Full Length ArticleTheoretical understanding of water adsorption on stepped iron surfaces
Graphical abstract
Introduction
The adsorption of water on iron metal is of widespread interest in many areas, such as corrosion, electrochemistry, catalysis, and groundwater remediation [1], [2], [3], [4], [5], [6], [7], [8], [9]. This is because the water adsorption and dissociation can lead to the oxidation of iron to form rust. For example, recent studies reveal that nanoscale zero valent iron (nZVI) have the potential for the remediation of groundwater contaminants. One of the main benefits of using nZVI is because Fe0 as a strong reductant can efficiently reduce the adsorbed contaminants [3]. Experimental studies have shown that nZVI is a hydrophilic surface with a combination of a small water contact angle, large H2 production and a substantial decrease in Fe0 content over time [10], [11]. As such, the nZVI particles can be easily oxidized with the decreased reactivity and shorter lifetime [10], [12]. To prolong the lifetime of nZVI for practical groudwater remdiation applications, the understanding of the water adsorption and dissoctiation on Fe surfaces play an essential role [7], [9], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23].
Theoretical modeling becomes particularly vital because it can reveal adsorption properties at the atomic level. Previously, theoretical studies using density functional theory (DFT) have focused on water adsorption on the three low-index zero-valent iron surfaces (1 0 0), (1 1 0), and (1 1 1). Eder et al. studied water adsorption properties on Fe (1 0 0) and Fe (1 1 0) surfaces using the DFT method at the generalized gradient approximation (GGA) [18]. The dissociation process was found to occur spontaneously on both Fe surfaces, with a reaction energy of −1.38 eV and −1.13 eV on Fe(1 1 0) and Fe(1 0 0) surfaces, respectively. The negative value suggests an exothermic process here. Lazar studied Fe (1 0 0) and Fe (1 1 1) using both GGA and the hybrid functional (HSE06) approaches [9]. They found that the adsorbed H2O molecule first dissociated into OH and H. At higher temperatures, OH can further dissociate into O and H. However, the HFeOH configuration is very stable, which indicates that the OH on the surface plays an important role in the reactivity of iron nanoparticles and pollutants in wastewater. Unlike the spontaneous dissociation found by Eder et al. [18], Lazar et al. showed that (1 0 0) and (1 1 1) have activation barriers of 0.68 eV and 0.58 eV for water dissociation into OH and H, respectively. Both water dissociation reactions were exothermic with dissociation energies of −1.73 eV for Fe(1 1 1) and −1.30 eV for Fe(1 0 0). Liu et al. studied water dissociation on Fe(1 1 0) using the spin-polarized DFT method with the Perdew–Burke–Ernzerhof functional (PBE) at the generalized gradient approximation (GGA) level [22]. An activation barrier of water on Fe(1 1 0) was found at 0.68 eV, with a dissociation adsorption energy of −1.28 eV.
Recent studies have demonstrated that the higher stepped index surfaces show higher catalytic activity than those of the more common stable lower index surfaces [24], [25], [26], [27], [28], [29]. Higher index surfaces are also better candidates to represent the active sites of nanoscale zero-valent iron (nZVI) due to their specific stepped surface structures [30]. In contrast to lower index surfaces, there have been no previous theoretical studies to our knowledge of water dissociation on stepped higher index surfaces. As such, understanding the adsorption of water on stepped surfaces is crucial for optimizing the practical applications of nZVI. From previous experimental x-ray diffraction (XRD) studies, three mainly exposed facets of nZVI particles are {1 1 0}, {2 0 0} and {2 1 1} [31], [32]. The {2 0 0} facet has the same structure of the low-indexed (1 0 0) surface. Thus, the {2 1 1} facet is the main exposed stepped facets in nZVI from the XRD data. In addition, the presence of (1 1 0) and (1 0 0) surfaces evidenced by the XRD data suggests the formation of the (2 1 0) surface since the (2 1 0) surfaced consists of (1 1 0) terraces with one atom wise, separated by step of (1 0 0) orientation of single-atom height [28], [31], [32]. Moreover, the (2 1 0) surface is one of the most open iron surfaces, which has the large surface relaxation due to the existence of low-coordinated surface atoms [33]. Additionally, the previous theoretical studies also demonstrated that both Fe(2 1 0) and (2 1 1) surfaces have relatively low surface energies, which suggests that both surface are thermally stable [30], [34]. In this end, both surfaces are selected as the model systems to represent the stepped features of nZVI particles.
The inclusion of suitable van der Waals forces has recently been demonstrated to be necessary to correctly describe the electronic, magnetic, and adsorption properties [34], [35], [36], [37], [38]. Here we apply DFT to undertake an analysis of molecular and dissociative water adsorption on the thermodynamically stable stepped Fe(2 1 0) and Fe(2 1 1) surfaces with the consideration of dispersion corrections. The water adsorption properties on the flat Fe(1 1 0) surface was also investigated as a reference. Our comparative results demonstrate that the stepped surfaces are more reactive towards water adsorption due to their low-coordination-number atoms, and, therefore, are more susceptible to oxidation.
Section snippets
Computational details
All DFT computations were performed using the Vienna Ab initio Simulation Package (VASP) based on the projector augmented wave (PAW) method with consideration of spin polarization [39], [40]. Vaspkit was used to build atomic models and post-process the data [41]. The optPBE exchange–correlation energy was employed with the consideration of vdW interaction correction [35], [36], [37], [42]. The electron–ion interaction was described using the PAW potentials, with the 3s23p63d74s1, 2s22p4, and 1s1
Surface properties
The optPBE computational method was chosen due to showing good performance for iron systems based on bulk and surface properties [49]. Firstly, the optPBE method was calculated against the bcc Fe crystal. Our previous study showed that the choice of computational method for Fe systems is essential to gain accurate results [34]. The lattice constant, magnetic moment, cohesive energy, and bulk modulus of 2.82 Å, 2.16 μB, and 5.19 eV were calculated, respectively, which are close to the
Conclusions
The theoretical understanding of the water with the nZVI is essential because nZVI is currently one of the most promising materials for groundwater remediation. In this study, we applied the DFT method to theoretically investigate the water adsorption and dissociation on the stepped surfaces because the stepped surface can mimic the geometry of nanoparticles. We investigated both molecular and dissociative water adsorption properties on stable stepped Fe(2 1 0), and Fe(2 1 1). The adsorption of
CRediT authorship contribution statement
Jessica Jein White: Methodology, Software, Formal analysis, Writing – original draft. Jack Jon Hinsch: Writing – review & editing. William W. Bennett: Writing – review & editing. Yun Wang: Project administration, Funding acquisition, Supervision, Conceptualization, Methodology, Resources, Writing – review & editing.
Declaration of Competing Interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: [Yun Wang reports equipment, drugs, or supplies was provided by National Computational Infrastructure].
Acknowledgements
This research was undertaken on the supercomputers in National Computational Infrastructure (NCI) in Canberra, Australia, which is supported by the Australian Commonwealth Government, and Pawsey Supercomputing Centre in Perth with the funding from the Australian government and the Government of Western Australia.
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