Au(hkl): Surface X-ray Diffraction Studies at the Electrochemical Interface By Joshua James Fogg Oliver Lodge Laboratory, Department of Physics University of Liverpool Thesis submitted in accordance with the requirements of the University of Liverpool for the degree of Doctor in Philosophy September 2018 i Abstract In-situ surface x-ray diffraction (SXRD) measurements have been performed to develop an increased understanding of electrocatalytic reactions and electrodeposition processes occurring on gold single crystal surfaces. The surfaces of gold exhibit a rich physical behaviour that is interesting not only from a structural perspective but also for applications in areas such as heterogeneous catalysis and electrocatalysis. The surface reconstructions of Au(111) and Au(100) have been found to undergo a potential dependent in plane surface compression in alkaline solution that is remarkably similar despite the underlying geometry. The compressibility is linked to the charge on the surface Au atoms with a simple free electron model. The surface compression and the reversible lifting of the reconstructions are determined by the interplay between surface charge and the adsorption of hydroxide species. Carbon monoxide adsorption is shown to supress both the potential-induced changes in surface compression and the lifting of the reconstruction leading to the promotion of electrocatalytic reactivity. Measurements of the Au(111)/Pb UPD system in acidic solution reveal a substitutional Au/Pb surface alloy of the ratio ~ 4:1 that forms at intermediate coverages of Pb during the stripping of Pb from the Au(111) electrode surface. Investigations into the adsorption of Acetonitrile (AcN) on the Au(111) electrode surface in sulphuric acid solution find that AcN has an enhancing effect on the adsorption of sulphate molecules. In the potential regions where sulphate is not adsorbed onto the Au(111) AcN is shown to weakly interact with the Au(111) electrode surface. The interaction between Cu and AcN ions in sulphuric acid solution and its effect on the Au(111)/Cu UPD process were investigated. AcN + was shown to form a [Cu(C2H3N)4] complex with copper that enhances the the deposition of copper on the Au(111) surface and suppress the characteristic honeycomb (√3x√3)R30° structure in sulphuric acid. i Acknowledgements First, I would like to thank my supervisor Prof. C. A. Lucas for introducing me to the field of surface x-ray diffraction and its combination with electrochemistry, his patience and support (and pints!) throughout my PhD. A big thank you to my secondary supervisor Dr. Yvonne Gründer, your insights have been invaluable over the last four years. Another big thank you to Paul Thompson for all his help with experiments, advice and welcoming me to Grenoble with a heavy drinking session that I still laugh about to this day. Kevin McCormick and Keith Williams deserve a mention for all their hard work building the electrochemical cells and crystal collets. Many thanks to Gary Harlow and Elizabeth Cocklin for making me feel welcome in the group and showing me the ropes. Liz, you still owe me that karaoke night! A thank you to the other members of the group for all their support sessions, scientific discussions and sanity checks, in no particular order: Nikolas Antonatos, Lisa Rhodes-Martin, Lena Reichenbach and Jack William Beane. Graeme O’Dowd deserves a mention for the nights out and putting up with me as a housemate for so long. Pete, Beth and Liam, I hope you know how invaluable you are. Thank you for the drinking sessions and lending an ear. Stephanie, although you still think a synchrotron is some kind of transformer, thank you for your patience and sticking with me through the years. I would like to thank my mum, Lindsey Heywood, for putting up with me during my write up over the last 6 months. Finally, my nana and grandad, Joan and Terry Kershaw. Thank you, without your unwavering support this thesis would not exist. ii Contents 1 Introduction ..................................................................................................................... 1 2 Theoretical Principles ...................................................................................................... 4 2.1 Introduction ............................................................................................................. 4 2.2 The Electrode/Electrolyte Interface ......................................................................... 4 2.2.1 Electrode Reactions ......................................................................................... 4 2.2.2 Underpotential Deposition ............................................................................... 6 2.2.3 The Electrochemical Double Layer ................................................................. 6 2.2.4 Cyclic Voltammetry .......................................................................................... 7 2.3 X-Ray Diffraction .................................................................................................... 10 2.3.1 Crystallographic Definitions .......................................................................... 10 2.3.2 Kinematical X-ray Diffraction ....................................................................... 11 2.3.3 Surface X-ray Diffraction ............................................................................... 18 2.3.4 Modelling Surface Structure .......................................................................... 19 2.3.5 Reciprocal Lattice for the Low-Index fcc Metal Surfaces .............................. 21 2.3.6 Additional contributions to the scattered intensity ........................................ 25 2.4 X-ray Voltammetry ................................................................................................. 29 3 Experimental Techniques .............................................................................................. 30 3.1 Sample Preparation ............................................................................................... 30 3.2 Electrochemistry .................................................................................................... 31 3.3 Synchrotrons and Beamlines ................................................................................. 36 3.4 Diffractometers ...................................................................................................... 38 3.5 Data Acquisition and Extraction ............................................................................. 41 3.5.1 Rocking Scans ................................................................................................ 41 3.5.2 Stationary Scans ............................................................................................. 42 3.5.3 Reciprocal Space Scan ................................................................................... 43 3.6 Data Analysis .......................................................................................................... 43 3.6.1 Correction Factors ......................................................................................... 43 3.6.2 Non-Linear Least Squares Fitting .................................................................. 44 4 The Au(hkl) Surface Reconstructions and the Effect of CO on the Surface/Electrolyte interface ................................................................................................................................. 45 4.1 Introduction ............................................................................................................ 45 4.1.1 The Au(hkl) Surface Reconstructions ............................................................. 46 4.2 Experimental .......................................................................................................... 50 4.3 0.1M KOH ............................................................................................................. 51 4.3.1 Au(111) .......................................................................................................... 51 iii 4.3.2 Au (100) ......................................................................................................... 56 4.3.3 Au(110) .......................................................................................................... 59 4.3.4 Au(111) and Au(100) in 0.1 M KOH .............................................................. 61 4.4 Au(hkl): the CO effect............................................................................................ 63 5 Formation of an Au/Pb Surface Alloy in the Au(111)/Pb UPD system ........................ 72 5.1 Introduction ........................................................................................................... 72 5.2 Experimental .......................................................................................................... 75 5.2 Characterisation of the Pb/Au(111) UPD system .................................................. 75 5.4 Formation of an Au/Pb surface alloy ..................................................................... 78 5.5 Au(100) in 0.1 M KOH +1 mM PbCO3 ................................................................ 85 5.6 Conclusions and Summary..................................................................................... 89 6 The Adsorption of Acetonitrile at the Au(111)/Electrolyte Interface ......................... 90 6.1 Introduction ..........................................................................................................