RESUMEN
Graphitic carbon nitride (g-C3N4) has been regarded as an intriguing photocatalyst applying to hydrogen generation but suffering rapid recombination of photoinduced electron-hole pairs and insufficient absorption under visible light. We developed a novel one-pot thermal copolymerization method of melamine as a precursor and 7,7,8,8-tetracyanoquinodimethane (TCNQ) as a comonomer to synthesize modified g-C3N4 (abbreviated as X% TCNQ) for the first time, aiming to directly incorporate TCNQ molecular into carbon nitride skeleton for the substitution of low-electronegative carbon for high-electronegative nitride atom. Results revealed that the as-prepared photocatalysts by copolymerization of melamine with TCNQ retained the original framework of g-C3N4, and dramatically altered the electronic and optical properties of carbon nitride. Various measurements confirmed that as-synthesized samples exhibited larger specific surface areas, faster photogenerated charge transfer and broader optical absorption by decreasing the π-deficiency and extending the π-conjugated system, thus facilitating the photocatalytic activity. Specifically, the 0.3% TCNQ exhibited as high as seven times than the pristine g-C3N4 on photocatalytic H2 generation and kept its photoactivity for five circles. This work highlights a feasible approach of chemical protocols for the molecular design to synthesize functional carbon nitride photocatalysts by copolymerizing appropriate g-C3N4 precursor and comonomers.
RESUMEN
Novel two-dimensional silicon-based material siloxene has been synthesized handily by a one-step method, which utilizes the characteristics of the topological exfoliation to simplify the process of synthesis and modification. It is worth mentioning that for the first time amino-modified derivative has been investigated. Amino modification can promote the oxidation of siloxene, enlarge the bandgap and extend the carrier lifetime of siloxene. The application of siloxene before and after modification in water-splitting has been investigated. In addition, the superiority of the resultant two-dimensional materials was concisely elaborated, which revealed that owing to more effective photogenerated carriers' separation in amino modification siloxene, hydrogen production could be greatly promoted.
RESUMEN
Photocatalytic hydrogen evolution by water splitting has become a very effective way to solve the energy crisis. For use in that process, graphitic carbon nitride (g-C3N4) has drawn much attention for its response in the visible region. However, its insufficient sunlight absorption efficiency and easy recombination of photoinduced carriers restrict its photocatalytic activity. Herein, we demonstrate a two-step liquid ultrasonic method in water to synthesize a series of tetracyanoquinodimethane (TCNQ)-C3N4 photocatalysts aiming to form a conjugated structure by 7,7,8,8-TCNQ. g-C3N4 was treated with APTES firstly on its surface in order to give a better interface contact with TCNQ. Benefiting from the conjugation effect between TCNQ and g-C3N4, the separation and transport efficiency of photogenerated carriers were significantly improved. Besides, introducing TCNQ also broadened the absorption region. Both of these points lead to the enhancement of photocatalytic H2 production rate, with the optimized 5% TCNQ-C3N4 giving a rate nearly 9.48 times that of pure g-C3N4. Also, 5% TCNQ-C3N4 (U) was prepared with unmodified g-C3N4, which exhibited a rate only 6.87 times that of pure g-C3N4, thus validating the necessity of surface modification. Our work reveals that the rational conjugated structure could modulate the electrical and optical properties of g-C3N4, yielding an improvement of photocatalytic activities.
RESUMEN
Plasmonic noble metal has been applied in photocatalytic materials, and TiO2 with plasmonic noble metal has been studied for a long time. In this work, we have fabricated incomplete covered Au/Ag alloy nanoshuttle-TiO2 nanomaterials with 268.7 µmol g-1 h-1 H2-evolution in a simple solution method. The considerable photocatalytic performance is mainly due to the enhanced surface plasmon resonance effect of Au/Ag alloy nanoshuttles. It has been found that TiO2 clusters attached to the Au/Ag nanoshuttles surface migrate under electrons irradiation and cover the exposed Au/Ag NS surface to achieve thermodynamic stability, which results in instability of photocatalytic performance. The mechanism has been discussed in detail.
RESUMEN
It is a conventional practice to adopt Weibull statistics with a modulus of 4 for characterizing the statistical distribution of cleavage fracture toughness of ferritic steels, albeit based on a rather weak physical justification. In this study, a statistical model for cleavage fracture toughness of ferritic steels is proposed according to a new local approach model. The model suggests that there exists a unique correlation of the cumulative failure probability, fracture toughness and yield strength. This correlation is validated by the Euro fracture toughness dataset for 1CT specimens at four different temperatures, which deviates from the Weibull statistical model with a modulus of four.
RESUMEN
Two-parameter Weibull statistics is commonly used for characterizing and modeling strength distribution of biomedical materials and its size dependence. The calibrated scale parameter and shape factor are usually sensitive to specimen size. Since Weibull statistics is subject to the weakest link postulate, this work proposed to directly resort to the weakest-link formulation for the cumulative failure probability to characterize size effect on strength distribution of quasi-brittle biomedical materials. As a preliminary examination, the approach was assessed by two sets of published strength data. It shows that the resultant expression for the cumulative probability follows either Weibull distribution or other type of distributions. The calibrated model parameters are independent of specimen size, so they can be used to transfer strength distribution from one set of specimens to another set of specimens with geometrical similarity under same loading mode. These initial results motivate a more comprehensive validation of the proposed approach to proceed via a larger set of case studies covering different quasi-brittle biomedical materials over a wider range of size variation.