RESUMO
Graphitic carbon nitride (g-C3N4) is a promising nonmetallic photocatalyst. In this manuscript, B-doped 3D flower-like g-C3N4 mesoporous nanospheres (BMNS) were successfully prepared by self-assembly method. The doping of B element promotes the internal growth of hollow flower-like g-C3N4 without changing the surface roughness structure, resulting in a porous floc structure, which enhances the light absorption and light reflection ability, thereby improving the light utilization rate. In addition, B element provides lower band gap, which stimulates the carrier movement and increases the activity of photogenerated carriers. The photocatalytic mechanism and process of BMNS were investigated in depth by structural characterization and performance testing. BMNS-10 % shows good degradation for four different pollutants, among which the degradation effect on Rhodamine B (RhB) reaches 97 % in 30 min. The apparent rate constant of RhB degradation by BMNS-10 % is 0.125 min-1, which is 46 times faster compared to bulk g-C3N4 (BCN). And the photocatalyst also exhibits excellent H2O2 production rate under visible light. Under λ > 420 nm, the H2O2 yield of BMNS-10 % (779.9 µM) in 1 h is 15.9 times higher than that of BCN (48.98 µM). Finally, the photocatalytic mechanism is proposed from the results of free radical trapping experiments.
RESUMO
The graphene oxide (GO) was found to be able to stabilize organic molecules including energetic compounds. However, the inherent mechanisms of such stabilization effects are still not well-known. Herein, various transition metal complexes of triaminoguanidine nitrate (TAGN) using GO as a dopant have been prepared and evaluated. It has been shown that the presence of GO could great improve the thermal stability of the resulted TAG-based complexes. The physical models governing their thermolysis for their initial rate-limiting decomposition steps are obtained using the state-of-the-art evaluation methods. These physical models are further supported by analyses of the overall gaseous products. In addition, the reaction pathways are proposed to explain the stabilization mechanisms of GO. For instance, by interaction of GO, the release of N2 from TAG-Ni was greatly postponed. There is a broad secondary peak at temperature of 378 °C due to decomposition of the nickel nitrides, as the primary thermolysis intermediates of TAG-Ni. The formation of cobalt nitrides plays a significant role on decomposition of TAG-Co and G-T-Co, which results in much less heat release and mass loss in comparison to TAG-Ni.
RESUMO
As cutting-edge emerging electromagnetic (EM) wave-absorbing materials, the Achilles' heel of graphenes is vulnerable to oxidation under high temperature and oxygen atmosphere, particularly at temperatures more than 600 °C. Herein, a graphene@Fe3O4/siliconboron carbonitride (SiBCN) nanocomplex with a hierarchical A/B/C structure, in which SiBCN serves as a "shield" to protect graphene@Fe3O4 from undergoing high-temperature oxidation, was designed and tuned by polymer-derived ceramic route. The nanocomplexes are stable even at 1100-1400 °C in either argon or air atmosphere. Their minimum reflection coefficient (RCmin) and effective absorption bandwidth (EAB) are -43.78 dB and 3.4 GHz at ambient temperature, respectively. After oxidation at 600 °C, they exhibit much better EM wave absorption, where the RCmin decreases to -66.21 dB and EAB increases to 3.69 GHz in X-band. At a high temperature of 600 °C, they also possess excellent and promising EW wave absorption, for which EAB is 3.93 GHz, covering 93.6% range of X-band. In comparison to previous works on graphenes, either the EAB or the RCmin of these nanocomplexes is excellent at high-temperature oxidation. This novel nanomaterial technology may shed light on the downstream applications of graphenes in EM-wave-absorbing devices and smart structures worked in harsh environments.
RESUMO
High-temperature stable and metal-free siliconboron carbonitride ceramics with high electromagnetic (EM) wave-absorbing efficiency were achieved through the structural design and pyrolysis of carbon-rich hyperbranched polyborosilazane precursors with pendent phenyl groups. The introduction of benzene rings into the precursors dramatically changes the microstructure and the EM wave-absorbing property of ceramics. It reveals that the ceramics pyrolyzed from the benzene ring-containing preceramic precursors have a higher carbon content and a larger number of sp2 carbons and generate crystalline carbons (graphitic carbons and tubular carbons) in situ, which lead to excellent EM wave-absorbing properties. The EM wave absorption efficiency and effective absorption bandwidth (EAB, reflection coefficient (RC) below -10 dB) can be tuned via annealing of the ceramics. The ceramics stable at 1320 °C exhibit their optimized EM wave-absorbing performance with a minimum RC (RCmin) of -71.80 dB and an EAB of 3.65 GHz (8.2-11.85 GHz). We believe that the research extends the design strategy of advanced EM wave-absorbing functional materials, which have great potential as promising absorbers in commercial or military applications.