Titanium dioxide has aroused motivating interests as a promising semiconductor material because of its unique properties such as non-toxicity, high catalytic efficiency and extensive band-gap . TiO2 has been largely utilized in different fields such as photo-catalysis, gas sensing, organic dye-sensitized solar cells, water-splitting and removal of air pollutants [2-5]. It is crystallized in three important crystallographic phases namely anatase, rutile, and brookite . The photocatalytic properties of TiO2 are restricted due to its wide band gap (3.2 eV), which makes the adsorption of the incoming solar light in a negligible percentage (3-5 %). Doping of TiO2 with some nonmetal elements such as nitrogen is an appropriate method to extend the optical response of TiO2 to the visible area [7, 8]. Recently, oxide supported Au nanoparticles have attracted noteworthy attention due to the catalytic activities of these materials at low temperatures [9, 10] for a progressively greater number of oxidation processes. Several experimental studies for excellent activity of Au nanoparticles have been published. Goodman et al.  reported size and effects of the band structure, and measured the onset of the reactivity to identify when the Au particles become as tinny as two monolayers so that they are changed from a metallic state to a separated state. Up to now, a great number of DFT calculations of O2 adsorption and CO+O2 reaction at different Au collections have been published. Hakkinen Landman and co-workers  reported the charging of Au particles as an efficient parameter. The adsorption of O2 and CO2 and reaction at gold nanoparticle models supported by TiO2 surfaces have been inspected by first principle studies . Several computational investigations on N-doped TiO2 anatase with Au nanoparticles have been reported, clarifying nearly the significant electronic and structural properties of these systems. For instance, Liu et al.  studied the adsorption of NO molecule on the undoped and N-doped TiO2 anatase nanoparticles. The increased ability of N-doped TiO2 anatase nanoparticles for adsorption of toxic NO2 molecules has been also addressed in our previous computational work . There are a few computational and experimental studies on the adsorption behavior of TiO2/Au nanocomposites. H2S molecule has been characterized as a toxic gas and control of the concentration of this injurious molecule is an important subject to public health and environmental protection .
In this study, the interactions of H2S molecules with TiO2/Au nanocomposites are investigated by DFT computations.We present the DFT results of the complex systems consisting of H2S molecule located between the TiO2 anatase nanoparticle and Au monolayer. The electronic structure of the adsorption systems is also analyzed using projected partial density of states (PDOS) and molecular orbital techniques. This work aims to provide an overall understanding on the adsorption behavior of TiO2/Aunanocomposites as potentially efficient gas sensors for H2S molecules.
Density functional theory
Density functional theory (DFT) calculations [17, 18] were performed using Open source Package for Material eXplorer (OPENMX) ver. 3.7 , which has been confirmed to be an effective software package for simulating the nanoscale materials specially solid state and crystalline materials [20, 21]. The cutoff energy of 150 Rydberg is considered regarding the energetics of the complex system. The considered pseudo atomic orbitals (PAO)s were made by using a basis set (of tree-s, three-p, one-d) for Ti atom, (three-s, three-p, two-d, one-f) for Au atom, (two-s and two-p) for O and N atoms, (three-s and three-p) for S atom within cutoff radii set to the values of 7 for Ti, 9 for Au, 5 for O and N, 8 for S (all in Bohrs). The generalized gradient approximation (GGA) in the Perdew–Burke–Ernzerhof form (PBE)was utilized for exchange-correlation energy functional. The relaxation of all atoms of the nanocomposites were set in the geometry optimization process. The selected pristine and N-doped nanocomposites are positioned in a 20 Ǻ × 20 Ǻ ×30 Ǻ box, which is much greater than the size of the composites. The open-source program XCrysDen  was employed for visualizing data such as molecular orbitals. In the simulation box, 86 atoms (14 Au, 48 O and 24 Ti atoms) of undoped or N-doped TiO2 nanoparticle with Au structure were contained. The adsorption energy of gas molecules adsorbed on the TiO2/Au nanocompopsite is determined as follows:
Ead = E (composite + adsorbate) - E composite – E adsorbate (1)
where E( composite + adsorbate ) is the total energy of the adsorption system, E composite is the energy of the TiO2/Au nanocomposite, and E adsorbate is the energy of isolated gas molecule. According to this definition, the adsorption energies of stable configurations are negative; the more negative the adsorption energy, the more energy-favorable the complex system.
Modeling TiO2/Au nanocomposites
The TiO2 anatase nanoparticles containing 72 atoms were constructed by putting 3×2×1 numbers of TiO2 unit cells along x, y and z axis, respectively. The unit cell is available at “American Mineralogists Database” webpage  and was reported by Wyckoff . Two appropriate oxygen atoms of TiO2 nanoparticle were replaced by nitrogen atoms to model the N-doped particles. Two dangling oxygen atoms were considered in the nanoparticle to set a 1:2 charge ratio. The vacuum spacing with the thickness of 11.5 Å was considered between the adjacent particles. Also, a gold structure consisting of 14 Au atoms was constructed in this work and coupled with TiO2 nanoparticle to model the TiO2/Au nanocomposites. The 86-atoms TiO2/Au nanocomposite positioned into a great cubic supercell is shown in Fig. 1. Gas-phase H2S has a trigonal planar geometry with H-S bond length of 1.34 Å and H-S-H bond angle of 92.1°, based on GGA method. The optimized geometries of the N-doped TiO2/Au nanocomposites are also displayed in Fig. 2.
RESULTS AND DISCUSSION
Structural and energetics aspects
The interaction of H2S molecule between TiO2/Au nanocomposites is shown in Fig. 3, as labeled by types A-E adsorption complexes. It was found that the interaction of H2S with TiO2 nanoparticle is more favorable in energy than that with Au nanoparticle, suggesting that the H2S molecule is more preferentially adsorbed on the TiO2 nanoparticle rather than Au nanoparticle. The reason may be due to the calculated adsorption energies and molecular orbital results of the complex systems. In configuration A, H2S molecule interacts with pristine nanocomposite, while configurations B and C show the H2S molecule adsorbed on the N-doped nanocomposites. Configuration D presents the H2S interaction with two contacting points from both Au and TiO2 nanoparticles. In configuration E, one H2S molecule is adsorbed on the two N-doped nanocomposite. Table 1 liststhe lengths for H-S bonds of the adsorbed H2S molecule and the newly-formed Au-S, Ti-S bonds, as well as H-S-H bond angles before and after the adsorption process. The results of this table indicate that the H-S bonds of the adsorbed H2S molecule are stretched after the adsorption process. The reason is that the electronic density is transferred from the Au-Au, Ti-O and H-S bonds to the newly-formed bonds in the middle of the nanocomposite and adsorbate molecule. The H-S-H bond angle of the H2S molecule was increased after the adsorption. This increase could be mostly attributed to the formation of a new bond and therefore transferring the electronic density from the old bonds of nanocomposite and adsorbed H2S moleculeto the newly-formed bonds. This formation of new bond also increases the p characteristics of bonding molecular orbitals of sulfur in the H2S molecule. The adsorption energy values for nanocomposites are reported in Table 1. Adsorption on the N-doped nanocomposite is found to be energetically more favorable than the adsorption on the undoped one, which means that the N doping has an activating role on the adsorption of H2S. In other words, N-doped nanocomposite can react with H2S molecule more efficiently. Also, two doped nitrogen atoms make the interaction of H2S on the nanocomposite very strong, compared to the one N-doped nanocomposite. It suggests that the adsorption on the two N-doped nanocomposite is energetically more favorable than that on the N-doped nanocomposite.
Fig. 4 presents the total density of states (TDOS) for pristine and two types of N-doped TiO2/Au nanocomposites before and after the adsorption process. The TDOS of nanocomposites only exhibit small differences in comparison with the isolated non-adsorbed nanocomposites. These differences included both small shifts in the energies of the peaks and presence of some peaks in DOS of the considered systems. However, these variations in energy of the states would affect the electronic transport properties of the nanocomposites and this feature can be beneficial for engineering H2S sensors. The projected density of states (PDOS) for the interaction of H2S molecule with TiO2/Au nanocomposites are displayed in Fig. 5. Panels (a, b) present the PDOS of the H2S sulphur atom of H2S molecule and the titanium atom of TiO2 nanoparticle for the configurations A and B. The great overlaps between the PDOS of these two atoms show that the sulphur atom of H2S molecule forms a chemical bond with the titanium atom of nanocomposite. The PDOS for configurations C, D and E are represented in panels (c-d and e-f), indicating high overlaps between the PDOS of sulphur atom of H2S molecule and the Ti and Au atoms of nanocomposite. These overlaps of the PDOS spectra declare that chemical bond is formed after the adsorption of H2S molecule. The isosurfaces of HOMO and LUMO molecular orbitals for H2S adsorption on the nanocomposites are shown in Fig. 6, indicating that the HOMO is greatly localized on the TiO2 nanoparticle, whereas the LUMO is mainly localized on the Au nanoparticle. To further describe the charge transfer between H2S molecule and TiO2/Au nanocomposites, in Table 1, we report the partial charge values based on Mulliken charge approach. The charge difference for the particle i after and before adsorption, was evaluated using the following formula:
∆Qj = Qi (in complex) – Qi (in vacuum) (2)
Where, Qi is the value of Mulliken charge of the particle(s) i. Subscript “i” denotes the TiO2/Au nanocomposites or H2S molecule. For complex E, the calculated charge value of nanocomposite is about -0.555 e and that of H2S adsorbate is +0.555 e, suggesting that H2S behaves as an electron acceptor. It was found that the charge transfer makes changes on the conductivity of the system.
In order to investigate the interaction of H2S molecules with undoped and N-doped TiO2/Au nanocomposites, we have carried out density functional theory calculations. The results indicate that the adsorption of H2S molecule causes the elongation of the H-S bonds of the adsorbed H2S molecule. The bond angles of the H2S molecule after the adsorption process are larger than those in the gas phase state, being attributed to the transfer of the electronic density from the Ti-O bonds of TiO2 nanoparticle and Au-Au bonds of Au nanoparticle, as well as the H-S bonds of H2S molecule to the newly-formed bond at the contacting point. The results also suggest that the N-doped nanocomposites have a higher capability to interact with harmful H2S molecule, compared to the undoped ones. The variations in the electronic structure and adsorption energies were found to be responsible for changing the conductivity of the system. Our reported results thus provide a theoretical basis for the possible application of TiO2/Au hybrid nanostructures as gas sensors for main air pollutants such as H2S in the environment.
This work has been supported by Azarbaijan Shahid Madani University.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.