Cong-Cong Shen

Enhanced visible-light photocatalytic activity of Z-scheme graphitic carbon nitride/oxygen vacancy-rich zinc oxide hybrid photocatalysts

With the objectives of enhancing the stability, optical properties and visible-light photocatalytic activity of photocatalysts, we modified oxygen vacancy-rich zinc oxide (Vo-ZnO) with graphitic carbon nitride (g-C3N4). The resulting g-C3N4/Vo-ZnO hybrid photocatalysts showed higher visible-light photocatalytic activity than pure Vo-ZnO and g-C3N4. The hybrid photocatalyst with a g-C3N4 content of 1 wt% exhibited the highest photocatalytic degradation activity under visible-light irradiation (λ ≥ 400 nm). In addition, the g-C3N4/Vo-ZnO photocatalyst was not deactivated after five cycles of methyl orange degradation, indicating that it is stable under light irradiation. Finally, a Z-scheme mechanism for the enhanced photocatalytic activity and stability of the g-C3N4/Vo-ZnO hybrid photocatalyst was proposed. The fast charge separation and transport within the g-C3N4/Vo-ZnO hybrid photocatalyst were attributed as the origins of its enhanced photocatalytic performance.

Carbon-Coated Fe3O4/VOx Hollow Microboxes Derived from Metal–Organic Frameworks as a High-Performance Anode Material for Lithium-Ion Batteries

As the ever-growing demand for high-performance power sources, lithium-ion batteries with high storage capacities and outstanding rate performance have been widely considered as a promising storage device. In this work, starting with metal-organic frameworks, we have developed a facile approach to the synthesis of hybrid Fe3O4/VOx hollow microboxes via the process of hydrolysis and ion exchange and subsequent calcination. In the constructed architecture, the hollow structure provides an efficient lithium ion diffusion pathway and extra space to accommodate the volume expansion during the insertion and extraction of Li+. With the assistance of carbon coating, the obtained Fe3O4/VOx@C microboxes exhibit excellent cyclability and enhanced rate performance when employed as an anode material for lithium-ion batteries. As a result, the obtained Fe3O4/VOx@C delivers a high Coulombic efficiency (near 100%) and outstanding reversible specific capacity of 742 mAh g-1 after 400 cycles at a current density of 0.5 A g-1. Moreover, a remarkable reversible capacity of 556 mAh g-1 could be retained even at a current density of 2 A g-1. This study provides a fundamental understanding for the rational design of other composite oxides as high-performance electrode materials for lithium-ion batteries.

Nanoheterostructured photocatalysts for improving photocatalytic hydrogen production

Abstract Rapid industrialization has accordingly increased the demand for energy. This has resulted in the increasingly severe energy and environmental crises. Hydrogen production, based on the photocatalytic water splitting driven by sunlight, is able to directly convert solar energy into a usable or storable energy resource, which is considered to be an ideal alternative energy source to assist in solving the energy crisis and environmental pollution. Unfortunately, the hydrogen production efficiency of single phase photocatalysts is too low to meet the practical requirements. The construction of heterostructured photocatalyst systems, which are comprised of multiple components or multiple phases, is an efficient method to facilitate the separation of electron-hole pairs to minimize the energy-waste, provide more electrons, enhance their redox ability, and hence improve the photocatalytic activity. We summarize the recent progress in the rational design and fabrication of nanoheterostructured photocatalysts. The heterojunction photocatalytic hydrogen generation systems can be divided into type-I, type-II, pn-junction and Z-scheme junction, according to the differences in the transfer of the photogenerated electrons and holes. Finally, a summary and some of the challenges and prospects for the future development of heterojunction photocatalytic systems are discussed.

Large improvement of visible-light photocatalytic H2-evolution based on cocatalyst-free Zn0.5Cd0.5S synthesized through a two-step process

Final metal sulfides Zn0.5Cd0.5S (ZnCdS-CH) are synthesized through a coprecipitation process followed by hydrothermal treatment. The morphological, structural and optical properties have been investigated extensively via diverse analytical techniques. The ZnCdS-CH solid solution without noble metal loading is employed in photocatalytic H2 evolution under visible light irradiation (λ ≥ 420 nm) and achieves a superior activity rate of 0.971 mmol h−1, which exceeds those of coprecipitated Zn0.5Cd0.5S (ZnCdS-C) samples by more than 13 times. Moreover, in the recycle test, the ZnCdS-CH photocatalyst shows a stable photocatalytic activity for H2 evolution under long-term visible-light irradiation. Characterization analyses demonstrate that the excellent photocatalytic H2-evolution performance of the ZnCdS-CH sample arises predominantly from the two-step processing procedure of coprecipitation followed by hydrothermal treatment at 200 °C, which makes it possess a hexagonal (wurtzite) structure, good dispersity, enhanced crystallinity, an appropriate band gap, a more negative conduction band, as well as a large number of surface defect states. This finding is of great significance for designing a facile, reproducible and inexpensive method to realise the potential of ZnxCd1−xS ternary metal sulfides in the field of H2 evolution by water splitting.

Synthesis of nanoporous structured iron carbide/Fe–N–carbon composites for efficient oxygen reduction reaction in Zn–air batteries

Large-scale industrial level applications of fuel cells and metal–air batteries have called for the development of highly efficient and low-cost oxygen reduction electrodes. Here we report the effective and simple preparation of iron carbide-embedded Fe–N-doped carbon (Fe3C/Fe–N/C) composites using an iron–phenanthroline (Fe–Phen) complex and dicyandiamide (DCA) as the precursors that are subsequently heat treated. The optimal catalyst pyrolyzed at 800 °C (Fe–Phen–N-800) exhibits superior oxygen reduction activity with onset and half-wave potentials of 0.99 and 0.86 V in 0.1 M KOH, respectively, which are higher than those of Pt/C (onset and half-wave potentials of 0.98 and 0.84 V) at the same catalyst loading. Moreover, the obtained Fe–Phen–N-800 displays comparable activity to Pt/C in 0.1 M HClO4 solution. Notably, the well-developed Fe–Phen–N-800 catalyst shows much higher long-term stability and better methanol tolerance than Pt/C. The results suggest that our catalyst is one of the most promising candidates to replace Pt catalysts toward oxygen reduction. Strikingly, a primary Zn–air battery using Fe–Phen–N-800 as the air cathode catalyst delivers higher voltages and gravimetric energy densities than those of a Pt/C-based system at the discharge current densities of 10 and 25 mA cm−2, thus demonstrating the potential applications of our catalyst for energy conversion devices.

Hydrogenation/oxidation induced efficient reversible color switching between methylene blue and leuco-methylene blue

In this paper, we present the use of graphitic carbon nitride (g-C3N4) supported palladium nanoparticles (Pd/g-C3N4) for reversible color switching of methylene blue (MB). g-C3N4 with a high polymeric degree could improve the dispersity of Pd nanoparticles, contributing to fast color switching of MB as the agglomeration of metal nanoparticles is significantly prevented. Moreover, strong metal-support interaction (SMSI) between Pd nanoparticles and g-C3N4 support promotes the adsorption and subsequent dissociation of molecular hydrogen and oxygen, thus leading to efficient reversible conversion between MB and leuco-methylene blue (LMB). Our obtained Pd/g-C3N4 nanocatalyst exhibits outstanding hydrogenation activity of blue MB to colorless LMB with a turnover frequency as high as 165 h−1 at room temperature, moreover, colorless LMB can quickly switch back to MB upon exposing the same reaction system to oxygen for oxidation. It is noted that our color switching system exhibits remarkable reversibility and stability without obvious fatigue even after 10 consecutive cycles. This novel redox-driven reversible color switching system could find potential in food packaging, sensing and organic transformations.

Oxygen deficient Pr6O11 nanorod supported palladium nanoparticles: highly active nanocatalysts for styrene and 4-nitrophenol hydrogenation reactions

The design and development of highly efficient and long lifetime Pd-based catalysts for hydrogenation reactions have attracted significant research interest over the past few decades. Rational selection of supports for Pd loadings with strong metal-support interaction (SMSI) is beneficial for boosting catalytic activity and stability. In this context, we have developed a facile approach for uniformly immobilizing ultra-small Pd nanoparticles (NPs) with a clean surface on a Pr6O11 support by a hydrogen thermal reduction method. The hydrogenations of p-nitrophenol and styrene are used as model reactions to evaluate the catalytic efficiency. The results show highly efficient styrene hydrogenation performance under 1 atm H2 at room temperature with a TOF value as high as 8957.7 h−1, and the rate constant value of p-nitrophenol reduction is 0.0191 s−1. Strong metal-support interaction and good dispersion of Pd nanoparticles, as demonstrated by XPS and HRTEM results, contribute to the excellent hydrogenation performance. Electron paramagnetic resonance (EPR) results suggest the presence of oxygen vacancies in the support, which serve as electron donors and enhance the adsorption and activation of reactants and subsequent conversion into products. Moreover, the catalyst can be recovered and reused up to 10 consecutive cycles without marked loss of activity. Overall, our results indicate that oxygen-deficient Pr6O11 nanorods (NRs) not only play a role as support but also work as the promoter to substantially boost the catalytic activities for organic transformations, therefore, providing a novel strategy to develop other high-performance nanostructured catalysts for environmental sustainability.

Ultrasmall Ni nanoparticles embedded in Zr-based MOFs provide high selectivity for CO2 hydrogenation to methane at low temperatures

Of great significance from an energy-saving viewpoint is the direct use of CO2 as a C1 source to mitigate the anthropogenic CO2 emission into the earths atmosphere and to produce methane that can be turned into chemicals and fuels. Herein, we report the use of UiO-66 metal–organic frameworks to anchor ultrasmall Ni nanoparticles (NPs), thus avoiding the sintering of Ni NPs protected by the frameworks. Transmission electron microscope images and EDX mappings show that Ni NPs with an average size of 2 nm are highly dispersed in UiO-66 (Ni@UiO-66). Moreover, the obtained catalyst with an optimal Ni loading of 20 wt% displays an outstanding activity (57.6% of CO2 conversion) and high selectivity (100%) for methane in long-term stability tests up to 100 h at a reaction temperature as low as 300 °C. Compared with Ni/ZrO2 and Ni/SiO2, the good dispersion of ultrasmall Ni NPs and the low activation energy (Ea = 68.9 kJ mol−1) facilitate a high catalytic activity, which makes Ni@UiO-66 a promising catalyst for CO2 methanation.

Hydrogen-bonding-assisted charge transfer: significantly enhanced photocatalytic H2 evolution over g-C3N4 anchored with ferrocene-based hole relay

Herein, a polymeric graphitic carbon nitride (for simplicity, g-C3N4) photocatalyst has been identified as a promising material for hydrogen production from water due to its comparatively low cost and facile modification of its electronic structure. However, to date, how to speed up the transfer rate of photogenerated charges, especially hole transfer rate, still remains a formidable challenge. Herein, a novel system of g-C3N4/1,1′-ferrocenedicarboxylic acid (FcDA) composites was developed for efficient visible light-driven H2 evolution, in which the redox mediator FcDA served as hole-transport molecules and platinum (Pt) acted as an electron sink. The FcDA molecules are anchored onto g-C3N4 by hydrogen-bonding interactions between the carboxylic groups and amino groups as well as π–π interactions between aromatic FcDA and graphitic C3N4. The matched energy-levels between g-C3N4 and FcDA facilitate photogenerated hole transfer from the g-C3N4 valence band to FcDA that results in the formation of FcDA+ radicals, and thus, photoinduced electrons and holes are efficiently separated. Due to the additional charge separation pathway, enhanced hole transfer kinetics, and the extremely rapid intermolecular radical reactions, charge recombination is effectively suppressed; therefore, more electrons can be released for hydrogen production. Under optimal experimental conditions, the developed g-C3N4/FcDA composite with 4 wt% FcDA exhibits high water splitting activity with a H2 evolution rate of up to 77.91 μmol h−1, which is nearly 6 times that of bare g-C3N4 (13.18 μmol h−1). Moreover, the obtained g-C3N4/FcDA photocatalyst displays excellent stability, and there is no obvious decrease in the H2-production rate after five test cycles. Thus, we anticipate that our simple modification strategy will offer an avenue to merge the polymeric g-C3N4 photocatalyst with surface organometallic chemistry for high-efficiency solar-to-fuel conversion.