Zhi-Wei Zhao

Superior adsorption capacity of g-C3N4 for heavy metal ions from aqueous solutions

In this work, graphitic-C3N4 (g-C3N4) was synthesized by a simple and environmentally friendly salt melt method, and characterized by using field-emission scanning and transmission electron microscopy, X-ray diffraction, Fourier transformed infrared spectroscopy, X-ray photoelectron spectroscopy and N2 adsorption-desorption analysis. The as-prepared g-C3N4 was used as an adsorbent to remove heavy metal ions from aqueous solutions. The adsorption kinetics of Pb(II) and Cu(II) followed the pseudo-second-order model. The g-C3N4 exhibited much higher adsorption capacity toward heavy metal ions (1.36 mmol/g for Pb(II), 2.09 mmol/g for Cu(II), 1.00 mmol/g for Cd(II) and 0.64 mmol/g for Ni(II)) than other adsorbents. The adsorption of Pb(II) and Cu(II) on g-C3N4 was slightly affected by ionic strength at pH<5.0 and increased with the increase of ionic strength at pH>5.0. The inner-sphere surface complexation mechanism was suitable to explain the interaction between heavy metal ions and the nitrogen- and carbon-containing functional groups of the g-C3N4. The experimental results reveal that g-C3N4 is a potential adsorbent for the removal of heavy metal ions from large volumes of aqueous solutions.

Plasmon enhanced visible light photocatalytic activity of ternary Ag2Mo2O7@AgBr–Ag rod-like heterostructures

In this work, a three-component system, Ag2Mo2O7@AgBr–Ag, has been successfully prepared through an in situ ion exchange reaction between Ag2Mo2O7 rods and Br−, followed by visible light irradiation. Using the as-prepared samples as photocatalysts, the visible light photocatalytic activity for degrading organic dyes, including methylene blue (MB) and Rhodamine B (RhB), is examined systematically. Among Ag2Mo2O7, Ag2Mo2O7/Ag, Ag2Mo2O7@AgBr, AgBr–Ag and Ag2Mo2O7@AgBr–Ag, the three-component Ag2Mo2O7@AgBr–Ag heterostructured photocatalyst is demonstrated to be superior compared to the others for dye photodegradation. The enhancements in visible light absorption and photocatalytic activity are ascribed to the synergistic effects originating from the surface plasmon resonance (SPR) of the Ag nanoparticles (NPs) and the cascade energy transfer for the effective separation of photogenerated carriers. Radical-trapping experiments demonstrate that holes (h+) are the major reactive species accounting for the degradation of MB and RhB molecules. Furthermore, five cycle tests indicate that the three-component hybrid photocatalyst Ag2Mo2O7@AgBr–Ag is highly stable and can be used repeatedly. Therefore, the Ag2Mo2O7@AgBr–Ag ternary heterostructures may be a promising candidate for use as a visible light photocatalyst for degrading organic contaminants in the near future.

Multifunctional flexible free-standing titanate nanobelt membranes as efficient sorbents for the removal of radioactive 90 Sr 2+ and 137 Cs + ions and oils

For the increasing attention focused on saving endangered environments, there is a growing need for developing membrane materials able to perform complex functions such as removing radioactive pollutants and oil spills from water. A major challenge is the scalable fabrication of membranes with good mechanical and thermal stability, superior resistance to radiation, and excellent recyclability. In this study, we constructed a multifunctional flexible free-standing sodium titanate nanobelt (Na-TNB) membrane that was assembled as advanced radiation-tainted water treatment and oil uptake. We compared the adsorption behavior of 137Cs+ and 90Sr2+ on Na-TNB membranes under various environmental conditions. The maximum adsorption coefficient value (Kd) for Sr2+ reaches 107 mL g−1. The structural collapse of the exchange materials were confirmed by XRD, FTIR and XPS spectroscopy as well as Raman analysis. The adsorption mechanism of Na-TNB membrane is clarified by forming a stable solid with the radioactive cations permanently trapped inside. Besides, the engineered multilayer membrane is exceptionally capable in selectively and rapidly adsorbing oils up to 23 times the adsorbent weight when coated with a thin layer of hydrophobic molecules. This multifunctional membrane has exceptional potential as a suitable material for next generation water treatment and separation technologies.

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.

Direct growth of cobalt-rich cobalt phosphide catalysts on cobalt foil: an efficient and self-supported bifunctional electrode for overall water splitting in alkaline media

The design of high-efficiency, economical and self-supported bifunctional electrodes for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is extremely crucial to renewable energy conversion processes, yet remains a long and arduous task. Here, we report the first example of cobalt-rich cobalt phosphide catalysts directly grown on cobalt foil (denoted as Co2P/Co-foil) as a novel non-noble metal and integrated electrode by one-step phosphorization of a pre-oxidized Co foil. Owing to the intrinsic catalytic properties of cobalt-rich cobalt phosphide and the intimate contact between Co2P and highly conductive Co foil, the resulting Co2P/Co-foil electrode exhibits excellent catalytic performances for both the HER and OER in basic solution, affording a current density of 10 mA cm−2 at low overpotentials of 157 mV for the HER and 319 mV for the OER, respectively. More importantly, these electrodes can be directly employed as both the anode and cathode in an alkaline electrolyzer, showing noble metal-like water splitting performances and long-term stability. Density functional theory (DFT) calculations suggest that the sites on the top of the P atoms in Co2P are the most active sites for the HER. This work would open an exciting new avenue to synthesize other metal-rich metal phosphide catalysts on conductive metal foil as self-supported electrodes using this facile, cost-effective and easy scale-up fabrication method for overall water splitting.

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.

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.