Influence of Nd3+ Doping on the Structure, Thermal Evolution and Photoluminescence Properties of Nanoparticulate TiO2 Xerogels

Journal Pre-proof 3+ Influence of Nd doping on the structure, Thermal Evolution and Photoluminescence Properties of Nanoparticulate TiO2 xerogels María T. Colomer, Carlos Roa, Angel L. Ortiz, Luz M. Ballesteros, Pablo Molina PII:

S0925-8388(19)34218-5

DOI:

https://doi.org/10.1016/j.jallcom.2019.152972

Reference:

JALCOM 152972

To appear in:

Journal of Alloys and Compounds

Received Date: 3 September 2019 Revised Date:

8 November 2019

Accepted Date: 9 November 2019

Please cite this article as: Marí.T. Colomer, C. Roa, A.L. Ortiz, L.M. Ballesteros, P. Molina, 3+ Influence of Nd doping on the structure, Thermal Evolution and Photoluminescence Properties of Nanoparticulate TiO2 xerogels, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.152972. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Influence of Nd3+ doping on the Structure, Thermal Evolution and Photoluminescence Properties of Nanoparticulate TiO2 Xerogels María T. Colomer a,*, Carlos Roa b, Angel L. Ortiz c, Luz M. Ballesteros b, Pablo Molinad a

Instituto de Cerámica y Vidrio, Consejo Superior de Investigaciones Científicas, CSIC, C/ Kelsen 5, 28049 Madrid, Spain.

d

b

Centro de Investigaciones en Catálisis (@CICATUIS), Escuela de Ingeniería Química, Universidad Industrial de Santander (UIS), Carrera 27 calle 9 ciudad universitaria, Bucaramanga, Colombia

c

Departamento de Ingeniería Mecánica, Energética y de los Materiales, Universidad de Extremadura, Avda. de Elvas S/N, 06006 Badajoz, Spain.

Departamento Física de Materiales, Instituto Nicolás Cabrera and Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, Madrid 28049, Spain. *Corresponding author: [email protected]

Abstract Undoped and Nd3+-doped xerogels (Nd3+-dopant content in the range 0.5-10 mol%) were obtained from colloidal sols in turn prepared by a “green sol-gel route” in aqueous media. The as-synthesized xerogels were characterized by X-ray diffractometry (XRD), high resolution electron microscopy (HREM) together with selected area electron diffractometry (SAED), and Raman spectroscopy (RS), finding that they crystallize as tiny nanocrystals of anatase with traces of brookite and rutile. The thermal stability of the as-synthesized xerogels was next studied in-situ by X-ray thermo-diffractometry (XRTD) as well as ex-situ by XRD, HREM/SAED, and RS on xerogels calcined at different temperatures (400-900 °C), concluding that Nd3+ doping results in the formation of substitutional solid solutions with enlarged unit-cell volume that (i) stabilizes the anatase phase retarding the onset of its transformation to rutile to ever higher temperatures and (ii) provides the anatase nanocrystals with superior resistance to crystal growth. It was also found that Nd4Ti9O24 eventually

precipitates at high temperatures when the Nd3+-dopant content is equal to or higher than 3 mol%, which leaves the still-existing anatase nanocrystals without stabilizer cations therefore causing their immediate transformation to rutile. Finally, the photoluminescence (PL) behaviour of the Nd3+-doped TiO2 xerogels with both anatase and rutile crystal structures was investigated and critically compared, observing that the PL emission (i) increases with increasing crystallinity of the host nanocrystals, (ii) decreases above a certain optimal Nd3+dopant content due to concentration quenching, and (iii) is much greater for rutile host matrices. In this scenario, the 3 mol% Nd3+-doped rutile xerogel maximized the PL emission, emitting at both ~915 and 925 nm due to the existence of two distinct local crystal fields for the Nd3+ cations.

Keywords:

Titania;

Luminescence.

Neodymium

doping;

Sol-gel;

Characterization

Techniques;

1. Introduction It is well known that the incorporation of lanthanide (Ln3+) cations like Eu3+, Sm3+, Ce3+, Er3+, Nd3+, or Yb3+ into TiO2 matrices results in effective luminescence in different wavelength ranges. Thus for example, Eu3+-, Sm3+- and Ce3+-doped TiO2 emit in the visible range [1-6], whereas Er3+-, Yb3+-, and Nd3+-TiO2 do it in the near infrared (NIR) [2,6,7]. Of these, there stands out Nd3+-doped TiO2 because it is a bifunctional material that, when excited, exhibits simultaneous photocatalytic activity (PCA) and NIR photoluminescent emission [8]. Nd3+-TiO2 could then be potentially used in environmental remediation, smart materials, and display technologies, to name a few applications. Of the two main TiO2 polymorphs, anatase is the most chosen as matrix to host the Nd3+ cations. Indeed, to the best of our knowledge, rutile has been used as matrix only once [9] despite it is known that the intensity of the photoluminescent emission depends on how the Ln3+ cations bond to their local environment. It then seems evident that new studies are needed aimed at comparing critically the photoluminescence of Nd3+-TiO2 with both anatase and rutile matrices prepared from a unique sol. It has not been studied before to the best of our knowledge. Ln3+ cations, besides their intrinsic luminescent interest, also restrict the growth of anatase TiO2 crystals, and stabilize this polymorph at higher temperatures [1,10-12]. Typically, the thermal stability is investigated using ex-situ characterization techniques, meaning that Ln3+-doped TiO2 is heat-treated at the desired temperature and then cooled down to room-temperature for its characterization. This type of studies is very useful, but does not provide a full description of the thermal evolution and stability. Indeed, there are currently both structural and compositional aspects of the Nd3+-TiO2 materials that still remains unclear, such as the type and range of solid solutions formed and the possible second phases present after calcination at high temperatures.

With these premises, the present study was aimed with two objectives in mind. One is to investigate the effect of Nd3+ doping on the thermal stability of TiO2 using a combination of both ex-situ and in-situ characterization techniques (i.e., X-ray diffractometry (XRD), X-ray thermo-diffractometry (XRTD), transmission electron microscopy (TEM), high-resolution electron microscopy (HREM), and Raman spectroscopy (RS)). The other is to investigate the effect of Nd3+ doping on the photoluminescence (PL) behaviour of TiO2 with both anatase and rutile crystal structures obtained from the same sol, as well as their critical comparison. In particular, the study was conducted on undoped and Nd3+-doped TiO2 xerogels (0.5, 1, 2, 3, 5, and 10 mol%) prepared from aqueous sols in turn synthesized by a colloidal sol-gel route since this fabrication route is more advantageous (for example, it yields narrower particle size distributions, it is environmental friendly, it is an easier synthesis procedure, etc.)

2. Experimental Procedure 2.1. Preparation of Xerogels and their Characterization The sols were synthesized following the synthesis procedure described elsewhere [13]. Briefly, the alkoxide hydrolysis was carried out by adding titanium (IV) isopropoxide (Ti(iPrO)4; 97%, Sigma-Aldrich, Germany) to a stirring mixture of deionized water (18.2 MΩ/cm, ultrapure Milli-Q, France) and nitric acid (65%, Panreac, Spain) in a molar ratio water/alkoxide of 50. In preparing the Nd3+/TiO2 sols, before the addition of the alkoxide neodymium (III) nitrate hydrate (Nd(NO3)3·xH2O; 99.99% Alfa Aesar, USA) was dissolved in the mixture of water and nitric acid to a molar ratio Nd+3/TiO2 of 0.5, 1, 2, 3, 5, and 10 mol%. The synthesis temperature and agitation were both kept constant during the entire process at 35 °C and 700 rpm, respectively. HNO3 was used as a catalyst and dispersing agent in a molar ratio H+/Ti4+ of 0.2. The iso-propanol byproduct was removed from the sol by a vacuum pump at room temperature.

The xerogels were obtained by drying the sols in air, first at room temperature and then in an oven (FN 50, Nüve, Turkey) at 80 °C. The resulting xerogels were next washed with deionized water and heat-treated in a furnace (HP 250, Mestra, Spain) at temperatures between 400 and 900 °C (reached using heating and cooling rates of 5 °C/min) for 10 min. After calcination at 400ºC, C analysis was performed by a LECO analyser (LECO CHNS-932 elemental analyser, USA). The xerogels were selectively characterized as needed using XRD, XRTD, TEM, HREM, and RS. In particular, XRD patterns were collected (D8 Advance, Bruker, Germany) at room-temperature over the angular range 20-70 º2θ, and the crystalline phases present were identified using the PDF2 database. The lattice parameters of selected xerogels were determined by the Pawley refinement of their XRD patterns (parameters were included to refine the instrumental shifts, the background, the peak profiles, and the unit cell dimensions). Also, the space groups I41/amd (141) of anatase, Pbca (61) of brookite, and P42/mnm (136) of rutile were used for the refinements. XRTD patterns were collected (D8 Advance, Bruker, Germany) over the angular range 20-45 º2θ, under non-isothermal heating conditions each 3 °C in the temperature range 30-900 °C (using an effective heating ramp of ~1 °C/min). TEM and HREM observations were made (JEM-2100F, JEOL, Japan) at 200 keV on some xerogels to corroborate the correctness of the XRD and XRTD analyses, and were complemented with the collection of selected area electron diffraction (SAED) patterns (also indexed using the PDF2 database) and of X-ray energy-dispersive spectroscopy (EDXS) spectra in point mode (i.e., on individual crystals).

2.2. Measurements of Raman and Photoluminescence Raman and PL spectra were acquired by means of a customized scanning confocal microscope (BX41, Olympus, Japan). An Ar+ laser (177-Series, Spectra Physics, USA) at 488 nm and a Ti:sapphire laser (3900S, Spectra Physics, USA) tuned at 805 nm were used as

excitation sources for RS and PL, respectively. These 488 and 805 nm laser beams were focused onto the sample by a x10 (NA = 0.3) microscope objective (MPLFLN, Olympus, Japan), resulting into 2 and 3.5 µm diameter spots onto the sample, respectively. Raman and PL signals were collected in backscattering geometry with the same objective, and directed by an optical fiber to a Peltier-cooled CCD detector (Synapse, Horiba, Japan) attached to a monochromator (iHR 550, Horiba, Japan). 3. Results and discussion 3.1. Characterization by C analysis, XRD, HREM, and XRTD First of all, the xerogels were analysed by a LECO analyser in order to check if residual C exists in the samples after calcination. These analysis indicated that the xerogels are free of C. Second, they were characterized by XRD at room-temperature. By way of example, Fig. 1 shows the XRD patterns of both the undoped TiO2 and Nd3+-doped TiO2 (1 mol% Nd3+) xerogels in their as-synthesized condition and after their calcination at 700 °C for 10 min. It can be seen that the XRD patterns of the as-synthesized xerogels exhibit weak, broad diffraction peaks of brookite, and also intense, broad diffraction peaks of anatase, some of which are indeed asymmetrical due to their partial overlapping with broad, weak diffraction peaks of rutile. Consequently, it can then be concluded that the as-synthesized xerogels are very nanocrystalline, and are formed essentially by anatase (major phase) plus some brookite (minor phase) and rutile (very minor phase). These conclusions were found to be extendable to the rest of as-synthesized xerogels independently of their Nd3+-dopant content. This phase composition is well understood considering that anatase is the thermodynamically stable polymorph when the TiO2 particles have sizes at the nanoscale [14], with brookite and rutile being common by-products of the sol-gel synthesis [15-17]. The TEM characterization of the as-synthesized xerogels confirms the deductions made by XRD. Thus for example, the HREM images and the SAED patterns of the undoped TiO2 and Nd3+doped TiO2 (3 mol% Nd3+) xerogels in their as-synthesized condition shown in Fig. 2

corroborate (i) the polycrystalline nature of these xerogels, (ii) the tiny size of the nanocrystals, and (iii) the predominance of anatase over brookite and rutile. The XRD patterns of the xerogels calcined at 700 °C differ substantially from those of the as-synthesized xerogels. Thus, it can be seen in Fig. 1 that the XRD pattern of the undoped TiO2 xerogel does no longer exhibit diffraction peaks of anatase and brookite, but only well-resolved, sharper diffraction peaks of rutile. The XRD pattern of the Nd3+-doped TiO2 (1 mol% Nd3+) xerogel exhibits however diffraction peaks of anatase, brookite, and rutile. Clearly, the diffraction peaks of rutile are comparatively much more intense than before in relation to the diffraction peaks of anatase and brookite. Moreover, the diffraction peaks of anatase and brookite are sharper than in the as-synthesized condition, although they are still broader than the diffraction peaks of rutile. Therefore, it can be concluded that the calcination at 700 °C for 10 min promoted the transformations of anatase and brookite to rutile, which went to completion for the undoped TiO2 xerogel but not for the Nd3+-doped TiO2 xerogels. Consequently, it is clear that doping with Nd3+ helped to stabilize the anatase and brookite phases to higher temperatures. In addition, it can also be concluded that the calcination caused growth of the nanocrystals, and that the rutile nanocrystals formed in the Nd3+-doped TiO2 xerogels are larger than the residual nanocrystals of anatase and brookite. According to an earlier study on TiO2 doped with Eu3+ and Nd3+ (1 and 5 mol%) [8], the transformation of anatase to rutile occurs for calcination temperatures above 500 °C. The TEM characterization of the calcined xerogels confirms this earlier observation. Thus for example, the HREM image and the SAED pattern of the Nd3+-doped TiO2 (3 mol% Nd3+) xerogel calcined at 400 °C for 10 min shown in Fig. 3 (i) rule out the presence of more rutile and (ii) corroborate the growth of the anatase nanocrystals. Lastly, it can also be seen in Fig. 1 that the calcination at 700 °C did not cause precipitation of neodymium oxides or neodymium titanates. In any case, the subsequent characterization by XRTD will shed more light on the thermal stability of the present undoped TiO2 and Nd3+-doped TiO2 xerogels.

The detailed comparison of the XRD patterns in Fig. 1 also reveals that doping with Nd3+ resulted in a slight shifting of the diffraction peak of the different phases present towards lower angles. In turns, this observation indicates that the Nd3+-doped TiO2 xerogels have crystal structures with larger unit cells relative to that of the undoped TiO2 xerogel. This was further confirmed by performing Pawley refinements of these XRD patterns. Thus for example, the Pawley refinements indicated that the undoped TiO2 and the 1 mol% Nd3+doped TiO2 have unit-cell volumes (i) of 138.7(8) and 140.7(9) Å3, respectively, for the anatase phase in their as-synthesized condition, and (ii) of 62.45(2) and 62.61(5) Å3, respectively, for the rutile phase when calcined at 700 °C for 10 min. The plotted outputs from these Pawley refinements are presented in Fig. 4. Consequently, taken together, the larger unit cells and the absence of Nd-rich oxides indicate necessarily that Nd3+ cations entered into the TiO2 lattice thus forming the corresponding solid solutions. In addition, the fact that the unit cell volumes of the three TiO2 polymorphs were larger in the Nd3+-doped TiO2 xerogels indicates that there was no preferential incorporation of Nd3+ in any given crystal structure (anatase, brookite, or rutile). Fig. 5 shows the two-dimensional intensity contours generated from the indexed XRTD patterns collected in-situ as a function of the calcination temperature for the undoped TiO2 and Nd3+-doped TiO2 (1, 2, 5, and 10 mol% Nd3+) xerogels. It can be seen that, regardless of their Nd3+-dopant content, these xerogels crystallize essentially as anatase phase, with some brookite and rutile. In addition, the as-synthesized xerogels are all nanocrystalline because the diffraction peaks are very broad, but with the nanocrystallite size decreasing as the Nd3+-dopant content increases. It can also be seen that with increasing calcination temperature there is little nanocrystallite growth up to a certain temperature that also dictates the onset of the transformations of anatase and brookite to rutile. This occurred at ~425 °C in the case of the undoped TiO2 xerogel, and however at ~490, 550, 740, and 770 °C in the case of the TiO2 xerogels doped with 1, 2, 5, and 10 mol% Nd3+, respectively. Therefore, it is clear

that doping with Nd3+ increasingly delayed the onset of the transformations to rutile, indicating that the Nd3+-doped TiO2 xerogels have ever higher thermal stability (the higher the Nd3+-dopant content the higher the thermal stability). The temperature at which the transformations to rutile are completed also increased by more than 275 °C for the Nd3+doped TiO2 xerogels, and in particular from ~550 °C in the undoped TiO2 xerogels to not less than 815 °C for the Nd3+-doped TiO2 xerogels. Nonetheless, it can also be seen in Fig. 5 that with increasing Nd3+-dopant content from 1 to 10 mol% the anatase/brookite phase is retained up to ever lower temperatures. Again, the retention of both phases indicates that there is no preferential incorporation of Nd3+ in anatase over brookite. Interestingly, in the case of the TiO2 xerogels doped with 2, 5, and 10 mol% Nd3+ the complete disappearance of the anatase/brookite phase came accompanied by the precipitation of Nd4Ti9O24. Consequently, the TiO2 xerogel doped with only 1 mol% Nd3+ retains the anatase/brookite phase up to a higher temperature than the rest of Nd3+-doped xerogels because it preserves its Nd3+ cations in solid solution, and this stabilizes the anatase/brookite phase. The precipitation of Nd4Ti9O24 can leave the anatase phase partially or totally without its Nd3+ stabilizer cations, moment at which the anatase/brookite phase readily transforms to the rutile phase. Logically, the formation of Nd4Ti9O24 is ever more favoured with increasing the Nd3+-dopant content, which explains well why the anatase/brookite phase completely disappeared at ever lower temperatures. The XRD patterns of the xerogels calcined at 400, 700, and 900 °C for 10 min (i.e., under isothermal heating) shown in Fig. 6 confirm the phase inventories extracted from the XRTD patterns (i.e., under non-isothermal heating). Thus, it can be seen that the all xerogels calcined at 400 °C are formed by anatase (major phase), brookite (minor phase), and rutile (very minor phase). It can also be seen that the undoped xerogel calcined at 700 °C contains only rutile, whereas the Nd3+-doped xerogels contain however anatase (major phase), brookite, and rutile, with the amount of this latter decreasing with increasing Nd3+-dopant

content. Finally, it can be observed that the undoped xerogel calcined at 900 °C is formed only by rutile, that the xerogel doped with 1 mol% Nd3+ contains rutile (major phase) and anatase (very minor phase), and that the xerogels doped with 3 and 5 mol% Nd3+ comprise rutile and Nd4Ti9O24. The TEM-EDXS observations, such as the one shown in Fig. 7 for the 3 mol% Nd3+-doped TiO2 xerogel calcined at 900 °C for 10 min, confirm the precipitation of Nd4Ti9O24 at high temperatures for Nd3+-dopant content above 1 mol%. Lastly, it can also be seen in Fig. 6 that, for the same temperature, the XRD peaks becomes broader with increasing Nd3+-dopant content, which reflects that the nanocrystals are increasingly smaller. Therefore, taken together, the XRD, XRTD, and TEM/HREM-SAED analyses indicate that by the present sol-gel synthesis it is possible to prepare nanocrystalline xerogels based on substitutional solid solutions with aliovalent Nd3+ solutes at the Ti4+ site of the anatase crystal lattice. This type of solid solutions forms despite the difference of ionic radius between the Ti4+ and Nd3+ cations (i.e., ~0.605 (VI) vs 0.983 Å (VI)), and implies (i) the enlargement of the lattice parameters of the anatase crystal structure to accommodate the larger Nd3+ cations in the cationic sublattice and (ii) the generation of oxygen vacancies in the vicinity of the anionic sublattice to preserve the charge neutrality (one O2- vacancy for each two Nd3+ cations). The chemical composition of these substitutional solid solutions is thus given by Ti1-xNdxO2-0.5x. In principle, the increase of the lattice parameters observed experimentally would also be compatible with the formation of interstitial solid solutions. However, this is ruled out for two reasons. The first is that the Nd3+ cations are too large to be accommodated at the interstitial sites of the anatase crystal lattice, and the second is that, to preserve the charge neutrality. In relation with the latter, (i) a larger number of O2- anions would also have to be accommodated at interstitial sites (three O2- anions for each two Nd3+ cations) in the vicinity of the Nd3+ cations thus resulting in a chemical composition given by TiNdxO2+1.5x, and the O2- anions have an ionic radius that is even larger (i.e., ~1.40 (VI) Å), or (ii) a large number of Ti4+ vacancies would have to be generated at the cation sublattice (three

Ti4+ vacancies per each four Nd3+ cations) collaboratively with the occupation of interstitial sites, which is not at all plausible. The formation of these substitutional solid solutions is responsible for the ever smaller nanocrystal sizes of the Nd3+-doped TiO2 xerogels and the retarded transformations of anatase and brookite to rutile. The former is due to the internal stresses in the crystal lattice induced by the larger Nd3+ cations together with the solute drag effect on the crystallite-boundary migration induced by the sluggish diffusion of the largest and heaviest Nd3+ cations [18]. The latter is because the slower kinetics of nanocrystallite size growth makes higher temperatures to be required to reach the critical nanocrystallite size of ~14 nm for which rutile is the thermodynamically more stable than the anatase and brookite [19]. Lastly, the preservation of these solid solutions is essential to retain anatase crystals up to ever higher temperature, with the precipitation of neodymium titanates causing the immediate transformation to rutile phase (because the cooperative diffusion of species is favoured by the high temperatures (and then high diffusion coefficients) and the small size of the nanocrystals (and then small diffusion distances)). It should be mentioned that those results from the present study are consistent with the survey of earlier studies on Nd3+-doped TiO2 prepared by different methods. Thus for example, Yildirim et al. [20] synthesized Nd3+-doped TiO2 nanoparticles by the flame spray pyrolysis method, reporting the presence of NdO2. Nassoko et al. [21] prepared Nd3+-doped TiO2 xerogels by the sol-gel method and calcined them at 400 °C, observing the formation of solid solutions (Nd3+ solutes in the TiO2 host) with increased lattice parameters. Tobaldi et al. [8] obtained Nd3+-doped TiO2 gels by the aqueous sol-gel route and calcined them at 450 °C, noting the formation of substitutional solid solutions with enlarged lattice parameters and the reduction in the crystal sizes with Nd3+ doping. Ghigna et al. [22] also prepared Nd3+-doped TiO2 nanopowders by the sol-gel method, concluding that Nd3+ enters into the TiO2 lattice as substitutional solute with the corresponding local disorder. D’Souza et al. [23] synthesized

prepared Nd3+-doped TiO2 powders by solid-state reaction at 600 °C, observing that the Nd3+ doping results in substitutional solid solutions with enlarged unit cells relative to TiO2. Yurtsever and Çiftçioglu [24] prepared Nd3+-doped TiO2 gels by the polymeric sol-gel route, reporting that Nd3+ doping retards the transformation of anatase to rutile and the precipitation of mixed oxides (i.e., Nd2Ti4O11 or Nd4Ti9O24) for calcination temperatures above 800 °C. Bokare et al. [25] synthesized Nd3+-doped TiO2 nanoparticles also using the polymeric sol-gel method and calcined at 500 °C, observing the enhanced stability of anatase with the Nd3+ doping. Lastly, Li et al. [26] prepared Nd3+-doped TiO2 nanoparticles by metalorganic chemical vapour deposition, identifying that the Nd3+ cations are situated at substitutional locations and cause lattice distortions (increase of the lattice parameters and local strain field at dopant sites).

3.2. Raman Spectroscopy Complementarily to the characterization by XRD, XRTD and TEM/HREM-SAED, micro RS was also used to achieve deeper insights on the thermal evolution and stability the present TiO2-based xerogels. In this context, Fig. 8A shows the normalized Raman spectra of undoped TiO2 in its as-synthesized condition and after calcination at temperatures from 400 to 900 °C. Earlier studies on undoped TiO2 and Ln3+-doped TiO2 xerogels have established the main Raman peaks corresponding to the different TiO2 polymorphs [27,28], whose positions have been marked by dashed lines in Fig. 8A for anatase, brookite and rutile. It can be seen that the main structure of Raman peaks observed in the as-synthesized, undoped xerogel at 150 (E1g), 397 (B1g), 519 (A1g) and 639 (E3g) cm-1 is due to the vibrational modes of the anatase phase. There are also other weak Raman peaks at 247 (A1g), 322 (B1g), and 366 (B2g) cm-1, attributable to the presence of minor brookite phase. Brookite should also exhibits a strong band at 153(A1g) cm-1, but this latter cannot be individually resolved due to its severe overlapping with the strongest Raman mode of the anatase phase observed at 150 cm-1. Indeed, this is the reason why the Raman peak observed at 150 (E1g) cm-1 is so broad and is

shifted relative to bulk anatase TiO2. These features are characteristic of highly nanocrystalline samples and are related to phonon confinement effects, while the sign of the frequency shifting compared to the bulk compound is determined by the dispersion ratio of the different vibrational modes [29,30]. It can also be seen in Fig. 8A that as the calcination temperature increases, the full width at half maximum (FWHM) of the ~150 cm-1 vibrational mode decreases and the frequency shifts down to lower values (redshift), recovering the reference value of ~144 cm-1 when the xerogel was calcined at 500 °C or above. This significant red-shifting, together with the FWHM reduction, are indicative of nanocrystal size growth, an observation that is in accordance with the conclusions reached by XRD. Additionally, the sequence of Raman spectra recorded for the undoped TiO2 xerogel clearly evidences the occurrence of a structural transition from anatase phase to rutile phase as the calcination temperature increases. In particular, it can be seen that the main Raman modes of the rutile phase (peaks at 235, 449 (Eg), 610 (A1g) cm-1) slightly arise when the xerogel was calcined at 600 °C, and that for calcinations at and above 700 °C the rutile vibrational modes already dominate the Raman spectra while those of the anatase phase vanish, evidencing a complete structural transformation from anatase to rutile in good qualitative agreement with the XRTD observations. Note however that the temperatures of both onset and end of the structural transformation from anatase to rutile determined by XRTD and RS are not expected to be the same due to the different calcination schedules applied (much slower heating in XRTD, and therefore transformations shifted to lower temperatures). Figs. 8B-C show now the Raman spectra of selected Nd3+-TiO2 xerogels (0.5, 3, and 10 mol% Nd3+) for the same calcination temperatures. The Raman spectra of the TiO2 xerogels with 1, 2, and 5 mol% Nd3+ are very similar, and therefore omitted for the sake of clarity and brevity. It can be seen that the structural transition from anatase to rutile is clearly retarded to higher temperatures with increasing Nd3+-dopant content in the TiO2 xerogels. This is something evident even for the xerogel with a Nd3+-doping content as low as only 0.5

mol% (Fig. 8B), for which the main rutile Raman peaks (at 235, 449 (Eg), and 610 (A1g) cm-1) are only observed for calcination temperatures above 700 °C. For higher Nd3+-dopant content the anatase-rutile transition is only detected for calcination temperatures above 800 °C. In addition, the Raman spectrum of the TiO2 xerogel doped with 10 mol% Nd3+ and calcined at 900 °C (Fig. 8D) is much more complex, including additional vibrational modes not attributable to the rutile phase. This is for example the case of the prominent Raman peak at 674 cm-1 (marked with a red arrow) and other minor Raman peaks located all along the spectrum. These are tentatively attributed to the vibrational modes of the Nd4Ti9O24 compound that precipitates at high temperatures for Nd3+-dopant contents above 1 mol%. A deeper analysis of the Raman features of Nd4Ti9O24 is however beyond the scope of this study, and is thus deferred for future work. Additional evidences of the effect of the Nd3+-dopant content on the TiO2 structures are presented in Fig. 9, which shows how the position (Fig. 9A) and the FWHM (Fig. 9B) of the anatase E1g vibrational mode vary with the composition for calcination temperatures of the xerogels up to 800 °C (the calcination temperature of 900 °C was excluded from the analysis because the Raman spectra are dominated by the rutile phase). It can be seen that in the case of the as-synthesized xerogels the E1g Raman mode is not affected by the Nd3+-dopant content, with the maximum location (~152 cm-1) and the FWHM (~33 cm-1) remaining essentially unchanged. This is attributed to that the high disorder inherent to the extremely nanocrystalline nature of the as-synthesized xerogels markedly dominates over the effect of Nd3+-dopant content. On the contrary, in the case of the xerogels with larger crystal sizes obtained by calcination at or above 400 °C, it is possible to observe that the E1g Raman peak underwent an ever significant blue-shift and broadening as the Nd3+-dopant content increases. In accordance with earlier studies [5,8,21], this corroborates the existence an increasing disorder in the TiO2 structure owing to the Nd3+ doping. Thus for example, substitution of Ti4+ by Nd3+ will distort the crystal lattice of anatase because both Ti4+ and Nd3+ will be

accommodated in D2d symmetry with shorter basal bonds with oxygen and longer apical bonds with oxygen, but (i) the ionic radii of Ti4+ and Nd3+ are ~0.605 (VI) and 0.983 (VI) Å, respectively, and (ii) the average Ti–O and Nd–O bond lengths are ~1.95 and 2.47 Å, respectively. Logically, the scenario is qualitatively similar for the crystal lattice of rutile.

3.3. Photoluminescence Another objective of the present study is to elucidate the effect of Nd3+ doping on the PL behaviour of TiO2 with both anatase and rutile matrices. When well stabilized, Ln3+ ions present 4f-4f electronic transitions arising from the shift and splitting of the

2S+1

LJ levels due

to the presence of an external static electric field (Stark effect). In the case of Ln3+ ions embedded in host crystals, the energy levels of the ions split owing to the crystal field of the host, which breaks the degeneracy of the quantum number of the total angular momentum J. (a)

(b)

The precise number and width of the Stark levels depend on the symmetry and the intensity of the crystal field. Thus, the energies and splitting of the Nd3+ electronic levels should be different for the different structural phases of the present TiO2 xerogels, since the site symmetry and distances to the first neighbours are different. Fig. 10A shows the PL emission spectra measured in the range 850-1010 nm for the Nd3+-doped xerogels calcined at 400 °C, in which anatase is the major phase. These spectra were obtained under pumping at 805 nm (4I9/2 → 4F5/2 + 2H9/2 transition), and show broad peaks centred at 925 nm that correspond to the 4F3/2 → 4I9/2 emission band of Nd3+. It can be seen that it is not possible to resolve the individual emission lines from the Stark splitting of the involved states. This is because there is an intense inhomogeneous broadening caused by the existence of a certain distribution of interatomic distances as well as by the presence of defects in the coordination sphere of Nd3+ ions. The emission spectra observed correspond well with earlier data reported for Nd3+ in anatase matrix [20,31]. Fig. 10B shows the PL emission spectra measured in the same spectral range for the Nd3+-doped xerogels calcined at 700 °C, in which anatase is still the major phase. The shape of these spectra is even more

complex than before, now with barely resolved emission lines according to the multiplicity of the 4F3/2 → 4I9/2 multiplet. Moreover, the shape of the spectra reveals the occurrence of nanocrystal growth, as was already observed by XRD, XRTD, and RS. Lastly, Fig. 10C shows the PL emission spectra acquired for the Nd3+-doped xerogels calcined at 900 °C, in which rutile is the major phase. It can be seen that the spectral shape depends on the Nd3+dopant content, with the spectra presenting a main peak centred at ~920 nm for Nd3+-dopant contents lower than or equal to 2 mol%, and however two main peaks at ~915 and 925 nm, respectively, for Nd3+-dopant contents equal to or higher than 3 mol%. This last suggests the presence of at least two non-equivalent Nd3+ centers, or, in other words, Nd3+ ions subjected to different local environments with two distinct average local coordination spheres or different charge compensation mechanism in order to accommodate Nd3+ in the rutile phase of TiO2. Alternatively, the change in the spectral shape observed for the higher Nd3+-dopant contents could be due to the emission from the precipitated crystals of Nd4Ti9O24, which have three non-equivalent centres for Nd3+ ions with highly distorted NdO6 square antiprisms and two different NdO6 octahedra as crystallographic basic units [32]. However, this last explanation is very unlikely because, according to both XRD and RS, Nd4Ti9O24 is indeed a very minor phase. As for the effect of the Nd3+-dopant content on the PL efficiency of the xerogels calcined at 400, 700, and 900 °C, Fig. 11 compares the integrated PL obtained from the 4F3/2 → 4I9/2 multiplet. It can be observed that the maximum emission yield for the Nd3+-doped TiO2 xerogels in anatase phase (i.e., xerogels calcined at both 400 and 700 °C) is attained when the Nd3+-dopant content is only 1 mol.%, whereas for Nd3+-doped TiO2 xerogels in rutile phase (i.e., xerogels calcined at 900 °C) is reached when the Nd3+-dopant content is 3 mol.%. It can also be seen than in the three cases the PL intensity drastically drops for the higher Nd3+-dopant contents, which is attributed to concentration quenching by a progressive migration of the Nd3+ ions favouring the formation of clusters among Nd3+ ions. This is not

exclusive of Nd3+ cations but also occurs for other Ln3+cations [31,33,34]. Lastly, it can be seen that the xerogels calcined at 900 °C have higher PL efficiency than those calcined at 700 °C, and these in turn are more efficient than those calcined at 400 °C. Therefore, it emanates that for PL applications the rutile nanocrystals with greater crystallinity are a much more desirable host matrix for the Nd3+ cations than the tiny nanocrystals of anatase. This conclusion is consistent with earlier observations on oriented films of anatase and rutile doped with 1, 2, or 5 at.% Nd3+ and growth by pulsed-laser-deposition at 700 °C [9], where it has also been observed that (i) the host matrix of highly-crystalline rutile maximizes the PL emission. Furthermore, it has been proposed in the same study that the Nd3+ cations could have different local environments with different local crystal fields [9], which is consistent with the present observation of two main peaks in the PL emission spectra. Indeed, this does not seem to be exclusive for rutile host matrices because Tobaldi et al. [8] observed peaks at 1090 and 1070 nm in the NIR emission spectra of 1 mol% Nd3+ doped-anatase (corresponding to the Nd3+: 4F3/2 → 4I11/2 emission band), which are attributed to the existence of at least two distinct Nd3+ average local coordination sites. Low-temperature high-resolution site selective spectroscopy and PL lifetimes could be used to determine the very weak structural changes around the emitting Nd3+ cations, but these studies are beyond the scope of this work and are therefore deferred for future study.

4. Conclusions Nanoparticulate xerogels of both undoped and Nd3+-doped TiO2 were obtained from colloidal sols in turn prepared by a “green sol-gel route” in aqueous media, and the effect of Nd3+ doping was investigated on the structure, thermal stability, and PL properties. Based on the present results and discussion, the following conclusions can be drawn: 1.

The as-synthesized xerogels all crystallize essentially as tiny nanocrystals of anatase, with minor abundance of nanocrystals of both brookite and rutile.

2.

Nd3+ cations introduce as solutes into the TiO2 crystal lattice thus forming substitutional solid solutions with enlarged unit-cell volumes.

3.

Nd3+ doping provides the Nd3+-doped TiO2 xerogels with increasing thermal stability, resulting in anatase nanocrystals that are much less prone to grow and to transform to rutile.

4.

Eventually, precipitation of Nd4Ti9O24 occurs for moderate and high Nd3+-dopant contents (3 mol% or higher). This leaves the still-existing anatase nanocrystals without stabilizer cations, causing their immediate transformation to rutile.

5.

PL emission of the Nd3+-doped TiO2 xerogels increases with increasing crystallinity of the host nanocrystals, decreases above a certain optimal Nd3+-dopant content, and is much greater for rutile host matrices than for anatase host matrices.

6.

The 3 mol% Nd3+-doped rutile xerogel maximizes the efficacy of the PL emission, emitting at both ~915 and 925 nm due to the existence of two distinct local crystal fields for the Nd3+ cations.

Acknowledgements. This work was supported by the Science, Innovation and Universities Ministry (Government of Spain) and FEDER Funds under the Grants nº MAT2016-76638-R, MAT2015-67586-C3-2-R, and MAT2016-78700-R.

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1. As-synthesized xerogels crystallize essentially as tiny nanocrystals of anatase 2. Nd3+ doping provides increasing thermal stability for anatase 3. Nd3+ introduces as solutes into TiO2 lattice forming substitutional solid solutions 4. 3 mol% Nd3+-doped rutile xerogel maximizes the efficacy of the PL emission 5. Emission at 915 and 925 nm can be due to two distinct local crystal fields for Nd3+

Declaration of interest statement

I declare on myself and behalf of all the authors that there is no any conflict of interest in relation with the paper entitled: Influence of Nd3+ doping on the Structure, Thermal Evolution and Photoluminescence Properties of Nanoparticulate TiO2 Xerogels written by María T. Colomer*, Carlos Roa, Angel L. Ortiz, Luz M. Ballesteros, and Pablo Molina.

*Corresponding author: [email protected]

Dr. M.T. Colomer