SnO2 Thin Films by SolâGel Method and Periodic

Article pubs.acs.org/JPCA

Preparation of TiO2/SnO2 Thin Films by Sol−Gel Method and Periodic B3LYP Simulations Emerson A. Floriano,†,‡ Luis V. A. Scalvi,† Margarida J. Saeki,§ and Julio R. Sambrano*,∥ †

Department of Physics, UNESP, São Paulo State University, CEP 17033-360, Bauru, São Paulo, Brazil Post-Graduate Program in Materials Science and Technology, UNESP, São Paulo State University, CEP 17033-360, Bauru, São Paulo, Brazil § Department of Chemistry and Biochemistry, UNESP, São Paulo State University, CEP 17033-360, Bauru, São Paulo, Brazil ∥ Modeling and Computational Simulations Group, Department of Mathematics, UNESP, São Paulo State University, CEP 17033-360, Bauru, São Paulo, Brazil ‡

ABSTRACT: Titanium dioxide (TiO2) thin films are grown by the sol−gel dip-coating technique, in conjunction with SnO2 in the form of a heterostructure. It was found that the crystalline structure of the most internal layer (TiO2) depends on the thermal annealing temperature and the substrate type. Films deposited on glass substrate submitted to thermal annealing until 550 °C present anatase structure, whereas films deposited on quartz substrate transform to rutile structure at much higher temperatures, close to 1000 °C, unlike powder samples where the phase transition takes place at about 780 °C. When structured as rutile, the oxide semiconductors TiO2/SnO2 have very close lattice parameters, making the heterostructure assembling easier. The SnO2 and TiO2 have their electronic properties evaluated by first-principles calculations by means of DFT/B3LYP. Taking into account the calculated band structure diagram of these materials, the TiO2/SnO2 heterostructure is qualitatively investigated and proposed to increase the detection efficiency as gas sensors. This efficiency can be further improved by doping the SnO2 layer with Sb atoms. This assembly may be also useful in photoelectrocatalysis processes. such as in solar cell development,8,9 optoelectronic devices,10 catalysis and photocatalysis,11,12 and gas detection sensors.13,14 SnO2, when crystallized as rutile, presents a wide and direct band gap transition, ranging from 3.6 to 4.1 eV.15,16 The electrical conductivity of undoped tin dioxide depends on deviation from stoichiometric composition, leading to oxygen vacancies, interstitial tin atoms, which are donors in the matrix,17 and oxygen adsorption at surface and grain boundary.18 The conductivity can also be modulated by doping introduction;19 for instance, doping SnO2 with pentavalent ions as Sb5+ leads to increased conductivity due to the substitution of Sn4+ in the SnO2 matrix, acting as donors.20 On the other hand, TiO2 also presents a wide band gap, about 3.3 eV,21 and the electronic transition may be of direct type22 or indirect type,23 depending on the crystalline structure.24 TiO2 may be found under three crystalline structures: anatase, rutile, and brookite, where anatase and rutile present tetragonal structure, whereas brookite is orthorhombic. The rutile structure presents band structure diagram with direct band gap transition, whereas the anatase has indirect band gap transition.22,23,25 The

1. INTRODUCTION Investigation on gas detection has grown considerably in recent years because of the requirement of measuring and controlling chemical species present in residues of industrial plants and hospitals and in gases emitted by combustion engines in the environment and control of several types of industrial processes.1,2 Thin films of oxide semiconductors are widely used for low level detection of gaseous pollutants in the air and other types of ambient substances.3,4 The operation of semiconductor-based sensors is related to the chemical reaction between surface adsorbed oxygen species and the gases present in distinct types of atmospheres.5 The oxygen has a fundamental role in the electrical conductivity of oxide semiconductors. The adsorption of oxygen species as negative ions takes place at semiconductor surface and grain boundary regions, reducing the concentration of free electrons, which are trapped at adsorbed oxygen species.6 Then when reactive gases recombine with oxygen species, the thin film electrical conductivity is considerably varied, allowing the monitoring of gaseous species concentration. In other words, the sensibility of the oxide sensor is strongly related to the response in the electrical conductivity as a consequence of the chemical and physical interaction with gases present in the environment at its surface.7 Oxide semiconductors, such as tin dioxide (SnO2) and titanium dioxide (TiO2), are generally transparent when prepared as thin films. These oxides have many applications © XXXX American Chemical Society

Special Issue: Energetics and Dynamics of Molecules, Solids, and Surfaces - QUITEL 2012 Received: November 30, 2013 Revised: May 9, 2014

A

dx.doi.org/10.1021/jp411764t | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

Article

sol is heated at 100 °C until the volume is reduced to 30%, providing a viscous sol with final concentration of about 0.2 mol/L. The titania precursor sol is prepared by complexing titanium isopropoxide Ti(C3H7O)4 with glacial acetic acid. The solution is diluted in isopropanol and kept stirring for 1 h. A solution formed by nitric acid and ethanol is added to the former, and the resulting solution is kept under stirring also for 1 h, leading to the TiO2 sol of about 0.75 mol/L. TiO2 films are deposited on soda-lime glass substrates or on quartz substrates by the dip-coating technique, with a dipping time of 10 s in titania precursor sol and withdrawal rate of 4.2 cm/min. Quartz substrate is used for thin film deposition when the thermal annealing step is carried out at temperatures higher than 600 °C, which would be impracticable for glass substrates. These films are submitted to hydrolysis (gelling) at room temperature and about 60% of relative humidity for 15 min and precalcined at 250 °C to eliminate volatile substances before the second deposition process. The process is then repeated, and the resulting film with two deposited layers is submitted to a final thermal annealing at 400 °C for 4 h. For the heterostructure assembling, SnO2 films are deposited on top of TiO2 under a dipping rate of 10 cm/min in SnO2 sol. After each deposition, the layer is fired at 400 °C for 10 min. This process is repeated 10 times, and the resulting film is submitted to a final annealing at 550 °C for 1 h. X-ray diffraction patterns are obtained using a Rigaku diffractometer, model D/MAX 2100PC operated with a Cu Kα radiation source (1.5405 Å) and a Ni filter for elimination of the undesirable Kβ radiation. The data are collected with 0.02° of step and scanning rate of 1°/min in the 2θ/θ mode for powder sample and 2θ mode for films (fixed incident angle of 1.5°) in the angular region of 20° ≤ 2θ ≤ 80°. UV−visible spectra are taken in a Varian (model Cary 300) or in a Shimadzu (model UVmini-1240) spectrophotometer. All the measurements are carried out in the range 190 to 900 nm.

occurrence of each structure is generally related to the temperature of thermal annealing carried out for samples. The anatase is obtained at annealing temperatures below 600 °C, whereas rutile generally occurs above 800 °C.26 The electrical properties of TiO2 thin films are also strongly related to the preparation method and processing. The application for gas detection is related mainly to the characteristics of surfaces (110) and (101) of SnO2 as well as TiO2. For SnO2, the surface (110) is thermodynamically the most stable when compared to other surfaces.27,28 However, the surface (101) also deserves being mentioned because this is the dominant surface in SnO2 samples composed by nanometric dimension crystallites.29 Concerning TiO2, Linsebigler and co-workers30 investigated the efficiency of TiO2 in photocatalytic processes and verified that samples with anatase structure are more efficient when compared to rutile structure, which was attributed to the lower electron−hole recombination rate in anatase TiO2. The evaluation of electronic structure of surfaces (101) and (001), from first-principles method calculation,31 shows that TiO2 surfaces with anatase structure show high oxygen adsorption, the surface (101) being the most efficient for this adsorption process. Concerning the combination of SnO2/TiO2 as a heterojunction, it has been shown that it is possible to improve the photocatalytic activity and also the sensor sensibility for gas detection by means of heterojunction assembling.32,33 TiO2 and SnO2, when crystallized under rutile structure, show very close lattice parameters, favoring the coupling and the heterostructure growth, even though the difference in band gaps, electronic affinity, and work function leads to valence and conduction band misalignment and band bending at the interface.34 According to Beltran and co-workers,22 the difference between lattice parameter a is 2.6%, whereas the difference between lattice parameter c is 5.4%. Lee and Hwang35 have investigated the properties of heterojunction SnO2/TiO2 as thin films deposited on SiO2 and Corning 1737 glass substrates and observed an improvement in the gas detection sensibility, compared to the oxides used individually. They attributed the obtained result to optically excited electron transfer from TiO2 to SnO2. In this way, the main goal of this work is the preparation of SnO2, TiO2, and the TiO2/SnO2 heterostructure and the evaluation of the structural and optical properties, as well as their electronic structure properties. This approach aims for the comprehension of the gas detection process in these materials. In addition, SnO2 is doped with Sb and its electronic structure is theoretically evaluated in order to understand the effect of doping the heterostructure to improve the efficiency of gas detection. In this sense, this paper deals with evaluation of the occurrence of crystalline structures and determination of preferential direction growth and surface electronic structure in order to obtain knowledge on the role of TiO2 as well as SnO2 for gas detection.

3. COMPUTING METHOD AND MODEL SYSTEM The computational simulations were made in the framework of density functional theory (DFT)36 with the B3LYP37 hybrid functional by means of the CRYSTAL0638 program. This program uses Gaussian basis set to represent crystalline orbitals as a linear combination of Bloch functions defined in terms of local functions (atomic orbitals). The level of precision of the calculation of Coulomb and the exchange series is controlled by five parameters, 7, 7, 7, 7, 14, chosen for the Coulomb overlap, Coulomb penetration, exchange overlap, first exchange pseudo-overlap, and second exchange pseudo-overlap, respectively. The integration in the reciprocal space was performed by sampling the Brillouin zone with a 4 × 4 × 4 pack-Monkhorst. The oxygen, tin, and titanium centers were described by 631G*,39 6-31G,40 and 6-31G,41 respectively. For the antimony center, the basis set proposed by M. Towler42 was used. The tetragonal rutile unit cells of SnO2 and TiO2 belonging to the P42/mnm space group are characterized by two lattice parameters a and c and internal parameter u.22 As a first step a full optimization procedure was carried out to determine both of the bulk equilibrium structures. The calculated lattice parameters a = 4.702 (4.707) Å and c = 3.194 (3.194) Å and internal coordinate u = 0.305 (0.306) for SnO2 and TiO2 (in

2. EXPERIMENTAL SECTION SnO 2 sol is prepared by dissolving tin tetrachloride pentahydrate (SnCl4·5H2O) in deionized water at a concentration of 0.5 mol/L. Sn4+ hydrolysis is performed by adding NH4OH under stirring, and the precipitate is submitted to dialysis with deionized water for about 10 days to eliminate Cl− and NH4+ ions. This procedure leads to the formation of SnO2 aqueous and transparent sol with a volume increase of about 10 times the initial volume (due to the dialysis process). Then this B



Article

As can be seen by comparison with the standard files, TiO2 powders treated at 650 and 700 °C present peaks related to anatase crystallographic planes, and a transition to rutile structure can be observed when it is annealed at 750 and 770 °C, presenting peaks related to the anatase and rutile structures simultaneously. The peak related to the plane (110) of rutile is the most intense. Further treatment at 780 °C leads to a predominance of rutile structure, as the peaks related to anatase crystallographic planes, (101) and (105), show rather reduced intensities when compared to the other rutile-related peaks. The temperature at which this phase transition from anatase to rutile is observed is in good agreement with that reported by Pookmanee and Phanichpnam.43 Figure 1b shows X-ray diffraction patterns of TiO2 thin films, which are deposited on soda-lime glass substrate, thermally annealed at 550 °C, and deposited on quartz substrate, treated at 700, 800, 900, 1000, and 1100 °C, for 1 h. The results show that the peaks become more intense as the annealing temperature is increased (higher crystallinity). This behavior becomes clearly evident up to 900 °C, at which the intensity of peak related to the plane (101) (TiO2 anatase) starts decreasing. The most remarkable aspect is that in both cases (powder and film) the material experiments the phase transition from anatase to rutile; however, in the case of supported films (Figure 1b) the transition temperature is much higher. The rutile structure in the case of thin films can be noticed just from 1000 °C, and the complete transition takes place at about 1100 °C. These results also suggest that the quartz substrate is responsible for the shift in the phase transition temperature because the quartz substrate possibly stabilizes the anatase phase for temperatures below 1000 °C. Kajitvichyanukul and co-workers44 immobilized TiO2 thin films on glass plates by a sol−gel technique associated with dip coating method and demonstrated that the appearance of rutile phase at 600 °C depends on the number of coating cycles, as the anatase phase is predominant up to five cycles but the rutile phase appears over five cycles of coatings. The thicknesses of these samples were 30 nm, 1 μm, and 3 μm for one, three, and five cycles, respectively. The thicknesses of the films investigated here, evaluated from transmittance data, are 320 nm for TiO2 deposited on glass substrate, thermally annealed at 550 °C (anatase phase), and 230 nm for the film deposited on quartz, thermally annealed at 1100 °C (rutile). It must be recalled that these possess two deposited layers, and probably the precursor sols are rather different, explaining these thicknesses. Anyway, as several material layers are deposited, the influence of the substrate must be minimized, and thus, the phase transition anatase−rutile may take place at lower temperature compared to that obtained here (about 1100 °C). It shows that the influence of the substrate can be minimized if TiO2 is deposited as many layers and the rutile phase can be obtained at lower temperature by our process (770−780 °C, as obtained in the case of powders). Table 1 presents the evaluation of texture coefficient (TC) calculated by the equation proposed by Moholkar and coworkers45 as a function of cystallographic planes (hkl), taking into account the respective intensities of experimental X-ray diffraction data for TiO2 thermally treated at 700 °C (anatase) and 1100 °C (rutile), deposited on quartz substrate, as well as the intensities of reference structure of TiO2 anatase (file PDF 21-1272) and rutile (PDF 21-1276). The evaluation of TC for anatase TiO2 films indicates the preferential for growth for the (101) direction, with a TC value of 1.89. As mentioned earlier, the application of anatase TiO2 as

parentheses), respectively, are in agreement with experimental and theoretical studies reported in the literature.22 From these optimized bulk structure, two models representing 4% (SnO2:4at%Sb) and 8% (SnO2:8at%Sb) of Sb doping in the SnO2 matrix were built with the supercell 2 × 3 × 2 (72 atoms) approach. Because of the use of the supercell approach, the computational time increases significantly, and because the lattice parameters for the doped supercell suffer small variations, the lattice parameters were not allowed to relax. The band structures of the bulk SnO2 and TiO2 and doping models have been obtained at 80 K and point along the appropriate high-symmetry paths of the adequate Brillouin zone. Diagrams of density of states (DOS) have been built for the analysis of electronic structures.

4. RESULTS AND DISCUSSIONS Figure 1 shows X-ray diffraction patterns of TiO2 powder (Figure 1a) and thin films deposited on soda-lime glass or quartz substrates (Figure 1b). TiO2 powders were submitted to thermal annealing at 650, 700, 750, 770, and 780 °C for 1 h, and the corresponding diffraction peaks are compared to the standard files of anatase (PDF 21-1272) and rutile (PDF 21-1276) structures, as shown in Figure 1a, which indicate that the thermal annealing temperature has strong influence on the crystalline structure.

Figure 1. X-ray diffraction pattern: (a) TiO2 powder submitted to thermal annealing at several distinct temperatures and TiO2 file data, PDF 21-1272 (anatase) and PDF 21-1276 (rutile); (b) TiO2 thin film on glass substrate, thermally treated at 550 °C, and TiO2 on quartz substrate, thermally treated at higher temperatures. The main crystallographic directions are labeled. C



Article

the heterojunction with the TiO2 layer treated at 1100 °C presents peaks related to SnO2 and TiO2, both of rutile structure, in good agreement with the diffraction pattern of individual TiO2 film submitted to this same annealing temperature (Figure 1b). Either favoring or changes in preferential orientation were noted. These results show that the type of crystal structure of the films in the TiO2/SnO2 heterojunction is determined by the temperature, as observed for individual films. Figure 3 shows the transmittance spectra for TiO2/SnO2 heterojunction films, deposited on glass substrates, with TiO2

Table 1. Texture Coefficient (TC) of TiO2 Thin Films Deposited on Quartz Substrates TiO2, 700 °C

referencea (hkl)

I/I0

I/I0

TC

(101) (004) (200) (105) (211)

1.00 0.20 0.35 0.20 0.20

1.00 0.08 0.22 0.08 0.10

1.89 0.76 1.18 0.76 0.95

(hkl)

I/I0

I/I0

TC

(110) (101) (211) (220)

1.00 0.50 0.60 0.20

1.00 0.16 0.22 0.09

1.85 0.59 0.68 0.83

TiO2, 1100 °C

referenceb

a

PDF 21-1272. bPDF 21-1276.

gas sensor is related mainly to the characteristics of surfaces (110) and (101). On the other hand, it is possible to verify that the rutile TiO2 film grows preferentially in the direction perpendicular to the plane (110). These results suggest that there is interaction between the substrate and crystallographic phases, since the preferential orientation appears for both crystallographic phases, but this orientation depends on the crystallographic structure. Figure 2 presents X-ray diffraction patterns of the TiO2/ SnO2 heterojunction. Although the general procedures of the Figure 3. UV−vis transmittance spectra of TiO2/SnO2 heterojunction, deposited on glass and quartz substrates. Inset: absorbance spectra.

thermally treated at 550 °C and on quartz substrates, with TiO2 thermally treated at 1100 °C. Results indicate that heterojunctions exhibit about 40% of transparency in the visible range independent of crystallographic structure type. This value is lower than those of typical SnO2 films and very close to those of TiO2 thin films (not shown here). The transparency of SnO2 is above 80% in the investigated range; thus, the lower transparency of the TiO2 films must predominate the transparency of heterostrusture. Figure 4 shows the calculated band structure diagram for several compounds. In this Figures, EF corresponds to the Fermi level. Figure 4a is the diagram for bulk SnO2. Figure 4b shows the primitive cell of tetragonal structure and high symmetric k-points in the first Brillouin zone. Figure 4c, Figure 4d, and Figure 4e show the band structure diagrams calculated for bulk SnO2:4at%Sb, SnO2:8at%Sb, and TiO2, respectively. Results presented in Figures 4 and 5 show that all of these semiconductors have direct band gap transition in the Γ−Γ direction. Energy values for the band gap (Eg) are 3.55 eV for SnO2, 3.27 eV for SnO2:4at%Sb, 3.13 eV for 8at%Sb, and 3.35 eV for TiO2. Eg values for SnO2:Sb are lower when compared to undoped material, indicating the presence of Sb 5+ substituting for Sn4+ in the SnO2 matrix, which decreases the band gap magnitude. Increasing the doping level additionally decreases the band gap magnitude. Besides, the high degeneracy induced by the high doping level (4 and 8 atom %) shifts EF to a position above the bottom of the conduction band, being 1.57 eV from the minimum of conduction band for SnO2:4at%Sb and 1.93 eV for SnO2:8at%Sb. The Eg value for

Figure 2. X-ray diffraction pattern of TiO 2/SnO2 thin film heterojunction treated at 1100 and 550 °C, respectively.

film preparation are described in the Experimental Section, some details must be emphasized here: (1) a two-layer TiO2 film was deposited on soda-lime glass substrate and treated at 550 °C, followed by a 10-layer film of SnO2, with a final thermal annealing of 550 °C for 1 h. (2) Another two-layer TiO2 film was deposited on quartz substrate and annealed at 1100 °C for 1 h, followed by a 10-layer SnO2 film, with a final thermal annealing at 550 °C for 1 h. The X-ray diffraction patterns show that the film treated at 550 °C presents peaks related to rutile SnO2 and anatase TiO2 because this annealing temperature leads to anatase TiO2, as discussed before (Figure 1b). The X-ray diffraction pattern of D



Article

Figure 4. Band structure diagram: (a) SnO2; (b) high symmetric k-points in the first Brillouin zone; (c) SnO2:4at%Sb; (d) SnO2:8at%Sb; (e) TiO2.

According to the model proposed by Anderson,47 the conduction band discontinuity (ΔEc) depends on the difference between electronic affinities (χSnO2 and χTiO2) of the two semiconductors, which is given by

TiO2 is in good agreement with the value reported in the literature.22 To better understand the influence and changes caused by the introduction of Sb atoms and the differences observed in the atomic orbitals, the total and projected density of states diagrams for the SnO2, TiO2, SnO2:4at%Sb, and SnO2:8at%Sb (Figure 5) were built for the analysis of the corresponding electronic structure. For the SnO2, the upper valence band (VB) is made up predominantly of the O 2p orbitals; there is also Sn 5s and 5p contributions in the range of −8 to −11 eV. The conduction band (CB) has more significant contributions of Sn 5s and 5p orbitals. The SnO2:4at%Sb shows the same behavior as SnO2:8at%Sb (Figure 5b and Figure 5c). The VB is constituted by O 2p orbitals, but these contributions are different and more affected if we compare with the bare system. The CB is made by 5s and 5p orbitals of Sn and Sb atoms. These statements are supported by the observed changes in the band structures. For TiO2 (Figure 5d), the result shows that the VB is formed by 2p orbitals of O atoms and the CB is formed by Ti 3d orbitals, in accordance with other theoretical results.40 Qualitative energy diagrams for the heterostrucutures TiO2/ SnO2 and TiO2/SnO2:Sb can be obtained from the band structure diagrams for SnO2, SnO2:Sb, and TiO2 shown in Figure 4. According to Pollman and Mazur,46 a discontinuity in the valence (ΔEv) and conduction (ΔEc) bands takes place at the heterostructure interface. The sum of these discontinuities depends on the band gap difference between the two materials, which is given by45 ΔEv + ΔEc = ΔEg

ΔEc = χTiO2 − χSnO2

(2)

ΔEc ≈ Ec1 − Ec2

(3)

or

From eq 1, the valence and discontinuity (ΔEv) can be obtained by ΔEv = ΔEg − Δχ

(4)

ΔEv ≈ Ev1 − Ev2

(5)

or

Table 2 presents the calculated values of the top of the valence band and the bottom of the conduction band for TiO2, SnO2, SnO2:4at%Sb, and SnO2:8at%Sb, as shown in Figure 4, as well as electronic affinities for SnO2 and TiO2.48 This table allows verifying that the bottom of the conduction band and the top of the valence band of TiO2 are above the corresponding energies of SnO2 and SnO2:Sb. Table 3 presents values for ΔEv and ΔEc discontinuities for TiO2/SnO2, TiO2/SnO2:4at%Sb, and TiO2/SnO2:8at%Sb heterostructures, obtained from data presented in Table 2, evaluated from eqs 3 and 5. Figure 6 shows qualitative energy band diagrams, built from data presented in Table 3, for the interfaces originating from the coupling in TiO2/SnO2 and TiO2/SnO2:Sb.

(1) E



Article

Figure 5. Total and projected density of states (DOS) for (a) SnO2, (b) TiO2, (c) SnO2:4at%Sb, and (d) SnO2:8at%Sb.

Table 2. Energy Values for the Top of Valence Band (Ev) and Bottom of Conduction Band (Ec), Obtained from SnO2, TiO2, SnO2:4at%Sb, and SnO2:8at%Sb, at Γ Point, and Electronic Affinity of TiO2 and SnO2

a

material

Ev (eV)

Ec (eV)

χ (eV)

SnO2 TiO2 SnO2:4at%Sbb SnO2:8at%Sbb

−4.25 −3.67 −4.22 −4.19

−0.70 −0.33 −0.95 −1.06

4.2a 4.6a

Figure 6. Schematic diagram showing the band structure at the interfaces: (a) TiO2/undoped SnO2; (b) TiO2/SnO2:4at%Sb; (c) TiO2/SnO2:8at%Sb. Ec and Ev are the energies for the bottom of the conduction band and the top of the valence band, respectively.

Wang and co-workers.48 bThis work.

Table 3. Discontinuities of ΔEv and ΔEc for TiO2/SnO2, TiO2/SnO2:4at%Sb, and TiO2/SnO2:8at%Sb Heterostructures heterostructure

ΔEv (eV)

ΔEc (eV)

TiO2/SnO2 TiO2/SnO2:4at%Sb TiO2/SnO2:8at%Sb

0.58 0.55 0.52

0.37 0.62 0.73

negative ions at the TiO2 surface. Concerning Figure 6, it can be observed that the doping increases the discontinuity in the conduction band. The lowest discontinuity in the conduction band occurs at the interface of TiO2/undoped SnO2 thin film heterostructure (Figure 6a). On the other hand, high Sb5+ doping in SnO2 induces a larger discontinuity in the conduction band, whereas the discontinuity in the valence band remains practically the same because of the decrease in the energy band gap of Sb-doped SnO2. It can be clearly seen for the interfaces TiO2/SnO2/4 atom % Sb and TiO2/SnO2/8 atom % Sb (Figure 6b and Figure 6c). The interface TiO2/SnO2:8at%Sb presents the largest discontinuity in the conduction band. Besides, the valence band and the conduction band of TiO2 are higher than those for SnO2. Although the coupling between these different compounds gives birth to a potential barrier whose magnitude may vary for these three heterojunctions, it

The formation of the heterojunction leads to a discontinuity in the conduction band and migration of free electrons from the TiO2 side to the SnO2 side, since the former has a lower conduction band level. It leads to a higher amount of incomplete bondings in the TiO2 side, which improves the efficiency as gas sensor, as long as the TiO2 is the side exposed to gas detection. The high electronic affinity of TiO2 compared to SnO2 favors the adsorption of gaseous species in the form of F



Article

lead to better sensibility for gas detection (related to the surface resistivity), increasing the electrical conductivity (due to charge carrier migration between these oxide semiconductors, from TiO2 to SnO2:Sb). It also may lead to unexplored new properties, when undoped heterojunction is compared to the Sb-doped material or other sort of doping, to be investigated in the future.

must be taken into account that even the undoped SnO2 presents a high degeneration and high free electron concentration, due to oxygen vacancies and interstitial tin atoms. It makes the existing barrier between layers to have a depletion layer thin enough to allow the electrons to tunnel through the barrier, which is independent of the barrier height. These characteristics also allow optical excitation using monochromatic sources with higher wavelength (lower energy) than the SnO2 band gap, leading to an electrical conductivity increase in the thin film arrangement, due to migration of optically excited electrons from the conduction band of TiO2 to the conduction band of SnO2. Figure 7 shows a schematic diagram of this process, which is obtained from the energy band diagram shown in Figure 4a and

5. CONCLUSIONS Evaluation of structural properties of TiO2 thin films revealed that the thermal annealing temperature is responsible for determining the crystalline structure. Powder samples presented a phase transition from anatase to rutile when treated at about 780 °C, whereas in thin films this phase transition takes place at about 1100 °C. We believe that the quartz substrate is responsible for this increase in the phase transition temperature. Besides, anatase TiO2 thin films grow preferentially in the crystallographic plane (101) whereas the rutile TiO2 films grow preferentially along the direction perpendicular to the plane (110). TiO2/SnO2 heterostructure presents the same sort of the crystalline structure as individual TiO2 films, which depends on the thermal annealing temperature of the material placed at the bottom of the heterostructure (TiO2 for the samples presented here). TiO2/SnO2, where the TiO2 layer is treated at 550 °C, presents diffraction peaks in planes related to SnO2 rutile and TiO2 anatase, whereas the heterojunction, where the bottom TiO2 film is treated at 1100 °C (quartz substrate) and the SnO2 film treated at 550 °C, exhibits diffraction peaks related to rutile SnO2 and TiO2. Band structure diagrams calculated from computational simulation based on the density functional theory, by using the program CRYSTAL06, lead to direct band gap energy transition for all the investigated materials, and the band gap values can be summarized as follows: 3.55 eV for SnO2, 3.27 eV for SnO2:4at%Sb, 3.13 eV for SnO2:8at%Sb, and 3.35 eV for TiO2. These diagrams of the evaluated band structures allow proposal of a simple model for the heterojunction TiO2/SnO2 interface, where it was verified that the conduction band and the valence band of TiO2 are above the corresponding values of undoped SnO2. These characteristics lead to interpretation of the effect of monochromatic light excitation, by using sources with energy below the band gap energy of SnO2, promoting increase in the heterostructure electrical conductivity. We believe that heterostructures formed by coupling of these semiconductors may lead to improvement in the electrical properties and increase the sensibility for gas detection.

Figure 7. Schematic diagram showing the heterostructure TiO2/SnO2 and the electron transfer process due to optical excitation with energy above the TiO2 band gap and below the SnO2 band gap.

Figure 4b, by taking EF as constant along the interface (equilibrium condition). Following this concept, adsorption centers, resulting from the electron transfer process, are preferentially formed at the TiO2 surface, modifying its resistivity. As already mentioned, the efficiency of the gas detection process is related to the surface resistivity variation of the material sensor. Then the combination of TiO2 and SnO2 may permit electrical conductivity and sensing efficiency increases in the gas detection process. Sb doping is responsible for band gap (Eg) decreasing in SnO2, probably due to the high doping concentration. Besides, this high doping increases the already high degeneracy and drives the Fermi level up to the bottom of the conduction band (Figures 4c and Figure 4d). SnO2:8at%Sb presents Eg slightly higher than that of TiO2, whereas SnO2:8at%Sb presents lower Eg than the undoped material. From Table 2, it is also possible to verify that the valence band top and the conduction band bottom of TiO2 are higher than the SnO2/4 atom % Sb and SnO2/8 atom % Sb corresponding energies. Besides, the relative energy difference between these values is higher when compared to undoped-SnO2. The conduction band discontinuity increases with concentration, whereas the valence band discontinuity remains practically the same. The energy band structure presented in Figure 4c and Figure 4d indicates that the electron transfer process, due to optical excitation and then the formation of adsorption centers at TiO2 surface, can be even more effective when the heterojunction is formed coupling TiO2 and SnO2:Sb instead of undoped SnO2. Therefore, the deposition and the investigation of these heterojunctions may be very attractive under a technological point of view, considering that this sort of arrangement may

■

AUTHOR INFORMATION

Corresponding Author

*Telephone: +55 14 3103 6086. Fax: +55 14 3103 6096. Email: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

This work is supported by Brazilian Funding Agencies FAPESP, CNPq, and CAPES. The computer facilities were supported by resources supplied by Molecular Simulation Laboratory and Center for Scientific Computing of the São Paulo State University. G



■

Article

(22) Beltrán, A.; Andrés, J.; Sambrano, J. R.; Longo, E. Density Functional Theory Study on the Structural and Electronic Properties of Low Index Rutile Surfaces for TiO2/SnO2/TiO2 and SnO2/TiO2/ SnO2 Composite Systems. J. Phys. Chem. A 2008, 112, 8943−8952. (23) Emori, M.; Sugita, M.; Ozawa, K.; Sakama, H. Electronic Structure of Epitaxial Anatase Tio2 Films: Angle-Resolved Photoelectron Spectroscopy Study. Phys. Rev. B 2012, 85, 35129−35135. (24) Mikami, M.; Nakamura, S.; Kitao, O.; Arakawa, H.; Gonze, X. First-Principle Study of Titanium Dioxide: Rutilo and Anatase. J. Appl. Phys. 2000, 39, 847−850. (25) Albuquerque, A. R.; Maul, J.; Longo, E.; Santos, I. M. G.; Sambrano, J. R. Hydrostatic and [001] Uniaxial Pressure on Anatase TiO2 by Periodic B3LYP-D* Calculations. J. Phys. Chem. C 2013, 177, 7050−7061. (26) Riyas, S.; Krishnan, G.; Mohan-Das, P. N. Polymorphism in TiO2 under the Influence of Cuo and MnO2. J. Ceram. Process. Res. 2006, 7, 301−306. (27) Yamaguchi, Y.; Tabata, K.; Yashima, T. First-Principles Calculations on the Surface Electronic and Reactive Properties of M/SnO2 (M = Ge, Mn) (110). J. Mol. Struct.: THEOCHEM 2005, 714, 221−233. (28) Matti, A.; Jaskari, M.; Rantala, T. T. Band Structure and Optical Parameters of the SnO2 (110) Surface. Phys. Rev. B 2001, 64, 407− 414. (29) Batzil, M.; Burst, J. M.; Diebold, U. Pure and Cobalt-Doped SnO2 (101) Films Grown by Molecular Beam Epitaxy on Al2O3. Thin Solid Films 2005, 484, 132−139. (30) Linsebigler, A. L.; Lu, G.; Yates, J. T. Photocatalysis on TiO2 surfaces: Principles, Mechanisms, and Selected Results. Chem. Rev. 1995, 95, 735−758. (31) Zeng, W.; Liu, T.; Wang, Z.; Tsukimoto, S.; Saito, M.; Ikuhara, Y. Oxygen Adsorption on Anatase TiO2 (101) and (001) Surfaces from First Principles. Mater. Trans. 2010, 51, 171−175. (32) Yang, J.; Li, D.; Wang, X.; Yang, X.; Lu, L.; Juan, Y.; Dan, L.; Xin, E. Rapid Synthesis of Nanocrystalline TiO2/SnO2 Binary Oxides and Their Photoinduced Decomposition of Methyl Orange. J. Solid State Chem. 2002, 165, 193−198. (33) Kanai, N.; Nuida, T.; Ueta, K.; Hashimoto, K.; Watanabe, T.; Ohsaki, H. Photocatalytic Efficiency of TiO2/SnO2 Thin Film Stacksprepared by DC Magnetron Sputtering. Vacuum 2004, 74, 723−727. (34) Sze, S. M. Semiconductor Devices, Physics & Technology; John Wiley & Sons: New York, 1985. (35) Lee, H. D.; Hwang, W. S. Substrate Effects on the Oxygen Gas Sensing Properties of SnO2/Tio2 Thin Films. Appl. Surf. Sci. 2006, 253, 1889−1897. (36) Kohn, W.; Becke, A. D.; Parr, R. G. Density Functional Theory of Electronic Structure. J. Phys. Chem. 1996, 100, 12974−12980. (37) Lee, C.; Yang, R. G.; Parr, R. G. Development of the Colle− Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785−789. (38) Saunders, V. R.; Dovesi, R.; Roetti, C.; Orlando, R.; Wilson, C. Z.; Harrison, N. M.; Doll, K.; Civalleri, B.; Bush, I. J. CRYSTAL06 User’s Manual; University of Turin: Torino, Italy, 2006. (39) Corno, M.; Busco, C.; Civalleri, B.; Ugliengo, P. Periodic ab Initio Study of Structural and Vibrational Features of Hexagonal Hydroxyapatite Ca10(PO4)6(OH)2. Phys. Chem. Chem. Phys. 2006, 8, 2464−2472. (40) Sambrano, J. R.; Nobrega, G. F.; Taft, C. A.; Andrés, J.; Beltrán, A. A Theoretical Analysis of TiO2/Sn Doped (110) Surfaces Properties. Surf. Sci. 2005, 580, 71−79. (41) Rassolov, V. A.; Pople, J. A.; Ratner, M. A.; Windus, T. L. J. 631G Basis Set for Atoms K through Zn. Chem. Phys. 1998, 109, 1223− 1229. (42) Towler, M. Theory of Condensed Matter Group. Antimony Basis Sets for the Crystal Program. http://www.tcm.phy.cam.ac.uk/ ~mdt26/basis_sets/Sb_basis.txt (accessed March 15, 2014).

REFERENCES

(1) Licznerski, B. Thick Film Gas Microsensors Based On Tin Oxide. Bull. Pol. Acad. Sci.: Tech. Sci. 2004, 4, 95−104. (2) Zeng, W.; Liu, T.; Wang, Z. Enhanced Gas Sensing Properties by SnO2 Nanosphere Functionalized TiO2 Nanobelts. J. Mater. Chem. 2004, 22, 3544−3548. (3) Gaidi, M.; Chenevier, B.; Labeau, M. Electrical Properties Evolution under Reducing Gaseous Mixtures (H2, H2S, CO) of the SnO2 Thin Films Doped with Pd/Pt Aggregates and Used as Polluting Gas Sensors. Sens. Actuators, B 2000, 65, 43−48. (4) Diéguez, A.; Romano-Rodriguez, A.; Alay, J. L.; Morante, J.; Bâr san, N.; Kappler, J.; Weimar, U.; Gopel, W. Parameter Optimization in SnO2 Gas Sensor for NO2 Detection with Low Cross Sensitivity to CO: Sol−Gel Preparation, Film Preparation, Powder Calcination, Doping and Grinding. Sens. Actuators, B 2000, 65, 166−168. (5) Ansari, S. G.; Ansari, Z. A.; Kadan, M. R.; Karekar, R. N.; Aiyer, R. The Effect of Humidity on an Sno2 Thick-Film Planar Resistor. Sens. Actuators, B 1994, 21, 159−163. (6) Watson, J.; Ihokura, K.; Coles, G. S. V. The Tin Dioxide Gas Sensor. Meas. Sci. Technol. 1993, 4, 711−719. (7) Yamaura, H.; Tamaki, J.; Moryia, K.; Miura, H.; Yamazoe, N. J. Selective CO Detection by Using Indium Oxide-Based Semiconductor Gas Sensor. J. Electrochem. Soc. 1996, 143, 36−37. (8) Fukai, Y.; Kondo, Y.; Mori, S.; Suzuki, E. Highly Efficient DyeSensitized SnO2 Solar Cells Having Sufficient Electron Diffusion Length. Electrochem. Commun. 2007, 9, 1439−1443. (9) Tachibana, Y.; Umekita, K.; Otsuka, Y.; Yasuhiro, S. Performance Improvement of CdS Quantum Dots Sensitized TiO2 Solar Cells by Introducing a Dense TiO2 Blocking Layer. J. Phys. D: Appl. Phys. 2008, 41, 1−5. (10) Cardoso, W. S.; Longo, C.; De Paoli, M. A. Preparaçaõ de Eletrodos Opticamente Transparentes. Quim. Nova. 2006, 28, 345− 349. (11) Vannice, M. A.; Sudhakar, C. A Model for the Metal-Support Effect Enhancing Carbon Monoxide Hydrogenation Rates over Platinum−Titania Catalysts. J. Phys. Chem. 1984, 88, 2429−2432. (12) Thompson, T. L.; Yates, J. T. Surface Science Studies of the Photoactivation Of TiO2New Photochemical Processes. J. Chem. Rev. 2006, 106, 4428−4453. (13) Sharma, R. K.; Bhatnagar, M. C.; Sharma, G. L. Effect of Nb Ion in TiO2 Oxygen Gas Sensor. Appl. Surf. Sci. 1996, 92, 647−650. (14) Smulko, J. M.; Ederth, J.; Li, Y.; Lish, L. B.; Kennedy, M. K.; Kruis, F. E. Gas Sensing by Thermoelectric Voltage Fluctuations in SnO2 Nanoparticle Films. Sens. Actuators, B 2005, 106, 708−712. (15) Mishra, K. C.; Johnson, K. H.; Schimidt, P. C. Electronic Structure of Antimony Doped Tin Oxide. Phys. Rev. B 1995, 51, 13972−13976. (16) Senguttuvan, T. D.; Malhotra, L. K. Sol Gel Deposition of Pure and Antimony Doped Tin Dioxide Thin Films by Non Alkoxide Precursors. Thin Solid Films 1996, 289, 22−28. (17) Rai, R.; Senguttuvan, T. D.; Lakshmikumar, S. T. Study of the Electronic and Optical Properties of Doped SnO2. Comput. Mater. Sci. 2006, 37, 15−19. (18) Bruneaux, J.; Cachet, H.; Froment, M.; Messad, A. Correlation between Structural and Electrical Properties of Sprayed Tin OxideFilms with and without Fluorine Doping. Thin Solid Films 1991, 197, 129−142. (19) Eremev, S. V.; Velikokhatnyi, O. I.; Kimta, P. N.; Potekaev, A. I. Effects of Antimony Doping on Electronic Structure of SnO2. Russ. Phys. J. 2004, 47, 701−704. (20) Kim, K. S.; Yoon, S. Y.; Lee, W. J.; Kim, K. H. Surface Morphologies and Electrical Properties Antimony-Doped Tin Oxide Films Deposited by Plasma-Enhanced Chemical Vapor Deposition. Surf. Coat. Technol. 2001, 138, 229−236. (21) Park, Y. R.; Kim, K. J. Structural and Optical Properties of Rutile and Anatase TiO2 Thin Films: Effects of Co Doping. Thin Solid Films 2005, 484, 34−38. H



Article

(43) Pookmanee, P.; Phanichphan, S. Titanium Dioxide Powder Prepared by a Sol−Gel Method. J. Ceram. Process. Res. 2009, 10, 167− 170. (44) Kajitvichyanukul, P.; Ananpattarachaia, J.; Pongpomb, S. Sol− Gel Preparation and Properties Study of TiO2 Thin Film for Photocatalytic Reduction of Chromium(VI) in Photocatalysis Process. Sci.Technol. Adv. Mater. 2005, 6, 352−358. (45) Moholkar, A. V.; Pawar, S. M.; Rajpure, K. Y.; Patil, P. S.; Bhosale, G. H. Properties of Highly Oriented Spray-Deposited Fluorine-Doped Tin Oxide Thin Films on Glass Substrates of Different Thickness. J. Phys. Chem. Solids 2007, 68, 1981−1988. (46) Pollmann, J.; Mazur, A. Theory of semiconductor heterojuntions. Thin Solid Films 1983, 104, 257−276. (47) Anderson, R. L. Experiments on Ge−GaAs Heterojunctions. Solid-State Electron. 1962, 5, 341−351. (48) Wang, C.; Shao, C.; Zhang, X.; Liu, Y. SnO2 Nanostructures− TiO2 Nanofibers Heterostructures: Controlled Fabrication and High Photocatalytic Properties. Inorg. Chem. 2009, 48, 7261−7268.

I


SnO2 Thin Films by SolâGel Method and Periodic

Recommend Documents

SnO2 Thin Films by SolâGel Method and Periodic