Adsorption on a metal oxide surface

Adsorption reaction

Water dissociation on a metal oxide surface produces protons and hydroxide ions. The protons and the hydroxides will adsorb preferentially onto Lewis basic and acid sites, respectively. Thus, the protons (H+) would likely anchor on anionic sites while the hydroxides (OH−_{) onto cationic sites of the NiO}

surface [58, 68, 135, 136] (Figure 2.15).

The electronic interaction between the adsorbate and the electrode surface can be of two types: physical adsorption (physisorption) and chemical adsorption (chemisorption). Physisorption designates weak interaction of the chemical species with the surface electrode due to the lack of a true chemical bond and would be seemingly associated with the van der Waals interaction. Physisorption is characterized by a narrow energy band (few meV) when measured e.g.: by EELS at low temperature (< 150 K) [69, 137]. Chemisorption (e.g.: electron exchange or electron pairing) induces electronic orbital coupling between the adsorbate and the anchoring site. It induces

Figure 2.15: Representation of adsorption of water molecules on a metal oxide surface. The protons and the hydroxides would anchor preferably onto the anionic and the cationic sites, respectively.

strong chemical bonds, for which the band width, contrary to physisorption, is about a few eV [69]. Thus, chemisorbed species can be detected by XPS. At room temperature, the sticking probability of non-dissociated species can be considered as 0, otherwise it can be taken equal to 1 [136]. It means that at room temperature the adsorbates, which are observed, are likely in strong interaction with the substrate. It should be mentioned that adsorbate are stable intermediates and cannot be compared to radical species.

For a neutral chemisorption process, protons and hydroxides are adsorbed in equal amount ([H_ads+ ] = [OH_ads− ]) and it can be assumed, at first sight, that this is the case when the metal oxide surface is exposed to a water vapour phase. If immersed in solution, the activity of the species on the surface can be tuned according to the pH of the solution. For instance, it can be imagined that strong alkaline solution would lead to the removal of adsorbed protons and [H_ads+ ] < [OH_ads− ]. This should result in an excess of negative charges on the electrode surface [135]. A strong acidic solution would leave a net positive charge and [H_ads+ ] > [OH_ads− ] [135]. The resulting charge on the metal oxide surface is compensated by a space-charge below the surface and by ionic species in the solution (K+_{, SO}2−

4 ...). The ionic species in the solution lead to

the formation of a charged double layer known as the Helmholtz double layer. The pH, for which the potential drop through the Helmholtz double layer is zero (Vh=0), is commonly called the isoelectronic point (IEP) or point of zero

zeta potential (PZZP) [135]. The pH for which protons and hydroxides are adsorbed in equal amounts ([H_ads+ ] = [OH_ads− ]) is termed point of zero charge (PZC) [135]. The two cases do not necessarily coincide at the same pH value because of the presence of others species in the solution, which can interact with the electrode surface [135].

The adsorption process happens when the metal oxide electrode is immersed in water or when the surface is exposed to a water vapour phase. For the latter, hydroxides and protons are likely adsorbed in equal quantities (neutral chemisorption process, with [H_ads+ ] = [OH_ads− ]). Ni 3d-bands are more narrow and less dispersive than O 2p bands [138,139]. Hammer et al. [138] pointed out that adsorption processes interacting with d-bands would lead to the formation of bonding and anti-bonding states as for a two-state problem (Figure 2.16). The resulting state of the adsorption process on d-bands would produce well

Figure 2.16: Density of state distribution for an adsorbate interacting (a) with a broad band (e.g.: O 2p) or (b) a narrow band (e.g.: Ni 3d), which leads to the formation of a bonding and anti-bonding state. Taken from Hammer et al. [138].

localized bonding and anti-bonding orbitals, whereas adsorption on O 2p bands would give rise to a broad feature [138]. Therefore, we assume that adsorbed species on Ni cationic sites would produce well defined electronic orbitals but it is very unlikely to measure the energy level of adsorbed species onto oxygen adsorbing site. Hydroxide (OH−) d-band interaction provides two distinctive electronic states in the valence region and are labeled 3σ (bonding orbital) and 1π (anti-bonding orbital) [68, 140]. The d-1π hybridization in the valence region leads to numerous electronic states, which are partially filled according to the position of the d-band center [140]. The closer the d-band center to the valence band maximum the higher the electron occupancy in the anti-bonding state, which in turn reduce the substrate-adsorbate interaction [140].

The Helmholtz double layer

The Helmholtz double layer designates two planar charged layers, where the layers are of opposite sign, which forms at a solid-liquid interface. It is actually made of charges from the solid (electrons, holes, trapped charges) facing charges from the solution (ions) [135]. The distance between the two layers is only few atomic units (∼ 3-5 A) but it might vary according to the polarization of the solid surface (e.g. with an external bias). The space between the two charged layers is supposedly filled with polarized water molecules for which the dielectric permittivity is ε = 5 (for non polarized water molecules, ε = 80). Eventually, the Helmholtz double layer can be addressed as a parallel plate capacitor:

C =∆Q ∆V =

Aεε0

d (2.25)

With A being the surface of the electrode, d the distance between the two layers, ε0the vacuum dielectric permittivity, ∆Q the net quantity of charges in

one layer and ∆V the potential drop through the double layer. From equation 2.25, one can derive the electric field arising at the Helmholtz double layer :

E =∆V d =

∆Q Aεε0

(2.26)

The electric field in the Helmholtz double layer can be extremely high. For instance, in considering an excess of charges equivalent to 0.01 to 0.1

monolayers on the surface (e.g.: [H_ads+ ] or [OH_ads− ]) [135], the electric field E can be as high as 109_{V/m [135]. It means the potential drop ∆V through the}

double layer is in the 0.1 to 1 V range, which is considerable.

It is important to point out that, for pure metallic electrodes and in the absence of electrochemical reaction, the Helmholtz double layer is driven by the difference in the workfunction between the surface of the solid and the electrolyte [135]. In equilibrium, for an uniform metallic electrode in contact with an electrolyte, anodic and cathodic current compensate each other and no net current flows through the interface. A Helmholtz potential drop built-up at the interface, which theoretically corresponds to the difference in workfunction on both side of the interface. The Helmholtz double layer can be, in theory, eliminated in polarizing the surface potential in order the Fermi energy position of the surface material and of the electrolyte are the same before contact.

In the case of a semiconductor, as for transition metal oxide materials, the Helmholtz double layer is controlled by the adsorption/desorption reactions on the surface, and not necessarily by the workfunction difference as for a metallic surface. Indeed, considering a free charge density of 1016-1017 charges m−2, the contribution of the free charges in the Helmholtz potential built-up is of the order of ∼ 10 mV [135]. Thus, the Helmholtz potential would arise from charged adsorbate site on the semiconductor surface as described in Section 2.4.1. Therefore, the nature of the adsorbing site on the semiconductor surface would define the properties of the Helmholtz double layer.

A schematic representation of a semiconductor/electrolyte interface is represented in Figure 2.17.

Figure 2.17: Typical band-diagram of a solid-liquid junction where the solid side is a n-type semiconductor. The thicknesses of the layers are taken from [135]. The space- charge region, surface state and the Helmholtz double layer have been detailed along this thesis. The Gouy-Chapman region corresponds to a charge gradient in diluted solution. This has been ignored in this thesis as the electrochemical experiments were realized in concentrated solution (pH=13-14).