Visualization 1: Making few-layer graphene photoluminescent by UV ozonation

Visualization 1 Originally published in Optical Materials Express on 01 November 2016 (ome-6-11-3527

Insights into the Oxidation Mechanism of sp²–sp³ Hybrid Carbon Materials: Preparation of a Water-Soluble 2D Porous Conductive Network and Detectable Molecule Separation

A thorough investigation of the oxidation mechanism of sp²–sp³ hybrid carbon materials is helpful for the morphological trimming of graphene. Here, porous graphene (PGN) was obtained via a free radical oxidation process. We further demonstrated the difference between traditional and free radical oxidation processes in sp²–sp³ hybrid carbon materials. The sp³ part of graphene oxide was oxidized first, and well-crystallized sp² domains were reserved, which is different from the oxidation mechanism in a traditional approach. The obtained PGN shows excellent performance in the design of PGN-based detectable molecule separation or other biomedical applications

Enhanced Crystallization from the Glassy State of Poly(l‑lactic acid) Confined in Anodic Alumina Oxide Nanopores

The crystallization behavior of poly­(l-lactic acid) (PLLA) infiltrated in anodic alumina oxide templates (AAO) was investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). During heating from the glassy state, the crystallization of infiltrated PLLA was unexpectedly enhanced as compared with bulk PLLA. The cold crystallization temperature of infiltrated PLLA from the glassy state was much lower than that of bulk PLLA. The half-crystallization time (<i>t</i>_1/2) of infiltrated PLLA at 75 °C decreased with the diameter of AAO nanopores. The glass transition temperature of PLLA was not influenced by the geometrical confinement. The enhanced crystallization from the glassy state was explained by surface-induced nucleation of AAO walls on PLLA. Our results provide the first observation of enhanced cold crystallization of polymers in confined geometry

Electrochemical Fabrication of High Quality Graphene in Mixed Electrolyte for Ultrafast Electrothermal Heater

High quality graphene sheets have been considered as a promising candidate in several industrial applications due to their excellent electronic and thermal conductivity. However, the mass production of high quality graphene sheets from graphite bulk is still facing great challenges. Here we demonstrated a new approach to prepare high quality graphene by mixing a solution of oxalic acid and hydrogen peroxide as the electrolyte. The reaction did not involve the oxidation of graphite and thus exfoliated graphene possesses a uniform lateral size (2–6 μm, 78.1%), low oxygen content (2.41 at. %), few structure defects, and high conductivity of 26 692 S m^–1. The optimized mixed electrolyte is environmental friendly, cheap and safe, and most importantly it is easy to be removed through low temperature heating, which facilitates graphene purification. An electrothermal heater, made from highly concentrated graphene ink (8.5 mg mL^–1) on A4-size paper or polyester, exhibits excellent performance: a rapid rise of temperature (up to 75.2 °C) in a short time (30 s) under a low voltage of 10 V. The as-made graphene is considered as a promising material for future application of printable electronics and wearable devices

Kinetically Enhanced Bubble-Exfoliation of Graphite toward High-Yield Preparation of High-Quality Graphene

Kinetically Enhanced Bubble-Exfoliation of Graphite toward High-Yield Preparation of High-Quality Graphen

Selenium Doped Graphene Quantum Dots as an Ultrasensitive Redox Fluorescent Switch

A new reversible fluorescent switch for the detection of oxidative hydroxyl radical (^•OH) and reductive glutathione (GSH) was designed based on the use of selenium doped graphene quantum dots (Se-GQDs). The Se-GQDs have a thickness of 1–3 atomic layers, a lateral size of 1–5 nm, a quantum yield of 0.29, and a photoluminescence lifetime of 3.44 ns, which ensured a high selectivity and stability for the fluorescent switch. The fluorescence of Se-GQDs was reversibly quenched and recovered by ^•OH and GSH, respectively, because of the reversible oxidation of C–Se groups and reduction of Se–Se groups. This brand-new GQD-based fluorescent switch gave a rapid response when tested in both aqueous solutions and living HeLa cells. In particular, the detection limit for ^•OH was only 0.3 nM, which was much lower than that in switches made from organic dyes

Facile and Highly Effective Synthesis of Controllable Lattice Sulfur-Doped Graphene Quantum Dots via Hydrothermal Treatment of Durian

Recently, the biomass “bottom-up” approach for the synthesis of graphene quantum dots (GQDs) has attracted broad interest because of the outstanding features, including low-cost, rapid, and environmentally friendly nature. However, the low crystalline quality of products, substitutional doping with heteroatoms in lattice, and ambiguous reaction mechanism strongly challenge the further development of this technique. Herein, we proposed a facile and effective strategy to prepare controllable sulfur (S) doping in GQDs, occurring in a lattice substitution manner, by hydrothermal treatment of durian with platinum catalyst. S atoms in GQDs are demonstrated to exist in the thiophene structure, resulting in good optical and chemical stabilities, as well as ultrahigh quantum yield. Detailed mechanism of the hydrothermal reaction progress was investigated. High-efficiency reforming cyclization provided by platinum was evidenced by the coexistence of diversified sp²-fused heterocyclic compounds and thiophene derivatives. Moreover, we also demonstrated that saccharides in durian with small molecular weight (<1000 Da) is the main carbon source for the forming GQDs. Because of the desulfurizing process, controllable photoluminescence properties could be achieved in the as-prepared GQDs via tuning doping concentrations

Green and Mild Oxidation: An Efficient Strategy toward Water-Dispersible Graphene

Scalable fabrication of water-dispersible graphene (W-Gr) is highly desirable yet technically challenging for most practical applications of graphene. Herein, a green and mild oxidation strategy to prepare bulk W-Gr (dispersion, slurry, and powder) with high yield was proposed by fully exploiting structure defects of thermally reduced graphene oxide (TRGO) and oxidizing radicals generated from hydrogen peroxide (H₂O₂). Owing to the increased carboxyl group from the mild oxidation process, the obtained W-Gr can be redispersed in low-boiling solvents with a reasonable concentration. Benefiting from the modified surface chemistry, macroscopic samples processed from the W-Gr show good hydrophilicity (water contact angle of 55.7°) and excellent biocompatibility, which is expected to be an alternative biomaterial for bone, vessel, and skin regeneration. In addition, the green and mild oxidation strategy is also proven to be effective for dispersing other carbon nanomaterials in a water system

Surface Modification of C₃N₄ through Oxygen-Plasma Treatment: A Simple Way toward Excellent Hydrophilicity

We developed a universal method to prepare hydrophilic carbon nitrogen (C₃N₄) nanosheets. By treating C₃N₄ nanosheets with oxygen plasma, hydroxylamine groups (N–OH) with intense protonation could be introduced on the surface; moreover, the content of N–OH groups increased linearly with the oxygen-plasma treatment time. Thanks to the excellent hydrophilicity, uniformly dispersed C₃N₄ solution were prepared, which was further translated into C₃N₄ paper by simple vacuum filtration. Pure C₃N₄ paper with good stability, excellent hydrophilicity, and biocompatibility were proved to have excellent performance in tissue repair. Further research demonstrated that the oxygen-plasma treatment method can also introduce N–OH groups into other nitrogen-containing carbon materials (NCMs) such as N-doped graphene, N-doped carbon nanotube, and C₂N, which offers a new perspective on the surface modification and functionalization of these carbon nanomaterials

Electrochemical Cutting in Weak Aqueous Electrolytes: The Strategy for Efficient and Controllable Preparation of Graphene Quantum Dots

The controllable and efficient electrochemical preparation of highly crystalline graphene quantum dots (GQDs) in an aqueous system is still challenging. Here, we developed a weak electrolyte-based (typically an ammonia solution) electrochemical method to enhance the oxidation and cutting process and therefore achieve a high yield of GQDs. The yield of GQDs (3–8 nm) is 28%, approximately 28 times higher than the yield of GQDs prepared by other strong electrolytes. The whole preparation process can be accomplished within 2 h because of the effective free radical oxidation process and the suppressed intercalation-induced exfoliation in weakly ionized aqueous electrolytes. The GQDs also showed excellent crystallinity which is obviously better than the crystallinity of GQDs obtained via bottom-up approaches. Moreover, amino-functionalization of GQDs can be realized by manipulating the electrolyte concentration. We further demonstrate that the proposed method can also be expanded to other weak electrolytes (such as HF and H₂S) and different anode precursor materials (such as graphene/graphite papers, carbon fibers, and carbon nanotubes)