Laser-Induced Anatase-to-Rutile Transition in TiO2 Nanoparticles

Article pubs.acs.org/JPCC

Laser-Induced Anatase-to-Rutile Transition in TiO2 Nanoparticles: Promotion and Inhibition Effects by Fe and Al Doping and Achievement of Micropatterning G. Cristian Vásquez,† M. Andrea Peche-Herrero,‡ David Maestre,*,† Alessandra Gianoncelli,§ Julio Ramírez-Castellanos,‡ Ana Cremades,† José María González-Calbet,‡ and Javier Piqueras† †

Departamento de Física de Materiales, Facultad de CC Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain Departamento de Química Inorgánica I, Facultad de CC Químicas, Universidad Complutense de Madrid, 28040 Madrid Spain § Elettra - Sincrotrone Trieste, Area Science Park, 34149 Basovizza-Trieste, Italy ‡

ABSTRACT: A study of the anatase to rutile transition (ART) induced by laser irradiation in TiO2 nanoparticles synthesized by a modified Pechini method has been carried out in this work, with special attention focused on the effects of doping with Al or Fe on the phase transition. Either promotion or inhibition of ART can be achieved by Fe or Al doping, respectively, as demonstrated by X-ray diffraction, Raman spectroscopy, photoluminescence, X-ray absorption, and transmission electron microscopy results presented in this work. The influence of dopants (Al, Fe) in the kinetics of the ART, the key role played in this process by the surface of the nanoparticles and the presence of oxygen vacancies, and the formation of rutile nucleation points at twinned regions have been discussed in this work. Finally, advantage has been taken of the ART controlled by laser irradiation, and an original laserinduced micropatterning based on spatial phase-controlled titania polymorphs has been developed, which assures significant progress in challenging microdevice design based on titania polymorphs.

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INTRODUCTION Increasing research is being carried out on TiO2 nanostructures, as during the last years this material has become one of the most promising semiconducting oxides regarding different applications. TiO2 (titania) has been widely exploited mainly in the field of photocatalysis1−5 where it is employed in environmentally beneficial processes such as hydrogen generation by water splitting and treatment of polluted water and air. In addition, it has also demonstrated its applicability in other areas such as gas sensing, solar cells, photovoltaics, medicine, chemical industry, or energy storage.6,7 In some of these applications, the suitability of TiO2 is strongly dependent on the ability to control its dimensions, structure of defects, doping, and also its crystallographic phase, which plays a key role in the functionalization of this material. TiO2 crystallizes in three polymorphs: anatase (tetragonal), rutile (tetragonal), and brookite (orthorhombic), with anatase and rutile being the most investigated and extensively used so far. The characteristic physical and chemical properties associated with rutile and anatase phases, i.e., different bandgaps, surface energies, dielectric constants, refractive indexes, or mechanical properties, among others, make them specifically suitable for different applications. For instance, anatase is more appropriate for photocatalytic performance, while rutile exhibits improved optical and electronic funcionality.8 In addition, it has been © XXXX American Chemical Society

recently found that a mixed anatase/rutile phase exhibits an enhancement of the TiO2 photocatalytic performance,9,10 with relevant consequences in the improvement of the production of hydrogen from water or the efficiency of dye-sensitized solar cells.7 Hence, optimized TiO2 applicability in this field can be achieved through precise control of the phase composition. In that way, efforts are directed toward the controlled growth of stabilized anatase and rutile single and mixed phases, as well as the understanding of the anatase to rutile transition. The anatase to rutile phase transition is a nonreversible process because of the higher thermodynamic stability of rutile, which occurs at temperatures between 600 and 700 °C for bulk TiO2 or at lower temperatures for nanosized TiO 2 . This reconstructive transformation consists of a nucleation and growth process involving diverse factors such as the presence of oxygen vacancies or titanium interstitials, the influence of dopants, the surface area, atmosphere, temperature, or the rutile nucleation at the interface of anatase nanoparticles.11,12 However, the physical and chemical processes involved in the ART, as well as the mechanisms to promote or inhibit it, have not been totally understood so far. Received: February 20, 2015 Revised: April 17, 2015

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DOI: 10.1021/acs.jpcc.5b01736 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) XRD pattern from anatase undoped TiO2. (b) Low magnification TEM image of undoped anatase TiO2 nanoparticles showing high homogeneity in size.

were collected with a step size of 0.04° and a collection time of 1 s/step. In the acquisition of XRD patterns and the corresponding cell parameters and Rietveld refinement analysis, a Panalytical X’Pert Pro Alpha1 instrument equipped with a primary fast X’Celerator detector operating at 45 kV and 40 mA, fitted with a primary curved Ge 111 monochromator in order to obtain Cu Kα1 radiation, was used. Conventional and high resolution transmission electron microscopy (TEM, HRTEM) were performed in a JEOL 3000 FEG electron microscope, fitted with a double tilting goniometer stage (±22°, ± 22°). An Oxford INCA analyzer system attached to the aforementioned microscope was used for the analysis of the local composition by means of energy-dispersive X-ray spectroscopy (EDS). Further HRTEM characterization was carried out with Digital Micrograph 3.9.3 software (Gatan Inc.) by using fast Fourier transform (FFT) and inverse fast Fourier transform (I-FFT) calculations, as well as a MacTempas 2.3.24 program (Total Resolution, Inc.) for the image simulation. Micro-Raman and micro-PL studies were carried out in a Horiba Jobin-Yvon LabRam Hr800 confocal microscope equipped with a 325 nm He−Cd laser, which was also used to induce the anatase to rutile transition by laser irradiation in selected locations of the samples. Prior to the laser irradiation, the nanopowders were pressed into pellets in order to achieve higher compaction and lesser roughness at the surface. Raman spectra were recorded at room temperature, and the scattered light was collected by using a charge coupled device (CCD). In this setup, the excitation light can be attenuated by using neutral density filters to reduce the total laser intensity of 20 mW (I0) by a factor 0.5, 0.25, and 0.1, thus involving different laser power densities. A laser power density of about 8 × 104 W cm−2 can be achieved if neutral filters are not used, as described in ref 15. Photoluminescence (PL) spectra have been acquired at room temperature with the 325 nm He−Cd laser using the lower laser intensity (0.1I0) in order to avoid a possible phase transition during the PL spectra acquisition. X-ray Absorption Spectroscopy (XAS) measurements coupled with soft-X-ray transmission microscopy (STXM) experiments were performed at the TwinMic beamline of the Elettra synchrotron facility in Trieste, Italy.16 TwinMic can work in the 400−2200 eV photon energy range with a spatial resolution down to sub-50 nm. For TEM and XAS-STXM characterization the as-grown nanopowders were previously dispersed in isopropanol and deposited in TEM grids.

The study of the anatase to rutile transition by means of a controlled laser irradiation is a main objective of this work, and it has been investigated as a function of the dopant (Al, Fe), its concentration, and the irradiation conditions in order to gain deeper comprehension and control of this phase transformation. The possibility of achieving an ART micrometric patterning is also studied, thus facing challenging performances of this material in different technological applications. This study has been performed on Ti1−xMxO2 (M = Fe, Al, and 0 ≤ x ≤ 0.3) anatase nanoparticles prepared by a Pechini modified method,12,13 which allows a precise control of the dimensions and the cationic concentration. The influence of Al and Fe doping in the structural and luminescent properties of anatase TiO2 nanoparticles has been also analyzed in this work. The structural, compositional, and optical characterization of the nanoparticles have been carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM), conventional and high-resolution transmission electron microscopy (TEM, HRTEM), selected area electron diffraction (SAED), X-ray energy dispersive spectroscopy (EDS), photoluminescence (PL), Raman spectroscopy, and X-ray absorption spectroscopy (XAS).

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EXPERIMENTAL SECTION The synthesis of the Ti1−xMxO2 (0 ≤ x ≤ 0.3; M = Al, Fe) nanopowders has been performed by means of a “Liquid-Mix method” using polymeric precursors.12,14 Ti(OBu)4 (Aldrich 97%), Al(NO3)3·9H2O (Aldrich 99%), Fe(NO3)3·9H2O (Aldrich 99%), citric acid, and ethylene glycol (Aldrich 99.99%) were used in the synthesis, as described in detail in a previous work.12 In a final step, this resin, which contains the cations in the desired stoichiometric amounts, was calcined, and a fine powder consisting of doped TiO2 nanoparticles was obtained for each dopant. The powders were treated at 350 °C for 30 h in order to completely remove organic residues. Finally, in order to obtain a well crystallized anatase single phase, Al-doped samples were treated at 550 °C for 15 h, while Fe-doped samples were treated at 450 °C for 15 h. A carboncontent analysis, performed on a PerkinElmer 2400 CHN analyzer with ±0.01% error, shows a carbon content lower than the quantification limit of the equipment; therefore, carbon impurities can be neglected. The synthesized nanopowders were studied by X-ray diffraction (XRD) using a Siemens D5000 diffractometer, with Cu(Kα) (λ = 1.5418 Å) as the working radiation. Data B


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RESULTS AND DISCUSSION In order to achieve understanding and control of the laserinduced anatase to rutile phase transformation, a precise characterization of the as-synthesized nanoparticles before and after irradiation is needed. As-Grown Samples. XRD measurements show that the asgrown nanoparticles correspond to TiO2 anatase (sp gr I41/ amd, z = 2 with lattice parameters a = b = 3.77 Å, c = 9.44 Å), as observed in Figure 1a where a XRD pattern from undoped TiO2 is shown. Anatase single-phase samples were obtained up to a cationic concentration of 30% for the Al-doped samples and only up to 20% for the Fe-doped samples, as a higher amount of dopant caused different ternary compounds to appear in the samples, which should be avoided in this case. The incorporation of dopants in the TiO2 lattice was confirmed by EDS measurements, which show compositional homogeneity, as well as atomic % values for the dopants similar to the expected ones according to the precursors.12 The average sizes of the undoped and doped TiO2 nanoparticles were calculated using the Scherrer formula: D = (Kλ)/(B cos θ), where θ is the peak position, λ is the X-ray wavelength, and B is the full width at half-maximum (fwhm) of the diffraction maxima, once the Rietveld analysis was performed on the monochromatic X-ray. Table 1 shows the average dimensions of the undoped and Fe, Al (20% cat.) doped TiO2 nanoparticles, with sizes around

spectroscopy and PL using the lowest laser intensity (0.1 Io) in order to avoid possible thermally induced phase transition during the measurements. Raman spectra from undoped and 10% cat. doped (Al, Fe) TiO2 nanoparticles are shown in Figure 2a. In all, the Raman spectra-only peaks corresponding to anatase active vibration modes17 are identified at 150 cm−1 (Eg), 198 cm−1 (Eg), 401 cm−1 (Eg), 523 cm−1 (B1g + A1g), 635 cm−1 (Eg). A weak wide contribution can be also observed at around 800−840 cm−1, the origin of which is not well-known, but it is commonly attributed to a B1g overtone scattering.12,18 Some variations in the relative intensities and positions of the Raman peaks can be appreciated as a function of the dopant, as observed in Figure 2a. These effects have been analyzed on the basis of the differences in the incorporation of Al or Fe in the TiO2 lattice, as well as the associated defects and stoichiometry changes,19 as described in ref 12. In both undoped and Aldoped TiO2, the dominant vibrational modes correspond to A1g+B1g (523 cm−1) and Eg (635 cm−1), while modes Eg (150 cm−1) and A1g + B1g (523 cm−1) are the most intense for Fedoped TiO2. The Raman-active modes A1g + B1g (523 cm−1) are associated with antisymmetric and symmetric bending vibrations O−Ti−O, whereas Eg (150 cm−1), the relative intensity of which is increased for Fe doped TiO2, is related to O−Ti−O symmetric stretching vibrations.20 Al3+ and Fe3+ substitution of Ti4+ in octahedral sites are the most common doping mechanisms in TiO2, which involves creation of oxygen vacancies and related Ti3+ interstitials due to charge imbalance.21,22 In the case of Fe doping, Fe2+ and Fe4+ should also be also considered, due to the multivalency of iron. In Figure 2a, slight differences can be appreciated in the position of some of the vibrational modes, mainly in the one centered at around 830 cm−1, which has been barely investigated so far. This mode, which decreases by Fe doping and increases when doping with Al, can be deconvoluted by two Gauss−Lorentzian contributions at about 800 and 851 cm−1, respectively. The former can be attributed to the first B1g overtone, while some works associated the latter with stretching modes of short apical Ti−O bonds at the TiO2 surface23 involving bond strength and short-range order. The behavior observed in the Raman spectra can be explained due to a relaxation in the Ti−O bonding at the surface of the Fe doped nanoparticles, as compared with undoped and Al-doped TiO2 for which the opposite behavior was observed. Normalized PL spectra from undoped, as well as doped (10% cat.), TiO2 nanoparticles are shown in Figure 2b. Similar

Table 1. Cell Parameters and Average Size (D) Corresponding to Anatase Undoped TiO2 and Highly Doped (Ti0.8Al0.2O2, Ti0.8Fe0.2O2) Nanoparticles TiO2 Ti0.8Al0.2O2 Ti0.8Fe0.2O2

a (Å)

c (Å)

D (Å)

3.79(1) 3.79(3) 3.81(2)

9.46(2) 9.47(8) 9.48(2)

35.38 ± 0.02 54.50 ± 0.02 42.22 ± 0.02

5 nm, as well as the corresponding lattice parameters. No remarkable variations in the lattice parameters due to the doping process were observed, as reported in ref 12. These results were confirmed by TEM measurements. As an example, the TEM micrograph in Figure 1b shows anatase-undoped TiO2 nanoparticles, with homogeneity in size and shape. Further information on the microstructural and compositional analysis of the as-grown nanoparticles can be found in ref 12. Prior to the analysis of the effects induced by irradiation, the as-grown nanoparticles were characterized by means of Raman

Figure 2. (a) Raman and (b) PL spectra acquired on anatase TiO2 nanoparticles doped with Al or Fe (10% cat.). Raman and PL spectra from undoped TiO2 are also included as a reference. C


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Figure 3. Raman spectra of anatase (a) undoped TiO2 and (b) Al (30% cat.) doped TiO2 nanoparticles before and after laser irradiation during 5 h. Peaks corresponding to rutile phase induced by irradiation are marked with arrows in (a).

Figure 4. (a) Raman spectra acquired on Fe (20% cat.) doped TiO2 nanoparticles before and after laser irradiation during 20 s. Peaks corresponding to anatase and rutile phases are marked with arrows in (a). Raman spectrum from rutile TiO2 nanoparticles is also included in (a) for comparison. Variation of the intensity corresponding to the anatase Eg mode as a function of the irradiation time is shown for Fe (10% cat.) and Fe (20% cat.) doped TiO2 nanoparticles in (b) and (c), respectively.

characterized, the effect of laser irradiation on the anatase TiO2 nanoparticles has been studied in situ by analyzing the evolution of the Raman spectra and the corresponding PL signal collected from undoped and doped (Al, Fe) TiO2 nanoparticles irradiated with different laser intensities (0.1I0, 0.25I0, 0.5I0, I0). XAS measurements were also performed on these samples. Once the phase transition was induced by means of high laser intensity, Raman analysis was performed using the lowest laser power density in order to avoid possible alteration of the samples during measurements. In the case of undoped TiO2, variations in the Raman spectra denoting the ART were observed only after extended laser irradiation using the highest laser intensity (I0). After some hours of continuous irradiation new peaks associated with TiO2 rutile phase (240, 447, 611, and 827 cm−1) start to appear in the Raman spectrum (marked with arrows in Figure 3a), which confirms that the initial anatase phase is progressively transformed into rutile. There are different approaches to quantify the proportion of rutile to anatase in samples containing anatase and rutile phases. As an example, Lee et al.33 calculated the phase conversion efficiency comparing the area under the Raman peaks of the corresponding phases. Following a similar procedure, in this work, Raman spectra from pure anatase and rutile nanoparticles were acquired, the intensity of the peaks was corrected as a function of the respective Raman cross section of each phase, and finally

luminescence spectra, formed by a broad emission in the visible range with two main contributions at 2.35 and 3 eV, have been measured for undoped and Al-doped TiO2 samples. As described in detail in ref 12, the emission at 2.35 eV has been usually attributed to surface defects mainly associated with oxygen deficiency and related intragap states24−27 as well as to Ti3+ associated with neighbor oxygen vacancies.28,29 The emission at higher energy (∼3 eV), which exhibits lower relative intensity, has been related to self-trapped excitons localized at TiO6 octahedra.30,31 In addition to the 2.35 eV band, Fe-doped samples show emissions at 2.1 eV, associated with oxygen vacancies related defects and undercoordinated Ti, and a narrower emission at 1.66 eV which has been recently related to Fe3+ in tetrahedral coordination.32 The intensity of this near-IR emission at 1.66 eV increases with the cationic amount of Fe, which suggests higher presence of Fe3+ in the probed samples. Differences as a function of the dopant have been also observed in the high energy region, which in this case is formed by two emissions at 3 and 3.3 eV, as confirmed by Gaussian deconvolution. The former (3 eV), also observed in undoped and Al-doped TiO2, is related to self-trapped excitons in anatase TiO2, while the latter (3.3 eV), which shows higher intensity in Fe-doped TiO2, can be associated with near bandedge emission. Anatase to Rutile Phase Transition Induced by Laser Irradiation. Once the as-grown nanoparticles have been D


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Figure 5. Variation of the anatase Eg mode as a function of the laser power density for (a) Ti0.9Al0.1O2 and (b) Ti0.9Fe0.1O2 nanoparticles. Evolution of the fwhm and Eg peak position as a function of the laser power reduction factor are shown for (c) Ti0.7Al0.3O2 and (d) Ti0.8Fe0.2O2.

anatase peak at 150 cm−1 reaches a minimum value and stop diminishing during further laser irradiation. Following this criterion, the phase transition of TiO2 (Fe 10% cat.) nanoparticles stabilizes after approximately 1 h irradiation, while for the TiO2 (Fe 20% cat.) ones the ART stabilizes after about 20 s, more than 2 orders of magnitude faster than in the case of the 10% Fe samples, as observed in Figure 4b and Figure 4c, respectively. In both cases, the ART is clearly faster than in undoped TiO2. Both in undoped and Fe doped TiO2 nanoparticles, the rutile phase induced by laser irradiation is stable, as the ART is irreversible, and it remains after the irradiation. In addition, the Eg mode (∼150 cm−1) characteristic of anatase, related to O−Ti−O symmetric stretching vibrations, has been carefully investigated as a function of the dopant and the laser irradiation conditions in order to analyze its evolution during the ART. As a first effect, peak shift and broadening are observed in the Raman spectra as the laser intensity is increased, which could be explained mainly due to thermal effects induced by laser irradiation. Figure 5a and Figure 5b show the normalized Eg mode (∼150 cm−1) corresponding to Al (10% cat.) and Fe (10% cat.) doped TiO2 nanoparticles irradiated with different laser power densities. If we focus on the fwhm of this Eg mode, it can be observed that Al-doped TiO2 samples show a continuous increase in the Raman peaks broadening as the laser intensity rises from 0.1I0 to I0 (Figure 5a), whereas for the Fe doped TiO2 nanoparticles the increment of the fwhm is observed only up to 0.5 I0, being constant at higher power densities for which the ART occurs (Figure 5b). Figure 5c and Figure 5d show the evolution of the fwhm and of the Eg peak position (∼150 cm−1) for the highly doped TiO2 nanoparticles. Again, a monotonically increase of these values with the intensity of the laser can be observed for the Al (30% cat.) doped samples, while for the Fe (20% cat.) doped ones a steady state can be appreciated for high laser power density. This effect could indicate that, contrary to the case of Al doped TiO2, laser intensities over 0.5 I0 would not induce a temperature increase for the Fe doped TiO 2 nanoparticles. In that case the energy transferred by laser irradiation would be invested on different physical processes, probably associated with bond-breaking and displacement of atoms involved in the reconstructive transformation from anatase to rutile, as confirmed for these samples by Raman

combined in a variable ratio in order to reproduce the experimental data. In the present work, this method has been previously verified in the estimation of the phase volume fraction in various reference samples formed by mixed anatase and rutile samples in a well-known composition. According to this procedure, the presence of a 40% rutile/60% anatase has been estimated in the undoped TiO2 nanoparticles after a continuous laser irradiation of 5 h. Hence, different anatase to rutile ratios can be promoted in the samples by controlling the irradiation time, which enables to achieve tuned mixed-phase samples. Similar studies have been carried out on the Al-doped TiO2 nanoparticles (Figure 3b). Contrary to the undoped TiO2, in this case, the ART was not observed in any of the analyzed samples (10, 20, and 30% cat.) even after laser irradiation with the maximum power density extended during more than 5 h. This inhibition of the ART is due to an improvement of the anatase TiO2 thermal stability induced by Al doping, together with a stabilization of surface states.34 Actually, in our previous work12 where ART has been thermally induced, the anatase phase has been stabilized up to temperatures above 900 °C by appropriate Al doping. Stabilization of anatase could be essential in the applicability of anatase TiO2 in high temperature processes and applications, where phase transformation may alter the performance of the device. In the case of Fe-doped TiO2 anatase nanoparticles, Raman spectra change rapidly with irradiation time by using the highest laser power density, as can be appreciated in Figure 4a. New peaks associated with the TiO2 rutile phase, marked with arrows in Figure 4a, are clearly identified in the irradiated samples. The Raman spectrum characteristic from TiO2 rutile nanoparticles synthesized in a previous work35 is also included in the figure as a reference. These results show that Fe doping favors ART induced by laser irradiation. In this case Raman spectra were also collected at different irradiation times in order to analyze the kinetic of the anatase to rutile transformation, with special attention focused on the evolution of the vibrational mode Eg (∼150 cm−1) characteristic of the anatase phase. As the phase transformation evolves, the relative intensity of this peak, considered as a fingerprint of anatase, diminishes while new peaks corresponding to TiO2 rutile vibration modes appear at 240, 447, 611, and 827 cm−1. In this case, the ART has been considered to be stabilized when the E


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the Fe2+ to Fe3+ ratio and the formation of rutile phase with rather low Ti3+ interstitials by laser irradiation are effects to be considered in the study of the ART process. Finally, X-ray absorption spectroscopy (XAS) measurements, highly sensitive to the chemical properties and electronic structure of the samples under study, have been performed at the TwinMic facility (Elettra synchrotron) on aggregates of Fe (20% cat.) doped TiO2 nanoparticles previously irradiated during different times with the maximum laser power density. The irradiated regions were localized by means of soft-X-ray transmission microscopy (STXM) at the TwinMic facility,36 and the spectra were collected in transmission mode by means of a photodiode detector. Figure 7 shows XAS spectra of the Ti

spectroscopy, where seven of the 24 Ti−O bonds per unit cell are broken and a cooperative displacement of both Ti and O atoms is required. PL spectra have been analyzed and correlated with Raman spectroscopy in order to achieve further information on the ART induced by laser irradiation. Figure 6 shows PL spectra

Figure 6. PL spectra from Fe (20% cat.) doped TiO2 nanoparticles before (A) and after (R) laser irradiation. PL spectrum from rutile TiO2 nanoparticles (rutile np) is also included for comparison.

corresponding to Fe-doped TiO2 nanoparticles (20% cat.) before (A) and after (R) laser irradiation with the maximum power density, as well as PL spectrum from as-synthesized rutile TiO2 nanoparticles used as a reference. The irradiation time has been controlled (90 s) in order to ensure the phase transition in the samples. After laser irradiation, the photoluminescence spectrum presents changes mainly in the band edge region, which is red-shifted, as observed in Figure 6. This effect can be explained by the different band gaps for anatase (3.2 eV) and rutile (3.0 eV) at 300 K, which confirms the anatase to rutile phase transition induced by laser irradiation. Moreover, the relative intensity of the emission at 1.66 eV, which has been attributed to Fe3+ in tetrahedral coordination, decreases in the irradiated rutile transited regions, while the emission at around 2.35 eV, associated with oxygen deficiency, is enhanced. Zhang et al.24 reported that the 1.66 eV emission, related to Fe3+ in tetrahedral coordination, can be quenched by formation of Fe2+. Therefore, the decrease of the relative intensity of this near-IR emission observed in the transitedrutile irradiated areas could indicate the formation of Fe2+ during the ART. Actually, different authors report the reduction of Fe3+ to Fe2+ by laser irradiation35 involving the excitation of valence electrons to vacant d-orbitals in Fe3+. PL signal from rutile nanoparticles, also shown in Figure 6 as a reference, is clearly dominated by an emission around 1.5 eV, characteristic of rutile, which is commonly associated with Ti3+ interstitials.35 This emission is not appreciated in the irradiation-induced rutile, which may suggest a low concentration of Ti 3+ interstitials in the irradiated regions. Previous studies35 also confirmed the decrease of the 1.5 V emission in rutile TiO2 nanoparticles doped with transition metals due to partially substitution of Ti3+ interstitials by the dopant cation. In our case, the quenching of the 1.5 eV emission should entail Ti3+ interstitials substitution not only by Fe3+ but also by Fe2+ (which ionic radius is closer to that of Ti3+), involving in this case the creation of oxygen vacancies. However, a decrease of the amount of Ti3+ by charge balance, or the presence of more effective radiative paths cannot be discarded. The increase in

Figure 7. XAS spectra of Ti L3 edge for the regions irradiated during 1, 10, 60, and 120 s.

L3 edge for the regions irradiated during 1, 10, 60, and 120 s. In the case of TiO2, the eg related peak of the L3 edge at around 460 eV is splitted into two peaks, the asymmetry of which is the fingerprint of the crystallographic phases.37 In particular, in the anatase phase the intensity of the peak at lower energy dominates, whereas a dominant peak at higher energy is representative of the rutile phase. In our case, rutile phase can be appreciated in the regions irradiated during more than 1 s, although the doublet asymmetry and the corresponding rutile phase is more evident as the irradiation time is increased, being complete for irradiation of 120 s. According to XAS results, the kinetic of this ART process seems to be slightly slower than the described by Raman spectroscopy, which can be explained by the different depth resolution associated with these techniques, being Raman spectroscopy more surface sensitivity than XAS. A phase transformation process initialized at the surface of the nanoparticles could be considered as a possible explanation, nevertheless the fact that in this case the nanopowders were not as compacted as for Raman measurements should be taken into account as it could modify the kinetics of the phase transformation. F


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PL techniques, which involve different excitation ranges, evidencing that rutile phase is not only formed at the surface but also throughout the irradiated nanoparticles. However, combined Raman and XAS results could point out to an initial formation of rutile at the surface of the nanoparticles. The surface of the anatase nanoparticles has demonstrated to play a main role in the ART, as different authors alter this phase transformation by surface pasivation. In our case, Raman measurements suggest variations in the surface order of the doped anatase nanocrystals, as Al and Fe doping generates contraction or relaxation of apical Ti−O bonds, respectively, which could inhibit or facilitate the phase transition. In the case of mixed phases, the interface rutile/anatase has been also reported to play a key role in the ART, as a directional heat dissipation through the interfaces toward the anatase phase has been observed.45 On the basis of XRD, HRTEM and molecular simulation techniques, different authors45−47 suggest that rutile nuclei originate from rutile-like structural elements created among anatase {112} twin easily formed at disordered surfaces and mainly at the interface between anatase nanoparticles, from which anatase converts into rutile. In that case, half of the Ti octahedral adjacent to the anatase {112} twin are displaced along the ⟨110⟩ direction to form straight rutile-like octahedral instead of zigzag anatase-like octahedral. Stable rutile nuclei grow and become stable by developing {101} facets. In this regard, HRTEM measurements have been performed in the irradiated samples in order to achieve results which can shed light to the understanding of the structural rearrangement involved in the ART. Figure 8a shows a TEM image of Fe (20%

Different authors have analyzed the ART and, in addition to the thermal effects, factors such as the initial particle size, the irradiation conditions, the presence of defects and impurities, or the chemical atmosphere have been also considered.11,38−40 Actually, it is widely accepted that the presence of defects in the oxygen sublattice is one of the most important factors affecting the kinetics of the ART, as a high concentration of oxygen vacancies enhances the lattice relaxation which eases the rearrangement of ions involved in the transition. On the other hand, the presence of interstitials, such as Ti3+, inhibits the phase transformation through lattice constrain. Previous XPS measurements12 revealed a high amount of Ti3+ on the near surface region of the Al doped TiO2 nanoparticles, which drastically diminishes in the Fe doped TiO2 ones. These results are in agreement with the inhibition of the laser-induced phase transition observed for Al doping. In our case, Raman and PL measurements indicate that oxygen vacancies are present not only in undoped and Fe doped samples, where promotion of ART has been observed, but also in Al-doped TiO 2 nanoparticles for which ART was hindered. In the case of Aldoped TiO2, the high presence of Ti3+, as Al3+ substitutional generates half an oxygen vacancy and a Ti3+ interstitial, produces lattice strain greater than lattice relaxation associated with oxygen vacancies, thereby hindering the ART during laser irradiation and stabilizing anatase phase. Moreover, the presence of Al3+ interstitials should be also considered in the inhibition of this phase transformation, which in this case would be only achieved by thermal treatment at high temperature (T > 900 °C).12 On the contrary, our results confirm that the presence of Fe greatly promotes the phase transformation by laser irradiation, being the ART faster as the amount of Fe increases. The creation of oxygen vacancies through the reduction of Fe3+ to Fe2+ by irradiation, or due to reaction of Fe3+ with Ti3+, has been reported by different authors.41,42 Our PL measurements indicate that Fe3+ could be partially reduced to Fe2+ by appropriate laser irradiation, which agrees with other works.43 Moreover, PL results also confirm that irradiation may not induce the formation of Ti3+ interstitials, but an increase of oxygen vacancies associated with reduction of iron. This process, which is not occurring in the Al-doped TiO2 due to the monovalence of aluminum, could accelerate the ART as compared to undoped TiO2. Besides, some authors33 also reported that Fe doping facilitates the ART due to an enhancement of the optical absorption during irradiation and photoinduced thermal heating effect owing to the formation of a defect band in the bandgap of the Fe-doped TiO 2 nanoparticles. Lee et al.33 suggest that the activation energy required for the structural anatase to rutile transformation can be decreased in Fe doped TiO2 due to the formation of defect levels caused by Fe doping, which enhances the thermal heating. According to our results, Fe doping does promote the ART, however strong evidence of thermal heating during the ART were not detected by Raman spectroscopy. Ricci et al.44 proposed an athermal anatase-to-rutile phase transition controlled by irradiation and related to oxygen adsorption and desorption phenomena at the surface of the TiO2. In that case the ART is thermally driven and the laser irradiation is only necessary to activate the formation of the first rutile nuclei, which tend to nucleate at faulted and twinned bicrystals. Xia et al.45 reported that during ART, the rutile phase is present mainly in the inner region of the agglomerates of anatase TiO2 nanoparticles. In our case, the rutile phase obtained by irradiation has been detected by means of Raman, XAS, and

Figure 8. (a) Both rutile (R) and anatase (A) nanocrystals can be observed in the TEM micrograph acquired on Ti0.8Fe0.2O2 irradiated during 10 s. (b) SAED pattern showing the spots (indicated by arrows) corresponding to both R and A phases and diffused rings at low angles, suggesting the presence of randomly oriented nanocrystals. (c) Enlarged TEM image from (a) corresponding to the rutile phase, where a twin is marked with arrows.

cat.) nanoparticles irradiated during 10 s with the maximum laser power density in order to partially induce the ART, obtaining a mixed anatase/rutile phase, as confirmed by Raman spectroscopy. SAED pattern in Figure 8b shows spots (marked with arrows) corresponding to rutile (R) and anatase (A) phases, and diffused rings at low angles, suggesting the presence of randomly oriented nanocrystals. Based on a carefully analysis of the SAED patterns, regions with anatase and rutile phases can be identified in Figure 8a, marked with A and R, respectively. In our case, twins can be occasionally observed in some of the rutile nanocrystals, as that shown in the enlarged G


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Figure 9. (a) Region of the Ti0.8Fe0.2O2 sample irradiated with the laser spot where rutile (green) and anatase (red) areas are identified on the basis of the A1g and Eg vibrational modes characteristic of rutile and anatase, respectively. Raman spectra corresponding to A1g rutile peak and Eg anatase peaks are shown in (b) and (c), respectively.

has potential applications in the field of photocatalysis, as optimized performance can be obtained through control of the phase composition. As an example, in this work a pattern with the ART motif has been designed by a controlled positioning of the laser spot at the desired locations on the surface of the sample, which is displaced with a two-axis translational stage, as depicted in the scheme shown in Figure 10.

image in Figure 8c, marked with arrows, with interplanar distances corresponding to rutile TiO2. These preliminary TEM measurements can point to a key role of twins during ART, in agreement with other authors.46−48 Anatase to Rutile Transition Patterning with Micrometer Resolution. Once the ART had been studied as a function of the dopant, its concentration, and the laser irradiation parameters, locally controlled phase transitions were performed by irradiation.49 Both for undoped and Fedoped TiO2, the laser-induced rutile phase is stable and remains after irradiation. Moreover, it has been confirmed that the ART was induced only in the irradiated area, which depends on the laser spot (∼2 μm in diameter). Figure 9a shows a region of the sample (anatase TiO2 with Fe 20% cat.) irradiated with the highest laser power density during 60 s with static laser beam well focused on the surface of the sample. After the laser irradiation, Raman spectra were collected with the lowest laser intensity (0.1I0) inside and outside the irradiated region. Results show that the ART has been locally achieved only in the irradiated area, where rutile phase is formed, while the surrounding region keeps the initial anatase phase, as observed in the image shown in Figure 9a formed by combining images acquired with the Raman peaks A1g (611 cm−1) and Eg (150 cm−1) characteristic of rutile and anatase respectively (marked in Figure 9b,c). After different measurements, it has been confirmed that the ART can be locally induced with a resolution ∼1 μm, even when the laser spot is ∼2 μm. This could be explained by the Gaussian intensity profile of the laser spot which results in a more effective ART in the central part of the irradiated area. It appears that this process enables a controlled modification of the anatase and rutile areas in the surface of the sample and the formation of periodic phase structures or more complicated phase patterns with micrometric resolution. It should be pointed out that as anatase and rutile exhibit different optical and electronic properties, these results can attract potential research interest in the fabrication of optoelectronic microdevices. Besides, the proportion of anatase transformed into rutile can be also controlled by varying the duration of the laser irradiation, thus inducing either total or partial ART. This effect

Figure 10. Scheme of the controlled positioning of the laser spot and the translational stage on which the sample is placed in order to perform micropatterning of a rutile “T“ on the surface of the anatase Ti0.8Fe0.2O2 sample.

Figure 11a shows the predesigned ART motif, as well as the image acquired with the A1g (611 cm−1) and Eg (150 cm−1) modes (Figure 11b), fingerprints of rutile and anatase, respectively. Figure 11c shows the image acquired with the A1g (611 cm−1)/Eg (150 cm−1) ratio in order to improve the identification of the regions transited to rutile. It can be observed that the laser-induced ART pattern has been successfully obtained. In some regions, as in the upper area at letter R, the phase transition seems to be incomplete, which could be explained by variations in the laser focusing due to the variable roughness of the irradiated area. Hence, a smaller surface roughness should lead to higher accuracy in the laserinduced ART micropatterning. An improved laser focusing together with a decrease in the laser spot could also lead to a higher accuracy in the patterning resolution. H


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been demonstrated, thus facing challenging performances of this material.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: davidmaestre@fis.ucm.es. Tel: (+34) 91 3945012. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by MINECO (Project Nos. MAT2011-23068, MAT 2012-31959, and Consolider Ingenio CSD 2009-00013). We are grateful to the National Centre for Electron Microscopy (CNME) at the Universidad Complutense de Madrid.

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REFERENCES

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Figure 11. (a) Predesigned “ART“ motif to be irradiated by the laser on the surface of an anatase Ti0.8Fe0.2O2 sample. (b) Raman image acquired with the A1g (611 cm−1) and Eg (150 cm−1) modes characteristic of rutile and anatase, respectively. (c) Raman image acquired with the A1g (611 cm−1)/Eg (150 cm−1) ratio.

Achieving a suitable control of the anatase to rutile phase transformation, as that described in this work, assures significant progress in the design of TiO2 microdevices.

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CONCLUSIONS Locally promoted ART and micropatterning based on titania polymorphs have been achieved by means of controlled laser irradiation on doped (Al, Fe) TiO2 nanoparticles synthesized by a modified Pechini method. This method allows us to obtain high homogeneity in size and composition for the doped anatase TiO2 nanoparticles, for which high dopant (Al, Fe) cationic concentration were obtained avoiding phase segregation. Raman spectroscopy and photoluminescence measurements point out to variations in the concentration of oxygen vacancies and Ti3+ induced by doping, as well as modification of Ti−O bonding at the surface of the nanoparticles as a function of the dopant, which could influence the ART. Control of the anatase to rutile transition was achieved by laser irradiation and the kinetic of this process has been studied as a function of the dopant and the irradiation conditions. Raman and PL results demonstrate that the ART can be either inhibited or promoted by doping with Al or Fe, respectively. The higher the concentration of Fe in the anatase TiO2 nanoparticles, the faster the phase transformation evolves. Stabilization of anatase due to the presence of Ti3+ at the surface of the nanoparticles, reduction of Fe3+ to Fe2+ involving oxygen deficiency during laser irradiation, and formation of rutile starting at the surface of the nanoparticles have been found to influence the ART process. TEM results point out the relevance of the presence of twins in the formation of rutile phase by laser irradiation. An improvement in the control of the ART has been described, as compared with other methods, such as thermal annealing for which micrometric control of local ART is not achieved. The possibility of achieving an ART micrometric patterning has I


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Laser-Induced Anatase-to-Rutile Transition in TiO2 Nanoparticles

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