The Influence of Brookite Impurities on the Raman Spectrum of TiO2

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C: Physical Processes in Nanomaterials and Nanostructures

The Influence of Brookite Impurities on the Raman Spectrum of TiO Anatase Nanocrystals 2

Maria Concepcion Ceballos Chuc, Carlos Manuel Ramos Castillo, Juan Jose Alvarado-Gil, Gerko Oskam, and Geonel Rodriguez Gattorno J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04987 • Publication Date (Web): 04 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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The Journal of Physical Chemistry

The Influence of Brookite Impurities on the Raman Spectrum of TiO2 Anatase Nanocrystals

M.C. Ceballos-Chuc*, C.M. Ramos-Castillo, J.J. Alvarado-Gil, G. Oskam and G. Rodríguez-Gattorno *Departamento de Física Aplicada, Centro de Investigación y de Estudios Avanzados del IPN, Unidad Mérida A.P. 73 Cordemex 97310 Mérida, Yucatán, México e-mail: [email protected]

Julio, 2018 ABSTRACT

Synthesis of TiO2 anatase nanocrystals by hydrothermal methods often results in formation of small quantities of brookite which is difficult to eliminate by tuning the reaction conditions and is usually present in the final nanomaterial. The effect of this impurity on the Raman spectrum in anatase nanomaterials has not been fully explored. In this work, a study on the effect of reaction temperature on the position and line-shape of the low-wavenumber Raman peak is presented. A comparison of the spectra of nanomaterials of pure anatase and anatase with brookite impurity (413 nm), synthesized by hydrothermal microwave heating (HMWH) is performed. It is shown that the low-wavenumber Raman peak (100-200 cm-1) for pure anatase nanocrystals is strongly dependent on the nanocrystal size, and the peak position is well described by the phonon confinement model. For anatase nanocrystals with 15% brookite impurity, the spectrum shows an asymmetric band, which is formed mainly by the contributions of the anatase Eg and brookite A1g modes, with the brookite B1g and B3g peaks further broadening the band. In addition, the phonon confinement model no longer describes the peak position. These results show that even a small amount of brookite can have a strong influence on the Raman spectra of anatase/brookite-impurity samples.

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1. Introduction

Raman spectroscopy is a powerful technique useful in the microstructural analysis of materials at the nanoscale1-3. In recent years, the optical Raman modes of titanium dioxide (TiO2) nanocrystals have been object of intense research, due to the wide range of applications of these materials in photocatalysis, solar cells, chemical sensors, identification of art heritage falsification, glass coating and self-cleaning materials4-12. TiO2 exists mainly in three different crystalline phases: rutile (tetragonal), anatase (tetragonal) and brookite (orthorhombic). Rutile is the thermodynamically stable form in the bulk, whereas bulk anatase and brookite are metastable13-15. However, in the synthesis of TiO2 nanomaterials the surface energy plays an important role, and anatase is the phase normally found in the sol-gel synthesis of nanocrystalline TiO2, with brookite often observed as a by-product when synthesis is carried out in acidic medium16-19. Previously, considerable attention has been focused on how the nanocrystallite size influences the position and width of the most intense Eg band located around Eg (144 cm-1)10, 12, 18-24. Anatase exhibits six Raman active modes: three Eg (144, 196, and 639 cm-1), two B1g (399 and 519 cm-1), and one A1g (513 cm-1)19. It has been established that phonon confinement, thermal effects and crystal defects are the main factors contributing to the peak position and broadening of the low-wavenumber Eg mode in anatase nanocrystals18-19, 24. In the case of phonon confinement, only the optical phonons at the zone center are involved in the first order Raman scattering process. However, in the case of nanocrystals, the effects of the interruption of the lattice periodicity at the particle surface have an increased influence, leading to phonon confinement. Thus, the phonon wave function decays to a small value close to the boundary, due to the fact that only q-vectors fulfilling the condition ∆q~π/D (where D is the particle


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size) in the Brillouin zone can contribute to Raman scattering, resulting in a blueshift and broadening of the Raman peak21-22, 24. On the other hand, the temperature effects on the Raman peak position and broadening are usually governed by two mechanisms: thermal expansion and anharmonic coupling of phonons to other phonon branches. The consequences of each one of these mechanisms on the line shape of anatase nanocrystals have been examined in detail18, 24. Additionally, the influence of lattice defects on the Raman spectra of anatase nanocrystals has been studied. Observed broadening and blue shift of 10 cm-1 in the Eg mode has been attributed to oxygen vacancies for nanocrystals with O/Ti stoichiometric ratio of 1.8918, 24-25. In general, sol-gel synthesis of TiO2 nanocrystals under acidic conditions results primarily in the anatase phase, however, a small impurity of brookite is difficult to eliminate by tuning the reaction conditions and is usually present in the final nanomaterial16-18, 25, the effect of this impurity on the Raman shift and broadening in anatase samples has not been fully explored26-27. In this work, we evaluate the low-wavenumber portion of Raman spectra (100-200 cm-1), comparing

phase-pure

anatase

and

anatase

impurified

with

brookite

nanocrystals in detail. We developed a new methodology to synthesize pure anatase and anatase impurified with brookite by fast hydrothermal microwave heating (HMWH) at different reaction temperatures (100-250 °C). With the aim of studying the relationship between composition, size and Raman spectrum of TiO2 nanocrystals, the microstructure of the as-prepared TiO2 nanopowders was characterized using diverse techniques such as: XRD, high resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS) and micro-Raman spectroscopy. To analyze the relationship between particle size distribution and average microstrain, with the size of the Raman shift and broadening of pure anatase nanocrystals, the phonon confinement model (PCM) is used.



2. Synthesis of TiO2 nanocrystals by microwave heating Several techniques have been developed to prepare TiO2 nanomaterials, such as sol-gel and hydro/solvo-thermal synthesis, physical/chemical vapor deposition, and flame-based synthesis methods26-34. In general, these conventional methods require applying a high temperature for long periods of time, and often they involve subsequent calcination to obtain nanoparticles with better crystallinity and moderately high surface area. Synthesis using microwave irradiation has the advantage of involving fast heat transfer and short synthesis times, which has converted this type of synthesis in an attractive alternative with reduced energy consumption18, 28-33, 35-39. For the preparation of anatase with brookite impurity the sol-gel method with microwave heating was used, following the steps depicted in Fig.1(a). A volume of 21 mL of Titanium tetraisopropoxide (Ti(OiPr)4, 97%, Aldrich) was added dropwise to 4 mL of glacial acetic acid under constant stirring. The resulting clear sol was added dropwise while stirring to 145 mL of water. Subsequently, the obtained sol was stirred continuously for 1 h. Adding 1.4 mL HNO3, fix temperature at 80°C for 40 min., then the solution was peptized for 75 min at 80°C. Then 25 mL of the obtained solution was transferred to a quartz vessel (30 mL) and irradiated in the microwave reaction system (Monowave 300; Anton Paar), at different temperatures (100-200 °C) to 0.5 MPa for 5 min with stirring at 300 rpm. The vessel was left to cool down to room temperature. After centrifugation at 6000 rpm for 30 min, the resulting precipitate was washed twice with deionized water and ethanol, and then dried under vacuum at 50 °C for 12 h, providing a white powder.


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In Fig.1(b) the synthesis of pure anatase nanocrystals is illustrated. Amorphous TiO2 was prepared by addition of a solution A (50 mL of water, Labconco WaterPro PS; 18 MΩ cm diluted in 210 mL of 2-propanol (C3H8O, J.T. Baker, 99.9%)) to a solution B (10 mL of titanium (IV) isopropoxide (C12H28O4Ti, Aldrich, TTIP 97%) in 210 mL of 2-propanol) under stirring and at room temperature. After 2 hours, the amorphous material was centrifuged and dried at 80 °C. This methodology was adapted from Reyes-Coronado et al.34. Phasepure nanocrystalline anatase was successfully synthesized by hydrothermal microwave heating (HMWH) using a MARS 6 (CEM Analytical, 600 W). First, 3 g of amorphous titania powder was diluted in 40 mL aqueous 1.5 M acetic acid (C2H4O2, AcOH, Sigma-Aldrich, 99.75 %) and transferred to a 60 mL Teflon reaction vessel. The dispersion was irradiated for 10 min at a fixed temperature without stirring. Additionally, synthesis experiments were carried out at different temperatures (100-250 °C) to explore the effect of temperature on the growth of pure anatase nanoparticles. 3. Analysis of the structure and microstructure Microstructural analysis of the TiO2 samples was carried out by observation with a high-resolution transmission electron microscope (HR-TEM), using a JEM-ARM200F-JEOL instrument in HAADF-STEM and BF-STEM modes. HRTEM, operating at 200kV, was used to measure dimensional lengths of crystallites. The composition of the samples was measured in a field emission scanning electron microscope (JEOL JSM-7600F) equipped with an energy dispersive X-ray detector (EDX X-Max OXFORD instruments). Compositional analyses were complemented with X-ray photoelectron spectroscopy (XPS) in a Thermo ScientificTM K-Alpha+TM X-ray Photoelectron Spectrometer, with a monochromatic Al-Kα X-ray source (1486 eV), at 10-8 Torr. The emitted photoelectrons were sampled from an area of 600 µm2. It has been reported that the binding peaks of the Ti 2p XPS spectrum can be altered by Ar ion



bombardment40. Thus, the XPS analyses were carried out using as-prepared samples on Si substrates without Ar bombardment to prevent reduction of Ti. All data were analyzed using the Thermo Avantage software v.5.967. The adventitious C 1s peak at 284.8 eV was used as an internal standard to compensate for any charging effects. Curve fits were performed using a Shirley background and a peak shape with 30 % Lorentzian/Gaussian character. XRD patterns were collected using a BRUKER D8 Advance with CuKα (λ=1.5418 Å) radiation equipped with a point detector (LYNXEYE (1D mode)), and measurements were carried out at 40 kV and 30 mA, with a step size of (2θ) 0.010°. Rietveld refinement was performed using the program of Bruker's Topas-4 for XRD pattern analysis, using a convolution-based profile fitting34 for refining the crystal structure and microstructure of the anatase samples. The crystallite size and strain effects on the peak broadening were analyzed using the Voigt approach. Topas-4 provides two refinement modes, GUI and Launch; the latter allows the user to input his own programming codes for specific refinement purposes. Initial data treatment was run in GUI mode to refine lattice parameters, preferred orientation and to correct for instrument broadening. The volume weighted column height (LVo-IB), which is predefined in Topas-4, was used as the dimensional length. Quantitative analysis was carried out using the software Bruker Diffrac.Suite EVA V4.1.15. The PDF entries JCPDS: 21-1272 (Anatase) and 29-1360 (Brookite) were used for phase identification. Nitrogen adsorption-desorption isotherms of the materials were determined at -196°C using a Belsorp (BEL JAPAN Inc.) analyzer. Each sample was degassed at 100°C for 12 h under 10-5 Pa. The surface areas were calculated by the Brunauer-Emmett-Teller (BET) method in the relative pressure range of P/P0 =0.05-0.25. The average pore sizes were obtained through Barrett-JoynerHalenda (BJH) method. The samples were characterized in detail by Raman spectroscopy; spectra were measured with a confocal Raman WiTec Alpha 300 (at 488 nm, 100x and


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grating 1800 grooves/mm, spectral resolution of 1 cm-1) to study the fine structure of the specimens. Measurements were carried out on powdered samples at low laser power (6 mW) in order to avoid transformation of the TiO2 phase and broadening of Raman bands by laser heating40. The spectra obtained were compared with database RRUFF (Raman spectroscopy of X-ray diffraction and mineral data of chemistry) []41. This technique allows to analyze morphological and structural properties of solids locally due to its high sensitivity to the characteristics of the phonons and the crystalline nature of material. To analyze the characteristics observed in the Raman spectra, we used the phonon confinement model, also known as the spatial correlation model. To determine the phase contents of anatase and brookite, all samples were examined by Raman in the range 0-400 cm-1. Due to overlap of the anatase (E1g) peak with the brookite (A1g and B1g) peaks, a numerical deconvolution technique was used to separate these peaks. The Raman spectrum was fitted by Lorentzian functions. The phase content of samples was calculated from the integrated intensities E1g (~148-153 cm-1) for anatase and A1g (~248 cm-1) for brookite.

4. Phonon confinement model The phonon confinement model has been widely used to explain the Raman line shape of various nanocrystals18-24. According to this model, the first order Raman scattering intensity due to a single nanocrystal of diameter (size) D can be approximated by:

, =

|0, | −

,

Γ + 0 2


(1)


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where 0, = !− /2# is the Fourier transform of the Gaussian

phonon wave function. The coefficient # takes different values depending on the

confinement regimes: # = 2 in the soft confinement model18-24, and # = 8π for the hard confinement model42 in which case the phonons are strongly confined,

thus only a small number of atoms in the central part of the sphere can vibrate. Previous work has shown that the hard confinement regime is adequate to reproduce the line shape of the Eg mode in anatase. Γ' is the full width at half

maximum (FWHM) of the zone-center optical phonon for the bulk material at the given temperature T. The term ω , represents, in fact, an average dispersion curve, which is usually expressed in terms of cosine functions.

ω , = ' + ) 1 − cos .,

(2)

where ω' is the wavenumber of the Raman mode of bulk material at a given

temperature (for anatase ω' = 144 123 at room temperature)43. The values of the coefficients Bi depend on the direction throughout the Brillouin zone. To

model the Raman shift of pure anatase nanocrystals, we have used the parametrization suggested by Šćepanović et al44 with B1 = 102 cm-1 and B2 = 28 cm-1 in Γ − Χ, B3 = 52 cm-1 and B4 = 15 cm-1 in Γ − Ν, and B5 = 18 cm-1 in Γ − Ζ

direction. The sum in Eq. (1) must be carried out over 2 dispersion curves, depending on mode degeneration.

The change of the lattice parameters (lattice volume) as a consequence of the decrease of the size of the nanoparticle (D) due to the effect of microstrain has been reported in various materials45(36,37). In order to include the influence of strain on the Raman spectra, the dispersion function ω , in Eq. (1) must be

replaced by ω , 1 − 7 Δ9/9' , where γ is the Grüninsen parameter of the Raman mode (4.23 for Eg mode in anatase) and Δ9 is the change in the volume

of unit cell, which has been determined for pure anatase nanocrystals by the Rietveld refinement analysis, presented in Fig. S1 (Supporting Information). Eq. (1) represents the line shape for a nanocrystal of size D. However, in

real materials, a wide distribution of particle sizes can be present, which significantly modifies the Raman line shape. The contribution of size distribution to the Raman line broadening can be obtained by:


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= ∑>=

The Influence of Brookite Impurities on the Raman Spectrum of TiO2

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