Thermal Annealing Approach Research Articles

Facile preparation of nitrogen-doped reduced graphene oxide as a metal-free catalyst for oxygen reduction reaction

The electronic and chemical properties of reduced graphene oxide (RGO) can be modulated by chemical doping foreign atoms and functional moieties. Nitrogen-doped reduced graphene oxide (N-RGO) is a promising candidate for oxygen reduction reaction (ORR) in fuel cells. However, there are still some challenges in further preparation and modification of N-RGO. In this work, a low-cost industrial material, urea, was chosen to modify RGO by a facile, catalyst-free thermal annealing approach in large scale. The obtained N-RGO, as a metal-free catalyst for oxygen reduction was characterized by XRD, XPS, Raman, SEM, TEM, and electrochemical measurements. It was found that the optimum synthesis conditions were a mass ratio of graphene oxide and urea equal to 1:10 and an annealing temperature of 800 °C. Detailed X-ray photoelectron spectrum analysis of the optimum product shows that the atomic percentage of N-RGO samples can be adjusted up to 2.6 %, and the resultant product can act as an efficient metal-free catalyst, exhibiting enhanced electrocatalytic properties for ORR in alkaline electrolytes. This simple, cost-effective, and scalable approach opens up the possibility for the synthesis of other nitrogen doping materials in gram-scale. It can be applied to various carbon materials for the development of other metal-free efficient ORR catalysts for fuel cell applications, and even new catalytic materials for applications beyond fuel cells.

Traps in Regioregular Poly(3-hexylthiophene) and its Blend with [6,6]-Phenyl-C61-Butyric Acid Methyl Ester for Polymer Solar Cells

Thermally stimulated current (TSC) technique is used to characterize traps in the regioregular poly(3-hexylthiophene) (rr-P3HT) and its blend with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). A hole trap in bulk rr-P3HT and an interfacial hole trap located at indium tin oxide (ITO)/rr-P3HT interface are revealed from the TSC measurement. Besides, molecular oxygen (O2) can form a deep electron trap with an onset of detrapping temperature at 225 K in rr-P3HT, in which O2 is located at the main chain region and the detrapping process is induced by chain motions under elevated temperature. In the blend of rr-P3HT:PCBM (1:1 w/w), additional hole trap states are generated in this blend system as compared to those of pure rr-P3HT and PCBM; however, these hole trap states can be reduced by thermal or solvent annealing approaches. Similar to rr-P3HT, a deep electron trap with an onset of detrapping temperature at 250 K can be formed in the blend after O2 exposure. In the case of low PCBM content in the blend (rr-P3HT:PCBM weight ratio of 4:1), an additional electron trap is generated.

Nitrogen-doped reduced-graphene oxide as an efficient metal-free electrocatalyst for oxygen reduction in fuel cells

The electronic and chemical properties of reduced-graphene oxide (RGO) can be modulated by chemical doping of foreign atoms and functional moieties. In this work, a low-cost industrial material, 5-aminotetrazole monohydrate (AM) was chosen to modify RGO by a facile, catalyst-free thermal annealing approach in largescale. The obtained nitrogen-doped reduced-graphene oxide (N-RGO), as a metal-free catalyst for oxygen reduction, was characterized by XRD, XPS, Raman, SEM, TEM and electrochemical measurements. It was found that the optimum synthesis conditions were a mass ratio of graphene oxide (GO) and AM equal to 1 : 25 and an annealing temperature of 700 °C. Detailed X-ray photoelectron spectrum analysis of the optimum product shows that the atomic percentage of the N-RGO samples can be adjusted up to 10.6%. Electrochemical characterizations clearly demonstrate excellent electrocatalytic activity of N-RGO toward the oxygen reduction reaction (ORR) in alkaline electrolytes via a four-electron pathway. The total content of graphitic and pyridinic nitrogen atoms is the key factor to enhance the current density in the electrocatalytic activity for ORR. This simple, cost-effective and scalable approach opens up the possibility for the synthesis of other nitrogen doping materials in gram-scale. It can be applied to various carbon materials for the development of other metal-free efficient ORR catalysts for fuel cell applications and even new catalytic materials for applications beyond fuel cells.

Synthesis of boron doped graphene for oxygen reduction reaction in fuel cells

Boron atoms, with strong electron-withdrawing capability, are doped into graphene frameworks forming boron doped graphene (BG) via a catalyst-free thermal annealing approach in the presence of boron oxide. Atomic force microscopic (AFM) and transmission electron microscopic (TEM) characterizations reveal that the as-prepared BG has a flake-like structure with an average thickness of ca. 2 nm. X-ray photoelectron spectroscopy (XPS) analysis demonstrates that boron atoms can be successfully doped into graphene structures with the atomic percentage of 3.2%. Due to its particular structure and unique electronic properties, the resultant BG exhibits excellent electrocatalytic activity towards oxygen reduction reaction (ORR) in alkaline electrolytes, similar to the performance of Pt catalysts. In addition, the non-metallic BG catalyst shows long-term stability and good CO tolerance superior to that of Pt-based catalysts. These results demonstrate that the BG, as a promising candidate in advanced electrode materials, may substitute Pt-based nanomaterials as a cathode catalyst for ORR in fuel cells as well as other electrochemical applications similar to the reported nitrogen doped graphene.

Facile thermal treatment process for assembling vertically aligned semiconductor nanorods in solution

Large-area films of vertically-aligned semiconductor nanorods have the potential to be useful, active materials for optoelectronic devices. We demonstrate here a highly facile thermal annealing approach for reversibly assembling 28 nm long CdSe nanorods into vertically aligned arrays in solution. Using temperature to control solvent strength, aggregated nanorods in a marginally poor solvent mixture were first dispersed at elevated temperatures and then reassembled into freely suspended, ordered sheets of aligned nanorods up to 24 μm in diameter upon slow cooling. The assembly method was tolerant of nanorod polydispersity and was effective over a wide range of solvents and nanorod concentrations. The pre-assembled nanorods could be directly drop-cast from solution onto a substrate and rapidly dried to obtain a film of vertically aligned nanorods.

Responses of 6FDA-based polyimide thin membranes to CO 2 exposure and physical aging as monitored by gas permeability

Membranes made from glassy polymers have been of great interest in the past decade for CO 2 removal from natural gas streams; however, strongly soluble gases, such as CO 2, can cause “plasticization” of polymer membranes, which greatly reduces the separation efficiency. This work examines the response of several 6FDA-based polyimides thin film membranes with thicknesses around 200 nm to CO 2 exposure and physical aging. DABA units are incorporated to create crosslinkable sites for such materials. Introducing DABA units to the 6FDA-DAM and 6FDA-mPDA polymers seems to result in materials even more prone to CO 2 plasticization. A unique thermal annealing approach is used to crosslink the polyimides via decarboxylation of the DABA units; the resulting crosslinked polymers appear to be much more plasticization resistant at high CO 2 pressures compared to their DABA containing counterparts prior to crosslinking. Prior thermal history plays a significant role in both the physical aging of the thin film membranes and their CO 2 plasticization resistance particularly for chemical structures that tend to lead to high free volume and permeability.

Catalyst-Free Synthesis of Nitrogen-Doped GrapheneviaThermal Annealing Graphite Oxide with Melamine and Its Excellent Electrocatalysis

The electronic and chemical properties of graphene can be modulated by chemical doping foreign atoms and functional moieties. The general approach to the synthesis of nitrogen-doped graphene (NG), such as chemical vapor deposition (CVD) performed in gas phases, requires transitional metal catalysts which could contaminate the resultant products and thus affect their properties. In this paper, we propose a facile, catalyst-free thermal annealing approach for large-scale synthesis of NG using low-cost industrial material melamine as the nitrogen source. This approach can completely avoid the contamination of transition metal catalysts, and thus the intrinsic catalytic performance of pure NGs can be investigated. Detailed X-ray photoelectron spectrum analysis of the resultant products shows that the atomic percentage of nitrogen in doped graphene samples can be adjusted up to 10.1%. Such a high doping level has not been reported previously. High-resolution N1s spectra reveal that the as-made NG mainly contains pyridine-like nitrogen atoms. Electrochemical characterizations clearly demonstrate excellent electrocatalytic activity of NG toward the oxygen reduction reaction (ORR) in alkaline electrolytes, which is independent of nitrogen doping level. The present catalyst-free approach opens up the possibility for the synthesis of NG in gram-scale for electronic devices and cathodic materials for fuel cells and biosensors.

Room temperature damage removal from InP by optical pumping

Removal of ion-implantation-induced damage in InP is observed to a considerable extent by optical pumping using argon ion laser irradiation at room temperature. The damage takes the form of recombination, scattering, and compensation centers, which were generated by Be implantation with dose of 1010–1012/cm2. We find that minority carrier injection by optical pumping is an attractive alternative to technologies based on a thermal annealing approach for the removal of these centers.