Meicheng Wen

Hydrogen Doped Metal Oxide Semiconductors with Exceptional and Tunable Localized Surface Plasmon Resonances

Heavily doped semiconductors have recently emerged as a remarkable class of plasmonic alternative to conventional noble metals; however, controlled manipulation of their surface plasmon bands toward short wavelengths, especially in the visible light spectrum, still remains a challenge. Here we demonstrate that hydrogen doped given MoO3 and WO3 via a facile H-spillover approach, namely, hydrogen bronzes, exhibit strong localized surface plasmon resonances in the visible light region. Through variation of their stoichiometric compositions, tunable plasmon resonances could be observed in a wide range, which hinge upon the reduction temperatures, metal species, the nature and the size of metal oxide supports in the synthetic H2 reduction process as well as oxidation treatment in the postsynthetic process. Density functional theory calculations unravel that the intercalation of hydrogen atoms into the given host structures yields appreciable delocalized electrons, enabling their plasmonic properties. The plasmonic hybrids show potentials in heterogeneous catalysis, in which visible light irradiation enhanced catalytic performance toward p-nitrophenol reduction relative to dark condition. Our findings provide direct evidence for achieving plasmon resonances in hydrogen doped metal oxide semiconductors, and may allow large-scale applications with low-price and earth-abundant elements.

Synthesis of Ce ions doped metal–organic framework for promoting catalytic H2 production from ammonia borane under visible light irradiation

A variety of physical properties such as optical, fluorescence, and guest absorption capacity integrated within hybrid MOF based materials may show synergetic effects for significantly enhancing the performance of many applications. In this study, new hybrid MOFs, cerium doped chromium based amine-functionalized metal–organic frameworks (CeMIL-101), were successfully synthesized for the first time, via hydrothermal treatment with the aim to enhance the photocatalytic activity of H2 production from ammonia borane (AB) under the assistance of visible light. The results analyzed by X-ray diffraction and X-ray adsorption near-edge structure strongly support the effective doping of Ce ions into the nodes of MIL-101. The catalytic activity of Pd/CeMIL-101 is remarkably higher than that of Pd/MIL-101 without Ce doping under visible light irradiation at ambient temperature. Pd/CeMIL-101 is highly efficient in generating the hydroxyl radicals and photo-excited charge transfer under visible light irradiation. Moreover, the presence of valence fluctuation of Ce3+/Ce4+ was the main factor for reducing the recombination rate of photo-excited charges. This study may bring an effective strategy for the design and development of highly active hybrid materials for achieving high photocatalytic activity.

Non-Noble-Metal Nanoparticle Supported on Metal–Organic Framework as an Efficient and Durable Catalyst for Promoting H2 Production from Ammonia Borane under Visible Light Irradiation

In this work, we propose a straightforward method to enhance the catalytic activity of AB dehydrogenation by using non-noble-metal nanoparticle supported on chromium-based metal-organic framework (MIL-101). It was demonstrated to be effective for hydrogen generation from ammonia borane under assistance of visible light irradiation as a noble-metal-free catalyst. The catalytic activity of metal nanoparticles supported on MIL-101 under visible light irradiation is remarkably higher than that without light irradiation. The TOFs of Cu/MIL-101, Co/MIL-101, and Ni/MIL-101 are 1693, 1571, and 3238 h(-1), respectively. The enhanced activity of catalysts can be primarily attributed to the cooperative promoting effects from both non-noble-metal nanoparticles and photoactive metal-organic framework in activating the ammonia borane molecule and strong ability in the photocatalytic production of hydroxyl radicals, superoxide anions, and electron-rich non-noble-metal nanoparticle. This work sheds light on the exploration of active non-noble metals supported on photoactive porous materials for achieving high catalytic activity of various redox reactions under visible light irradiation.

Synthesis of highly visible light active TiO2-2-naphthol surface complex and its application in photocatalytic chromium(VI) reduction

Photocatalysis is an effective approach for the removal of heavy metal ions present in the aquatic bodies. In this report, TiO2 nanoparticles were successfully functionalized with 2-naphthol (2-NAP) using simple and scalable condensation reaction. The prepared photocatalyst was demonstrated as superior visible light photocatalyst for the effective reduction of Cr(VI). The 2-NAP functionalized TiO2 displayed a remarkable enhancement in the photocatalytic reduction of Cr(VI) under visible light irradiation (λ > 400 nm). The maximum Cr(VI) reduction of about 100% (7 fold higher activity than bare TiO2) was achieved within 3 h. The discernible enhancement in the photocatalytic reduction of TiO2-2-NAP can be ascribed to improved optical absorption in visible region, high crystallinity of TiO2 and high surface area. In addition, the photogenerated electron transfer from 2-NAP to TiO2 (ligand to metal transfer) can significantly improved the photocatalytic performance than bare TiO2 counterparts. Therefore, the functionalization of metal oxides with organic ligands can open new directions to overcome the existing limitations in photocatalysis.

High-surface-area plasmonic MoO3−x: rational synthesis and enhanced ammonia borane dehydrogenation activity

Well-crystallized, high-surface-area plasmonic MoO3−x was synthesized by combining an evaporation induced self-assembly (EISA) process and a subsequent hydrogen reduction at a certain temperature. The intrinsic anisotropic crystal growth process of tiny MoO3 nuclei to MoO3 nanosheets was successfully inhibited. Detailed characterization by means of XRD, TEM, N2 physisorption, and XPS measurements revealed that the synthesized MoO3−x not only showed a strong localized surface plasmon resonance (LSPR) under incident light but also had a relatively large specific surface area. The specific surface area (SBET) of MoO3−x after reduction at 200 °C was 30.0 m2 g−1, which was 22.7 and 9.1 times higher than those of commercially available MoO3 and our previously reported MoO3−x nanosheets, respectively. We also demonstrate that such a semiconductor with a large surface area could be used as a highly efficient catalyst that dramatically enhances the dehydrogenation activity for ammonia borane (NH3BH3; AB) under visible light irradiation.

Palladium nanoparticles supported on titanium doped graphitic carbon nitride for formic acid dehydrogenation

Pd nanoparticles (NPs) supported on Ti-doped graphitic carbon nitride (g-C3 N4 ) were synthesized by a deposition-precipitation route and a subsequent reduction with NaBH4 . The features of the NPs were studied by XRD, TEM, FTIR, XPS, EXAFS and N2 -physisorption measurements. It was found that the NPs had an average size of 2.9 nm and presented a high dispersion on the surface of Ti-doped g-C3 N4 . Compared to Pd loaded on pristine g-C3 N4 , the Pd NPs supported on Ti-doped g-C3 N4 exhibited a high catalytic activity in formic acid dehydrogenation in water at room temperature. The enhanced activity could be attributed to the small Pd NPs size, as well as the strong interaction between Pd NPs and Ti-doped g-C3 N4 .

Visible‐Light‐Responsive Carbon Dioxide Reduction System: Rhenium Complex Intercalated into a Zirconium Phosphate Layered Matrix

The intercalation of CO2 reduction catalyst [Re(bpy)(CO)3Cl] (bpy=2,2′‐bipyridine) into a layered zirconium phosphate (Zr(HPO4)2⋅H2O, α‐ZrP) with an interlayer distance of 10.3 Å was performed. XRD, IR spectroscopy, and extended X‐ray absorption fine structure measurements provided strong evidence that the Re complexes were intercalated into the layered interspace without significant structural change. The visible‐light photoredox system for the catalytic conversion of CO2 was constructed by using Re/ZrP as catalyst, Rhodamine B as photosensitizer, and triethanolamine as a sacrificial reagent. The emission intensity of Rhodamine B near λ=580 nm decreased with increasing amount of intercalated Re complex, suggesting the occurrence of an electron transfer from Rhodamine B to the Re complex. Performing the reaction with isotopic 13CO2 demonstrated that the CO produced originated mostly from CO2. Furthermore, this CO2 reduction system showed remarkable durability and could be used repeatedly without significant loss in activity.

Enhancement of Catalytic Activity Over AuPd Nanoparticles Loaded Metal Organic Framework Under Visible Light Irradiation

In this study, monometallic gold and palladium, and bimetallic gold/palladium were respectively deposited on chromium based amine-functionalized metal–organic framework (NH2-MIL-101) by using the impregnation method, with the aim to enhance the activity of Pd-catalyzed chemical reaction under the assistance of visible light at ambient temperatures. The oxidation state, size and location of metal nanoparticles were determined by XAFS and TEM. The physical properties of as-synthesized catalysts were also well characterized by UV–vis spectroscopy, BET and XRD. The catalytic activities of those metal nanoparticles deposited MOFs were studies for hydrolysis of ammonia borane and Suzuki–Miyaura coupling reaction. Compared with monometallic counterpart, the bimetallic gold/palladium showed much higher catalytic activity for hydrolysis of ammonia borane and Suzuki–Miyaura coupling reaction under visible light irradiation. The enhancement of the catalytic activity might be attributed to the synergistic effect between plasmonic bimetallic nanoparticles and metal organic framework in effective conversion of solar energy for generating electron rich palladium species to activate the electrophilic compounds as well as nucleophilic partner compounds. This investigation offers a new opportunity to explore the high-performance metal nanoparticle loaded MOFs photocatalysts for efficient utilization of sunlight in a wide range of chemical reactions.

Uniform anatase single-crystal cubes with high thermal stability fully enclosed by active {010} and {001} facets

In order to improve the photocatalytic activity of TiO2, synthesis of well-faceted single crystals of anatase TiO2 with a high percentage of reactive facets exposed has attracted much attention due to its fascinating surface atomic configuration. In this study, large anatase single crystal cubes with {001} and {010} facets exposed were prepared for the first time via a facile microwave-assisted method. The preparation involved an aqueous solution of titanium tetrachloride, ionic liquid (1-methyl-imidazolium tetrafluoroborate), and sodium dodecyl benzene sulfonate (SDBS) as titanium precursor, facet-directing agent, and surfactant, respectively. It is demonstrated that these single-crystalline TiO2 crystals with cubic morphology showed better photocatalytic performance than decahedral anatase TiO2 due to the high density of unsaturated surface Ti5c atoms on cubic TiO2.

Room-Temperature and Aqueous-Phase Synthesis of Plasmonic Molybdenum Oxide Nanoparticles for Visible-Light-Enhanced Hydrogen Generation.

A straightforward aqueous synthesis of MoO3-x nanoparticles at room temperature was developed by using (NH4 )6 Mo7 O24 ⋅4 H2 O and MoCl5 as precursors in the absence of reductants, inert gas, and organic solvents. SEM and TEM images indicate the as-prepared products are nanoparticles with diameters of 90-180 nm. The diffuse reflectance UV-visible-near-IR spectra of the samples indicate localized surface plasmon resonance (LSPR) properties generated by the introduction of oxygen vacancies. Owing to its strong plasmonic absorption in the visible-light and near-infrared region, such nanostructures exhibit an enhancement of activity toward visible-light catalytic hydrogen generation. MoO3-x nanoparticles synthesized with a molar ratio of Mo(VI) /Mo(V) 1:1 show the highest yield of H2 evolution. The cycling catalytic performance has been investigated to indicate the structural and chemical stability of the as-prepared plasmonic MoO3-x nanoparticles, which reveals its potential application in visible-light catalytic hydrogen production.