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Semiconductor/Electrolyte Interface Dr. Katarzyna Skorupska Electrical double layer • • • IHL (Inner Helmholtz Layer) electrical centers of the specifically adsorbed ions. Contains solvent molecules and sometimes other species (ions or molecules) that are said to be specifically adsorbed (Stern layer). In order of few Angstroms. OHL (Outer Helmholtz Layer) centers of nearest solvated ions. long-range electrostatic forces, independent of the chemical properties of the ions diffuse layer, extends from the OHP into the bulk of the solution Space charge layer 0.1Å semiconductor metal in nm scale The charge is distributed over certain distance below the surface Semiconductor surface • Specific adsorption at the surface by chemical bond: -OH-F• Physical adsorption by electrostatic forces Processes at the electrode – solution interface Faradaic (the amount of chemical reaction caused by the flow of current is proportional to the amount of electricity passed)) - electron transfer (ox : red) non-Faradaic • adsorption and desorption at electrode surface • charge does not cross the interface, external currents can flow when the potential, electrode area, or solution composition changes • Mass transfer (e.g., from the bulk solution to the electrode surface). • Electron transfer at the electrode surface. • Chemical reactions preceding or following the electron transfer. • homogeneous processes (e.g., protonation or dimerization) • heterogeneous ones (e.g., catalytic decomposition) on the electrode surface. • Other surface reactions, • adsorption, • desorption, • crystallization (electrodeposition). When a steady-state current is obtained, the rates of all reaction steps in a series are the same. semiconductor - electrolyte Characterized by: - energy bands (ECB, EVB, EF) The electrochemical potential of electrons is given by Nernst equation In the bulk it depends on doping: Cox, Cred – concentration of oxidized and reduced components theoretical absolute scale with vacuum level as given in relation to standard hydrogen electrode – reference electrode a reference point Reference electrodes * Standard hydrogen electrode (SHE) (E=0.000 V) activity of H+=1 * Normal hydrogen electrode (NHE) (E ≈ 0.000 V) concentration H+=1 The platinized platinum electrode is dipped in an acidic solution and pure hydrogen gas (1 bar) is bubbled through it. * Saturated calomel electrode (SCE) (E=+0.244 V saturated) The reaction between elemental mercury and mercury(I) chloride (Hg2Cl2) in saturated solution of KCl in water * Copper-copper(II) sulfate electrode (E=+0.314 V) * Silver chloride electrode (E=+0.197 V saturated) The redox electrode based on silver metal (Ag) and its salt — silver chloride (AgCl, also called silver(I) chloride). * pH-electrode (in case of pH buffered solutions, see buffer solution) The Gerischer Model Charge transfer at SC electrolyte interface Frank-Condon principle • e- transfer 10-15s • relaxation time for solvent 10-9-10-13s λ- reorganization energy (~1eV) energy of product with respect to its equilibrium state when its solvent coordinate is still the same as that of reactant state R. Memming, “Semiconductor Electrochemistry”, WILEY-VCH 2001 Model based on: • Electronic state in solid • Energy level in solution Electronic energies in electron energy diagram (simplified model, no motion of the solvent) Distribution function (depending on electronic energy) Wox(E), Wred(E) Wox, Wred- probabilities of finding the empty and occupied electronic state at energy E only single electronic state of redox system is considered DOX E0ox E0F, redox DRED E0red W0-constant distance x density of states Dox, Dred- distribution function E0ox, E0red- electron energy of redox system (empty and occupied states) E0F, red- standard Fermi level of a redox system Distribution of electronic levels of the redox system DOX E0ox E0F, redox Wox- distribution of empty electronic levels Wred- distribution of occupied electronic levels DRED Density of states DOS is proportional to concentration of species distance x E0red density of states R. Memming, “Semiconductor Electrochemistry”, WILEY-VCH 2001 DOX ΔE1/2 E0ox E0F, redox E0red DRED distance x density of states as the λ- reorganization energy (~1eV) ΔE1/2 ~ 0.5 eV Negative bias applied to the SC DOX E0ox E0F, redox DRED distance x E0red density of states - Flattening of the bands - Increase concentration of e- at the surface - Continuous increase of cathodic bias – accumulation situation at the surface n-Si : electrolyte at the equilibrium for three redox couples - before contact - after contact formation b) SC in depletion c) High barier height a) SC in accumulation electrons available at the surface deficit of electrons at the surface exchange by EVB above the EF below the EF in the middle of the gap below the EF close to EVB b) SC in depletion c) SC in inversion (pa) SC in accumulation type at the surface electrons available at the surface deficit of electrons at the surface - via ECB less probable high barrier height e- available at the surface via ECB excluded e- exchange via ECB exchange by EVB Charge transfer via surface states - empty above EF - occupied below EF n-Si : redox couple in equilibrium and under illuminations Vph Illumination - Excess carriers are generated VPh = VOC kT jL = ln + 1 q js Decomposition of SC thermodynamic Decomposition potentials for reduction of semiconductor for oxidation of semiconductor F – Faraday constant ΔG – known change in free Gibbs energy for each reaction favorable decomposition stable against cathodic reaction stable against anodic reaction PROTECTION In flat band situation by introducing into electrolyte a redox couple - below nEdecomp - above pEdecomp Appling an external potential in order to create depletion situation at the surface During illumination - minority carriers will participate in the decomposition process Stability rules: system has not enough energy to perform the decomposition to noticeable rate Photoexited charges can be used by: - Photoelectrocatalytic process - Corrosion of photoelectrode hν electrolyte p-Si Si + 2H2O + 4h+ => SiO2 +4H+ DESIRED PROCESS competition 2H2O + 4h+ => O2 + 4H+ ELECTRODE CORROSION R. Memming, “Semiconductor Electrochemistry”, WILEY-VCH 2001