battery operation. This study demonstrates a strategy of using small GOQDs with tunable properties to effectively modulate ion-solvent coordination in GPEs and thus improve the performance and lifespan of LIBs.