Structure–charge relationship – the case of hematite (001)

Abstract

We present a multidisciplinary study on the hematite (001)-aqueous solution interface, in particular the relationship between surface structure (studied via surface diffraction in a humid atmosphere) and the macroscopic charging (studied via surface- and zeta-potential measurements in electrolyte solutions as a function of pH). Upon aging in water changes in the surface structure are observed, that are accompanied by drastic changes in the zeta-potential. Surprisingly the surface potential is not accordingly affected. We interpret our results by increasing hydration of the surface with time and enhanced reactivity of singly-coordinated hydroxyl groups that cause the isoelectric point of the surface to shift to values that are reminiscent of those typically reported for hematite particles. In its initial stages after preparation the hematite surface is very flat and only weakly hydrated. Our model links the entailing weak water structure with the observed low isoelectric point reminiscent of hydrophobic surfaces. The absence of an aging effect on the surface potential vs. pH curves is interpreted as domination of the surface potential by the doubly coordinated hydroxyls, which are present on both surfaces.

The Effects on the Response of Metal Oxide and Fluorite Single Crystal Electrodes and the Equilibration Process in the Interfacial Region

“…The value of pHstirr should not be confused with pHpzp of crystal itself which is defined by the thermodynamic equilibrium constants of interfacial reactions (Equation 1). For most investigated metal oxide crystal planes [37,38,40,41] the broad electroneutrality region were obtained in which case the accurate value of pHpzp was hard to obtained, but rather pHpzp zone. The value of pHiep for the same metal oxides crystal planes obtained from streaming potential results is found to be in the narrow pH region between pH = 2.0 and pH = 4.5 (Table 3).…”

“…The potential developed at the certain distance from the metal oxide and fluorite surface is under the influence of accumulated and weakly bounded ions in diffuse part of electrical interfacial layer which surface concentrations depend of the stirring speed. During SCrE potential measurements it was noticed that some rutile, TiO2 (001) 2.5 6 [72] 5.6 [72] hematite, Fe2O3 (0001) 3.0 8 [41] 3.5-4.0 [72] sapphire, Al2O3 (0001) 2.3 6.5 [72] 4.1 [73] ceria, CeO2 (111) 3.4 5.9 [74] electrodes behave generally differently than another, especially in terms of time of equilibrium, and hysteresis which sometimes occurs upon two-way titration. Such behavior might be a consequence of the electrical and structural properties of the crystals which define the distribution of the ions within the EIL, as well as the kinetics of surface reactions.…”

“…The samples were cleaned as described earlier. [41] All solutions were prepared using deionized and decarbonized water (> 18 MΩ cm) and analytical grade chemicals. The aqueous electrolyte solutions were prepared by dilution of standard acid and base solutions (NaOH, HCl: Fluka, fixanal, c = 0.1 mol dm -3 ); and dissolution of weighted ammount of salt (NaCl: Fluka, puriss p.a.)…”

“…[18][19][20][21][22][23][24][25] The ISFET electrode was already used as a sensitive electronic device for determining the concentration of H + ions in the solutions. Twelve years ago, the first metal oxide single crystal electrode (SCrE) was constructed ( Figure 2c) [26] and since then have been developed (Figure 2d) [9,[27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] A special case of the single crystal electrode is the ice electrode, [43,44] i.e. a metal wire covered by a thin layer of ice (Figure 2b).…”

The Synthesis and Mechanism of (001)-Orientated Hematite Nano Rings: A Combined Theoretical and Experimental Investigation

“…In the past years, several experimental and computational studies were performed on the termination and chemistry of Hematite surfaces, such as (001) using the three-index scheme with reference to the hexagonal cells, [16][17][18][19] (012), 20,21 and (113). 22,23 Since a nanoparticle usually consists of several di®erent surfaces, a complete understanding of the surface chemistry should include all of the enclosing surfaces.…”