Mechanistic insights into water adsorption and dissociation on amorphous -based catalysts

Despite having defects, amorphous titanium dioxide (aTiO2) have attracted significant scientific attention recently. Pristine, as well as various doped aTiO2 catalysts, have been proposed as the potential photocatalysts for hydrogen production. Taking one step further, in this work, the author investigated the molecular and dissociative adsorption of water on the surfaces of pristine and Fe2+ doped aTiO2 catalysts by using density functional theory with Hubbard energy correction (DFT+U). The adsorption energy calculations indicate that even though there is a relatively higher spatial distance between the adsorbed water molecule and the aTiO2 surface, pristine aTiO2 surface is capable of anchoring H2O molecule more strongly than the doped aTiO2 as well as the rutile (1 1 0) surface. Further, it was found that unlike water dissociation on crystalline TiO2 surfaces, water on pristine aTiO2 catalyst experience the dissociation barrier. However, this barrier reduces significantly when aTiO2 is doped with Fe2+, providing an alternative route for the development of an inexpensive and more abundant catalyst for water splitting.


Introduction
Among various oxide semiconductor photocatalysts, crystalline forms of titanium dioxide (cTiO 2 ) have attracted significant attention in last decades as promising photocatalysts due to its biological and chemical inertness, strong oxidizing power and long-term stability against photocorrosion and chemical corrosion [1,2]. Where many crystalline oxide surfaces [3] including cTiO 2 surfaces [4][5][6] are extensively studied for their electronic structures, defect levels and polaron formation, amorphous TiO 2 (aTiO 2 ) despite having actual technical applications such as use as an active photocatalyst, a substrate, or a protection layer [7][8][9] was CONTACT Kulbir Kaur Ghuman kulbirghuman@i2cner.kyushu-u.ac.jp lacking many such investigations until recently. It is now well known that there is a possibility of using aTiO 2 as an inexpensive and more abundant alternative to cTiO 2 [10,11] which may lead to relatively simple and inexpensive technology in future. Despite having qualitatively similar electronic structure as of cTiO 2 [10,11], aTiO 2 forms large localized bound state on several O and Ti atoms due to the strong electron interaction with the lattice distortion [10,12,13]. These localized states in aTiO 2 leads to selftrapping of holes, excess electrons and excitons which play important roles in the radiation-induced processes [14][15][16] such as water splitting. Moreover, very recently many studies showed that the photoactivity of aTiO 2 can further be enhanced by doping it with suitable dopants [17][18][19][20][21][22][23]. The synergetic role that the amorphousness and dopants play in the photoactivity of aTiO 2 was attributed to the unique position of the midgap states, high self-trap energy, low mobility and weak chemical bonds of doped aTiO 2 [24].
Inspite of these numerous studies proposing aTiO 2 -based catalysts for water splitting, the fundamental insights into the reaction pathway for water dissociation on aTiO 2 -based catalysts are still lacking. This motivated the author to analyse the associative and dissociative water adsorption on pristine and doped aTiO 2 surfaces using first-principles based theoretical investigations. Since our recent study [24] clearly indicated that the visible light absorption of aTiO 2 can be improved by doping it with Fe 2+ due to its unique properties, the author have considered Fe 2+ doped aTiO 2 (Fe(II)-aTiO 2 ) as a model for doped aTiO 2 catalyst. Further in this work, the performance of aTiO 2 -based catalysts is also compared with cTiO 2 , by analysing water splitting on rutile (1 1 0) surface with the same level of theory.

Computational models and methods
The melt-quenching method was used to prepare aTiO 2 models [12]. The properties of the prepared aTiO 2    Figure 1. This doping level can be readily achieved for 3d transition metals in rutile TiO 2 [26,27]. The electronic characteristics of aTiO 2 sample has been investigated using ab initio method. The plane wave pseudopotential approach, together with the Perdew-Burke-Ernzerhof [28] exchange-correlation functional, and Vanderbilt ultrasoft pseudopotentials [29] was utilized throughout. The kinetic energy cutoffs of 544 and 5440 eV were used for the smooth part of the electronic wavefunctions and augmented electron density, respectively. The Quantum-ESPRESSO code, PWSCF package [30], was used to perform the calculations. All calculations are spin polarized. The structures were relaxed by using a conjugate gradient minimization algorithm until the magnitude of residual Hellman-Feynman force on each atom was less than 10 −2 Ry/Bohr. In all electronic density of states (DOS) and projected density of states (PDOS) plots a conventional Gaussian smearing of 0.007 Ry was utilized.
Appreciable underestimation of band gap and delocalization of d and f electrons are well-known limitations of DFT. Therefore, density functional theory with Hubbard energy correction (DFT+U) formalism was used in this study with U = 4.2 eV applied to Ti 3d electrons and U = 6.00 eV applied to Fe 3d electrons for analysing electronic properties of pristine and doped aTiO 2 . The value of U for Ti has been chosen not solely on the basis of band gap but also depending on the property of interest [31], which in the current study, is the photocatalytic behaviour of TiO 2 that in-turn depends upon the position of band gap states and their effect on the electronic structure [32]. This value of U for Ti is consistent with theoretical investigations by Morgan et al. [33], who calculated it by fitting the peak positions for surface oxygen vacancies to experimental X-ray photoelectron spectroscopy data. Further the value of U for Fe is taken from Ref. [34] which showed that the DOS of FeO show characteristics similar to experiments [35] with U = 6.0 eV. Lastly, in order to evaluate the minimum energy pathways (MEPs) and transition states (TSs) for the water splitting reaction and estimate the relative activation energy barriers faced during dissociation of H 2 O molecule, non-spin-polorized climbing image nudged elastic-band (CI-NEB) DFT calculations with 7 images [36][37][38] were performed.

Results and discussion
First, the electronic properties of pristine aTiO 2 and Fe(II)-aTiO 2 surfaces were analysed and compared with the electronic properties of rutile (1 1 0) surface. Then the active sites for the adsorption of H 2 O, OH, and H species on pristine aTiO 2 and Fe(II)-aTiO 2 surfaces were investigated followed by the study of reaction mechanism for water splitting on them. Water splitting on rutile (1 1 0) surface was also analysed for comparison. One water molecule per unit cell was used for all the calculations.

Electronic properties of aTiO 2 , Fe(II)-aTiO 2 , rutile (1 1 0) surfaces
To understand the catalytic properties of amorphous and crystalline surfaces, the author first studied the electronic DOS and PDOS for the d electrons of Fe and Ti atoms and p electrons of O atoms for pristine aTiO 2 , Fe(II)-aTiO 2 , and rutile (1 1 0) surfaces. The DOS and PDOS are represented in Figure 2 for all the surfaces. It can be seen from Figure 2 that the magnitude of the point electronic gap, defined as the difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), is about 2.2, 1.8, and 1.6 eV for pristine aTiO 2 , Fe(II)-aTiO 2 and rutile (1 1 0) surfaces, respectively. Further, for all the surfaces the valence band mainly consists of the O 2p states and that the conduction band is dominated by Ti 3d states. In addition, the DOS analyses also reveals that there exist a midgap state for aTiO 2 surface due to O 2p orbitals (Figure 2(a)). However, it disappears when aTiO 2 is doped with Fe(II) (Figure 2 (b)). The midgap states can act as a recombination or a trapping center [24] and hence can decrease or increase the catalytic activity of aTiO 2 surface accordingly. The band tail states for Fe(II)-aTiO 2 surface however act as a trapping center and improve the photoactivity of Fe-aTiO 2 by absorbing light in visible region as reported in our previous study [24].

Adsorption energy calculation for H 2 O, OH and H species
The adsorption energies of H 2 O, OH and H species on the amorphous surfaces were calculated as where E tot (E bare ) is the energy of the surface with (without) adsorbate and E ad is the energy of the isolated adsorbate species calculated in the same supercell. Hence, a negative δH ads indicates stable adsorption whereas a positive value indicates unstable adsorption. The author also calculated the adsorption of H 2 O molecule and dissociated H 2 O on rutile (1 1 0) surface for comparison. Adsorption energies for the most stable site for all the surfaces are represented in Table 1.
As a first step, author has investigated the possibility of H 2 O, H and OH adsorption on the pristine aTiO 2 surface. For H 2 O and OH adsorption a total of three surface Ti atom sites based on their different coordination number were identified: fourfold coordinated Ti (Ti-4c), fivefold coordinated Ti (Ti-5c) and sixfold coordinated Ti (Ti-6c). For H adsorption, all the O atom sites neighbouring to these three surface Ti atoms were investigated. The three sites for H 2 O and OH adsorption and the ten sites for H adsorption are highlighted in Figure 3 in blue and yellow, respectively. While it is possible that the other adsorption sites exist on the aTiO 2 surface, the sites that author identified are enough to provide an understanding of the relative strength of binding and the spread of adsorption energies.
By analysing the interaction of H 2 O molecule on the three adsorption sites, surprisingly, it was found that binding of H 2 O with Ti-6c (δH ads = −1.07 eV) is energetically preferred over under-coordinated Ti-5c (δH ads = −0.24 eV) and Ti-4c sites (δH ads = −0.30 eV). This result is contrary to the H 2 O adsorption on cTiO 2 surfaces [39]. On crystalline surfaces water prefers to adsorb on under-coordinated site. To explain this, aTiO 2 surfaces before and after water adsorption on all the sites were analysed. It was found that the atoms neighbouring to Ti-6c rearrange themselves after water adsorbs on this site ( Figure 4). For instance, the O atom (circled) neighbouring to Ti-6c site and showing charge accumulation (represented by spin density diagram, Figure 5(a)) gets rearranged after water adsorption on Ti-6c site. However, such behaviour is absent when H 2 O is adsorbed on Ti-5c or Ti-4c site of aTiO 2 surface. This reconstruction of the surface due to the presence of water in the vicinity of Ti-6c site is probably the reason H 2 O is more stable on Ti-6c site compared to Ti-5c and Ti-4c site of aTiO 2 surface. By comparing water adsorption on pristine aTiO 2 with water adsorption on rutile (1 1 0) surface (Table  1), it is clear that water is slightly more stable on sixfold coordinated Ti atom of aTiO 2 surface then on fivefold coordinated Ti atom of rutile (1 1 0) surface (Table 1), with a difference of about 0.13 eV in their adsorption energies. Further, the bond distance analysis shows that water adsorbs on the Ti-6c site with a 3.52 Å distance between the surface Ti atom and the O atom of water, which is higher than the existing experimental measurements (2.21±0.02 Å) [40] as well as our theoretical calculations for water adsorption on rutile (1 1 0) surface. The strongest adsorption energy and higher distance of water from the aTiO 2 surface indicate that aTiO 2 can be used to achieve highly stable anchoring of water contrary to crystalline surface on which water is comparatively less stable but chemically bonded. Furthermore, after dissociation of water on aTiO 2 surface, the most stable configration is the one in which the OH group binds to the fourfold coordinated Ti atom while the remaining H atom binds with the O atom coordinated to fourfold Ti. This indicates that even though the molecular adsorption of water is more probable at sixfold coordinated Ti atom, the dissociative adsorption of water might take place at undercoordinated surface Ti atom as on cTiO 2 catalyst.
In order to understand water dissociation on Fe(II)-aTiO 2 surface author then optimized three different configurations representing Fe(II)-aTiO 2 surface: Fe(II) doped at Ti-4c site (Fe(II)-4c-aTiO 2 ), Fe(II) doped at Ti-5c site (Fe(II)-5c-aTiO 2 ) and Fe(II) doped at Ti-6c site (Fe(II)-6c-aTiO 2 ) ( Figure 3). In all these surface +2 oxidation state of Fe is achieved by substituting one Ti and one O atom with one Fe atom. Out of these three surfaces Fe(II)-6c-aTiO 2 was the most stable surface with total energy difference of −1.37 and −1.13 eV from Fe(II)-5c-aTiO 2 and Fe(II)-4c-aTiO 2 surfaces, respectively. Further, the spin density plot for the doped system ( Figure 5(b)) shows that the spin densities are strongly localized on the Fe atom and O sites in the nearest-neighbour position relative to the Fe dopant.
Next, water adsorption on the most stable, Fe(II)-6c-aTiO 2 , surface was analysed. The adsorption of H 2 O molecule was analysed by placing it on the top of Fe atom, resulting in the binding energy of −0.79 eV. Comparing with pristine aTiO 2 and rutile (1 1 0) surfaces, H 2 O is energetically least stable on Fe(II)-aTiO 2 surface. However, the distance (2.26 Å) between the Fe atom of the surface and the O atom of the water molecule is similar to the distance (2.21 Å) of adsorbed water on rutile (1 1 0) surface. Further, the author performed calculations for OH and H adsorption on Fe(II)-6c-aTiO 2 surface by placing OH on the top of Fe atom and H on top of four different O atom sites indicated by g, c, d and e in Figure 3. As for H 2 O adsorption, OH and H adsorption is also less stable when compared to OH and H adsorption on pristine aTiO 2 surface. The binding energy and vertical distance analysis indicate the Fe(II)-aTiO 2 surface might behave similarly to cTiO 2 surfaces for water adsorption and dissociation.

Reaction pathway for water dissociation
In order to gain an atomic scale understanding of the fundamental reaction pathway and get a sense of the relative activation energy barriers faced during water dissociation, CI-NEB calculations with 7 images were performed for all the surfaces. The preliminary results for the MEP and the TS energies for water dissociation on aTiO 2 , Fe(II)-aTiO 2 and rutile (1 1 0) surfaces are  represented in Figures 6-8, respectively, along with the initial state (IS), TS, and final state (FS) geometries. The chosen TS configration corresponds to the highest energy point along the MEP. The activation energy barrier, E a , is calculated as E a = E TS -E IS , and the reaction energy, E is calculated as E = E FS -E IS , where E TS is the energy of the TS, E IS is the energy of IS and E FS is the energy of the FS. The negative E indicates an exothermic reaction and a positive E represents an endothermic one. Based on the most stable water adsorption sites (Table 1), Ti-4c, Ti-6c and Ti-5c sites were chosen for water dissociation on aTiO 2 , Fe-aTiO 2 and rutile (1 1 0) surfaces, respectively. The IS corresponds to surface with chemically adsorbed H 2 O molecule, and the FS corresponds to the surfaces having dissociated water molecule with OH and H species adsorbed on the surface.
The reaction path was found to have similar TSs for both Fe(II)-aTiO 2 (Figure 7), and rutile (1 1 0) ( Figure 8) surfaces [39], where the H 2 O molecule gets distorted resulting in elongated OH-H bond. However, for pristine aTiO 2 surface TS consists of dissociated H 2 O molecule with OH and H group bonded to the surface Ti and O atoms, respectively. Furthermore, our calculations of reaction energy, E, indicate that water dissociation is exothermic on aTiO 2 ( E = −3.09 eV) and Fe(II)-aTiO 2 ( E = −1.27 eV) surfaces just like that on rutile (1 1 0) ( E = −0.84) surface and other cTiO 2 surfaces [41]. It should also be noted that, water splitting on Fe(II)-aTiO 2 surface is less exothermic than on aTiO 2 surface and more exothermic than on cTiO 2 surfaces. The negative reaction energies for aTiO 2 -based catalysts point towards favorable kinetics.
Further, the H 2 O dissociation is spontaneous on the rutile (1 1 0) surface but faces certain barrier for dissociation on amorphous surfaces. Activation energy barrier (E a ) analyses shows that water dissociation on pristine aTiO 2 and Fe(II)-aTiO 2 surfaces has E a of 0.92  and 0.18 eV, making Fe(II)-aTiO 2 surface energetically more favourable for water splitting compared to pristine aTiO 2 surface. The smaller activation barrier for water dissociation on the Fe(II)-aTiO 2 surface is due to its unique band structure Figure 2. The tail states present in Fe(II)-aTiO 2 band structure (Figure 2(b)) might lead to better hybridization of energy states of the H 2 O molecule with the energy states of Fe(II)-aTiO 2 surface than the energy states of the pristine aTiO 2 surface. In other words, the existence of these resonating energy states in Fe(II)-aTiO 2 catalyst might make it easier for the H atom of the H 2 O molecule to bind with the neighbouring O atom of the surface upon H 2 O molecule interaction with the Fe-aTiO 2 surface.

Conclusions
In this study, DFT+U analysis was conducted for providing the mechanistic insights into the water splitting reaction on the aTiO 2 -based catalysts. Our analysis showed that in contrast to crystalline surfaces H 2 O molecule tends to be more stable on sixfold coordinated Ti site then on under-coordinated sites of aTiO 2 surface. Further, the interaction between H 2 O and sixfold Ti site of aTiO 2 surface is stronger (δH ads = −1.07 eV) than the interaction between H 2 O and fivefold Ti site of rutile (1 1 0) surface (δH ads = −0.94 eV). The higher H 2 O stability on the pristine aTiO 2 surface could be attributed to the difference in the surface states of the adsorption sites where the spatial arrangement of the O atoms with reference to the Ti atom plays a major role. Furthermore, the density of states analyses and the CI-NEB analysis conducted in this study points towards better photocatalytic activity of doped aTiO 2 as compared to pristine aTiO 2 for water splitting. Overall, the present study reinforces the usefulness of doping in amorphous TiO 2 catalysts providing an alternative path to prepare an efficient and cost-effective visible light photocatalysts.