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First principle study of HF molecule adsorption on TiO2 (110) surface

Titanium and its alloy components are one of the most important technological materials, which has found extensive application in various industries. However, surface defects play a key role in the mechanical properties of these components. Currently, wet chemical etching is one of the most important procedure for surface processing due to the presence of HF since it can etch metal oxide. Therefore, there still a need to investigate the etching mechanism. In this work, adsorption of HF on TiO2(110) surface has been studied using density functional theory to investigate the fundamental process of etching. HF molecule is adsorbed on the TiO2 surface by dissociation to form Ti-F and O-H species. The interaction between HF and TiO2surface become more favorable at high HF coverage. The pre-adsorbed of water molecule is favorable for HF adsorption process, which is in good agreement with experimental results. Fluorination processes show the formation and desorption of water intermediate at 0.50 ML coverage. We also investigated the relation between the work function and Mulliken charge for HF adsorption. The results indicate that the interaction of F on the surface attracts electrons due to its higher electronegativity than oxygen. Our results suggest that adsorption of HF is considered chemisorption process.


Introduction
Titanium and its alloys are one of the most important materials used in various industries such as aerospace, biomedical and automotive [1]. During the investment casting, oxygen is absorbed from the ceramic mould, which results in the formation of an alpha case layer and is very brittle [2]. Formation of this alpha case layer causes deterioration of mechanical properties that result in material micro-failure [3]. Surface modification is required due to spontaneously growth of a native oxide layer. Recently, wet chemical/ion etching process has been widely employed as a surface modification using hydrogen fluoride (HF) as an etchant, to remove alpha case layer and improve the surface quality [4,5]. Metal surface etching by HF is one of the most important material tailoring techniques in the manufacturing of metal-based alloyed components. The main purpose of chemical etching is to remove alpha case layer by breaking down titanium oxide layer [6]. The presence of fluorine ion significantly influences the electrochemical behaviour of a metal surface, which is a major relevance for etching and corrosion [7]. There are many theoretical and experimental studies on surface etching but the mechanism of hydrogen fluoride etching process is still not well-understood [8]. In order to get a better understanding of atomic etching mechanism, a density functional theory (DFT) approach has become an attractive theoretical method to provide chemical bonding, atomic interaction and hybridisation [9]. Recently, research interest has emerged driven by the demand of a new method that is capable of etching metal surface oxides. However, previous studies focused mainly on the Al2O3 surface [10] and the adsorption behaviour on a single atom or probe molecules on metal oxides [11]. Wang et al, [12] investigated the selective etching phenomenon on anatase-TiO2 surfaces. Their results demonstrated that the etching processes is energetically preferable and merely occur on the (100) surface under high HF concentrations. Recently, work done by Yang et al [13] demonstrated that the adsorption of fluorine (F) ion on the surfaces of anatase-TiO2 makes (100) surface energetically preferable to (101) surface.
To investigate deeply on the fluorination process of titanium or metal oxides surface, in this work DFT is employed to study the adsorption behavior of HF on rutile TiO2 (110) surface. These present work greatly improve our understanding on electronic properties of titanium dioxide and reaction behavior of HF. The structure of HF on TiO2 (110) surface with and without pre-adsorbed water at different coverage were systematically investigated. The adsorption energies, the relative energy, Mulliken charge analysis and work function of the systems were calculated. The main goal of the study is to identify the key steps to remove oxygen atoms from the metal oxide surface.

Methodology
The study employed DFT to investigate adsorption behavior of HF on TiO2 surface using planewave/pseudopotential approach as implemented in Cambridge Serial Total Energy Package (CASTEP) [14]. The electron-exchange and correlation were described by the Perdew-Burke-Ernzerh functional of the generalized gradient approximation (GGA) [15]. The GGA-PBE functional underestimate the band gap, which reduces photo-induced electron transfer during reactions [16]. In order to correct the strong electronic correlation problem, DFT+U method is utilized with a U parameter of 5.0 eV for Ti in TiO2 that was first proposed by Dudarev et al [17]. The Brillouin-zone integrations are using the Monkhorst-Pack approach [18] with K-point sampling grids 4x4x1 for geometry calculation with cut-off energy of 400 eV. The adsorption energy is defined as: Where / is the total energy of the molecule-surface system, and represent the energy of the pure surface and free HF molecule, respectively. A is negative for exothermic adsorption process and positive for endothermic.

Structural geometry and adsorption energy
Surface model is an essential basis precision for adsorption calculations. In figure 1, the optimized structure of TiO2 (110) surface is presented wherein figure 1 (a) shows the side-view and figure 1 (b) top-view. The TiO2 (110) surface consist of three-fold oxygen atoms (O3c), five-and six-fold titanium atoms (Ti5c and Ti6c, respectively). The resulting of geometry of the surface slabs are relaxed to obtain energetic minima. The surface energies of the two slabs with four and six layers relaxed are 1.39 and 1.93 Jm -2 ,which were found to be in good agreement with the results obtained Idriss et al [19]. The surface energy depends on the atomic number of layers; 7-layer slab was found to be more stable with lower surface energy than 13-layer. Therefore, 7-layer slab model was selected to describe the adsorption of HF molecule on TiO2 (110) surface.  Table 1 presents the adsorption energies and geometric parameters of HF interacting with TiO2 (110) surface. Firstly, HF molecule was placed on Ti5c and Ti6c site as shown in figure 1 (b) and the systems were relaxed to determine their adsorption energy. Comparing their adsorption energies, it was found that HF molecule thermodynamically favors adsorption site of Ti5c (-4.36 eV) interaction than that of Ti6c (-4.16 eV) on the TiO2 (110) surface. This clearly reveals that bonding is crucial for HF to react with TiO2 surface and that could drive HF dissociation. The adsorption energy values are negative, entailing that initial fluorination process of TiO2 (110) surface is energetically favorable.  It was found that the Mulliken charge increases with HF coverage. This suggest that exposing TiO2 surface to various HF molecules results in surface saturation. Figure 2 This suggests a stronger chemical interaction of HF and TiO2 (110) surface. The maximum number of HF molecule coverage was four. Similar adsorption mechanism was observed for all HF coverage and this suggests that HF molecules dissociate completely to the TiO2 (110) surface.

Relative Energy
To further understand the fluorination process of titanium dioxide, the successive relative energy of two dissociated HF molecules in the TiO2 (110) surface are also calculated and their results are shown in figure 3. The total relative energies of the systems for the water formation and desorption were found to be 2.371 eV and 0.291 eV, respectively; which suggest that the reaction is not spontaneous.

Co-adsorption of H2O and HF on the TiO2 (110) surface
Previous result revealed that etching titanium oxide surfaces through a wet fluorination process is more favourable than dry etching [20], whereby the existence of water plays a crucial role. One-way to establish this theory, co-adsorption behaviour of H2O and HF on the TiO2 (110) surface was also investigated. The same technique was done by Quan et al [10] on aluminium oxide surfaces. Figure 4 present the co-adsorption of H2O-HF molecules on TiO2 (110) surface. When the water molecule is adsorbed on the TiO2 (110) surface, a Ti-O bond formed with the bond length of 1.906 Å. The calculated adsorption energy of -4.83 eV was found, this is in good agreement with the value of -4.796 eV found by Dai et al [21]. When HF molecule is adsorbed on the TiO2 (110) surface with the present of H2O molecule, HF dissociate completely to form Ti-F and O-H with the bond length of 1.989 Å and 1.001 Å, respectively. Adsorption energy of HF with pre-adsorbed H2O (-6.02 eV) is more stable than the adsorption energy (-4.36 eV) without pre-adsorbed water molecule. When the second HF molecule is introduced on the surface, the adsorption energy is (-7.28 eV) more favourable than the adsorption energy without H2O molecule. This suggest that the pre-adsorption of water or wet HF is favourable for fluorination of TiO2 surface than adsorption of HF gas. This correspond with the experimental observation of dry and wet etching on TiO2 and SiO2 surfaces [22].

Work function and Mulliken charge
Etching mechanism is often electrochemical in nature and this concept arises from corrosion [23]. The process involves electron charge transfer hence Mulliken charge analysis were calculated and summarized in table 3. The trend seen is that the Mulliken charge increases with an increasing HF molecules which implies that more charges are depleted from the surface upon the Ti-F interaction. The changes in surface charge shown to affect surface work function. When a single HF molecule is adsorbed, the work function slightly increases from 2.651-2.815 eV. It was also found that the work function is coverage depended as shown in table 3. The work function offers a good measure of how prone is the corrosion of a certain material [24]. In this study change in the work function has been found to be coverage-depended see figure 5. The large relative increase in work function with increasing HF coverage is due to the high electronegativity of fluorine atoms. However, for 0.75 ML coverage the work function decreases as shown in figure 5 due to the repulsive interaction between closed adsorbed fluorine-ion. Different changes in the work function are cause by charge transferred associated with fluorine ion.