Research

Summary of Interests 

Montage

  • Multifunctional materials including metal-organic frameworks (MOFs), hybrid gels, polymers etc
  • Nanocomposites — processing, characterisation and engineering novel applications
  • Nano- and micro-mechanics of 2D materials, such as exfoliated nanosheets, thin films, and coatings
  • Highly porous network materials, including metallic fibres and foams
  • Computational modelling and theoretical simulations: quantum mechanical simulations (DFT) and finite-element modelling
  • In-situ studies at large national science facilities — synchrotron light source and neutron spectroscopy

 



Research Highlights

  • Photophysical Tuneability and Chemical Sensing Abilities of a Newly Discovered "OX-1" MOF Nanosheet Material 

Outstanding functional tunability underpinning metal-organic framework (MOF) confers a versatile platform to contrive next-generation chemical sensors, optoelectronics, energy harvesters and converters. We report a rare exemplar of a porous 2D nanosheet material, constructed from an extended 3D MOF structure. We develop a rapid supramolecular self-assembly methodology at ambient conditions, to synthesize readily-exfoliatable MOF nanosheets, functionalized in situ by adopting the Guest@MOF (Host) strategy. Nanoscale confinement of light-emitting molecules (as functional guest) inside the MOF pores generates unusual combination of optical, electronic, and chemical properties, arising from the strong host-guest coupling effects. We show highly promising photonics based chemical sensing opened up by the new Guest@MOF composite systems. By harnessing host-guest optochemical interactions of functionalized MOF nanosheets, we have accomplished detection of an extensive range of volatile organic compounds (VOCs) and small molecules important for many practical applications.

Read the full article: Advanced Materials, 29, 1701463 (2017) [DOI] [pdf] [Journal Cover]


  • Existence of Terahertz Rotors Detected in a Metal-Organic Framework [2017]

Vibrations with a frequency of 1,000,000,000,000 oscillations per second — which scientists called the “Terahertz modes” — are playing a fundamental role in controlling the structural sturdiness and functions of porous materials, down to the molecular level. In this study published in PRL, we present the outcomes of an international collaborative effort, comprising large-scale experiments conducted at the high-resolution pulsed neutron source and third generation synchrotron radiation facility, in tandem with computational modeling studies to pinpoint the existence of Terahertz modes in a porous MOF structure. We show and explain, for the first time, the basis for a number of exhilarating vibrational motions intrinsic to MOF: (i) Rotational actions reminiscent to that of a hindered propeller (see clips above: computed by density-functional theory, DFT); (ii) Bouncing motions akin to that of a trampoline, and, (iii) Rocking movements of an unhinged framework produced by a shearing action. We have discovered that collective Terahertz motions are indeed ubiquitous in MOF materials, which could serve as a unique nanoscale platform not only to probe core physical mechanisms, but also to interrogate new phenomena previously thought to be intractable.

Read the full article: Phys. Rev. Lett., 118, 255502 (2017) [DOI] [PDF]

 
  • Photoemissive and photostable host-guest MOF for energy applications [2016]

Porous crystalline MOFs offer long-range ordering and well defined void spaces, thereby enabling nano-caging of guest molecules to achieve unconventional opto-electronic properties. We have recently demonstrated that host-guest non-covalent interaction can be utilised to yield exciting photonic MOF systems, useful for lighting, sensing and electronic applications. Using an in-situ guest trapping strategy, we have achieved nano-caging of a bulky emissive metal complex guest (ZnQ – an OLED emitter) within the internal large voids of the zeolitic imidazolate framework-8 (ZIF-8) hosts. We have characterized their photoluminescence characteristics by means of spectroscopic techniques, which are substantiated by density functional (DFT) simulations to understand the possible electronic pathways arising from donor-acceptor energy transfer mechanisms. We discovered that, the extensive host-guest intermolecular interactions caused by spatial nano-confinement of ZnQ in ZIF-8 pores led to a strong red-shifted emission spectra evidenced in the encapsulated hybrid crystals. This translates into a warmer color rendering potentially useful for LED lighting applications. Another striking result concerns, the major improvement observed in the photophysical stability of the luminescent guest species upon caging inside ZIF-8 pores. We established that, the nanoscale cavity of MOF could act as a protective vessel thus shielding light-sensitive fluorophores against rapid photochemical decomposition processes. It is envisaged that the host-guest nano-confinement methodology presented in this work may be extended to the vast family of MOF systems available in the literature.

Read the full article: Nanoscale, 8, pp. 6851–6859 (2016) [DOI] [PDF]


  • Soft Matter Encompassing Fibrous Gels Coexisting with Metal-Organic Framework (MOF) Nanoparticles [2015]

In our recent Advanced Materials paper, we reported the discovery of a brand new family of gel-like smart materials exhibiting counterintuitive physical and chemical properties. These supramolecular gels are in fact hybrid materials constructed from the process of self-assembly, starting from metal and organic components. Scrutinising the resultant hybrid gels under the scanning electron microscope, we were astounded to see exquisite fine-scale fibre architectures, which are completely different from those known in any other contemporary gel materials. Intertwined amongst these microscopic fibres are numerous MOF nanoparticles (ca. 100 nm) that have been created in situ. The MOF nanoparticles can be easily harvested and used as a precursor to fabricate thin-film sensing devices (see Figure). Noteworthy, this newly discovered family of hybrid gels is highly tuneable, enabling us to engineer bespoke materials with the desired properties to fit a specific application. To illustrate this, we have demonstrated that by simply switching the types of solvent used in material synthesis, we can tailor its electrical conduction property and mechanical resilience of such hybrid systems. Our methodology has opened the doors for exploiting MOF-based supramolecular gels as a new 3-D scaffolding material useful for engineering bespoke optoelectronics and smart micro-mechanical devices.

Read the full article: Adv. Mater. 27(30) pp. 4438-4446 (2015) [DOI - Open Access] [PDF]
Journal Cover: [Adv. Mater. Issue 30/2015 pp.4523]
Press Release: Oxford Science Blog - "Shape-shifting gels get smarter"

 

  • Unravelling Terahertz Vibrations in Metal-Organic Frameworks (MOFs) [2014]

Metal-organic frameworks (MOFs) are a promising new class of next-generation materials. With nanoscale cage-like structures featuring exceedingly large internal surface areas, MOFs can be used to capture and store molecules, giving them a wide range of potential applications from gas storage and capturing CO2 to microelectronics, drug encapsulation and use in sensors. In order to turn potential into reality, it’s necessary to understand their physical structure at a fundamental level, and how this determines their properties on the macroscopic scale. In our recent PRL paper,  we used a combination of inelastic neutron scattering and synchrotron radiation far-infrared absorption spectroscopy, in conjunction with density functional theory (DFT), to examine low-frequency terahertz (THz) vibrations in three prototypal ZIF materials (an important subset of MOFs).  Using this combination of techniques, the team discovered that such vibrations are intrinsically linked to observed physical phenomena in the ZIFs. For example pore breathing, where the cage structure opens and closes, and gate opening, where the material undergoes a step change in the amount of molecules it is able to capture, can be linked back to THz vibrational modes and the underpinning collective lattice dynamics.

Read the full article: Phys. Rev. Lett. 113, 215502 (2014) [DOI] [PDF]

Press Release: [Oxford Science Blog] [ISIS Science News] [Diamond Annual Review 2014/15 + PDF] [Phys.org] [Technology.org]

 

  • Mechanical Properties of Nanoporous Metal-Organic Framework Materials [2012]

zif8-shearGiven their vast potential for gas separations and storage, sensing, and drug delivery applications, detailed knowledge of the mechanical behaviour of MOFs is highly desirable from both a scientific and technological perspective.

Here, for the first time, we use laser Brillouin scattering to measure the single-crystal elastic constants (Cij’s) of an inorganic-organic (hybrid) framework material: ZIF-8, a prototypical imidazole-based MOF exhibiting a large accessible pore volume. The elastic constants are fundamentally important as they reflect the intrinsic interatomic bonding that governs the structure and stability of solids. 

Tensorial analysis of the elastic constants (Figure) reveals the complete picture of the anisotropic elasticity in cubic ZIF-8. We discover that ZIF-8 has a remarkably low shear modulus (~1 GPa), which is the lowest yet reported for a single-crystalline extended solid. Using ab initio computations, we further demonstrate that ZIF-8’s elastic constants can be reliably predicted. The underlying elastic deformation mechanism is governed by the pliant coordination tetrahedra, whose distortion is being accommodated by the large porosity. Our results shed new light on the role of elastic constants for establishing the fundamental structural stability of MOF-type materials, and thus their suitability for commercial applications. 

Read the full articles: Phys. Rev. Lett. 108, 095502 (2012) [DOI] [PDF] | Proc. Nat. Acad. Sci. USA 107(22), 9938-9943 (2010) [DOI] [PDF]

 

  • Nanomechanical Characterisation of Two-Dimensional Layered Materials [2012]

Nanosheets-ACSNanoWe report the first example of a 2-D inorganic-organic (hybrid) framework material bonded by weak van der Waals interactions, which can be ultrasonically exfoliated into nanosheets. 

The fully exfoliated nanosheets correspond to a unilamellar thickness of ~1 nm, while the partially exfoliated multilayer films exhibit a typical thickness on the order of 10 nm. It can be seen that such a colloidal suspension exhibits the Tyndall effect (Figure), whereby the finely dispersed nanosheets in solution are capable of scattering an incident laser light beam.

We used atomic force microscopy to characterize their surface topography and to map the variation of nano-mechanical properties across the surface of the delaminated nanosheets. The morphology and crystallographic orientation of the exfoliated layers were further studied by transmission electron microscopy. Additionally, we investigated the elastic anisotropy underlying the bulk host material by means of nanoindentation performed on single crystals, from which the critical resolved shear stress needed for micromechanical delamination of individual layers was determined to be relatively small (ca. 0.4 GPa).

Nanosheets generated by such a "top-down" approach are extremely promising because they can be used as precursors for creating continuous thin films, or to be further integrated with various polymer matrices to afford multi-functional composite materials.

Read the full article: ACS Nano 6(1), 615–621 (2012) [DOI] [PDF]

 

  • Polymer-MOF Nanocomposite Materials [2012]nanocomposite

Nanoparticles of Metal-Organic Frameworks (MOFs) (Fig.a) can be incorporated into a polymeric matrix to afford nanocomposite membranes with enhanced functional properties. Gas-separation membranes for energy-efficient commercial applications must exhibit exceptional selectivity and permeability, as well as an excellent combination of thermo-mechanical properties to survive severe operating conditions. To date no known MOF material fulfils all of these prerequisites simultaneously.

Recently a novel route has been developed in the Cavendish Laboratory at Cambridge (Q. Song & E. Sivaniah, see DOI) for fabricating a polymer-MOF nanocomposite membrane (Fig.b) that exhibits excellent separation capabilities. The breakthrough lies in the use of a new colloidal processing route to homogeneously disperse MOF nanoparticles. This groundbreaking work has opened up new possibilities for designing a variety of novel nanocomposites incorporating different polymer matrices and MOF nanostructures, whose mechanical and multifunctional properties both can be combined and tailored to match the required applications.

Such nanocomposite materials also have significant potential for use in drug delivery and desalination applications.

Read the full article: Energy Environ. Sci. 5(8), 8359-8369 (2012) [DOI] [PDF]

 

  • Fundamental Structure–Mechanical Property Correlations of Hybrid Framework Materials [2011]

ZIFs-void-morphologiesThis is the first review article focussing on the mechanical properties of inorganic-organic framework materials, including MOFs and dense hybrid frameworks.

In contrast to the sheer number of recent publications which dealt with synthesis of novel framework structures and characterisation of their diverse functional properties, studies devoted to understanding the mechanical properties of hybrid frameworks are comparatively few. The aim of this critical review is thus to bridge this gap by highlighting the recent advances with a view to stimulate further research in this important field. Undoubtedly, the structural robustness and resilience of hybrid framework materials under (static and/or dynamic) mechanical stresses are central to the optimal performance of the wide range of technological applications envisaged.

Here we present the elasticity of hybrid materials by considering their Young’s modulus, Poisson’s ratio, bulk modulus and shear modulus; this is followed by their hardness, plasticity, yield strength and fracture behaviour. Experimental work on single crystals and amorphised monoliths involved primarily the application of nanoindentation, AFM, and high-pressure X-ray crystallography. Theoretical studies, on the other hand, encompassed the application of first-principles density functional (DFT) calculations and molecular dynamics (MD) simulations.

Read the full article: Chem. Soc. Rev. 40(2), 1059-1080 (2011) [DOI] [PDF]