Formation of OH groups and Hydrogen Peroxide Molecules on the TiO 2 Anatase Surface : Pseudopotential Calculations

The density functional pseudopotential simulation was carried out to study dissociation of the H2O molecule on the TiO2 anatase surface (pure and W doped). Formation and desorption of the OH groups were studied, and it was shown that the adding of tungsten into titanium dioxide leads to reduction of the desorption energy of OH groups from 6.06 eV to 4.74 eV. Creation of the hydrogen peroxide H2O2 molecules was also investigated. Substitution of Ti with W on the TiO2 anatase surface decreases the formation energy of hydrogen peroxide molecules and moves it up to the range of visible sun light. Decrease of the formation energy of free OH groups and H2O2 molecules, which are fissile oxidizers, increases their quantity in water and promotes increase in effectiveness of organic pollutants decomposition. Article History Received: 16 April 2018 Accepted: 11 May 2018

introduction Titanium dioxide (TiO 2 ) in the form of anatase has a wide range of the functional properties.It is one of the most often used semiconductors for cleaning of sewage and air, for solar elements and generators of hydrogen.The effectiveness of TiO 2 anatase in these applications is caused by combination of its unique properties: a biological indifference (not toxicity), low cost and high resistance to photocorrosion in aqueous solutions [1][2][3][4][5] .The high photocatalytic activity of TiO 2 anatase usually is explained by the fact that it has the suitable width of the forbidden region (from 2.4 eV to 2.8 eV) and the considerable positive potential of a valence band (+3.1 eV) which allow it efficiently to photolise water to such strong oxidizers as OH radicals and H 2 O 2 molecules, which are capable to deep oxidation of organic molecules into CO 2 and H 2 O 3 .However, pure TiO 2 anatase possesses a small quantum yield because of the fast electron-hole recombination; it is not stable because its structure is inclined to transform into a more stable but not photoactive TiO 2 rutile phase; the edge of characteristic absorption of anatase lies at the efge of the UF range, thus it is not capable to use efficiently solar energy.
Addition of some transitional metals is used to eliminate these shortcomings.It is known that introduction of Nb, Mo, and especially W leads to improvement of photocatalytic activity [6][7][8][9] .Authors of those works connect this fact with large valences and oxygen attraction of above metals in comparison with Ti.
As it is noted by many authors 3,[10][11][12][13] , main factors of photocatalytic effect of the titanium dioxide on decomposition of organic pollutants in water are following ones: As it is noted above, usually at the descriptions of photocatalytic activity of TiO 2 anatase the main attention is directed on electronic transitions under the influence of light; however we consider that these transitions serve only as the mechanism for absorption of energy, which can come to the system in different ways.Therefore, we build our research of mechanisms of the water decomposition near the anatase surface (pure and doped) only through study of energy of the system, without the direct consideration of electron transitions.As a doping element we chose tungsten because it is the most efficient dopant.

Methods and Models
The present investigation was carried out in the framework of the density functional theory 14,5 with using the well-known computer code FHI96md 16 .Exchange-correlation effects were calculated in the general gradient approach GGA 17 ; the plane wave energy cutoff was taken of 50 Ry; pseudopotentials were constructed using the FHI98pp code 18 .Studied surfaces were modeled by slabs placed into a supercell with a size of 14.5248×14.5248×40.0a.u.3 (a.u.=0.529Ǻ).The pure TiO 2 slab contained 16 Ti atoms and 32 O atoms.To study the dopant influence one of the surface Ti atoms was replaced with a W atom; an additional O atom was placed near the W atom to saturate its large valence.Figure 1 demonstrates a placement of atoms in the slabs Ti 16 O 32 и Ti 15 W 1 O 33 and on their surfaces.

Formation of OH groups
First of all we studied water adsorption on the pure TiO 2 surface.We have found that positions just above metal atoms are the most favorable places for the molecule H 2 O.This result agrees with data of the known works [19][20][21] .The H 2 O molecule adsorption energy on the pure TiO 2 has been found equal to 1.2 eV.Ti and W atoms situated on the doped surface have approximately the same adsorption activity and the H 2 O adsorption energy above them is about 2.0 eV.There are published data only for pure TiO 2 .It was reported [19] that the H 2 O adsorption energy on the TiO 2 rutile depends on the water covering and lies in the range of 0.95-1.08eV.The H 2 O adsorption energy on the TiO 2 anatase surface was found of 0.73 eV 20 .In our case the final state of the dissociation reaction is practically the same.The OH group remains connected with atom of metal (W), however the H atom goes to the bridge position between two Ti atoms (Fig. 1).The barrier height is 0.3 eV for the pure TiO 2 and 0.8 eV for the doped with W (Fig. 2).
The energy gain of this reaction is 2.5 eV for the pure TiO 2 anatase and 0.9 eV for the doped one.
We see that the W atom accompanied by the additional atom of oxygen increases the barrier height of the H 2 O dissociation reaction (0.8 eV instead of 0.3 eV).However, the energy spent for overcoming the barrier (0.8 eV) is compensated by the final energy gain of 0.9 eV; thus this reaction can happen without additional inflow of energy.
The aim of our investigation is to find the energy needed for desorption of the OH group.Let us pay an attention to the fact that the energy gain of the reaction in the doped system is much less than in the pure one.It means that products of the reaction in the doped system are in less bound state than in pure one.Our calculations confirm this conclusion.
The energy for desorption of an OH group has been found to be 6.06 eV for the pure system and 4.74 eV for the W doped one.
Therefore, the presence of W atoms really stimulates the emergence in water of free OH radicals and thereby promotes more intensive decomposition of organic substances which are present in aqueous solutions.

Formation of Hydrogen Peroxide
Hydrogen peroxide is a rather stable substance, in which atoms of hydrogen are weakly bound.Owing to this fact this substance acts as a strong oxidizer.In usual conditions it may be produced through reaction of oxygen with water: The energy scheme of this reaction is shown in Figure 3.The energy barrier of the reaction is caused by requirement of some additional energy for dissociation of the oxygen molecule.The reaction product (a hydrogen peroxide molecule) is less favorable than initial products (molecules of oxygen and water) therefore, this reaction goes with energy absorption.
Our calculations show that dissociation of oxygen molecules on the surface of titanium dioxide happens spontaneously, without any barrier.However the both atoms of oxygen are bound with the TiO 2 surface, taking so-called "bridge" positions between atoms of titanium (Fig. 4).
If titanium dioxide is within water, single atoms of oxygen can interact with water molecules.The scheme of such reactions is presented in Fig. 5.
The calculation procedure consisted in the following steps.A molecule of water was located above the TiO 2 surface at the distance of 10 a.e., what provided its weak interaction with surface atoms; the oxygen atom of this molecule was fixed, i.e. its coordinates remained invariable in the course of this study.The initial system represented on the left panel of Fig. 5 was brought to the equilibrium state, the equilibrium energy was calculated.Then the O 1 oxygen atom marked with an arrow was moved step-by-step in the direction of the water molecule, and the equilibrium energy of the whole system was again found in the every step.When the atom O 1 approached the oxygen atom of the H 2 O molecule on such distance, at which the total energy of the system corresponded to a certain minimum, the O 1 atom remained in this situation, and we began to shift the closest atom of hydrogen towards the O 1 atom until a new minimum of total energy was obtained.
The energy scheme of these calculations for the pure (not doped) TiO 2 is presented in Fig. 6.We see that the total value of the energy needed for formation of a molecule of hydrogen peroxide near the surface of the pure titanium dioxide, consists of energies needed for overcoming two barriers.First, the barrier (ΔE O ) for the oxygen atom separation from the surface of pure TiO 2 and its accession to the H 2 O water molecule; second, the barrier (ΔE H ) for the hydrogen atom separation from the oxygen atom of the water molecule and its joining with the already associated oxygen atom with final formation of the hydrogen peroxide H 2 O 2 molecule.As a summary result we obtained: ΔE(pure TiO 2 )= 9.88 eV.This is a very big value even for the ultra-violet radiation!Thus formation of hydrogen peroxide acts very slowly at the sun light using pure TiO 2 as a catalyst.
We have done the same calculations for the case when titanium dioxide was doped with tungsten.In this case spontaneous dissociation of the oxygen molecule was also observed on the titanium dioxide surface near the W atom (Fig. 7).
The geometrical formation scheme of a hydrogen peroxide molecule near the surface of TiO 2 doped with tungsten is presented in Fig. 8.The energy scheme is plotted in Fig. 9.
Our calculations show that the total value of the energy needed for formation of a hydrogen peroxide molecule near the surface of titanium dioxide doped  with tungsten is equal to 4.0 eV.This value is much less than the barrier energy for the case of pure titanium dioxide (9.88 eV) and corresponds to the conventional demarcation between ultra-violet radiation and visible light.

Conclusion
The density functional pseudopotential simulation shows that the barrier height for dissociation of the H 2 O molecule to the OH group and atomic H on the W doped surface of TiO 2 anatase is 0.8 eV.The energy gain of this reaction is 0.9 eV for the doped one.The energy spent for overcoming the barrier of 0.8 eV is compensated by the final energy gain of 0.9 eV; thus this reaction can happen without additional inflow of energy.
The calculated energy for desorption of an OH group is 6.06 eV for the pure TiO 2 anatase and 4.74 eV for the W doped system.
The energy needed for formation of a hydrogen peroxide H 2 O 2 molecule near the surface of titanium dioxide doped with tungsten is 4.0 eV.This value is much less than the barrier energy for the case of pure titanium dioxide (9.88 eV) and corresponds to the conventional demarcation between ultra-violet radiation and visible light.
Summarizing results, we can say that at the same conditions the number of OH radicals and hydrogen peroxide molecules near the W/TiO 2 catalyst has to be much larger than near the surface of pure TiO 2 .As the value of energy necessary for creation of OH radicals and hydrogen peroxide molecules approaches the range of visible light, the effectiveness of decomposition of organic pollutants in aqueous solutions significantly increases in accordance to experiments.We believe that results of this work will be useful as to understanding of fundamental mechanisms of reactions on the surfaces of catalysts, and to concrete industrial applications.

Fig. 1 . A scheme of study the behavior of the H 2 O
Fig. 1.A scheme of study the behavior of the H 2 O molecule on the TiO 2 surface.Ti atoms are presented as big white balls; O atoms are shown as small white balls; H atoms are imagined as small black balls.The grey colored balls present Ti atoms replaced with W atoms and an addition O atom.The reaction path for the H 2 O molecule dissociation is demonstrated by a dotted line.The numbers 1, 2, 3, 4 correspond to: (1) the clear surface; (2) the adsorption of the H 2 O molecule; (3) the H 2 O dissociation to OH and H; (4) the desorption of the OH group.

Fig. 2 .
Fig. 2. The energy barrier for the H 2 O dissociation on the TiO 2 anataze surface: pure and doped with W.

Fig. 4 .Fig. 5 :
Fig. 4. The scheme of dissociation of a molecule of oxygen on the TiO 2 surface.Hereinafter, only two atomic layers of the TiO 2 slab are shown for simplicity.
21r second step is to investigate dissociation of the H 2 O molecule to the OH group and the H atom.This process was studied recently for the H 2 O molecule adsorbed above the Ti adatom placed upon the TiO 2 rutile surface21.Authors have found that the OH the group remains to be bound with the Ti atom while hydrogen atom moves to other metal atom.The reaction path is characterized by a small barrier (about 0.2 eV) and the energy gain of 1.2 eV.