Introduction

Titanium dioxide (TiO₂) 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 TiO₂ 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 [1-5]. The high photocatalytic activity of TiO₂ 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 H₂O₂ molecules, which are capable to deep oxidation of organic molecules into CO₂ and H₂O [3].

However, pure TiO₂ 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 photo-active TiO₂ 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 [6-9]. 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, 10-13], main factors of photocatalytic effect of the titanium dioxide on decomposition of organic pollutants in water are following ones:

1. formation of hydroxyl OH groups;
2.  formation of molecules of hydrogen peroxide H₂O₂.

The hydroxyl OH group is a strong oxidant; thus it is important to study mechanisms of dissociation of the H₂O molecule to an OH group and an H atom, and to make clear how W affects desorption the OH group from the surface of TiO₂. Hydrogen peroxide H₂O₂ also is a rather strong oxidant, and formation of its molecules at the TiO₂ surface in water has to be studied too.

anatase the main attention is directed on electronic transitions under the influence of light; ₂As it is noted above, usually at the descriptions of photocatalytic activity of TiO 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.however we consider that these transitions serve only as the mechanism for absorption of energy, which can come to the system in different ways.

Methods and models

Titanium dioxide (TiO₂) 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 TiO₂ 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 [1-5]. The high photocatalytic activity of TiO₂ 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 H₂O₂ molecules, which are capable to deep oxidation of organic molecules into CO₂ and H₂O [3].

However, pure TiO₂ 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 photo-active TiO₂ 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 [6-9]. 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, 10-13], main factors of photocatalytic effect of the titanium dioxide on decomposition of organic pollutants in water are following ones:

1. formation of hydroxyl OH groups;
2. formation of molecules of hydrogen peroxide H₂O₂.

The hydroxyl OH group is a strong oxidant; thus it is important to study mechanisms of dissociation of the H₂O molecule to an OH group and an H atom, and to make clear how W affects desorption the OH group from the surface of TiO₂. Hydrogen peroxide H₂O₂ also is a rather strong oxidant, and formation of its molecules at the TiO₂ surface in water has to be studied too.

anatase the main attention is directed on electronic transitions under the influence of light; ₂As it is noted above, usually at the descriptions of photocatalytic activity of TiO 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.however we consider that these transitions serve only as the mechanism for absorption of energy, which can come to the system in different ways.

Results and discussions

Formation of OH groups

First of all we studied water adsorption on the pure TiO₂ surface. We have found that positions just above metal atoms are the most favorable places for the molecule H₂O. This result agrees with data of the known works [19-21]. The H₂O molecule adsorption energy on the pure TiO₂ 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 H₂O adsorption energy above them is about 2.0 eV. There are published data only for pure TiO₂. It was reported [19] that the H₂O adsorption energy on the TiO₂ rutile depends on the water covering and lies in the range of 0.95-1.08 eV. The H₂O adsorption energy on the TiO₂ anatase surface was found of 0.73 eV [20].

Increase of the adsorption energy of a water molecule under the influence of impurity demonstrates that binding energy of the molecule atoms with atoms of the surface increases. As the change of binding energy happens due to "flowing" of the electronic density, a qualitative conclusion follows: this increase in bonding with a surface has to decrease in bonding within the H₂O molecule.

Our second step is to investigate dissociation of the H₂O molecule to the OH group and the H atom. This process was studied recently for the H₂O molecule adsorbed above the Ti adatom placed upon the TiO2 rutile surface [21]. 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.

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 TiO₂ and 0.8 eV for the doped with W (Fig. 2). The energy gain of this reaction is 2.5 eV for the pure TiO₂ anatase and 0.9 eV for the doped one.

Figure 1: A scheme of study the behavior of the H2O molecule on the TiO2 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   Click here to view figure

Figure 2: The energy barrier for the H2O dissociation on the TiO2 anataze surface: pure and doped with W   Click here to view figure

We see that the W atom accompanied by the additional atom of oxygen increases the barrier height of the H₂O 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:

O₂ + 2 H₂O => H₂O₂.

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.

Figure 3: Scheme of the reaction of the hydrogen peroxide formation   Click here to view figure

spontaneously, without any barrier. happensOur calculations show that dissociation of oxygen molecules on the surface of titanium dioxide surface, taking so-called "bridge" positions between atoms of titanium (Fig. 4).However the both atoms of oxygen are bound with the TiO₂

Figure 4: The scheme of dissociation of a molecule of oxygen on the TiO2 surface. Hereinafter, only two atomic layers of the TiO2 slab are shown for simplicity   Click here to view figure

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 TiO₂ 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 O₁ 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 O₁ approached the oxygen atom of the H₂O molecule on such distance, at which the total energy of the system corresponded to a certain minimum, the O₁ atom remained in this situation, and we began to shift the closest atom of hydrogen towards the O₁ atom until a new minimum of total energy was obtained.

Figure 5interaction of single atoms of oxygen with a  water molecule near the surface of the pure titanium dioxide:The scheme of   Click here to view figure

The energy scheme of these calculations for the pure (not doped) TiO₂ 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 TiO₂ and its accession to the H₂O 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 H₂O₂ molecule. As a summary result we obtained: DE(pure TiO₂)= 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 TiO₂ as a catalyst.

Figure 6: Change of the total energy of the TiO2 system + H2O + O1 + O1 in the course of formation of the molecule H2O2. ΔEO is a barrier for the oxygen atom separation from the surface of pure TiO2 and its accession to the H2O water molecule; ΔEH is a barrier for the hydrogen atom separation from the oxygen atom in the molecule of water and its accession to already joined oxygen atom with final formation of the hydrogen peroxide H2O2 molecule   Click here to view figure

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).

Figure 7: The scheme of dissociation of the oxygen molecule on the surface of TiO2 alloyed by tungsten   Click here to view figure

The geometrical formation scheme of a hydrogen peroxide molecule near the surface of TiO₂ doped with tungsten is presented in Fig. 8. The energy scheme is plotted in Fig. 9.

Figure 8: The scheme of interaction of single atoms of oxygen with water molecule near the surface of titanium dioxide alloyed by tungsten   Click here to view figure

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.

Figure 9: Change of the total energy of the TiO2 system + H2O + O1 + O1 in the course of formation of the molecule H2O2. ΔEO is a barrier for the oxygen atom separation from the surface of TiO2 doped with tungsten and its accession to the H2O water molecule; ΔEH is a barrier for the hydrogen atom separation from the oxygen atom in the molecule of water and its accession to already joined oxygen atom with final formation of the hydrogen peroxide H2O2 molecule.   Click here to view figure

Conclusion

The density functional pseudopotential simulation shows that the barrier height for dissociation of the H₂O molecule to the OH group and atomic H on the W doped surface of TiO₂ 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 TiO₂ anatase and 4.74 eV for the W doped system.

The energy needed for formation of a hydrogen peroxide H₂O₂ 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. As the value of energy necessary for ₂ catalyst has to be much larger than near the surface of pure TiO₂ near the W/TiOcreation 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.

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