Coupling of Surface Chemistry and Electric Double Layer at TiO₂ Electrochemical Interfaces

Abstract

Surfaces of metal oxides at working conditions are usually electrified due to the acidbase chemistry. The charged interface compensated with counterions forms the so-called electric double layer. The coupling of surface chemistry and electric double layer is considered to be crucial but poorly understood because of lacking the information at the atomistic scale. Here, we used the latest development in density functional theory based finitefield molecular dynamics simulation to investigate pH-dependence of the Helmholtz capacitance at electrified rutile TiO 2 (110)-NaCl electrolyte interfaces. It is found that, due to competing forces from surface adsorption and from electric double layer, water molecules have a stronger structural fluctuation at high pH and this leads to a much larger capacitance. It is also seen that, interfacial proton transfers at low pH increase significantly the capacitance value. These findings elucidate the microscopic origin for the same trend observed in titration experiments.Graphical TOC Entry 1 arXiv:1902.00802v3 [cond-mat.soft] 30 Jun 2019The surface chemistry of metal oxides is more involved than that of metals because the surface charge depends on proton exchange driven by pH of the environment. 1,2 The condition in which the surface has no net proton charge is called the point of zero proton charge (PZPC). [3][4][5] However, working conditions for most earth-abundant metal oxide based photocatalysts are not at PZPC and the corresponding solid-liquid interface is highly electrified. 6 By attracting counterions from the solution side, the electric double layer (EDL) is formed at the interface in which the electric field can be as high as 10 9 V/m. 7 Therefore, including the coupling of surface chemistry and EDL at metal oxide-electrolyte interfaces is necessary.The differential capacitance C EDL of the EDL at oxide materials-electrolyte interfaces has been long regarded as a key probe of its structure. For semiconducting oxides, the capacitance can be resolved into three distinct components connected in series: 8,9 The first component is the result of the depletion or accumulation of electrons in the space charge (SC) layer of the semiconductor electrode, which can be 10 to 100 nm thick. The second component C H is the Helmholtz capacitance due to specific adsorption of hydroxide groups or protons and corresponding counter ions. The last component C GC called the Gouy-Chapman capacitance, stems from the diffusive electrolyte and depends on the ionic strength.The relationship between surface structure and ion complexation of oxide-solution interface is a long-standing topic in colloid chemistry. Experimental interest in these metal oxide systems dates back to early 60s. 10,11 In titration experiments, the variation of surface charges due to adsorption/desorption of H + is measured against the pH in solution. To interprete experiments, 2-pK model including a phenomenological neutral site (SOH 0 ), dissociate site (SO − ) and associate site (SOH + 2 ) was introduced ("S" for the su...

Electrosorption at metal surfaces from first principles

“…Notable factors are the approximate DFT exchange-correlation functional and their errors in adsorption energies, the neglect of explicit water 58,59,86 , or missing co-ions 93 . In addition, the present analytical derivations indicate that discrepancies between allimplicit and all-explicit interfacial capacitances 39,55,92,99 and work functions 58 can have an impact as well. We expect many of these limitations to be overcome by FGC-type schemes with implicit/ explicit hybrid descriptions of interfacial water, an approach we will pursue in the future.…”

“…As shown convincingly in a recent study by Cheng et al 84 , an increasing distance between metallic surface and interfacial water for higher pH values lead to a decrease in the adsorption energy and thereby to a peak shift of correct magnitude on the RHE scale. In general, any such influence of surface-specific interfacial water 58,59,[85][86][87][88][89][90][91][92] can not be captured in calculations based only on implicit solvation. This highlights that present-day FGC calculations still need to be seen as a numerically efficient approximation-that as a next step might need to be refined by appropriately extending them to mixed explicit/implicit solvation models 39,58 .…”

The impact of non-ideal surfaces on the solid-water interaction: a time-resolved adsorption study

“…However, also intrinsically electrochemical properties of the solid-liquid interface, such as the capacitance of the Helmholtz-layer, are affected by alterations of the interaction of the surface with water. [28]…”

Band positions of anatase (001) and (101) surfaces in contact with water from density functional theory

“…1,2 Here, the application case crucially depends on the energetic alignment of the bands with respect to the electrolyte or the underlying photoabsorber. Furthermore, titanium dioxide is the model system par excellence for a wide-gap metal-oxide to study the electronic properties of a semiconductor in contact with water, both experimentally [3][4][5] and theoretically, 4,[6][7][8][9][10][11][12][13][14][15][16] hereby also playing an important role in method development and validation.…”