Dr Yiyang Li

Abstract:

Understanding structural responses of metal–organic frameworks (MOFs) to external stimuli such as the inclusion of guest molecules and temperature/pressure has gained increasing attention in many applications, for example, manipulation and manifesto smart materials for gas storage, energy storage, controlled drug delivery, tunable mechanical properties, and molecular sensing, to name but a few. Herein, neutron and synchrotron diffractions along with Rietveld refinement and density functional theory calculations have been used to elucidate the responsive adsorption behaviors of defect-rich Zr-based MOFs upon the progressive incorporation of ammonia (NH3) and variable temperature. UiO-67 and UiO-bpydc containing biphenyl dicarboxylate and bipyridine dicarboxylate linkers, respectively, were selected, and the results establish the paramount influence of the functional linkers on their NH3 affinity, which leads to stimulus-tailoring properties such as gate-controlled porosity by dynamic linker flipping, disorder, and structural rigidity. Despite their structural similarities, we show for the first time the dramatic alteration of NH3 adsorption profiles when the phenyl groups are replaced by the bipyridine in the organic linker. These molecular controls stem from controlling the degree of H-bonding networks/distortions between the bipyridine scaffold and the adsorbed NH3 without significant change in pore volume and unit cell parameters. Temperature-dependent neutron diffraction also reveals the NH3-induced rotational motions of the organic linkers. We also demonstrate that the degree of structural flexibility of the functional linkers can critically be affected by the type and quantity of the small guest molecules. This strikes a delicate control in material properties at the molecular level.

Abstract:

TiO2-based powder materials have been widely studied as efficient photocatalysts for water splitting due to their low cost, photo-responsivity, earthly abundance, chemical and thermal stability, etc. In particular, the recent breakthrough of nitrogen-doped TiO2, which enhances the presence of structural defects and dopant impurities at elevated temperatures, exhibits an impressive visible-light absorption for photocatalytic activity. Although their electronic and optical properties have been extensively studied, the structure-activity relationship and photocatalytic mechanism remain ambiguous. Herein, we report an in-depth structural study of rutile, anatase and mixed phases (commercial P25) with and without nitrogen-doping by variable-temperature synchrotron X-ray powder diffraction. We report that an unusual anisotropic thermal expansion of the anatase phase can reveal the intimate relationship between sub-surface oxygen vacancies, nitrogen-doping level and photocatalytic activity. For highly doped anatase, a new cubic titanium oxynitride phase is also identified which provides important information on the fundamental shift in absorption wavelength, leading to excellent photocatalysis using visible light.

Abstract:

Conspectus

Heterogeneous catalysis is an area of great importance not only in chemical industries but also in energy conversion and environmental technologies. It is well-established that the specific surface morphology and structure of solid catalysts exert remarkable effects on catalytic performances, since most physical and chemical processes take place on the surface during catalytic reactions. Different from the widely studied faceted metallic nanoparticles, metal oxides give more complicated structures and surface features. Great progress has been achieved in controlling the shape and exposed facets of transition metal oxides during nanocrystal growth, usually by using surface-directing agents (SDAs). However, the effects of exposed facets remain controversial among researchers. It should be noted that high-energetic facets, especially polar facets, tend to lower their surface energy via different relaxation processes, such as surface reconstruction, redox change, adsorption of countercharged species, etc. These processes can subsequently lead to surface defect formation and break the surface stoichiometry, and the resulting changes in electronic configurations and charge migration properties all play important roles in heterogeneous catalysis. Because different materials prefer different relaxation methods, various surface features are created, and different techniques are required to investigate the different features from facet to facet. Conventional characterization techniques such as X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, etc. appear to be insufficient to elucidate the underlying principles of the facet effects. Consequently, an increasing number of novel techniques have been developed to differentiate the surface features, enabling greater understanding of the effects of facets on heterogeneous catalysis.

In this Account, on the basis of previous studies by our own group, we will focus on the effects of tailored facets on heterogeneous catalysis introduced by engineered simple binary metal oxide nanomaterials primarily with exposed polar facets, in combination with detailed surface studies using a range of new characterization techniques. As a result, fundamental principles of the effects of facets are elucidated, and the structure–activity correlations are demonstrated. The surface features introduced by different relaxation processes are also investigated using a range of characterization techniques. For example, electron paramagnetic resonance spectroscopy is used to detect the oxygen vacancies, while probe-assisted solid-state NMR spectroscopy is shown to be facet-sensitive and able to evaluate the surface acidity. It is also shown that such different features influence the heterogeneous catalytic performances in different ways. With the help of first-principles density functional theory calculations, unique properties of the faceted metal oxides are discussed and unraveled. Besides, other materials such as transition metal chalcogenides and layered double hydroxides are also briefly discussed with regard to their application in facet-dependent catalysis studies.

Abstract:

Solar light-driven water splitting provides a promising way to store and use abundant solar energy in the form of gaseous hydrogen which is the cleanest chemical fuel for mankind; therefore this field has been attracting increasing attention over the past decades. The fundamental steps for efficient photocatalyst for water splitting include uptake of photons of targeted energy range by appropriate electronic band structure, excited electrons and holes (excitons) migration, as well as recombination and selective conversion excited electrons for H+ reduction to H2 and holes and OH− to O2 on catalyst surface. Each step if not efficiently taken place could hamper the overall photocatalytic activity. Numerous semiconductors with appropriate bandgaps have mainly been developed as candidates for effective solar energy capture, whereas at present, their low quantum efficiency still remains as the major obstacle in further applications. In this minireview, we will disentangle the progress to develop photocatalysts with good photon uptake from photocatalytic water splitting performance. In accordance with the thermodynamic and kinetic considerations of the photocatalytic water splitting reaction, different strategies for improving the fundamental processes have been briefly reviewed. Some recent advances in facilitating charge carriers separation have also been presented. Photocatalytic water splitting at elevated temperatures is emphasized as a novel approach to suppress photo-excitons recombination on catalyst surface owing to adsorption of enhanced concentration of ionic species including H+ and OH− to create their local polarization to the excitons. Stronger polarization to hinder the excitons recombination can also be obtained by using polar-faceted support materials to the active phase of semiconductor. It is clearly demonstrated in this minireview that such high temperature–promoted photocatalytic water splitting systems could open up a new direction and provide a new innovation to this field.

Abstract:

Photocatalytic water splitting is attracting enormous interest for the storage of solar energy but no practical method has yet been identified. In the past decades, various systems have been developed but most of them suffer from low activities, a narrow range of absorption and poor quantum efficiencies (Q.E.) due to fast recombination of charge carriers. Here we report a dramatic suppression of electron-hole pair recombination on the surface of N-doped TiO2 based nanocatalysts under enhanced concentrations of H+ and OH−, and local electric field polarization of a MgO (111) support during photolysis of water at elevated temperatures. Thus, a broad optical absorption is seen, producing O2 and H2 in a 1:2 molar ratio with a H2 evolution rate of over 11,000 μmol g−1 h−1 without any sacrificial reagents at 270 °C. An exceptional range of Q.E. from 81.8% at 437 nm to 3.2% at 1000 nm is also reported.