Monitoring the Simultaneous Implantation of Ti and Tb Cations to a Sacrificial Template and the Sol-Gel Synthesis of Tb-Doped TiO2 (Anatase) Hollow Spheres and Their Transition to Rutile Phase

Citation: Colomer, M.T.; Vattier, F. Monitoring the Simultaneous Implantation of Ti and Tb Cations to a Sacrificial Template and the Sol-Gel Synthesis of Tb-Doped TiO2 (Anatase) Hollow Spheres and Their Transition to Rutile Phase. Int. J. Mol. Sci. 2022,23, 13162. https://doi.org/ 10.3390/ijms232113162 Academic Editor: Iolanda Francolini Received: 30 September 2022 Accepted: 24 October 2022 Published: 29 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). International Journal of Molecular Sciences Article Monitoring the Simultaneous Implantation of Ti and Tb Cations to a Sacrificial Template and the Sol-Gel Synthesis of Tb-Doped TiO2(Anatase) Hollow Spheres and Their Transition to Rutile Phase María Teresa Colomer 1,* and Florencia Vattier 2 1Instituto de Cerámica y Vidrio (ICV), Consejo Superior de Investigaciones Científicas (CSIC), c/Kelsen 5, E-28049 Madrid, Spain 2Instituto de Ciencia de Materiales de Sevilla (ICMS), Consejo Superior de Investigaciones Científicas (CSIC)-Universidad de Sevilla, c/Américo Vespucio 49, E-41092 Sevilla, Spain *Correspondence: [email protected]; Tel.: +34-91-735-5840; Fax: +34-91-735-5843 Abstract: Tb-doped TiO 2 (anatase) micro-hollow spheres (HSs) with nano-shells, in the range 0.00–3.00 at.% Tb , were successfully synthesized by a simultaneous chemical implantation route of both Ti and Tb cations from chlorides to a poly-styrene (PST)-co-poly-divinyl benzene (PDVB) sacrificial template, followed by controlled hydrolysis and polycondensation reactions. After water addition to the mixture of the precursors with the template, a decrease in the intensity and a shift to lower wavenumbers of the C=O absorption band in the IR spectra can indicate not only the anchoring of Ti and Tb ions to the carbonyl group of the template but also the hydrolysis of the implanted precursors. This latter process can involve a proton attack on the Ti–Cl, Tb–Cl and C=O bonds, the occupation of a vacant site by a water molecule, and then the dissociation of the dangling Ti–Cl, Tb–Cl ligands and C=O bonds. It gives rise to Ti 1−x Tb x [(OH) 4−u Cl v ]@PST–PDVB and Ti 1−x Tb x [(OH) 4−y ]@PST–PDVB complexes (x = 0.00, 0.0012, 0.0170 and 0.030). Finally, polycondensation of these species leads to Ti 1−x Tb x O 2−w0 @PST–PDVB compounds. After subsequent thermal removal at 550 ◦ C of the template, the IR bands of the core (template) totally vanished and new bands were observed in the 400–900 cm −1 region which can be attributed to the metalloxane bondings (M–O, M’–O, M–O–M, M–O–M’ and/or M’–O–M’, being M and M’ = Ti and Tb, respectively, i.e., mainly vibration modes of anatase). Then, micron-sized HSs of TiO 2 and Tb-doped-TiO 2 (anatase) were obtained with nano-shells according to field emission gun scanning electron microscopy (FEG-SEM) and transmission electron microscopy (TEM) observations. Furthermore, X-ray photoelectron spectroscopy (XPS) measurements confirmed the presence of Tb 4+ (38.5 and 41.2% for 1.70 and 3.00 at.% Tb, respectively) in addition to Tb 3+ in the resulting HSs, with increasing Tb 4+ content with both Tb doping and higher calcination temperatures. Then, these HSs can be considered as rare earth (RE) co-doped systems, at least for 1.70 and 3.00 at.% Tb contents being the transition to rutile phase favored by Tb doping for those compositions. Finally, diffusion of Tb from the inner parts to the surface of the HSs with the calcination treatments was also observed by XPS. Keywords: hollow sphere; Tb; TiO2; implantation; sol-gel; X-ray methods 1. Introduction Titanium dioxide has excellent physico-chemical properties and unique applications in photocatalysis, gas sensors, solar cells, etc. Extensive research studies have been focused on controlling the microstructure, size and morphology of TiO 2 to achieve novel and enhanced properties [ 1 – 4 ]. In particular, TiO 2 HSs attract great interest due to their low density, higher surface area, better separation of electrons and holes, surface permeability, etc. For those reasons, the synthesis of RE-doped TiO 2 materials as HSs with improved Int. J. Mol. Sci. 2022,23, 13162. https://doi.org/10.3390/ijms232113162 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2022,23, 13162 2 of 18 photoluminescent and/or photocatalytic properties is in focus [ 5 – 8 ]. From a simple point of view, hollow micro-nanospheres can be easily fabricated through digging void space inside solid spheres. According to the preparation process and formation mechanism three major methods are the most widely used to prepare hollow structures: (i) hard template, (ii) soft template, and (iii) self-template method. Among all three commonly employed protocols the first strategy has been extensively adopted in the fabrication of hollow micro and nanospheres as it is straightforward to use. This route is the approach considered by many researchers to be the most robust and reliable pathway to date for the preparation of hollow particles with a well-defined shape, size, and particle-size distribution in relatively large quantities [ 9 – 11 ]. This method is based on the implantation of a metal ion from a metalorganic compound or an anhydrous salt dissolved in a nonpolar solvent to penetrate the template surface and locate preferentially underneath the surface [ 10 ]. Hollow particles can be obtained by subsequent removal of the template by calcination in air. This type of synthesis has been used in previous works to obtain HSs of different compositions (e.g., Al 2 O 3 [ 10 ], TiO 2 [ 11 ] and In 2 O 3 [ 12 ]) and also for the synthesis of RE-doped anatase and rutile HSs such as Er-doped TiO 2 [ 5 ] and Tb-doped TiO 2 HSs [ 7 ]. In this study, a simultaneous chemical implantation of the two cations has been established with the aim of developing and optimizing the synthesis of the Tb-doped anatase HSs. In previous works where two cations are needed Ti 4+ , the major cation is first implanted to the sacrificial template and the RE cation is added to the suspension obtained after implantation of the major cation [ 5 , 7 ]. In this research, we have used for the first time to the best of our knowledge a simultaneous chemical implantation of both cations not only the major one. In this way, a better Tb distribution in a faster and more easily conducted procedure is expected to be achieved. After implantation controlled hydrolysis was performed. In addition, it is also the first time to the best of our knowledge that the reactions that take place during this type of HSs synthesis have been monitored. Furthermore, it will allow us the control of the reaction times and to extend this type of synthesis to any type of doped TiO 2 HSs. Then, the present work aims at monitoring the steps that occur during the implantation of the cations to the polymer and during the sol-gel synthesis of Tb-doped TiO 2 (anatase) HSs through the preparation of Ti 1−x Tb x O 2−w0 @PST–PDVB core-shells. We have studied the formation process, the morphology, the texture, the structure, the chemical composition, and the thermal evolution of the as-obtained micro-nano TiO 2 and Tb-doped TiO 2 HSs. Furthermore, the anatase–rutile transition (ART) in these types of materials is also studied and discussed since controversial results have been found regarding the enhancement or the inhibition of this transformation in the previous literature of Tb-doped TiO2materials [13–16]. 2. Results and Discussion 2.1. Monitoring Ti and Tb Simultaneous Implantation and Sol-Gel Synthesis of Ti[(OH) 4−y Cl y ]@PST–PDVB, Ti 1−x Tb x [(OH) 4−u Cl v ]@PST–PDVB, TiO 2 and (Ti , Tb)O 2−w0 HSs (IR, FEG-SEM-XEDS and XPS) A PST–PDVB copolymer was chosen as a sacrificial template since both anhydrous chlorides, i.e., TiCl 4 and TbCl 3 employed in the synthesis can be incorporated to the copolymer via the formation of a coordination bond between each chloride and the PST and/or the PDVB polymers. It is due to the ability of chlorides to accept PST moiety and the π electrons donated from the electron rich PDBV. It can lead to PST–PDVB–TiCl 4 [ 17 ], PST–PDVB–TbCl3and/or PST–PDVB–TiCl4–TbCl3complexes. The template used here shows as its main features the following bands (Figure 1a): a slight band centered at 3446 cm −1 and one at approximately 1623 cm −1 (they are indicated with arrows in Figure 1a), corresponding to the stretching and bending vibration of hydroxyl groups, respectively [ 18 ]. Peaks in the region of 1640–1680 cm −1 are related to C=C vibrations and those in the region of 2873–2960 cm −1 are associated with =C–H bonds. The band at 2964 cm −1 can correspond to the stretching and antisymmetric vibration of the C–H in the carbonyl group. In addition, the characteristic absorption peak at 1735 cm −1 Int. J. Mol. Sci. 2022,23, 13162 3 of 18 is associated with a C=O stretching vibration of an ester (it is indicated with an arrow in Figure 1a) [ 19 ]. The bands at 1262 and 2930 cm −1 are also attributed to the stretching and antisymmetric vibrations, respectively, in the carbonyl group of the C–H [ 20 ]. The spectrum also shows strong intensity absorption at 1163 cm −1 associated with a C–O bond, suggesting a carboxilyc acid ester that has been grafted with the PDVB polymer (it is indicated with an arrow in Figure 1a). The peaks observed at 1616 and 901 cm −1 can be a result of the conjugation of the aromatic ring present in the DVB molecule. The bands at the 700–870 cm −1 range are attributed to the aromatic C–H out-of-plane bending vibrations and absorption bands of unsubstituted styryl residues [ 18 ]. The presence of para-crosslinks from the PDVB polymer is demonstrated by the peak at 840 cm −1 . Moreover, the band at 712 cm −1 (it is indicated with an arrow in Figure 1a) can be attributed to the skeletal C–C stretching vibration of tri-substituted benzene rings [ 21 ]. Furthermore, in the IR of Figure 1a, one characteristic absorption band of mono-substituted vinyl groups can be also recognized (900 cm −1 ), likely due to unreacted double bonds during the polymerization reaction. Those of the vinyl aromatic bonds at 1440 and 1490 cm −1 and those that match the skeletal vibration at 1603 cm −1 can be also detected in the spectrum. In addition, the surface of the pure template contains an appreciable amount of complex functional groups, including hydroxyl and carbonyl groups (Figure 1a), which provide ideal binding sites for the chlorides. After addition of the chlorides and before the water was added to the mixture (template + chlorides), any new peak is observed over the examined wavenumbers by IR. However, shrinkage (4.2%) of the average diameter size of the template after mixing with the TiCl 4 or TiCl 4 + TbCl 3 mixture is detected by FEG-SEM (please compare Figures 2and Figure S1). This fact, together with the color change of the template particles from white to light brown, suggests that a reaction between the chloride/s and the spacer occurs before hydrolysis. However, Tb is not observed by XPS even when the surface of the spheres was etched with Ar ions at 10 −6 torr, 5 KV and 10 mA, during 50 min. It may be because Tb is located underneath the surface of the template and with these etching conditions is not observable. However, the presence of Tb is confirmed by FEG-SEM-XEDS (Figure 2). Those analyses also showed a homogeneous distribution of Tb on the HSs. Int. J. Mol. Sci. 2022, 23, x FOR PEER REVIEW 4 of 20 Figure 1. Normalized IR spectra of the template (a), of the template + TiCl4 and TbCl3 mixture + HClaq after 2 h of reaction (b) and after 5 h of reaction (c), i.e., of the Ti1−xTbx[(OH)4−uClv]@PST– PDVB intermediate compound (Tb content in (b) and (c) is 0.12 at.%). Arrows are added in order to follow the evolution of the IR bands being the related reactions explained in the text. Figure 2. FEG-SEM picture of the mixture of the template + TiCl4 and TbCl3 precursors together with its characteristic XEDS analysis before the water addition, i.e., before hydrolysis and polycondensation reactions (HSs with 0.12 at.% Tb). After the addition of water, IR spectra revealed a decrease of the intensity of the bands from 500 to 800 cm−1 for benzene ring C–H bending, indicating the donor–acceptor interaction of carbon–carbon (π) bond on the benzene rings with TiCl4 and TbCl3 (Figure 1a,b). A decrease wavenumber of 5 cm−1, when compared with the value of the band for the pristine template, i.e., 1740 vs. 1735 cm−1 and a shift of the C=O absorption band in the 0 0.5 1 1.5 2 2.5 3 3.5 40080012001600200024002800320036004000 Transmittance (arb. units) Wavenumber (cm−1) a b c Figure 1. Normalized IR spectra of the template (a), of the template + TiCl 4 and TbCl 3mixture + HClaq after 2 h of reaction (b) and after 5 h of reaction (c), i.e., of the Ti 1−x Tb x [(OH) 4−u Cl v ]@PST–PDVB intermediate compound (Tb content in (b) and (c) is 0.12 at.%). Arrows are added in order to follow the evolution of the IR bands being the related reactions explained in the text. Int. J. Mol. Sci. 2022,23, 13162 4 of 18 Int. J. Mol. Sci. 2022, 23, x FOR PEER REVIEW 4 of 20 Figure 1. Normalized IR spectra of the template (a), of the template + TiCl4 and TbCl3 mixture + HClaq after 2 h of reaction (b) and after 5 h of reaction (c), i.e., of the Ti1−xTbx[(OH)4−uClv]@PST– PDVB intermediate compound (Tb content in (b) and (c) is 0.12 at.%). Arrows are added in order to follow the evolution of the IR bands being the related reactions explained in the text. Figure 2. FEG-SEM picture of the mixture of the template + TiCl4 and TbCl3 precursors together with its characteristic XEDS analysis before the water addition, i.e., before hydrolysis and polycondensation reactions (HSs with 0.12 at.% Tb). After the addition of water, IR spectra revealed a decrease of the intensity of the bands from 500 to 800 cm−1 for benzene ring C–H bending, indicating the donor–acceptor interaction of carbon–carbon (π) bond on the benzene rings with TiCl4 and TbCl3 (Figure 1a,b). A decrease wavenumber of 5 cm−1, when compared with the value of the band for the pristine template, i.e., 1740 vs. 1735 cm−1 and a shift of the C=O absorption band in the 0 0.5 1 1.5 2 2.5 3 3.5 40080012001600200024002800320036004000 Transmittance (arb. units) Wavenumber (cm−1) a b c Figure 2. FEG-SEM picture of the mixture of the template + TiCl 4 and TbCl 3 precursors together with its characteristic XEDS analysis before the water addition, i.e., before hydrolysis and polycondensation reactions (HSs with 0.12 at.% Tb). After the addition of water, IR spectra revealed a decrease of the intensity of the bands from 500 to 800 cm −1 for benzene ring C–H bending, indicating the donor–acceptor interaction of carbon–carbon ( π ) bond on the benzene rings with TiCl 4 and TbCl 3 (Figure 1a,b). A decrease wavenumber of 5 cm −1 , when compared with the value of the band for the pristine template, i.e., 1740 vs. 1735 cm −1 and a shift of the C=O absorption band in the IR spectra occur. Both facts can be associated with the anchoring of Tb and Ti ions to the carbonyl group (Figure 1b,c). The band intensity centered at 3025 cm −1 (=CH– stretching of benzene ring) decreases (marked with an arrow in Figure 1a–c), indicating a reduction in the electron density on the carbon–carbon ( π ) double conjugated bond. This can be due to the donation of electron density from π bond to the vacant d orbital of Ti 4+ in TiCl 4 and partial vacant f orbital of Tb 3+ in TbCl 3 . Furthermore, those bands related to the stretching and antisymmetric vibration of the C–H in the carbonyl group ( 2873–2960 cm−1 ) also decreased [ 18 ] (Figure 1b,c) compared with the bands of the spectrum for the template (Figure 1a). Both facts indicate not only that the carbonyl group of the template can have some kind of interaction with the chlorides and with Ti 1−x Tb x [(OH) 4−u Cl v ] δ+ and Ti 1−x Tb x [(OH) 4−y ] ρ+ species [ 18 ] but also that the carbonyl group takes part during the hydrolysis process as an electron donor for the H 3 O + (electrophile attack). Furthermore, the bands in the range 1150–1085 cm −1 of the template (C–O stretching and bending vibrations) [ 18 ] also decreased with the hydrolysis process (Figure 1a–c). A new band at 615 cm −1 (Figure 1b,c) is also noted which can imply the formation of metalloxane bonds (it is marked with an arrow in Figure 1b,c). Then, the IR spectra indicate that the hydrolysis reaction mechanism involves proton attack on the Ti–Cl, Tb–Cl and C=O bonds, the occupation of a vacant site by a water molecule, and then the dissociation of the dangling Ti–Cl, Tb–Cl ligands and C=O bonds. For samples calcined at 550 ◦ C and greater, the IR bands of the template totally vanished (Figure 3a–d). Moreover, new bands were observed in the 400–900 cm −1 region that can be attributed to the metalloxane bondings (M–O, M’–O, M–O–M, M–O–M’, and/or M’–O–M’, being M and M’ = Ti and Tb, respectively), i.e., mainly vibration modes of anatase (Figure 3a,b). In particular, the band centered at 480 cm −1 is observed for both undoped and doped HSs that can be attributed to the transverse optical vibrations of the Ti–O bonds [ 22 – 24 ] in anatase. Only bands of rutile are observed for HSs calcined at 1000 ◦ C (523 and 661 cm −1 for the undoped, and 520 and 658 cm −1 for the doped ones) (Figure 3c,d, respectively). The shape of these bands varied in accordance with the different crystal phases; anatase has less-defined Ti–O stretching bands as it possesses a less ordered structure (Figure 3a,b) compared to rutile (Figure 3c,d), respectively. For the HSs calcined at 800 ◦ C vibration modes of both anatase and rutile are observed (not shown here) in accordance with XRD results (see below in Section 2.5). Int. J. Mol. Sci. 2022,23, 13162 5 of 18 Int. J. Mol. Sci. 2022, 23, x FOR PEER REVIEW 6 of 20 Figure 3. Normalized IR spectra of 0.00%_550 and 1.70%_550 (a) and (b), respectively, and 0.00%_1000 and 1.70%_1000 (c) and (d), respectively. After hydrolysis and polycondensation, another color change from light brown to dark brown is observed for the particles. Figure 4 shows FEG-SEM pictures of the template + TiCl4 and TbCl3 mixture + HCl (aq) after 5 h of reaction for the 0.12 at.% Tb sample (a) and the same HSs after 7 h of reaction and calcination at 550 °C for 30 min (b). The presence of Cl− in the intermediate compounds before washing and calcination as well as the presence of both Ti and Tb was confirmed by XEDS (Figure S2). Figure 4b shows a uniform hollow Tb-doped TiO2 microsphere with an outer average diameter size of 1.8 ± 0.2 µm, confirmed by TEM (Figure 5a). The size of the spheres after calcination at 550 °C suffers a shrinkage process of approximately 37% when compared with uncalcined spheres. A rough surface is noted related to the presence of aggregates of nanoparticles. In addition, the average shell thickness of the HSs is 64.0 ± 0.2 nm (Figure 5a). The SAED diagram shown in Figure 5b provides a direct evidence of the structure of the material which corresponds to anatase phase according to XRD results (see below in Section 2.2.) 0.5 1 1.5 2 2.5 3 400900140019002400290034003900 Intensity (arb. units) Wavenumber (cm−1) a b c d Figure 3. Normalized IR spectra of 0.00%_550 and 1.70%_550 (a) and (b), respectively, and 0.00%_1000 and 1.70%_1000 (c) and (d), respectively. After hydrolysis and polycondensation, another color change from light brown to dark brown is observed for the particles. Figure 4shows FEG-SEM pictures of the template + TiCl4 and TbCl 3 mixture + HCl (aq) after 5 h of reaction for the 0.12 at.% Tb sample (a) and the same HSs after 7 h of reaction and calcination at 550 ◦ C for 30 min (b). The presence of Cl − in the intermediate compounds before washing and calcination as well as the presence of both Ti and Tb was confirmed by XEDS (Figure S2). Figure 4b shows a uniform hollow Tb-doped TiO 2 microsphere with an outer average diameter size of 1.8 ±0.2 µm , confirmed by TEM (Figure 5a). The size of the spheres after calcination at 550 ◦ C suffers a shrinkage process of approximately 37% when compared with uncalcined spheres. A rough surface is noted related to the presence of aggregates of nanoparticles. In addition, the average shell thickness of the HSs is 64.0 ± 0.2 nm (Figure 5a). The SAED diagram shown in Figure 5b provides a direct evidence of the structure of the material which corresponds to anatase phase according to XRD results (see below in Section 2.2.) Int. J. Mol. Sci. 2022, 23, x FOR PEER REVIEW 7 of 20 Figure 4. FEG-SEM pictures of the template + TiCl4 and TbCl3 mixture + HClaq after 5 h of reaction (a) (it corresponds to the sample of IR in Figure 1c) and FEG-SEM picture of the 0.12 at.% Tb HSs after 7 h of reaction and calcination at 550 °C (0.12%_550) (b). Figure 5. TEM picture of a 0.12%_550 HS where the shell thickness of the capsule can be observed (a). SAED pattern corresponding to the HS is shown in (b). 2.2. Texture and Structure of Ti[(OH)4−yCly]@PST–PDVB, Ti1−xTbx[(OH)4−uClv]@PST–PDVB, TiO2 and (Ti,Tb)O2−w′ The HSs were analyzed by a LECO analyzer in order to check whether residual C and N are present in the samples after calcination at 550 °C for 30 min. These analyses indicated that the HSs are free of both C and N. Figure 6a shows the N2 gas adsorption– desorption isotherms of the template and the intermediate compound obtained at the end of the polycondensation reaction and before calcination, i.e., of Ti0.988Tb0.12[(OH)4−uClv]@PST–PDVB intermediate product. The template is a non-porous solid according to its isotherm (Figure 6a). A slight hysteresis in the adsorption– desorption cycle found can indicate non-rigid aggregates of plate-like particles or a pore network of macropores between the spheres [25]. In this case, the latter option is the plausible one. In addition, the N2 gas adsorption–desorption isotherms of the Figure 4. FEG-SEM pictures of the template + TiCl4and TbCl3mixture + HClaq after 5 h of reaction ( a ) (it corresponds to the sample of IR in Figure 1c) and FEG-SEM picture of the 0.12 at.% Tb HSs after 7 h of reaction and calcination at 550 ◦C (0.12%_550) (b). Int. J. Mol. Sci. 2022,23, 13162 6 of 18 Int. J. Mol. Sci. 2022, 23, x FOR PEER REVIEW 7 of 20 Figure 4. FEG-SEM pictures of the template + TiCl4 and TbCl3 mixture + HClaq after 5 h of reaction (a) (it corresponds to the sample of IR in Figure 1c) and FEG-SEM picture of the 0.12 at.% Tb HSs after 7 h of reaction and calcination at 550 °C (0.12%_550) (b). Figure 5. TEM picture of a 0.12%_550 HS where the shell thickness of the capsule can be observed (a). SAED pattern corresponding to the HS is shown in (b). 2.2. Texture and Structure of Ti[(OH)4−yCly]@PST–PDVB, Ti1−xTbx[(OH)4−uClv]@PST–PDVB, TiO2 and (Ti,Tb)O2−w′ The HSs were analyzed by a LECO analyzer in order to check whether residual C and N are present in the samples after calcination at 550 °C for 30 min. These analyses indicated that the HSs are free of both C and N. Figure 6a shows the N2 gas adsorption– desorption isotherms of the template and the intermediate compound obtained at the end of the polycondensation reaction and before calcination, i.e., of Ti0.988Tb0.12[(OH)4−uClv]@PST–PDVB intermediate product. The template is a non-porous solid according to its isotherm (Figure 6a). A slight hysteresis in the adsorption– desorption cycle found can indicate non-rigid aggregates of plate-like particles or a pore network of macropores between the spheres [25]. In this case, the latter option is the plausible one. In addition, the N2 gas adsorption–desorption isotherms of the Figure 5. TEM picture of a 0.12%_550 HS where the shell thickness of the capsule can be observed ( a ). SAED pattern corresponding to the HS is shown in (b). 2.2. Texture and Structure of Ti[(OH)4−yCly]@PST–PDVB, Ti1−xTbx[(OH)4−uClv]@PST–PDVB, TiO2and (Ti,Tb)O2−w0 The HSs were analyzed by a LECO analyzer in order to check whether residual C and N are present in the samples after calcination at 550 ◦ C for 30 min. These analyses indicated that the HSs are free of both C and N. Figure 6a shows the N 2 gas adsorption–desorption isotherms of the template and the intermediate compound obtained at the end of the polycondensation reaction and before calcination, i.e., of Ti 0.988 Tb 0.12 [(OH) 4−u Cl v ]@PST–PDVB intermediate product. The template is a non-porous solid according to its isotherm (Figure 6a). A slight hysteresis in the adsorption–desorption cycle found can indicate non-rigid aggregates of plate-like particles or a pore network of macropores between the spheres [ 25 ]. In this case, the latter option is the plausible one. In addition, the N 2 gas adsorption–desorption isotherms of the Ti(OH) 4 @PST–PDVB and Ti 1−x Tb x [(OH) 4−u Cl v ]@PST–PDVB composites are characteristic of mesoporous materials. In particular, their isotherms correspond to type IV with a loop of type H2 according to the IUPAC classification [ 25 ]. Figure 6b shows the N 2 gas adsorption–desorption isotherms of two calcined Tb-doped TiO 2 capsules (1.70 and 3.00 at.% Tb) after their calcination at 550 ◦ C for 30 min. A very slight hysteresis in the adsorption–desorption cycles found for these calcined spheres can indicate a pore network of macropores between HSs [ 25 ], as in the case of the template. The specific surface areas found ranged from 44 ± 3 m 2 /g for the undoped to ~34 ± 4 m 2 /g for doped HSs calcined at 550 ◦ C for 30 min, respectively (Table 1). After calcination of the HSs at 1000 ◦C, the specific surface areas found were approximately 0.1 ±0.5 m2/g. Table 1. Specific surface areas of the HSs calcined at 550 ◦C. SSA (m2/g) 0.00%_550 44 ±3 0.12%_550 35 ±4 1.70%_550 34 ±4 3.00%_550 32 ±4 Int. J. Mol. Sci. 2022,23, 13162 7 of 18 Int. J. Mol. Sci. 2022, 23, x FOR PEER REVIEW 8 of 20 Ti(OH)4@PST–PDVB and Ti1−xTbx[(OH)4−uClv]@PST–PDVB composites are characteristic of mesoporous materials. In particular, their isotherms correspond to type IV with a loop of type H2 according to the IUPAC classification [25]. Figure 6b shows the N2 gas adsorption–desorption isotherms of two calcined Tb-doped TiO2 capsules (1.70 and 3.00 at.% Tb) after their calcination at 550 °C for 30 min. A very slight hysteresis in the adsorption–desorption cycles found for these calcined spheres can indicate a pore network of macropores between HSs [24], as in the case of the template. The specific surface areas found ranged from 44 ± 3 m2/g for the undoped to ~34 ± 4 m2/g for doped HSs calcined at 550 °C for 30 min, respectively (Table 1). After calcination of the HSs at 1000 °C, the specific surface areas found were approximately 0.1 ± 0.5 m2/g. Table 1. Specific surface areas of the HSs calcined at 550 °C. SSA (m2/g) 0.00%_550 44 ± 3 0.12%_550 35 ± 4 1.70%_550 34 ± 4 3.00%_550 32 ± 4 0 2 4 6 8 10 12 14 16 18 20 0 0.2 0.4 0.6 0.8 1 N2sorption (cm3/g) Relative Pressure (P/Po) Template (Adsorption) Template (Desorption) 0.12%@PST-PDVB (Adsorption) 0.12%@PST-PDVB (Desorption) a Int. J. Mol. Sci. 2022, 23, x FOR PEER REVIEW 9 of 20 Figure 6. (a) N2 adsorption–desorption isotherm curves of the template and of the Ti0.988Tb0.012 [(OH)4−uClv]@PST–PDVB composite. (b) N2 adsorption–desorption isotherm curves of 0.12%_550 HSs and 1.70%_550. Figure 7 shows the XRD diffraction patterns of the template (a) after the simultaneous implantation of both TiCl4 and TbCl3 (0.12 at.% Tb) to the carbonyl groups of the template, followed by the addition of HClaq, i.e., when the hydrolysis and polycondensation reactions are taking place (b), and after calcination at 550 °C for 30 min (c). The virgin template exhibited a broad diffraction band at 2θ of the range 15.00–30.00° suggesting the amorphous structure of the polymeric scaffold (Figure 7a). However, in the case of the intermediate product formed after implantation and the partial hydrolysis and polycondensation reactions (Figure 7b), the XRD pattern shows a type of semi-crystalline structure in which the intensity of the characteristic reflection band at 2θ = 15.00–30.00° has diminished without altering its position. The reason for the decrease in intensity is likely due to the accommodation of TiCl4 and TbCl3 molecules in the PST–PDVB matrix forming complexes with the template. In addition, new peaks at 18.18, 18.74 and 25.26° (indicated with arrows) could confirm the implantation of both chlorides in the polystyrene matrix (Figure 7b). In fact, in a previous work, Rahmatpour et al. [17] found a peak at 2θ = 18.06° that assigned to the implantation of TiCl4 onto the PST–PDVB which is in good agreement with that found in this study (18.18°). After calcination at 550 °C for 30 min pure anatase phase was completely crystallized for all compositions (Figure 7c is shown as a way of example). Neither brookite nor rutile phases were noted for any composition calcined at that temperature. In addition, neither metal clusters of Tb nor impurity phases such as Tb oxides were detected. 0 50 100 150 200 250 300 350 400 0 0.2 0.4 0.6 0.8 1 N2sorption (cm3/g) Relative Pressure (P/Po) 0.12%_550 (Adsorption) 0.12%_550 (Desorption) 1.70%_550 (Adsorption) 1.70%_550 (Desorption) b Figure 6. ( a ) N 2 adsorption–desorption isotherm curves of the template and of the Ti 0.988 Tb 0.012 [(OH) 4−u Cl v ]@PST–PDVB composite. ( b ) N 2 adsorption–desorption isotherm curves of 0.12%_550 HSs and 1.70%_550. Int. J. Mol. Sci. 2022,23, 13162 8 of 18 Figure 7shows the XRD diffraction patterns of the template (a) after the simultaneous implantation of both TiCl 4 and TbCl 3 (0.12 at.% Tb) to the carbonyl groups of the template, followed by the addition of HCl aq , i.e., when the hydrolysis and polycondensation reactions are taking place (b), and after calcination at 550 ◦ C for 30 min (c). The virgin template exhibited a broad diffraction band at 2 θ of the range 15.00–30.00 ◦ suggesting the amorphous structure of the polymeric scaffold (Figure 7a). However, in the case of the intermediate product formed after implantation and the partial hydrolysis and polycondensation reactions (Figure 7b), the XRD pattern shows a type of semi-crystalline structure in which the intensity of the characteristic reflection band at 2 θ = 15.00–30.00 ◦ has diminished without altering its position. The reason for the decrease in intensity is likely due to the accommodation of TiCl 4 and TbCl 3 molecules in the PST–PDVB matrix forming complexes with the template. In addition, new peaks at 18.18, 18.74 and 25.26 ◦ (indicated with arrows) could confirm the implantation of both chlorides in the polystyrene matrix (Figure 7b). In fact, in a previous work, Rahmatpour et al. [ 17 ] found a peak at 2 θ = 18.06 ◦ that assigned to the implantation of TiCl 4 onto the PST–PDVB which is in good agreement with that found in this study (18.18 ◦ ). After calcination at 550 ◦ C for 30 min pure anatase phase was completely crystallized for all compositions (Figure 7c is shown as a way of example). Neither brookite nor rutile phases were noted for any composition calcined at that temperature. In addition, neither metal clusters of Tb nor impurity phases such as Tb oxides were detected. Int. J. Mol. Sci. 2022, 23, x FOR PEER REVIEW 10 of 20 Figure 7. Indexed XRD patterns of the template (a), the Ti1−xTbx[(OH)4−uClv]@PST–PDVB (x = 0.012) intermediate product after 5 h of reaction (b) and Ti1−xTbxO2−w′ (x = 0.012) after calcination at 550 °C for 30 min (0.12%_550) (normalized XRD pattern) (c). The peaks at 18.18, 18.74 and 25.26° that can confirm the implantation of TiCl4 and TbCl3 in the PST–PDVB matrix are indicated with arrows. 2.3. Thermal Evolution of the Ti1−xTbx[(OH)4−uClv]@PST–PDVB followed by DTA-TG Analysis The thermal behavior of the PST–PDVB was analyzed in a previous work [10]. The behavior of the Ti1−xTbx[(OH)4−uClv]@PST–PDVB composite obtained at the end of the hydrolysis and polycondensation reactions and before washing was studied by DTA-TG in air (Figure 8). The TG curve of the template displays, as was observed before [10], a weight loss of 100% during the calcination process, which is the burning of the spheres. According to those and in line with our DTA-TG results the sacrificial template begins to degrade at temperatures above 250 °C and shows two main weight loss regions. At the temperature range of 300–400 °C, a pronounced exothermic peak occurs in accordance with a significant weight loss of approximately 60%. This is due to the oxidation of the low melting, pendant alkyl groups in the template polymers. A second exothermic peak with a weight loss of approximately 40% follows at the temperature region 450–530 °C. This resulted from the decomposition of the aromatic rings [26]. The TG curve of the Ti1−xTbx[(OH)4−uClv]@PST–PDVB composite shows four loss stages: below 200 °C a weight loss of 12.2 wt.% can be due to the dehydration of the microspheres and OH− and the physisorbed volatile organic removal. This weight loss is accompanied by an endothermic peak centered at 99.3 °C that is not observed for the free template. Four pronounced exothermic peaks occur at 232.2, 365.8, 436.0 and 484.4 °C in accordance with four significant weight losses of a total amount of approximately 55.4 wt.%. The first, second and fourth peaks can be associated with the decomposition, oxidation and burning of the template, respectively [10], as is indicated above. The third peak detected at 436.0 °C and indicated with a star in Figure 8 is related to a weight loss that can be due 0 0,5 1 1,5 2 2,5 3 3,5 4 10 20 30 40 50 60 70 80 Intensity (arb. units) 2θ(º) 101 004 200 105 211 220 215 116 204 a b c Figure 7. Indexed XRD patterns of the template (a), the Ti 1−x Tb x [(OH) 4−u Cl v ]@PST–PDVB ( x = 0.012 ) intermediate product after 5 h of reaction (b) and Ti 1−x Tb x O 2−w0 (x = 0.012) after calcination at 550 ◦ C for 30 min (0.12%_550) (normalized XRD pattern) (c). The peaks at 18.18, 18.74 and 25.26 ◦ that can confirm the implantation of TiCl4and TbCl3in the PST–PDVB matrix are indicated with arrows. 2.3. Thermal Evolution of the Ti 1−x Tb x [(OH) 4−u Cl v ]@PST–PDVB followed by DTA-TG Analysis The thermal behavior of the PST–PDVB was analyzed in a previous work [ 10 ]. The behavior of the Ti 1−x Tb x [(OH) 4−u Cl v ]@PST–PDVB composite obtained at the end of the Int. J. Mol. Sci. 2022,23, 13162 9 of 18 hydrolysis and polycondensation reactions and before washing was studied by DTA-TG in air (Figure 8). The TG curve of the template displays, as was observed before [ 10 ], a weight loss of 100% during the calcination process, which is the burning of the spheres. According to those and in line with our DTA-TG results the sacrificial template begins to degrade at temperatures above 250 ◦ C and shows two main weight loss regions. At the temperature range of 300–400 ◦ C, a pronounced exothermic peak occurs in accordance with a significant weight loss of approximately 60%. This is due to the oxidation of the low melting, pendant alkyl groups in the template polymers. A second exothermic peak with a weight loss of approximately 40% follows at the temperature region 450–530 ◦ C. This resulted from the decomposition of the aromatic rings [ 26 ]. The TG curve of the Ti 1−x Tb x [(OH) 4−u Cl v ] @PST–PDVB composite shows four loss stages: below 200 ◦ C a weight loss of 12.2 wt.% can be due to the dehydration of the microspheres and OH − and the physisorbed volatile organic removal. This weight loss is accompanied by an endothermic peak centered at 99.3 ◦ C that is not observed for the free template. Four pronounced exothermic peaks occur at 232.2, 365.8, 436.0 and 484.4 ◦ C in accordance with four significant weight losses of a total amount of approximately 55.4 wt.%. The first, second and fourth peaks can be associated with the decomposition, oxidation and burning of the template, respectively [ 10 ], as is indicated above. The third peak detected at 436.0 ◦ C and indicated with a star in Figure 8is related to a weight loss that can be due to residual Cl − elimination [ 27 ]. The presence of Cl − was also confirmed by FEG-SEM-XEDS, as is mentioned above. The percentage of residual weight is approximately 32.0%, which accounts for the final (Ti,Tb)O 2−w0 product, indicating that the HSs were prepared in a considerably high yield by this method. We observed that the degradation temperature of the polymer slightly increases when the polymer is part of the Ti 1−x Tb x O 2−z0 @PST–PDVB composite, i.e., a shifting in the temperature events for both TG and DTA curves is detected for the composite with respect to the PST–PDVB template [ 10 ]. This is plausible since the polymer is covered by a coating of Ti 1−x Tb x [(OH) 4−u Cl v ] which slightly hinders the calcination of the template. Int. J. Mol. Sci. 2022, 23, x FOR PEER REVIEW 11 of 20 to residual Cl− elimination [27]. The presence of Cl− was also confirmed by FEG-SEM-XEDS, as is mentioned above. The percentage of residual weight is approximately 32.0%, which accounts for the final (Ti,Tb)O2−w′ product, indicating that the HSs were prepared in a considerably high yield by this method. We observed that the degradation temperature of the polymer slightly increases when the polymer is part of the Ti1−xTbxO2−z′@PST–PDVB composite, i.e., a shifting in the temperature events for both TG and DTA curves is detected for the composite with respect to the PST–PDVB template [10]. This is plausible since the polymer is covered by a coating of Ti1−xTbx[(OH)4−uClv] which slightly hinders the calcination of the template. Figure 8. Thermogravimetric and differential thermal analyses curves of the Ti0.988Tb0.012 [(OH)4−uClv]@PST–PDVB composite (a and a′, respectively) in air (Tb content is 0.12 at.%). The star indicates the exothermic peak related to Cl− removal. 2.4. Compositional Analysis of the Calcined HSs by XPS The nature and composition of the HSs surfaces were analyzed in selected samples by XPS. As is mentioned above, in the analysis of the HSs before calcination, Tb was not detected for any sample unlike in the calcined HSs. The survey spectra and the spectral regions of Tb 3d, Ti 2p and O 1s peaks for the samples calcined at 550, 800 and 1000 °C are depicted in Figure S3. The Ti 2p photoemission signals consist in a doublet at approximately 465 and 459 eV which can be attributed to Ti 2p1/2 and Ti 2p3/2, respectively. These photoemission signals are separated by 5.8–5.7 eV according to the values of ΔBE reported for Ti4+ in TiO2 nanoparticles with both anatase and rutile structures [15,28,29]. As is expected for air-exposed samples they do not show any signal corresponding to Ti3+. The surface composition of the samples was obtained from the relative intensities of each element and they are collected in Table 2. As is mentioned above, since Tb content is very low for the sample with a nominal 0.12 at.% content, its determination is within the detection limit of the technique. For that reason, the spectrum of Tb is not deconvoluted. -1 0 1 2 3 4 5 6 7 0 20 40 60 80 100 120 25 225 425 625 825 1025 Heat Flow (W/g) Weight (%) Temperature (ºC) exo endo a a Figure 8. Thermogravimetric and differential thermal analyses curves of the Ti 0.988 Tb 0.012 [(OH) 4−u Cl v ]@PST–PDVB composite (a and a 0 , respectively) in air (Tb content is 0.12 at.%). The star indicates the exothermic peak related to Cl−removal. Int. J. Mol. Sci. 2022,23, 13162 16 of 18 3.2.6. XPS XPS experiments were performed in a PHOIBOS-100 spectrometer (SPECS, Berlin, Germany) with a non-monochromatic Al K α source (h ν = 1486.6 eV) and the power of the X-ray source was 230 W (11.5 kV and 20 mA). The electron energy hemispherical analyzer was operated in the constant pass energy mode (SPECS PHOIBOS 100DLD). Low resolution survey spectra were obtained with a pass energy equal to 50 eV, whereas high energy resolution spectra of the main photoemission peaks from the detected elements (i.e., Tb 3d, Ti 2p, O 1s) were obtained with 35 eV as pass energy. The spectra were analyzed with the “CASA XPS” software, version 2.3.16.Dev52 (by Neal Fairly, UK). Shirley-type backgrounds were used to determine the areas under the peaks. The Tb 3d 5/2 spectra were fitted with two components, for both Tb 3+ and Tb 4+ , using Gaussian–Lorentzian functions (GL = 30). Each component was simulated by a main function with the same width ( 5.7–5.9 eV ) and its associated satellite separated 2 eV and slightly wider (7.9 eV). Component positions and widths were kept constant in all fittings. The O 1s signal was fitted with two different components separated by 1.5 eV (GL = 30) and were assigned to oxo ligand titanium-bounded (fwhm = 1.7 eV) and hydroxyl groups (fwhm = 2.4 eV). 3.2.7. FEG-SEM-XEDS and TEM-SAED-XEDS The microstructure of the samples was analyzed during different steps of the reactions and before and after calcination by means of field-emission scanning electron microscopyenergy dispersive X-ray (FEG-SEM-XEDS) (Hitachi S-4700 type I, Tokyo, Japan). Calcined powders were also characterized by transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were carried out on a Jeol 2000FX (Tokyo, Japan) microscope working at 200 keV, equipped with an ISIS analyzer system that characterized the local composition by X-ray energy-dispersive spectrometry (XEDS). 4. Conclusions It has been established and monitored a protocol of synthesis to obtain homogeneous Tb-doped TiO 2 (anatase) HSs based on a simultaneous implantation of Ti and Tb species on a PST–PDVB polymer first, followed by a sol-gel process. The carbonyl groups of the copolymer template take part during the hydrolysis according to IR results. The mechanism of this process involves proton attack on the Ti–Cl, Tb–Cl and C=O bonds, the occupation of a vacant site by a water molecule and then, the dissociation of the dangling Ti–Cl, Tb–Cl ligands and C=O bonds, according to the observed decrease of both the band intensity and of the wavenumbers of the carbonyl group. Ti 1−x Tb x [(OH) 4−y Cl z ]@ PST–PDVB and Ti 1−x Tb x [(OH) 4−u0 ]@PDVB intermediate complexes are formed during this step. Finally, polycondensation leads to amorphous Ti 1−x Tb x O 2−w0 @PST–PDVB compounds. This synthesis route allows us to prepare TiO 2 and Tb-doped TiO 2 HSs with pure anatase phase structure after calcination according to XRD. XPS demonstrated the presence of Tb 4+ with a fraction of 38.5 and 41.2% in the HSs, with 1.70 and 3.00 at.% Tb, respectively, in addition to Tb 3+ . Then, our doped HSs doped with these compositions can be considered as codoped systems and as is observed in previous studies for co-doped RE systems the ART is favored. An increasing fraction of Tb with calcination temperature for a given composition confirmed a Tb diffusion process from the inner part to the surface of the HSs. Finally, the synthesis method described in this work could be extended to the preparation of other RE-doped anatase systems. Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/ijms232113162/s1. Author Contributions: Conceptualization: M.T.C.; methodology: M.T.C.; formal analysis: M.T.C. and F.V.; investigation: M.T.C. and F.V.; resources: M.T.C. and F.V.; data curation: M.T.C. and F.V.; writing-original draft preparation—review and editing: M.T.C.; funding acquisition: M.T.C. All authors have read and agreed to the published version of the manuscript. Int. J. Mol. Sci. 2022,23, 13162 17 of 18 Funding: This work was supported by Science and Innovation Ministry (Government of Spain) and FEDER Funds under Grant PID-2019-104118RB-C21. 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