Article pubs.acs.org/JPCC
Structural and Electronic Effects of Incorporating Mn in TiO2 Films Grown by Sputtering: Anatase versus Rutile André L. J. Pereira,*,†,‡ Lourdes Gracia,‡ Armando Beltrán,‡ Paulo N. Lisboa-Filho,† José H. D. da Silva,† and Juan Andrés‡ †
Grupo de Materiais Avançados, Universidade Estadual Paulista Unesp/Bauru, São Paulo, Brazil Departament de Química Física i Analítica, Universitat Jaume I, Castello De La Plana, Spain
‡
ABSTRACT: Pure and Mn-doped TiO2 films have been deposited by sputtering technique onto SiO2 substrates. The films display a compact columnar morphology, as revealed by scanning electron microscopy. X-ray diffraction and Raman scattering results provide evidence that the pure TiO2 films are predominantly anatase phase, but the increase in Mn concentration favors the rutile phase. The optical characterization shows a systematic decrease in the value of band gap and an increase in the tail states with the increase in Mn concentration. Magnetization measurements display purely diamagnetic behavior in the undoped TiO2 film and substrate and paramagnetic behavior in the Mn-doped films. No indication of ferromagnetic signature has been evidenced. First-principle calculations based on density functional theory and periodic models were employed to calculate the band structure and the density of electronic states to investigate the influence of Mn incorporation in the electronic structure of TiO2. Both experimental data and electronic structure calculations evidence the fact that the presence of Mn produces important modifications in the electronic states, mainly related to the 3d Mn orbitals in the inside the gap and in the vicinity of the band edges. implantation of transition metals in TiO2 films prepared by RF magnetron sputtering or increasing the substrate temperature during the deposition of undoped films, there is an increase in efficiency of absorption for wavelengths up to 550 nm. Li et al.20 have presented a novel method that is suitable for fabricating multivalent Mn-TiO2 nanospeheres doped with other ions and tuned size to be used as a high-performance visible-light photocatalyst. The effects of incorporation of transition metals in the TiO2 matrix extend beyond the photocatalysis. When used to convert solar light into chemical and electric energy in a solar cell, these effects increase the conversion efficiency.21 Very recently, the effects of Mn doping on the dielectric properties of TiO2 ceramics was studied by Chao and Dogan.22 When Mn concentration was low, they observed an increase in the generation of electron traps and a consequent enhancement of the dielectric properties.22 In addition, several experimental and theoretical works published recently confirm the possibility of ferromagnetic behavior above room temperature in various semiconductors doped with transition metals. These semiconductors include TiO2 doped with Co,23,24 Fe,25,26 V,27 Ni,25 Cu,28 Cr,29,30 and Mn31−34 prepared by various techniques.6 Some studies demonstrate that this ferromagnetism behavior would be directly related to structural disorder,35−37 oxygen vacancies,38,39 clusterization of the transition metal,40 and, some-
1. INTRODUCTION Titanium dioxide (TiO2), also known as titania, has been studied extensively because of its unique physical and chemical properties, including a high refractive index (n = 2.7), excellent optical transmittance in the visible and near-infrared frequency range, and high dielectric constant (∼80 at 1 MHz).1−3 Because its electrical resistance decreases significantly with increasing oxygen vacancies, TiO2 is also is an excellent candidate for the material used in oxygen sensors.4 Currently, there is a great interest in increasing the efficiency of solar cells by using TiO2 in a photovoltaic cell.5−8 When used as an electrode in photocatalysis, TiO2 increases the efficiency of separating water by electrolysis. However, once its bandgap is ∼3.2 eV, the application of TiO2 in photocatalysis is limited because it is not possible to use the visible range of sunlight. In an effort to increase the electronic properties of TiO2, many processes have been developed where transition metals are incorporated in its structure.6,9−11 In photocatalysis, for example, the TiO2 is doped with a transition metal to extend the light absorption to the visible region and increase the catalytic activity.10−12 Introducing transition metals into TiO2 can change the coordination environment of Ti in the lattice and modify the electronic band structure of TiO2.11 In general, the metal sites act as trapping sites by accepting the photogenerated electrons from the TiO2 valence band (VB), mitigating the charge recombination and finally improving the photocatalytic activity of the catalysts.13−18 In this context, the overview of the recent advances achieved on visible lightresponsive TiO2-based photocatalysts presented by Ji et al.19 needs to be cited. These authors highlight that by ion © 2012 American Chemical Society
Received: November 7, 2011 Revised: March 14, 2012 Published: March 22, 2012 8753
dx.doi.org/10.1021/jp210682d | J. Phys. Chem. C 2012, 116, 8753−8762
The Journal of Physical Chemistry C
Article
times, to a magnetic contamination of the samples.41 Therefore, the origin of the ferromagnetism is still a subject of great controversy and the theme of intense research.36,42 Espinosa et al.43 have combined experimental results with ab initio density functional calculations to find the origin of the magnetism in undoped and Mn-doped SnO2 thin films obtained by RF magnetron sputtering. Considering the great potential of application of TiO2 doped with transition metals, the aim of the present report is to contribute to a better understanding of the effects of Mn doping on the structural, electronic, and magnetic properties in TiO2 films prepared by the RF magnetron sputtering technique. To assist in interpreting the results, we also carried out complementary theoretical calculations of the electronic structure of Ti1−xMnxO2 based on the density functional theory (DFT) and periodic models. Band structures and density of states were analyzed in detail to find an electronic structure−property correlation to provide some significant perspectives regarding future directions for research.
scanning electron microscope (SEM) and selected area energydispersive X-ray spectroscopy (EDX) (JEOL-JMS-7001F-fieldemission SEM). Unpolarized micro-Raman scattering experiments were performed at room temperature using a HeNe laser (6328 Å line) with power set below 5 mW to not burn the sample. The signal was collected by a Horiba Jobin Yvon LabRAM HR microspectrometer equipped with a thermoelectric (TE)-cooled, multichannel charge-coupled device (CCD) detector and a spectral resolution better than 2 cm−1. The transmittance measurements in the absorption edge (λ = 190−1100 nm) were performed by an UVmini-1240 Shimadzu spectrophotometer to determine the absorption coefficients and energy gap of the films. Cisneros’s method for transmittance44 was used to determine the refractive index, absorption coefficient, and thickness in the less-absorbent (low Mn content) samples, with the help of a computational routine. For a better interpretation of the experimental results, we carried out first-principle calculations using the CRYSTAL09 program package.45 To study Mn substitution in anatase (I41/ amd) and rutile (P42/mnm) TiO2 structures, we performed periodic calculations using a supercell 2 × 2 × 2 model with 48 atoms.46 Becke’s three-parameter hybrid nonlocal exchange functional47 combined with the Lee−Yang−Parr gradientcorrected correlation functional, B3LYP,48 was used. The O and Mn atoms were represented by the 6-31G(d1) and 8-41G Gaussian basis set, respectively, available at the Crystal web site.49 For the titanium atom, the 6-31G basis set developed by Rassolov et al.50 was selected, as in a previous work.51 To improve the accuracy of our results, we applied the empirical correction scheme for energy in periodic systems that considers the long-range dispersion contributions proposed by Grimme52 and implemented by Bücko et al.53 A note of caution is mandatory here: debate continues about the difficulty of obtaining accurate property predictions for transition-metal oxides from DFT calculations, and it is well known that DFT underestimates the value of the band gap.54,55 Despite attempts to use hybrid functional and dynamical mean field theory to treat the problem, DFT with the Becke threeparameter (exchange), Lee, Yang, and Parr (B3LYP) functional remains an economical choice.56,57 Studies of doped TiO2 tend to focus on band gap modulation by doping with aliovalent cations, which presents issues with the reliability of DFT in describing the electronic structure of the dopants in metal oxides.43,58−62 In the present study, we are interested in the changes upon doping of the electronic and structural properties as well as the band gap.
2. EXPERIMENTAL SECTION Ti1−xMnxO2 thin films were deposited by reactive RF magnetron sputtering using a Ti-metal target (99.999%) and a mixture of Ar+O2. The residual pressure of the sputtering chamber before and after the film depositions was smaller than 1 × 10−6 Torr. Before each deposition, the target was sputtered with Ar for 10 min to ensure that the target was clean at the moment of the films growth. Manganese was added to the films by a cosputtering process; that is, pure Mn slabs (99.99%) were placed onto the Ti target and also suffered the sputtering process, resulting in incorporating Mn into the films. The concentration of Mn incorporated into the films was controlled by covering different fractions of the target area with Mn slabs. The TiO2 films were deposited on heated (∼450 °C) amorphous SiO2 substrates. The substrate heating was measured by a thermocouple incorporated into the substrate holder. The main deposition parameters of the films studied in this work are presented in Table 1. The structure of the films was analyzed by X-ray diffraction (XRD) analysis using a Rigaku Ultima 2000+. The microscopy images and the composition of the films were collected by a Table 1. Deposition Parameters of Ti1−xMnxO2 Films Grown by Radio Frequency (RF) Magnetron Sputtering onto aSiO2a λgap
Egap
sample
AMn/ATi
x
thickness (nm)
(nm)
(eV)
KL35 KL47 KL43 KL39 KL37 KL41 KL48
0.000 0.0016 0.003 0.006 0.008 0.020 0.060
0.000 0.002 0.03 0.06 0.11 0.19 0.30
449 401 450 434 472 500 466
364.7 385.1 435.1 459.3 539.1 632.7 775.0
3.40 3.22 2.85 2.70 2.30 1.96 1.60
3. RESULTS AND DISCUSSION SEM images of Ti1−xMnxO2 films grown by sputtering are shown in Figure 1. Figure 1a corresponds to a low concentration of Mn (x ≈ 0.002), Figure 1b corresponds to an intermediate concentration of Mn (x ≈ 0.11), and Figure 1c corresponds to a higher content of Mn (x ≈ 0.19). Figure 1d−f shows the respective film surfaces. As can be observed in Figure 1a−c, the films present a columnar morphology, with the columns aligned perpendicular to the substrate’s surface. This behavior was already observed in films of other materials, like Ga1−xMnxN,63 pure64 and Fedoped65 ZnO, and pure Mo66 grown by sputtering. Increasing the Mn from x ≈ 0.002 (Figure 1a) to x ≈ 0.11 (Figure 1b) apparently does not produce any significant change in the shape of the columns. The film with a high concentration
a Substrates constant parameters: O2 flow, 0.2 sccm; Ar flow, 40.0 sccm; total pressure, 5 × 10−3 Torr; residual pressure,
