ES2525737B2 - Nanocrystalline titanium dioxide with anatase and rutile phase mixing in proportion and / or spatial distribution controlled by laser irradiation - Google Patents

Abstract

Nanocrystalline titanium dioxide with mixture of anatase and rutile phases in proportion and / or spatial distribution controlled by laser irradiation. # Titanium dioxide is a material with a wide range of applications for which a certain polymorphism is required that makes it necessary to find methods control of the anatase-rutile phase transition that allows to regulate the mixing of both phases. It is proposed to obtain nanocrystalline titanium oxide with a mixture of anatase and rutile phases quantitatively and / or spatially controlled with micrometric resolution by laser stamping. This phase control allows to design multifunctional and versatile devices based on nanocrystalline titanium oxide and the different properties of its anatase and rutile phases.

Description

Nanocrystalline titanium dioxide with anatase and rutile phase mixing in proportion and / or spatial distribution controlled by laser irradiation

Technique Sector The invention falls within the sector of semiconductor devices and functional coatings. More specifically, it refers to the manufacture of (itanium dioxide with proportion and / or spatial distribution at micrometric scale of anatase / rutile phases on demand for application in functional devices of various kinds such as the production of hydrogen by decomposition Photocatalytic water or solar cells.

State of the art Titanium dioxide (Ti02) is currently one of the most investigated materials due to its multiple technological and industrial applications, covering sectors such as photocatalysis, gas sensors, solar cells, energy storage, pigments or medicine, among others (Asahi, R. et al., VisiblewLight Photocatalysis in NitrogenwDoped Titanium Oxides, Science, 293, 269w 271 (200 1); Roy, SC et al. Toward Solar Fue / s: Pholocatalytic Conversion olCarbon Dioxide lo Hydrocarbons, ACS Nano 4,1259-1278 (2010); Gratzel, M., Photoelectrochemical celJ. ~. Nature, 414, 338344 (2001); Fabbri, F. et al., ThermaJ

w

Processing and Characterizations 01 DyewSensilized Solar Ceils Based on Nanostructured TiO; F, J. Phys. Chem. C. 11 7. 3729w 3738 (2013 ». In fact, today, Ti02 is one of the reference materials in applications based on photocatalysis and is being used in the resolution of environmental problems such as water purification or the reduction of pollutant gases.Therefore, there is a growing interest in expanding its functionality, promoting the development of new technologies and progressing in research. and optimization of its properties.Most of these applications require a comprehensive control of the dimensions, defect structure, doped or polymorphism of Ti02, so, for example, the surface-volume ratio increases considerably when manufactured in the form of nanoparticles, which improves its applicability in systems such as gas sensors or catalysts, the response of which can be optimized by means of adequate doping. At the same time, the photocatalytic activity of Ti02 improves in samples doped with Fe allowing, in addition, its use in magnetic devices, while doped with Al favors applications in thermal shock devices.

TiO¡ can crystallize in three structures; rutile (tetragonal), anatase (tetragonal) and brookite (orthorhombic). The metastable anatase and brookinta phases are irreversibly transformed into rutile (the most stable phase) when the temperature rises. Acutally, the rutile and anatase phases are the most studied and used in various industrial applications depending on their different physicochemical properties, since these phases have different values of forbidden energy range, dielectric constant, refractive index, surface energy or mechanical properties (Hanaor, DA H el al., Review 01 (he anatase lo rutile phase transforma / ion, J. Mater. Sci. 46, 855-874 (2011). Consequently, the rutile phase is commonly used in applications optoelectronics, while the anatase phase is frequently used as a catalyst.Also, it has recently been found that in some cases the controlled mixture of anatasalutile phases allows optimizing the applicability of this material in systems based on photocatalysis (Ohno, T. et al ., Synergism between rutile and ana / ase TiO; partie / es in pholoca / alytic oxida / ion oinaphthalene, Appl. Catal. A. 244, 383391 (2003)) in comparison with the Pure phase response. In addition, some of the 15 applications of the anatase phase are limited to being able to transfer it to rutile when raising the temperature, so it is essential to know the processes that control the transition. In general, the anatase-rutile transition is a non-reversible process that occurs between 600 and 700 ° C, this temperature being somewhat lower in the case that anatase is present in the form of nanoparticles. Recently, the study of this transition has been resumed, giving 20 relevance that, in addition to the need to provide a certain thermal energy, there are other factors such as the presence of oxygen vacancies, titanium interstitials, presence of dopants, or area superficial, which are fundamental in the transition mechanism. At present, the chemical and physical processes involved in the transition, or the mechanisms that inhibit or promote it, have not yet been fully understood, although it has been proven that the doping can modify the anatase-rutile transition (a by way of example, doped with Fe favors it while doped with Al inhibits it). Therefore, it is especially important to find anatase-rutile transition control methods that allow the phase mixing to be regulated so that they can be exploited in the design of functional devices of various kinds such as hydrogen production by photocatalytic decomposition of the

30 high efficiency water or solar cells based on dyes, thus expanding the functionalization of this material.

SIj have proposed techniques that make the transition from anatas to rutile in a general way and, for

Therefore, without local control. Some refer to a control by annealing temperature

(JP200513 7988, eN I 01698503), solid phase reaction (JP2005089213) or phase selection

by anodizing (JPS63297592). In most cases the transition achieved is not homogeneous and regions may appear throughout the mixture with phases without any control.

In the present invention a method is proposed to control the anatase-rutile transition using laser irradiation producing prints with quantitative and spatial phase control simultaneously. Phase monitoring is done using the Raman technique.

Until now, some authors have investigated the anatase-rutile transition through irrigation

10 laser analyzing the influence of particle size, the presence of different atmospheres, surface reactivity or doping (Ricci, PC al., Analase lo rutile phase lransition in TiO] nanoparticles irradialed by visible Iighl, Joumal of Physical Chemistl) 'C 117, 7850 · 7857 (2013); Zhang, J. et al., UV Raman speclroscopic study on TiO]. 1. Phase transformation allhe Surface and in lhe bu / k, Journal of Physical Chemisll) 'B 11 O, 927 ~ 935 (2006)). However, it has not been possible to establish a control in the local generation of rutile phase or mixed phases.

On the other hand, EP0767222 describes a control method that incorporates the

monitoring of the quantitative distribution of the phases using Raman technique. Without

However, the method of the present invention incorporates distribution monitoring

20 spatial phases thus expanding its functionalization.

Detailed description of the invention

Titanium dioxide nanoparticles are synthesized using a variant of the method of

Pechini that has great control over the size and doping of nanoparticles

25 (P20 \ 400722). The precursors used in the synthesis process are Ti (OBU) 4, citric acid, ethylene glycol and M (N03) J'9H20 in the case of nanoparticles doped with an M metal (for example, Al or Fe). First, a solution of citric acid with deionized water (in a 1: 3 ratio) is prepared. Second, the stoichiometric amounts of Ti (OBu) 4 and the precursor of the selected dopant are added to the aqueous solution, while

30 that the mixture is kept under continuous stirring at a temperature below 100 ° C (without boiling). Subsequently, the amount of ethylene glycol is added to the citric acid in a 2: 3 ratio, a gel being formed as the water evaporates at a temperature higher than 100 ° C, which generates a resin that is then calcined to obtain a fine powder of nanoparticles of carbon dioxide. titanium. In order to eliminate organic waste, the powder obtained is subjected to a

The technical treatment at 350 ° C for 30 hours_ Finally, the samples doped with iron are treated at 450 ° C for 1 S hours and the doped with aluminum are treated at 550 ° C for 15 hours to complete the crystallization of the nanoparticles.

In both cases, nanoparticles with a carbon content below the detection limit of the analyzer used and a controlled doping of up to 30% cationic are obtained. The dimensions of the nanoparticles range between 3 and 10 nm depending on the dopant and its concentration. Specifically, the non-doped TiOl nanoparticles have a size of 3.5 nm while those doped with Al show sizes between approximately 5 and 10 nm, depending on the cationic content of Al which varies between 10 and 30%; Samples doped with Fe have sizes around 4.5 nm with little variation depending on the cationic content of Fe.

The nanoparticles obtained are single phase anatase. A laser stamping technique is used to generate the anatasarutil transition in a spatially controlled manner. This technique can be performed, for example, by a focal microscope equipped with a 325 nm He-Cd laser to irradiate the sample locally. In this case, the penetration range of the 325 nm laser radiation in titanium oxide is approximately 200 nm in size that the phase transfusion is performed in a surface region, which is sufficient for devices whose interaction with The medium is made through its surface. If it is desired to manufacture a device in which the phase transfonation is more volumetric, it is possible to use a 633 nm laser, whose penetration exceeds the micron, or even lasers with infrared wavelengths. Other sources of excitation can also be used.

The laser irradiation is carried out on a sample in the form of a tablet foonated in a press from powders containing anatase nanoparticles doped with Al or Fe in different concentrations depending on the application temperature, the desired final mixture of phases and its spatial distribution

The amount of phase that transits is controlled in silu by Raman spectroscopy technique. The Raman control spectra are taken with the minimum laser power to avoid changes in the state of the sample during the measurement and are adjusted by calibrating the resulting spectrum to a weighted mixture of the proper spectrum of the anatase phase and the rutile phase that indicates the proportion of phases generated in the irradiated area. In this way, the anatasarutilic transition can be controlled locally with micrometric resolution, which makes the elaboration

of rutile phase patterns and / or mixing on demand of phases (whether periodic or more complex) on controlled areas of the sample.

In the case of pure anatase, the anatase-rutile transition is achieved using times of

5 high irradiation and maximum laser power (after 5 hours of irradiation the foonation of 60% rutile is achieved). In the case of anatase doped with Alla anatase-rutile transition, it is inhibited so that the transition is not achieved even if high rdition times and maximum power are used. On the contrary, in the case of Anatase doped with Fe the anatase-rutile transition is promoted and takes place rapidly. Table 1 summarizes

10 some parameters necessary to promote the transition of a certain percentage of anatase phase to rutile depending on the duration of the laser irradiation at maximum power and the cationic percentage of dopant of the starting material.

Table I

CATIONIC DOPANT CONTENT 0% 10% AI 20% At 30% AI 10% Fe 20% Fe

IRRADLATION TIME (% OF ROAD TRANSITION) 5h (60%) 5h (0%) 60 min (64%) 100 mi (82%) 20 s (88%)

15 As the Fe content increases, the anatase-rutile transition is achieved in a shorter time, requiring only a few seconds in the case of nanoparticles doped with 2 (Y% Fe. During irradiation the laser focuses on the sample at a point of approximately 2 microns in diameter; due to the Gaussian intensity distribution the

The effective transition zone is limited to a region of 1 micron in diameter, which allows to control the regions in which the anatase phase transits to rutile with micrometric resolution. In addition, the irradiation region can be monitored by establishing the laser position, the time the laser radiates each point, as well as the pattern to follow during irradiation. This allows the elaboration of rutile phase patterns or mixing of phases on demand, whether of type

25 newspaper or more complex.

Due to the control of the parameters, especially the irradiation time (dependent, in turn, on the Fe content), the transition can be complete or partial, so that by means of laser stamping you can choose between micrometric zones of anatase phase pure (the 30 not lit) areas of pure rutile phase (lit enough to transit

completely) and areas with a mixture of phases (illuminated for a limited time to partially transit), depending on the needs and functionality required.

EMBODIMENT OF THE INVENTION The present invention is further illustrated by the following examples which are not intended to limit its scope.

Example 1: This example shows the local control of the transition induced by laser irradiation depending on the doping and the irradiation time.

The phase transition is induced locally by using a Kimmon / K Hc-Cd laser

Series with a wavelength of 325 Dm and an output power of 25mW in a microscope

confocal laser Horiba Jobin-Yvon LbRam lfR800. The phase identification is done in situ by

15 means of the Raman spectroscopy technique reducing the output power of the laser by factors 0.5, 0.25 and 0.1 by means of neutral filters. The software used in the acquisition and analysis of the data (LabSpec 5) allows, in addition to taking specific spectra, to take spectra in meshes with a lincal, square, circular or polygonal shape by adjusting the number of points of the mesh and the exposure time of the To be.

In a first stage, the characteristic Raman spectra of samples of undoped nanoparticles are obtained for each phase (rutile and anatase) grown by the same method, which are used as reference. To do this, a map of Raman spectra is made on each sample in a 15x 15 mesh square spaced 0.5 11m, setting the acquisition time to

25 10 seconds and 2 repetitions at each point and using a neutral filter of factor 0.1, thus minimizing the laser power in order not to promote the anatase · rutile transition during measurement. Once the map is obtained, all the spectra are averaged to obtain a characteristic Raman spectrum of the anatase phase (SA) and another of the rutile phase (SR) that have been corrected to take into account the different effective Raman dispersion section of both

30 phases

In the second stage, the different nanoparticle samples (without doping, doped with Fe or doped with Al in different concentrations) are irradiated with the maximum laser power (without neutral filter) during controlled times ranging from 30 seconds to 6 hours , depending on the sample and the time required by the phase transition. Through periodic acquisition

of Raman spectra, using [at minimum laser power density, the evolution of [a anatase-rutile mucus transition can be monitored so that the time needed to partially or totally generate the anata transition can be established! '; a-rulilo in each mue! '; lra.

In the third stage the final spectra of the irradiated samples are reproduced, according to the conditions previously established, by means of a linear combination of the spectra obtained in the first stage.

Using this method you can calculate the percentage of generated rutile phase Iras irradiation (Sexp) during a time t determined by the equation: S ~ xp "" aSA + pSII

where or. and p are the parameters of the linear combination used to reproduce the S "X (» spectrum from the reference spectra SA and SI /. These parameters indicate the volume fraction of each of the phases as a function of the irradiation time that determines the proportion of rutile / anatase phase generated in each sample.

Thus, when irradiating a sample of Ti02 without doping for 5 hours with the maximum laser power, the analysis of the Raman spectra indicates coefficients a = OA and p = O.6; that is, 40% of the anatase phase and 60% of the rutile phase are generated. In the case of samples doped with Fe (20% cationic), after 20 seconds of irradiation, it is possible to generate 88% of n.til (Table 1).

Example 2: Stamping of micrometric patterns by means of laser irradiation.

Stamping by laser irradiation is performed on samples of Ti02 nanoparticles doped with Fe (20% cationic).

To know the spatial resolution, the phase transition is generated locally by irradiating the sample at a single point without using neutral filters and keeping the laser well focused on the surface of the sample. Posterionnenle acquires a map of spectra using a reduced laser power with a Neutral filter of factor 0.1 in a square mesh of approximately 5x5 ~ m2 of area in steps of 0.2 ~ m (in both x and y directions) keeping the irradiated point in the center. Control raman spectra are acquired at each point. Finally, the resulting map of the intensities of the Raman peaks characteristic of each phase is represented (El: in J50 cm, l for the anatase phase and AI ~ in 611 cm-1 for the rutile phase) indicating the spatial distribution of the anatase and rutile phases in the analyzed region. Laser radiation allows locally generated regions of rutile transited from anatase with a spatial resolution of diameter d = 1 Jlm (Figure la).

To print a specific motif on the surface of the sample, a polygonal mesh with the desired shape and dimensions is generated. In this example, a mesh of dimensions 15 jlm wide and lS¡lIn high has been generated, establishing a separation of points within the mesh of 0.5 Jlm (approximately half of the spatial resolution determined above) in the x and y directions.

The irradiation pattern that the laser will follow is designed (in this case a letter "T", as shown in Figure lb) and 120 seconds of continuous exposure are set at each point, ensuring a transition from anatase phase to complete rutile in the sample . Subsequently, the Raman intensities map is acquired, using the minimum laser power, in a square mesh of 20x20 ~ m2 and 0.25) step. 1m keeping the patterned motif centered in order to identify the regions where rutile phase has been generated by controlled irradiation of the laser. In this way, the rutile phase is generated with great precision following the preset stamping pattern (Figure Ic).

Example 3: Obtaining on demand samples of titanium oxide with quantitative controlled phases and / or spatially

A square mesh of nnxn irradiation points is made, in areas of LxL} ~ m2, as shown in the diagram in Figure 2. The area irradiated by the laser in each of the nmxm steps corresponds to 1 [(d / 2) 2, where d is the diameter of the micrometric circular region irradiated at each point by laser focused on the sample. In this way, areas with a spatially controlled phase distribution are obtained by irradiating at each point for a sufficient time to fully promote the phase transition, or the percentage of rutile phase at each point determined by the irradiation time (O <R ¡(%) <100), under the condition that the passage of the laser is greater than the spatial resolution, that is, L / ni :::: d. The percentage of rutile phase generated in the study area (L, xL) ~ m2) can be calculated according to:

Description of the figures 5 Figure I shows: (a) the distribution of rutile phase (gray) generated locally by point laser irradiation in an area composed of anatase phase (black), (b) the letter-shaped irradiation pattern ' 'T' to be followed by the laser and (e) the distribution of anatase (black) and rutile (gray) phase over a previously irradiated region according to the indicated pattern.

10 Figure 2 depicts the scheme of a mesh of nxn) points irradiated over an area of

L, xL} ~ m2.

Claims (10)

1. Procedure for obtaining nanocrystalline titanium dioxide with a mixture of anatase and rutile phases controlled in proportion and spatial distribution by laser irradiation that understands: Obtain the characteristic Raman spectra of undoped nanoparticles for each phase (anatase and rutile) Design an irradiation pattern that will follow the laser on the irradiated sample. Irradiate with laser, following the designed pattern, the single-phase anatase titanium dioxide nanoparticles in the form of a tablet. Control the evolution of the anatase-rutile transition by acquiring Raman spectra of the irradiated material. Calculate the percentage of the rutile phase generated by comparison between the spectra obtained in the monitoring and the characteristic spectra of each phase. Maintain the irradiation process until the desired percentage of rutile phase is achieved.

2.

Procedure for obtaining nanocrystalline titanium dioxide with a mixture of anatase and rutile phases controlled in proportion and distribution, according to claim 1, wherein the irradiated titanium dioxide nalloparticles are pure or doped with Al and / or Fe.

3.

Procedure for obtaining nanocrystalline titanium dioxide with a mixture of anatase and rutile phases controlled in proportion and distribution, according to claims 1 and 2, where irradiation is performed locally by means of a focal microscope equipped with a He-Cd laser with a wavelength of 325 nm and an output power of

25mW

Four.

Procedure for obtaining nanocrystalline titanium dioxide with a mixture of anatase and rutile phases controlled in proportion and distribution, according to previous claims, where the characteristic spectra of each phase (anatase and rutile) and the control spectra are taken with the minimum laser power and they are adjusted by calibrating the resulting spectrum to a weighted mixture of the spectrum characteristic of the anatase phase (SA) and the rutile phase (SR).

5.

Procedure for obtaining nanocrystalline titanium dioxide with a mixture of anaphase and rutile phases controlled in proportion and distribution, according to claim 3, where the laser output power is reduced by factors 0.5, 0.25 and 0.1 using neutral filters.

6.

Procedure for obtaining nanocrystalline titanium dioxide with a mixture of anaphase and rutile phases controlled in proportion and distribution, according to previous claims, where the irradiation of the doped or undoped nanoparticles is carried out on a micrometric circular region of diameter d with the maximum power of laser during controlled times ranging between 30 seconds and 6 hours depending on the composition of the sample and the desired rutile phase percentage.

7.

Procedure for obtaining nanocrystalline titanium dioxide with a mixture of anatase and rutile phases controlled in proportion and distribution, according to previous claims, wherein the percentage of rutile phase generated at a point after irradiation (Se. ~ P) for a time (t) It is calculated by the equation:

S ~ Jp "" aSA + pSn where he and P are the linear combination parameters used to reproduce the St ~ p spectrum, from the reference spectra SA and Sil, which represent the fraction in volume of each of the phases depending on the irradiation time.

8.

Procedure for obtaining nanocrystalline titanium dioxide with anatase and rutile phase mixture controlled in proportion and distribution, according to claim 1, wherein the irradiation pattern is a mesh of nxm irradiation points in areas of

Lxx21lm2 being L / ni? D, where d is the diameter of the micrometric circular region irradiated at each point by the laser focused on the sample

9.

Procedure for obtaining nanocrystalline titanium dioxide with phase mixing

anatase and rutile controlled in proportion and distribution, according to claim 8, wherein the area irradiated by the laser in each of the n / Xn] steps corresponds to 1t (d / 2) 2. 10. Procedure for obtaining nanocrystalline titanium dioxide with anatase and rutile phase mixing controlled in proportion and distribution, according to claims 8 and 9, where the percentage of rutile phase generated in the study area, L, XL2 (Ilm2) is calculated according to the equation: