Second Harmonic Generation Studies of Fe(II) Interactions with Hematite (α-Fe₂O₃)

Abstract

Iron oxides are a ubiquitous class of compounds that are involved in many biological, geological, and technological processes, and the Fe(III)/Fe(II) redox couple is a fundamental transformation pathway; however, the study of iron oxide surfaces in aqueous solution by powerful spectroscopic techniques has been limited due to "strong absorber problem". In this work, atomic layer deposition (ALD) thin films of polycrystalline α-Fe 2 O 3 were analyzed using the Eisenthal χ (3) technique, a variant of second harmonic generation that reports on interfacial potentials. By determining the surface charge densities at multiple pH values, the point of zero charge was found to be 5.5 ± 0.3. The interaction of aqueous Fe(II) at pH 4 and in 1 mM NaCl with ALD-prepared hematite was found to be fully reversible and to lead to about 4 times more ferrous iron ions adsorbed per square centimeter than on fused-silica surfaces under the same conditions. The data are consistent with a recently proposed conceptual model for net Fe(II) uptake or release that is underlain by a dynamic equilibrium between Fe(II) adsorbed onto hematite, electron transfer into favorable surface sites with attendant Fe(III) deposition, and electron conduction to favorable remote sites that release and replenish aqueous Fe(II).

Computational Molecular Simulation of the Oxidative Adsorption of Ferrous Iron at the Hematite (001)–Water Interface

“…1 In turn, charge carrier mobility can lead to the coupling of redox reactions between remote surface sites via bulk charge transport. 14 These observations suggest that there can be interaction between Fe(II) and iron oxide/oxyhydroxide minerals without any net change in the overall apparent iron concentration in solution. [3][4][5][6][7] Interestingly, although Fe(II) adsorption onto iron oxide/oxyhydroxide surfaces is not expected to occur at pH 3 based on macroscopic observations, 8- 12 Catalano et al 13 showed evidence, based on X-ray reflectivity measurements, of significant reaction between Fe(II) and three hematite surfaces at this pH.…”

“…Scanning tunneling microscopy 40 and crystal truncation rod [28][29] studies have indicated that the two terminations can occur on a same crystal surface in distinct domains but that the hydroxylated termination, which will be considered in this work, is 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 5 predominant. Previous work 37 demonstrated that the surface charge of the (001) surface in aqueous solutions is close to neutral over a wide pH range (4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14) because of the stability of the doubly-coordinated surface hydroxyls. In addition, the first two hydrolysis constants of aqueous Fe(II) are 9.5 and 20.6.…”

Development of ultrafast broadband electronic sum frequency generation for charge dynamics at surfaces and interfaces

“…This process is also called electric-field induced SHG (EFISH) process. 13,15,48,[91][92][93][94][95][96][97][98][99][100][101] The former was excluded since the two surface states exhibit almost the same population rate in the early time upon the photoexcitation, as displayed in Fig. 5(c).…”

The electronic, chemical and electrocatalytic processes and intermediates on iron oxide surfaces during photoelectrochemical water splitting

“…These pairs are localized and so far stable, but when exposed to ambient water molecules, their protons are transferred to adjacent oxygen rows, thereby forming single hydroxyl groups for the case of TiO 2 (110), for example [23]. In bulk aqueous solution at high pH, the surface is characterized by adsorbed OH-and negative surface charge density; this has been the subject of acid-base titration of hematite and other oxide surfaces for decades, and more recently has been the subject of nonlinear optical studies [24][25][26]. The negative surface charge density and attendant surface potential is most likely also a function of crystal face exposed to solution as exemplified by the substantial surface potential differences between crystal faces found at low pH [27].…”

“…This will structure the interfacial water molecule dipoles in an "oxygen-up, H-down configuration", but one which is nevertheless hydrogen bonded and relatively well structured. There are various forms of evidence for this assertion, including recent optical second harmonic generation studies on corundum and hematite [24][25][26]. The hematite flatband potential at pH 12, vs. Ag + /AgCl, is approximately -710 to -600 mV, depending on doping and other factors -so that application of any potential positive of these values imposes positive band bending on the hematite, which enhances charge separation -electrons going to the interior and holes going to the surface.…”