Electrifying Metal-Organic Frameworks: Absolute Electron Energies Prof. Aron Walsh Chair in Materials Design Department of Materials Imperial College London https://wmd-group.github.io @lonepair hν e- MOF Detector ΔE = ?

Metal-Organic Frameworks (MOFs) Crystals of Organic & Inorganic Building Blocks Metal Ligand 0 1 2 3 0 Molecular complexes O0I0 Hybrid chains O0I1 Hybrid layers O0I2 Hybrid framework O0I3 1 Coordination polymer O1I0 Mixed layers O1I1 Mixed framework O1I2 2 Coordination layer O2I0 Mixed framework O2I1 3 Coordination framework O3I0 Rule: n+m ≤ 3 Dimensionality of Inorganic Connectivity (In) I – O – I Connectivity (Om) Adapted from A. K. Cheetham et al, Chem. Comm. 4780 (2006)

Emergence of Conductive MOFs

Electroactive MOFs Materials Platform for Exciting Physical Properties This is an isotherm-free presentation Hendon, Butler & Walsh, MRS Bulletin 41, 870 (2016)

Electroactive MOFs Materials Platform for Exciting Physical Properties This is an isotherm-free presentation Hendon, Butler & Walsh, MRS Bulletin 41, 870 (2016)

Absolute Band Energies are Important Relative Electron Energies Band gaps and valence / conduction band widths Absolute Electron Energies Dictate stability and transfer of charge • Redox chemistry: photocatalysis to batteries • Defect physics: controlling n-type / p-type • Device engineering: band alignment & barriers

Talk Outline: Ionisation Potentials 1. History of Absolute Electron Energies 2. Bridging Theory and Experiment 3. Workfunctions of Porous Materials

(Semiconductor) Terminology 1. Ionisation Potential – Energy to remove electron from valence band (VB) to the vacuum level (VL) 2. Workfunction – Energy to remove electron from the Fermi level (EF ) to the vacuum level 3. Electron Affinity – Energy to add an electron to the conduction band (CB) from the vacuum level VB CB VL EF

19th Century: Photoelectric Effect Ultraviolet Light “Excites” Certain Materials Annalen der Physik (1887); DOI: 10.1002/andp.18872670827

Bulk Electron Binding Energy Binding Energy = E[N-1] – E[N] Energy of system with N electrons

Surface Electrostatic Double Layer Workfunction = E[N-1] – E[N] + ED Energy due to surface dipole

Surface Electrostatic Double Layer Workfunction = E[N-1] – E[N] + ED !+ !- Electrons spill at a surface

Calculations for Polar Crystals Long-range Polarisation of Crystal: IP and EA

Emergence of Semiconductor Devices From Electron Energies to Devices Semiconductor Metal

Emergence of Semiconductor Devices From Electron Energies to Devices

Enter Density Functional Theory Self-Consistent Electronic Structure

Enter Density Functional Theory Surface Dipole can be Quantified

Exit Density Functional Theory Arbitrary Reference for Electronic Eigenvalues of Solids – Potential Zero is Shape Dependent

Periodic Boundary Conditions An Infinite Crystal Has No Vacuum Level

Talk Outline: Ionisation Potentials 1. History of Absolute Electron Energies 2. Bridging Theory and Experiment 3. Workfunctions of Porous Materials

Phenomenological Theory Geometric Mean of the Electronegativity This approximates the mid-gap energy (for atoms the Mulliken electronegativity is the mean of the IP and EA) Nethercot (1974); Butler & Ginley (1978); Xu & Schoonen (2000) Used in many recent high-throughput screening studies, but is clearly limited: no account of local structure, oxidation state, or chemical bonding

Phenomenological Theory Actually a Decent Guess… Butler & Ginley, J. Electrochem. Soc. 125, 229 (1978) Calculated

Phenomenological Theory

Phenomenological Theory

Screening New Photoactive Materials Sustainability index From Searching over 4 Trillion Compounds… D. W. Davies et al, Chem 1, 617 (2016); https://github.com/WMD-group/SMACT

Screening New Photoactive Materials Sustainability index From Searching over 4 Trillion Compounds… D. W. Davies et al, Chem 1, 617 (2016); https://github.com/WMD-group/SMACT

Quantitative Techniques Experiment Simulation Photoelectron Spectroscopy (secondary electron cutoff) Surface Termination (vacuum alignment) Electrochemical Response (flat-band potential) Molecular Dynamics (explicit solvent) Photoelectron Spectroscopy (core level binding) Heterojunction (core level alignment) Electrochemical Response (redox energies) Molecular Dynamics (explicit solvent / charge)

Embedded Crystal Approach Quantum Mechanical (QM) / Molecular Mechanical (MM) Embedding of Crystalline Solids Region I: QM Region II: MM (active) Region III: MM (frozen) Region IV: Point Charges Aim to reproduce electrostatic, chemical, and mechanical environment of crystal in Region I Daresbury Laboratory (UK): www.chemshell.org

Embedded Crystal Approach D. O. Scanlon et al, Nat. Mater. 12, 798 (2013); www.chemshell.org Calculate Ionisation Potentials with Long-range Polarisation of Crystal (ΔSCF procedure)

Effect of Local Coordination J. Buckeridge et al, Chem. Mater. 17, 3844 (2015); www.chemshell.org Range of Oxygen Environments (2,3,4) in TiO2

Effect of Local Coordination J. Buckeridge et al, Chem. Mater. 17, 3844 (2015); www.chemshell.org Range of Electron Energies in TiO2 Polymorphs Ti is 7-fold coordinated!

Effect of Local Coordination J. Buckeridge et al, Chem. Mater. 17, 3844 (2015); www.chemshell.org Range of Electron Energies in TiO2 Polymorphs Ti is 7-fold coordinated!

Talk Outline: Ionisation Potentials 1. History of Absolute Electron Energies 2. Bridging Theory and Experiment 3. Workfunctions of Porous Materials

Emergence of Conductive MOFs

Limitations of Current Atomistic Theory Technique Limitations Surface Termination (vacuum alignment) Values depend on surface Molecular Dynamics (explicit solvent) Requires large scale simulations – convergence Heterojunction (core level alignment) Requires compatible crystal structures Embedded Clusters (direct IP calculation) Requires complex setup procedure and forcefield

Frameworks with Large Pores a d b e c f r = 6 r = 5, 7, 8 r = 9 r = 7.5 r = 7.5 r = 10 Pore Radius () MOF-5 HKUST-1 ZIF-8 COF-1M CPO-27 MIL-125

Use the Internal Vacuum Level Eigenvalues Aligned to Pore Potential https://github.com/WMD- group/MacroDensity • Integrate sphere at center of pore • Check that potential has reached a plateau • Implemented in Python code (for 1D, 2D and 3D averages):

Band Alignment of MOFs Place MOFs and Inorganic Semiconductors on a Common Energy Scale Butler, Hendon and Walsh, JACS 136, 2703 (2014) Good agreement with first measurements of MOF workfunctions

Band Alignment of MOFs Explain Electron Localisation in d0 Frameworks Nasalevich et al, Sci. Rep. 6, 23676 (2016)

Electroactive MOFs Materials Platform for Exciting Physical Properties Hendon, Butler & Walsh, MRS Bulletin 41, 870 (2016)

Electroactive MOFs Materials Platform for Exciting Physical Properties Hendon, Butler & Walsh, MRS Bulletin 41, 870 (2016)

Conclusions 1. Solid absolute electron energies are simple to define but challenging for theory and experiment 2. Electroactive MOF knowledge is increasing towards rational control of functionality Collaborators: Keith Butler (Bath); Chris Hendon (MIT); Yu Kumagai & Fumiyasu Oba (Tokyo Tech); Su-Huai Wei (Beijing CSRC); Alexey Sokol & John Buckeridge (UCL); Jorge Gascon (Delft) Funders: JSPS; ERC; EPSRC; Royal Society Slides: https://speakerdeck.com/aronwalsh