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Large scale and facile synthesis of Sn doped TiO2 aggregates using hydrothermal synthesis
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Large scale and facile synthesis of Sn doped TiO2 aggregates using hydrothermal synthesis ⁎
Saida Mehraza,b, Peerawas Konsongc, Abdelhafed Talebd,e, , Nahed Dokhanea, Lek Sikongc a
Unité de Recherche Matériaux, Procédés et Environnement, M’hamedBougara de Boumedès University, 35000 Boumerdés, Algeria Research Centre in Industrial Technologies CRTI, BP 64, route de Dely-Ibrahim, Chéraga, Algiers 16033, Algeria c Department of Mining and Materials Engineering, Faculty of Engineering, Prince of Songkla University Hat yai, Songkhla 90112, Thailand d PSL Research University, Chimie ParisTech - CNRS, Institut de Recherche de Chimie Paris, 75005 Paris, France e Université Pierre et Marie Curie, 4 place Jussieu, 75231 Paris, France b
A R T I C L E I N F O
A B S T R A C T
Keywords: Hydrothermal Sn doping TiO2 Nanoparticles assembly Optical properties
Sn doped TiO2 aggregates have been successfully prepared via one pot hydrothermal technique. Different methods were used to characterize prepared Sn doped TiO2 aggregates such as DRX, XPS, N2 adsorption (BET), FEGSEM and UV–Vis spectroscopy. It was illustrated that the size, the morphology and the phase of prepared TiO2 aggregates is strongly influenced by the amount of added Sn doping. In addition, it was demonstrated that the prepared aggregates properties is influenced by the synthesis temperature. Furthermore, it was shown that the prepared Sn doped TiO2 aggregates are of high crystallinity. The influence of added Sn dopant amount on the optical and structural properties of the prepared Sn doped TiO2 aggregates have been investigated.
1. Introduction In material science, recent progress is mainly devoted to develop innovative strategies to prepare nanomaterials with desired properties, which are a coupling of both its intrinsic and extrinsic properties. Among the strategies reported for extrinsic material properties control, nanoparticles agglomeration appears to be a promising approach to obtain materials with controlled architectures and desired properties for targeted applications. In fact, combining a mixture of nanoparticles with different physical and chemical properties offers a large number of possibilities to tailor the properties of agglomerated materials. In addition to the properties of individual nanoparticles, their assembly gives rise to collective properties due to nanoparticles interaction. Compared to bulk material, this new configuration is more flexible for material preparation and extrinsic properties control. For example, it allows the combination of different properties even conflicting ones, such as large submicrometer-sized particles for light harvesting and large surface area at nanoscale for dye loading [1]. For lithium ion batteries, it alleviates strongly the electrodes stress induced by their volume variation during the insertion and desertion of lithium ion [2]. The agglomeration of nanoparticles is a consequence of nanoparticles destabilization and it depends both on the nanoparticles properties (size, shape, surface modification, concentration) and those of their embedding media. However, a combination of both of these
⁎
properties is required to be considered for the prediction of nanoparticles agglomeration [3,4]. Different agglomerated metallic and semiconductor nanoparticles were reported in the literature and it has shown an accurate control of the pore size and specific surface area [5,6]. However, most of the reported nanoparticles aggregates suffer from the lack of size and morphology control. Furthermore, to control the intrinsic material properties, different strategies were reported in the literature such as quantum dots and metallic nanoparticles decoration [7], dyes loading [8], doping with different elements [9,10], nanoparticles assembly [11,12] etc … Among these strategies, the doping approach with metal ions has been proven to be an effective and promising way to control the intrinsic properties of semi-conductor to be used as performing anode materials. Recently, tremendous efforts have been devoted to develop doped materials with optimized properties for different applications. Among the studies semiconductor materials, TiO2 nanostructures have received particular attention due to their use as anode materials for potential applications in different areas such as photovoltaic [13], photo catalysis [14], gas sensing [15], photo-splitting of water [16] and batteries [17]. However, TiO2 nanostructures suffer from some limitations, due to its large band gap which is around 3.2 eV and low electron transfer rate which impedes its application. As a consequence of its large band gap, it is sensitive only to UV light excitations, which reduce strongly the efficiency of solar cell based on TiO2 nanostructures. In addition to the
Corresponding author at: PSL Research University, Chimie ParisTech - CNRS, Institut de Recherche de Chimie Paris, 75005 Paris, France. E-mail address: [email protected] (A. Taleb).
http://dx.doi.org/10.1016/j.solmat.2017.06.048 Received 31 March 2017; Received in revised form 16 June 2017; Accepted 19 June 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Mehraz, S., Solar Energy Materials and Solar Cells (2017), http://dx.doi.org/10.1016/j.solmat.2017.06.048
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Fig. 3. XPS spectra of Sn doped TiO2 aggregates at synthesis temperatures of 150 °C and different Sn doping contents as indicated (a) XPS survey (b) high resolution of Sn 3d core level.
nanostructure is extended to the visible light spectrum, which improves its photocatalytic activity [22] and solar conversion efficiency [23]. Additionally, it is well established that a dopant could improve the electron transfer rate, which limits the charges recombination process [24,25]. For lithium ion batteries application, dopant is expected to keep high capacity and improved cycling performance [26]. Among all the metal ions used for TiO2 doping, Sn is considered as the most effective, thanks to the fact that the ionic radii of Ti4+ (0.605 A) and Sn4+ (0.690 A) are roughly similar, which favors the substitution of Ti4+ by Sn4+ in the TiO2 crystal lattice to form a stable solid solution [27]. Different amounts of incorporated Sn dopant in TiO2 lattice were reported in the literature ranging from 1% to roughly 5 mol% [28,29]. This value depends both on the precursor and the synthesis method [30]. Various hybrid structures of SnO2 and TiO2 were studied in literature such as TixSn1−xO3 solid solution [31], Ti2/ 3Sn1/3O2 [32] etc … Furthermore, several methods were used to prepare Sn doped TiO2 such as sol-gel method [33–35], hydrothermal [36] and chemical vapor deposition [37]. Among the methods reported in the literature, the hydrothermal method presents many advantages such as the ability to produce nanoparticles with narrow size distribution, dispersed in polar or nonpolar solvents. In the present work, Sn doped TiO2 aggregates made by TiO2 nanoparticles as a building unit have been successfully prepared hydrothermally. It was shown that the size of prepared TiO2 aggregates can be controlled from few tens nanometers to few hundred nanometers depending on the synthesis temperature. Depending on the synthesis temperature Sn dopant promotes TiO2 phase and morphology transformation. Additionally, it has been shown that the increasing of added Sn dopant increases the size of TiO2 nanoparticles building unit of prepared aggregates and reduces significantly its band gap energy at certain Sn dopant amount threshold.
Fig. 1. XRD pattern of Sn doped TiO2 aggregates prepared at different synthesis temperatures and Sn doping as indicated.
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lithium ion batteries application, the low theoretical specific capacity of TiO2 impedes its extensive use [18]. To improve the efficiency of TiO2 nanostructure as anode materials for different applications, the doping approach by different metal ions such as Al 3+, Cr 3+, Fe 3+, Co 3+ etc … was used [19,20]. In most cases, the dopant induces the modification of TiO2 electronic structure through the introduction of band-gap states [21], which reduces the band gap energy. As a consequence, the absorption of TiO2 2
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Fig. 4. High resolution XPS spectra of Ti2p core level (a) O1s core level (b) at synthesis temperatures of 150 °C and different Sn doping contents as indicated.
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Binding Energy (eV) 2. Experimental
microscope (FEGSEM) operating at an acceleration voltage of 10 kV. The film thicknesses were measured with a Dektak 6 M stylus profiler. The crystalline structure of TiO2 was determined by an X-ray diffractometer (Siemens D5000 XRD unit) in 2θ range from 20° to 80° by 0.07° s−1 increasing steps operating at 40 kV accelerating voltage and 40 mA current using Cu Kα radiation source with λ = 1.5406 Å. The chemical compositions of all the samples were determined by Xray photoelectron spectroscopy (XPS) and for the measurements we used a RIBER MACII analyzer system equipped with a AL Kα X-ray source (hυ = 1486.6 eV; spot size 20 mm, power = 11 kVx50 w). The optical measurements were performed using a Cary 50 (Varian) UV–Vis–NIR spectrophotometer equipped with unpolarized light and operating at normal incidence in the wavelength range from 300 nm to 850 nm. Nitrogen adsorption - desorption isotherms were measured at liquid nitrogen temperature on a Micromeritics ASAP 2020 apparatus. Before analysis, all the samples were degassed at 120 °C for 10 h. The specific surface area (SBET) was evaluated using the Brunauer-Emmett-Teller (BET) method in the P/P° range of 0.05–0.25. The pore size distribution was determined from the desorption branch of the isotherm using the Barret-Joyner-Halender (BJH) method. The total pore volume was determined from the amount of N2 adsorbed up to P/P° = 0.98.
2.1. Synthesis of Sn doped TiO2 aggregates The synthesis of Sn doped TiO2 aggregates has been performed using a hydrothermal method. The titanium (IV) oxysulfate hydrate (TiOSO4, Sigma Aldrich) precursor was prepared in aqueous solution by dissolving 6.4 g (2.5 M) of TiOSO4 in 16 ml of distilled water under constant stirring for 25 min to get a clear red solution ([Ti] = 0.4 M). Then Sn tin (IV) chloride (SnCl4, Sigma Aldrich) was added to TiOSO4 solution at different weight ratios of Sn and Ti: 1%, 3%, 5%, 7% and 10%. The prepared solution was placed in a PTFE lined autoclave (volume 25 ml) and heated with a rate of 2.5°/min. During the synthesis the temperature was maintained at 200 °C for 6 h with an accuracy of ± 2 K. Then the prepared TiO2 nanoparticles were washed six times in distilled water and two times in ethanol. The obtained powder by centrifugation is then was dried overnight in the oven. Finally, the dried nanoparticles were annealed in air at 500 °C for 30 min (heating rate is 5 °C/min). All the chemicals are of analytical grade and used without further purification. Water used in all the experiments was purified by Milli Q System (Millipore, electric resistivity 18.2 MΩ cm). 2.2. Characterization
3. Results and discussion The morphological investigations of the prepared films were achieved with a high-resolution Ultra 55 Zeiss FEG scanning electron
After the aggregates synthesis a white powder is obtained, which is 3
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Fig. 5. FEGSEM images of Sn doped TiO2 aggregates at synthesis temperatures of 100 °C and indicated Sn doping content.
powder crystalline phase changes under the synthesis conditions. For a synthesis temperature of 100 °C and lower, as the Sn doping content increases, the peak corresponding to rutile phase appears, whereas that of anatase phase disappear; which suggests that Sn was incorporated in the crystalline structure of rutile TiO2 powder. Different authors reported that the addition of Sn promotes the transformation of anatase to rutile phase [39,40]. This is due to fact that the phase of SnO2-cassiterite is similar to that of rutile phase [41]. Furthermore, to induce such a phase transition using annealing process and used synthesis conditions, the temperature must be higher than 700° as it is shown in our previous work [42]. However, for synthesis temperature higher than 100 °C, the anatase remains the main phase whatever the Sn dopant content. Additionally, for synthesis temperatures higher than 150 °C, the comparison of XRD patterns reveals the appearance of a new peaks at roughly 2theta = 27.34, which correspond to (110) crystallographic plane of SnO2- cassiterite (JCPDS Card No 41-1445), a signature of solid
characterized with different techniques. The crystalline phase and structure of prepared powder at different temperatures (80, 100, 150, 180, 190, 200 and 220 °C) and Sn doping contents (wt%, wt%, wt%, wt % and wt10%), were investigated by X-ray diffraction method. The obtained XRD patterns for some temperatures are presented in Fig. 1, and it has revealed a crystalline structure with all the peaks assigned to TiO2 anatase phase (JCPDS No. 89-4921) or TiO2 rutile phase (ICDD No. 01-087-0710), which is a signature of material with high purity. In addition, the average crystallite sizes were calculated using Scherer analysis of the full width at half-maximum of the intense peak corresponding to (101) crystallographic plane [38]. For the synthesis temperatures mentioned above, calculated TiO2 nanoparticles sizes are found to increase from 9.8 to 30.4 nm with the synthesis temperature enhancement. From Fig. 1 it can be observed that the crystalline phase of prepared powder depends on the synthesis temperature and the content of Sn doping. A comparison of obtained XRD patterns shows the
4
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Fig. 6. FEGSEM images of Sn doped TiO2 aggregates at synthesis temperatures of 200 °C and indicated Sn doping content.
solution formation of Ti4+ and Sn4+. Furthermore, it can be seen that the appearance onset of this peak depends on the temperature and amount of Sn doping (Fig. 1). A closer analysis of XRD patterns on Fig. 2 and particularly the peaks corresponding to crystallographic plane (110) of SnO2-cassiterite and (101) of TiO2 shows a slight shift to a larger angle when the Sn doping content increases. This could be attributed to the substitution of Ti4+ by Sn4+ in the TiO2 crystal lattice, and the fact that the ionic radii of Ti4+ (0.605 A) is slightly smaller than that of Sn4+ (0.690 A), which induces a formation of more compact crystal structure and a decrease of the corresponding lattice parameters [43]. The chemical composition of prepared powder is analyzed by using XPS and the obtained spectra are depicted in Fig. 3. It can be seen from XPS survey in Fig. 3(a) that the prepared powders show the presence of intense peaks corresponding to O1s, Ti2p and Sn3d core levels. The high resolution spectra of Sn3d core levels show two peaks located at 496,6 eV and 488,4 eV corresponding respectively to Sn3d3/2 and
Sn3d5/2. In addition, the high resolution XPS spectra in Fig. 3(b) and Fig. 4 show one peak located at 529.2 eV assigned to O1s core level, which position is shifted toward high binding energy with the addition of Sn doping. Additionally, the two peaks located at 464.1 eV and 458.4 eV assigned respectively to Ti2p1/2 and Ti2p3/2 core levels show slight shift toward high binding energy with the addition of Sn doping. Different shifts observed from XPS spectra can be attributed to chemical environment changes of Ti and O when adding Sn doping, which confirm the formation of Ti-O-Sn bonding in the Sn doping TiO2 through the substitution of Ti by Sn. From the XPS results corresponding to synthesis temperature of 150 °C (Fig. 4), it can be noted that the atomic ratio of Ti and Sn for prepared powder corresponds to a solid solution of Ti0.92 Sn0.08 O2, Ti0.85 Sn0.15 O2, Ti0.89 Sn0.11 O2 and Ti0.89 Sn0.11 O2 for respectively Sn doping content of wt%Sn, wt%Sn, wt%, wt10%Sn. This confirms that Sn doping content increases with the increasing of added amount of Sn to reach a limit from which the incorporated Sn in TiO2 lattice structure 5
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Fig. 7. Plots of specific surface area determined from BET experiments and average crystallite size determined from XRD experiments versus the Sn doping amount by considering the weight of added Sn for the synthesis for indicate temperature.
content. For lower temperature than 100 °C, nanoparticles of more or less spherical shapes are obtained and their size and shape are not affected by the Sn doping content (Fig. 5). At higher temperature than 200 °C, the size and shape of nanoparticles building units depend strongly on the Sn doping content (Fig. 6). It can be observed that the shape of nanoparticles is of more or less spherical shape, and it changes to large hexagonal plat at high Sn doping content (10% Sn). From these results it can be concluded that the temperature favors the
remains constant whatever the added amount of Sn. Furthermore the discrepancy between the atomic ratio of Ti and Sn with the ratio of the respective added weight is ascribed mainly to their loss during the synthesis process and to the synthesis temperature. Furthermore, the morphology of prepared Sn doped TiO2 aggregates is characterized by the FEGSEM experiments and it is depicted in Figs. 5 and 6. It can be observed that the aggregates are made of nanoparticles, which size and shape depend on the temperature and Sn doping 6
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Fig. 8. Schematics illustration of particles assembly compactness depending on their morphology (a) spherical particles (b) polyhydric faceted particles.
(b) 3.5
nanoparticles size increase and the morphology change. The specific surface area and the average pore size of prepared Sn doped TiO2 aggregates are subsequently estimated using the Brunauer, Emmett and Teller method, through the nitrogen adsorption/desorption isotherms. From hysteresis loop of the nitrogen isotherm curves, the corresponding BET specific surface areas are calculated to be 93.78 m2 g−1, 83.84 m2 g−1, 70.05 m2 g−1, 18.54 m2 g−1 and 41 m2 g−1 for respectively the Sn doping content of about 0%, 1%, 3%, 5% and 7% for the synthesis temperature of 100 °C (Fig. 7c and d). Furthermore, based on the Barrett-Joyner-Halenda (BJH) method and the desorption branch of the nitrogen isotherms, the average calculated pore-size is about 9.8 nm, 10.1 nm, 11.6 nm, 11.1 nm and 11.4 nm for respectively the same mentioned Sn doping contents and synthesis temperature (Fig. 7d). Similar results are obtained for Sn doped TiO2 aggregates powder prepared at temperatures of 150 °C and higher (Fig. 7e and f). The BET and BJH characteristics of porous Sn doped TiO2 aggregates are in opposition to the results obtained for TiO2 powder with 0% Sn doping content prepared at different temperatures. This is due to the fact that, when the TiO2 nanoparticles size increases, the pore size of TiO2 aggregates increases too, and the specific surface area decreases (Fig. 7a and b). When the Sn doping content increases, TiO2 nanoparticles size increases and their morphology changes from more or less spherical nanoparticles to hexagonal plat nanoparticles (Fig. 6). The sticking of these plate nanoparticles reduces drastically the specific surface area and the pore size. In fact, the assembly process of nanoparticles without any energy barriers and within a certain facet is strongly dependent on the area and frequency of this facet. Furthermore, particles follow different mechanisms to assemble, based mainly on the properties of their size and morphology [44]. These mechanisms induce different compactnesses of nanoparticles assembly as it is illustrated in the schematics of Fig. 8(a) and (b). It can be observed that the more the particles is faceted, the more the structure of their assembly is compact with less structure void. These results are opposite to those reported recently [45] on the Sn doped TiO2 particles prepared by the sol gel technique. Those authors reported that the specific surface area and the TiO2 nanoparticles size increase with the added Sn doping amount. The comparison of these results point out clearly that the effect of added Sn doping depends strongly on the synthesis technique. To study the optical properties of prepared Sn doped TiO2 aggregates, a qualitative evaluation of the band gap was performed with the Tauc plot [46] (Fig. 9a) for different synthesis conditions of TiO2 powder. The optical band gap of TiO2 film can be determined from the
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Sn (mol%) Fig. 9. (a) The (αhν)2 versus photon energy curve of representative pure TiO2, 1%Sn/ TiO2, 3%Sn/TiO2, 5%Sn/TiO2, 7%Sn/TiO2 and, 10%Sn/TiO2 samples hydrothermal at 150 °C. (b) Plot of band gap energy obtained from Tauc plot versus the Sn doping content of Sn doped TiO2 aggregates prepared at different synthesis temperatures as indicated.
sharply falling transmission region. According to Tauc equation below [46], the absorption coefficient has the following energy dependence: 1
α=
B (hν − Eg) r hν
where B is a constant which does not depend on hυ, and for most r−1 semiconductors it is in the range of 105–106 cm−1 eV r [47]; r is the power coefficient whose value indicates the type of predominant electronic transitions, it takes the following values of 1/2, 2, 3/2 or 3 respectively for allowed direct, allowed indirect, forbidden direct and forbidden indirect electronic transitions [48,49]. The best fit of (αhυ)r versus hυ gives the value of r = 1/2 in the case of the present Sn doped TiO2 aggregates. The extrapolation of the linear part of the Tauc curve as shown in Fig. 9a allows the determination of the band gap energy. In Fig. 9b, the obtained band gap energy is plotted for prepared powders at different conditions of temperature and Sn doping content. It can be observed that the behavior of the band gap energy versus the Sn doping content depends on the synthesis temperature (Fig. 9b). 7
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Gomi, TixSn1−xO3 solid solution as an anode material in lithium-ion batteries, Electrochim. Acta 72 (2012) 186–191. [32] I. Issac, M. Scheuermann, S.M. Becker, E.G. Bardaji, C. Adelhelm, D. Wang, C. Kubel, S. Indris, Nanocrystalline Ti2/3Sn1/3O2 as anode material for Li-ion batteries, J. Power Sources 196 (2011) 9689. [33] X. Li, R.C. Xiong, G. Wei, Preparation and photocatalytic activity of nanoglued Sndoped TiO2, J. Hazard. Mater. 164 (2009) 241–246. [34] S. Mahanty, S. Roy, S. Sen, Effect of Sn doping on the structural and optical properties of sol-gel TiO2 thin films, J. Cryst. Growth 261 (2004) 77–81. [35] I. Rangel-Vazquez, G. Del Angel, V. Bertin, F. Gonzalez, A. Vazquez-Zavala, A. Arrieta, J.M. Padilla, A. Barrera, E. Ramos-Ramirez, Synthesis and characterization of Sn doped TiO2 photocatalytic effect of Sn concentration on the textural properties and on the photocatalytic degradation of 2, 4-dicholophenoxycetic acid, J. Alloy. Compd. 643 (2015) S144–S149. [36] Y. Ks, Synthesis of TiO2 Materials Using Ionic Liquids and Its Applications for Sustainable Energy and Environment, J. Nanosci. Nanotechnol. 16 (2016) 4302–4309. [37] Y. Cao, W. yang, W. Zhang, G. Liu, P. Yue, Improved photocatalytic activity of Sn4+ doped TiO2 nanoparticulate films prepared by plasma enhanced chemical vapor deposition, New. J. Chem. 28 (2004) 218–222. [38] H.P. Klugm, L.E. Alexander, X-ray Diffraction Procedures, Wiley Interscience Publication, New York, 1974. [39] F. Fresno, J.M. Coronado, D. Tudela, J. Soria, Influence of the structural characteristics of Ti1−xSnxO2 nanoparticles on their photocatalytic activity for elimination of methylcyclohexanevapors, Appl. Catal. B Environ. 55 (2005) 159–167. [40] K.N.P. Kumar, D.J. Fray, J. Nair, F. Mizukami, T. Okubo, enhancement anatase to rutile phase transformation without exaggerated particle growth in nanostructured titania-tin oxide comosites, Scr. Mater. 57 (2007) 771–774. [41] K.N.P. Kumar, D.J. Fray, J. Nair, F. Mizukami, T. Okubo, enhancement anatase to rutile phase transformation without exaggerated particle growth in nanostructured titania-tin oxide comosites, Scr. Mater. 57 (2007) 771–774. [42] A. Taleb, F. Mesguish, Th Onfroy, X. Yanpeng, Design of TiO2/Au Nanoparticle films with controlled crack formation and different architectures using centrifugal
At temperatures of 100 °C and lower, the addition of Sn doping at a lower amount of 1% Sn induces the reduction of the band gap energy and the phase transition from anatase to rutile but after, the band gap energy remains constant for higher Sn doping content (Fig. 9b). For temperature of 150 °C and higher, the band gap energy depends on the effective Sn doping content and the obtained results are in accordance with those of XPS and XRD, which confirms the incorporation of Sn doping in TiO2 structure lattice at certain threshold of Sn doping added weight. From Fig. 9b, it can be observed than this threshold depends on the synthesis temperature as it is confirmed by XRD experiment in terms of the appearance of the peak corresponding to crystallographic plane (110) of SnO2-cassiterite (Fig. 1) and by XPS in terms of the shift of O1s core level toward high binding energy (Fig. 4). 4. Conclusion A large scale production, of Sn doped TiO2 aggregates was achieved using the hydrothermal method. Furthermore, it was demonstrated that the prepared Sn doped TiO2 aggregates are made of TiO2 nanoparticles as a building unit. In addition, it has been demonstrated that TiO2 nanoparticles size and shape depend strongly on the synthesis temperature and the Sn doping content. It was illustrated that the addition of Sn doping induces strongly the reduction of specific surface area as a consequence of TiO2 nanoparticles size increasing and shape change. At low synthesis temperature of 80 °C, it was demonstrated that the addition of Sn doping promotes the phase transition of TiO2 from anatase to rutile. At high temperatures it was shown that the Sn doping addition induces the TiO2 nanoparticle shape change from more or less spherical shape to hexagonal plat shape. Furthermore, the effective Sn doping incorporated in TiO2 structure lattice, was shown to depend on the synthesis temperature and the weight of added Sn doping amount. From Tauc curves, it was shown that a significant reduction of the TiO2 band gap energy takes effect from a certain Sn doping amount threshold which depend on the synthesis temperature. Acknowledgement The authors would like to thank the Thailand Research Fund (TRF) and the Algerian Ministry of High Education and Scientific Research (PNE 451, 2016–2017) for supporting this work. References [1] A. Taleb, F. Mesguich, X. Yanpeng, C. Colbeau-Justin, P. Dubot, Optimized TiO2 nanoparticles packing for photovoltaic applications, J. Solmat 148 (2016) 52. [2] Y. Cai, H.E. Wang, S.Z. Huang, M.F. Yuen, H.H. Cai, C. Wang, Y. Yu, Y. Li, W.J. Zhang, B.L. Su, Porous TiO2 urchins for high performance Li-ion battery electrode: facile synthesis, characterization and structural evolution, Electrochem. Acta 210 (2016) 206–214. [3] Y.T. He, J. Wan, T. Tokunaga, Kinetic stability hematic nanoparticles the effect of particle size, J. Nanopart. Res. 10 (2007) 321–332. [4] D. Zhou, Z. Ji, X. Jiang, D.R. Dunphy, J. Brinker, A.A. Keller, Influence of material properties on TiO2 Nanoparticle Agglomeration, PLoS ONE 8 (2013) e81239. [5] Q. Zhang, T.P. Chou, B. Russo, S.A. Jenekhe, G. Cao, Polydisperse aggregates of ZnO nanocystallites: a method for energy conversion efficiency enhancement in dye sensitized solar cells, Adv. Funct. Mater. 16 (2008) 1654–1660. [6] J. Xi, Q. Zhang, K. Park, Y. Sun, G. Cao, Enhanced power conversion efficiency in dye sensitized solar cells with TiO2 aggregates/nanocrystallites mixed photoelectrodes, Electrochim. Acta 56 (2011) 1960–1966. [7] S. Mukhopadhyay, D. Maiti, S. Chatterjee, S. Devi, G. Suresh Kumar, Design and application of Au decorated ZnO/TiO2 as a stable photocatalyst for wide spectral coverage, Phys. Chem. Chem. Phys. 18 (2016) 31622–31633. [8] V. Dhas, S. Muduli, S. Agarkar, A. Rana, B. Hannoyer, R. Banerjee, S. Ogale, Enhanced DSSC performance with high surface area thin anatase TiO2 nanoleaves, Sol. Energy 85 (2011) 1213–1219. [9] O. Carp, C.I. huisman, A. rerller, photoinduced reactivity of titanium dioxide, Progress. Solid State Chem. 32 (2004) 33–177. [10] M. Durr, S. Rosselli, A. Yasuda, G. Nelles, band-gap engineering of metal oxides for dye sensitized solar cells, J. Phys. Chem. B 110 (2006) 21899–21902. [11] C. Petit, A. Taleb, M.P. Pileni, 3D self-organization of cobalt nanoparticles: magnetic properties, J. Phys. Chem. B 103 (1999) 1805. [12] A. Taleb, V. Russier, A. Courty, M.P. Pileni, Collective optical properties of silver
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S. Mehraz et al. strategy, RSC Adv. 5 (2015) 7007. [43] S. Li, M. Ling, J. Qiu, J. Han, S. Zhang, Anchoring ultra-fine TiO2-SnO2 solid solution particles onto graphene by one pot ball milling for long life lithium ion batteries, J. Mater. Chem. A 3 (2015) 9700–9706. [44] Q. Zhou, B. Wang, P. Wang, C. Dellago, Y. Wang, Y. Fang, Nanoparticle-based crystal growth via multistep self-assembly, Cryst. Eng. Comm. 15 (2013) 5114. [45] I. Rangel-Vazquez, G. Del Angel, V. Bertin, F. Bertin, F. Gonzalez, A. VazquezZavala, A. Arrieta, J.M. Padilla, A. Barrera, E. Ramos-Ramirez, Synthesis and characterization of Sn doped TiO2 photocatalysts: effect of Sn concentration on the
[46] [47] [48] [49]
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textural properties and on the photocatalytic degradation of 2,4-dichlorophenoxyacetic acid, J. Alloy. Compd. 643 (2015) S144–S149. J. Domaradzki, Structural, optical and electrical properties of transparent V and Pddoped TiO2 thin films prepared by sputtering, Thin Solid Films 497 (2006) 243. N.F. Mott, E.A. Davis, Electronic Processes in Non-crystalline Materials, second ed., Clarendon Press, Oxford, 1979. J. Tauc, A. Menth, J. Non-Cryst. Solids 8–10 (1972) 569. J. Kane, H.P. Schweizer, Chemical vapor deposition of transparent electrically conducting layers of indium oxide doped with tin, Thin Solid Films 29 (1975) 155.
Contents lists available at ScienceDirect
Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
Large scale and facile synthesis of Sn doped TiO2 aggregates using hydrothermal synthesis ⁎
Saida Mehraza,b, Peerawas Konsongc, Abdelhafed Talebd,e, , Nahed Dokhanea, Lek Sikongc a
Unité de Recherche Matériaux, Procédés et Environnement, M’hamedBougara de Boumedès University, 35000 Boumerdés, Algeria Research Centre in Industrial Technologies CRTI, BP 64, route de Dely-Ibrahim, Chéraga, Algiers 16033, Algeria c Department of Mining and Materials Engineering, Faculty of Engineering, Prince of Songkla University Hat yai, Songkhla 90112, Thailand d PSL Research University, Chimie ParisTech - CNRS, Institut de Recherche de Chimie Paris, 75005 Paris, France e Université Pierre et Marie Curie, 4 place Jussieu, 75231 Paris, France b
A R T I C L E I N F O
A B S T R A C T
Keywords: Hydrothermal Sn doping TiO2 Nanoparticles assembly Optical properties
Sn doped TiO2 aggregates have been successfully prepared via one pot hydrothermal technique. Different methods were used to characterize prepared Sn doped TiO2 aggregates such as DRX, XPS, N2 adsorption (BET), FEGSEM and UV–Vis spectroscopy. It was illustrated that the size, the morphology and the phase of prepared TiO2 aggregates is strongly influenced by the amount of added Sn doping. In addition, it was demonstrated that the prepared aggregates properties is influenced by the synthesis temperature. Furthermore, it was shown that the prepared Sn doped TiO2 aggregates are of high crystallinity. The influence of added Sn dopant amount on the optical and structural properties of the prepared Sn doped TiO2 aggregates have been investigated.
1. Introduction In material science, recent progress is mainly devoted to develop innovative strategies to prepare nanomaterials with desired properties, which are a coupling of both its intrinsic and extrinsic properties. Among the strategies reported for extrinsic material properties control, nanoparticles agglomeration appears to be a promising approach to obtain materials with controlled architectures and desired properties for targeted applications. In fact, combining a mixture of nanoparticles with different physical and chemical properties offers a large number of possibilities to tailor the properties of agglomerated materials. In addition to the properties of individual nanoparticles, their assembly gives rise to collective properties due to nanoparticles interaction. Compared to bulk material, this new configuration is more flexible for material preparation and extrinsic properties control. For example, it allows the combination of different properties even conflicting ones, such as large submicrometer-sized particles for light harvesting and large surface area at nanoscale for dye loading [1]. For lithium ion batteries, it alleviates strongly the electrodes stress induced by their volume variation during the insertion and desertion of lithium ion [2]. The agglomeration of nanoparticles is a consequence of nanoparticles destabilization and it depends both on the nanoparticles properties (size, shape, surface modification, concentration) and those of their embedding media. However, a combination of both of these
⁎
properties is required to be considered for the prediction of nanoparticles agglomeration [3,4]. Different agglomerated metallic and semiconductor nanoparticles were reported in the literature and it has shown an accurate control of the pore size and specific surface area [5,6]. However, most of the reported nanoparticles aggregates suffer from the lack of size and morphology control. Furthermore, to control the intrinsic material properties, different strategies were reported in the literature such as quantum dots and metallic nanoparticles decoration [7], dyes loading [8], doping with different elements [9,10], nanoparticles assembly [11,12] etc … Among these strategies, the doping approach with metal ions has been proven to be an effective and promising way to control the intrinsic properties of semi-conductor to be used as performing anode materials. Recently, tremendous efforts have been devoted to develop doped materials with optimized properties for different applications. Among the studies semiconductor materials, TiO2 nanostructures have received particular attention due to their use as anode materials for potential applications in different areas such as photovoltaic [13], photo catalysis [14], gas sensing [15], photo-splitting of water [16] and batteries [17]. However, TiO2 nanostructures suffer from some limitations, due to its large band gap which is around 3.2 eV and low electron transfer rate which impedes its application. As a consequence of its large band gap, it is sensitive only to UV light excitations, which reduce strongly the efficiency of solar cell based on TiO2 nanostructures. In addition to the
Corresponding author at: PSL Research University, Chimie ParisTech - CNRS, Institut de Recherche de Chimie Paris, 75005 Paris, France. E-mail address: [email protected] (A. Taleb).
http://dx.doi.org/10.1016/j.solmat.2017.06.048 Received 31 March 2017; Received in revised form 16 June 2017; Accepted 19 June 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Mehraz, S., Solar Energy Materials and Solar Cells (2017), http://dx.doi.org/10.1016/j.solmat.2017.06.048
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Fig. 3. XPS spectra of Sn doped TiO2 aggregates at synthesis temperatures of 150 °C and different Sn doping contents as indicated (a) XPS survey (b) high resolution of Sn 3d core level.
nanostructure is extended to the visible light spectrum, which improves its photocatalytic activity [22] and solar conversion efficiency [23]. Additionally, it is well established that a dopant could improve the electron transfer rate, which limits the charges recombination process [24,25]. For lithium ion batteries application, dopant is expected to keep high capacity and improved cycling performance [26]. Among all the metal ions used for TiO2 doping, Sn is considered as the most effective, thanks to the fact that the ionic radii of Ti4+ (0.605 A) and Sn4+ (0.690 A) are roughly similar, which favors the substitution of Ti4+ by Sn4+ in the TiO2 crystal lattice to form a stable solid solution [27]. Different amounts of incorporated Sn dopant in TiO2 lattice were reported in the literature ranging from 1% to roughly 5 mol% [28,29]. This value depends both on the precursor and the synthesis method [30]. Various hybrid structures of SnO2 and TiO2 were studied in literature such as TixSn1−xO3 solid solution [31], Ti2/ 3Sn1/3O2 [32] etc … Furthermore, several methods were used to prepare Sn doped TiO2 such as sol-gel method [33–35], hydrothermal [36] and chemical vapor deposition [37]. Among the methods reported in the literature, the hydrothermal method presents many advantages such as the ability to produce nanoparticles with narrow size distribution, dispersed in polar or nonpolar solvents. In the present work, Sn doped TiO2 aggregates made by TiO2 nanoparticles as a building unit have been successfully prepared hydrothermally. It was shown that the size of prepared TiO2 aggregates can be controlled from few tens nanometers to few hundred nanometers depending on the synthesis temperature. Depending on the synthesis temperature Sn dopant promotes TiO2 phase and morphology transformation. Additionally, it has been shown that the increasing of added Sn dopant increases the size of TiO2 nanoparticles building unit of prepared aggregates and reduces significantly its band gap energy at certain Sn dopant amount threshold.
Fig. 1. XRD pattern of Sn doped TiO2 aggregates prepared at different synthesis temperatures and Sn doping as indicated.
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lithium ion batteries application, the low theoretical specific capacity of TiO2 impedes its extensive use [18]. To improve the efficiency of TiO2 nanostructure as anode materials for different applications, the doping approach by different metal ions such as Al 3+, Cr 3+, Fe 3+, Co 3+ etc … was used [19,20]. In most cases, the dopant induces the modification of TiO2 electronic structure through the introduction of band-gap states [21], which reduces the band gap energy. As a consequence, the absorption of TiO2 2
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microscope (FEGSEM) operating at an acceleration voltage of 10 kV. The film thicknesses were measured with a Dektak 6 M stylus profiler. The crystalline structure of TiO2 was determined by an X-ray diffractometer (Siemens D5000 XRD unit) in 2θ range from 20° to 80° by 0.07° s−1 increasing steps operating at 40 kV accelerating voltage and 40 mA current using Cu Kα radiation source with λ = 1.5406 Å. The chemical compositions of all the samples were determined by Xray photoelectron spectroscopy (XPS) and for the measurements we used a RIBER MACII analyzer system equipped with a AL Kα X-ray source (hυ = 1486.6 eV; spot size 20 mm, power = 11 kVx50 w). The optical measurements were performed using a Cary 50 (Varian) UV–Vis–NIR spectrophotometer equipped with unpolarized light and operating at normal incidence in the wavelength range from 300 nm to 850 nm. Nitrogen adsorption - desorption isotherms were measured at liquid nitrogen temperature on a Micromeritics ASAP 2020 apparatus. Before analysis, all the samples were degassed at 120 °C for 10 h. The specific surface area (SBET) was evaluated using the Brunauer-Emmett-Teller (BET) method in the P/P° range of 0.05–0.25. The pore size distribution was determined from the desorption branch of the isotherm using the Barret-Joyner-Halender (BJH) method. The total pore volume was determined from the amount of N2 adsorbed up to P/P° = 0.98.
2.1. Synthesis of Sn doped TiO2 aggregates The synthesis of Sn doped TiO2 aggregates has been performed using a hydrothermal method. The titanium (IV) oxysulfate hydrate (TiOSO4, Sigma Aldrich) precursor was prepared in aqueous solution by dissolving 6.4 g (2.5 M) of TiOSO4 in 16 ml of distilled water under constant stirring for 25 min to get a clear red solution ([Ti] = 0.4 M). Then Sn tin (IV) chloride (SnCl4, Sigma Aldrich) was added to TiOSO4 solution at different weight ratios of Sn and Ti: 1%, 3%, 5%, 7% and 10%. The prepared solution was placed in a PTFE lined autoclave (volume 25 ml) and heated with a rate of 2.5°/min. During the synthesis the temperature was maintained at 200 °C for 6 h with an accuracy of ± 2 K. Then the prepared TiO2 nanoparticles were washed six times in distilled water and two times in ethanol. The obtained powder by centrifugation is then was dried overnight in the oven. Finally, the dried nanoparticles were annealed in air at 500 °C for 30 min (heating rate is 5 °C/min). All the chemicals are of analytical grade and used without further purification. Water used in all the experiments was purified by Milli Q System (Millipore, electric resistivity 18.2 MΩ cm). 2.2. Characterization
3. Results and discussion The morphological investigations of the prepared films were achieved with a high-resolution Ultra 55 Zeiss FEG scanning electron
After the aggregates synthesis a white powder is obtained, which is 3
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Fig. 5. FEGSEM images of Sn doped TiO2 aggregates at synthesis temperatures of 100 °C and indicated Sn doping content.
powder crystalline phase changes under the synthesis conditions. For a synthesis temperature of 100 °C and lower, as the Sn doping content increases, the peak corresponding to rutile phase appears, whereas that of anatase phase disappear; which suggests that Sn was incorporated in the crystalline structure of rutile TiO2 powder. Different authors reported that the addition of Sn promotes the transformation of anatase to rutile phase [39,40]. This is due to fact that the phase of SnO2-cassiterite is similar to that of rutile phase [41]. Furthermore, to induce such a phase transition using annealing process and used synthesis conditions, the temperature must be higher than 700° as it is shown in our previous work [42]. However, for synthesis temperature higher than 100 °C, the anatase remains the main phase whatever the Sn dopant content. Additionally, for synthesis temperatures higher than 150 °C, the comparison of XRD patterns reveals the appearance of a new peaks at roughly 2theta = 27.34, which correspond to (110) crystallographic plane of SnO2- cassiterite (JCPDS Card No 41-1445), a signature of solid
characterized with different techniques. The crystalline phase and structure of prepared powder at different temperatures (80, 100, 150, 180, 190, 200 and 220 °C) and Sn doping contents (wt%, wt%, wt%, wt % and wt10%), were investigated by X-ray diffraction method. The obtained XRD patterns for some temperatures are presented in Fig. 1, and it has revealed a crystalline structure with all the peaks assigned to TiO2 anatase phase (JCPDS No. 89-4921) or TiO2 rutile phase (ICDD No. 01-087-0710), which is a signature of material with high purity. In addition, the average crystallite sizes were calculated using Scherer analysis of the full width at half-maximum of the intense peak corresponding to (101) crystallographic plane [38]. For the synthesis temperatures mentioned above, calculated TiO2 nanoparticles sizes are found to increase from 9.8 to 30.4 nm with the synthesis temperature enhancement. From Fig. 1 it can be observed that the crystalline phase of prepared powder depends on the synthesis temperature and the content of Sn doping. A comparison of obtained XRD patterns shows the
4
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Fig. 6. FEGSEM images of Sn doped TiO2 aggregates at synthesis temperatures of 200 °C and indicated Sn doping content.
solution formation of Ti4+ and Sn4+. Furthermore, it can be seen that the appearance onset of this peak depends on the temperature and amount of Sn doping (Fig. 1). A closer analysis of XRD patterns on Fig. 2 and particularly the peaks corresponding to crystallographic plane (110) of SnO2-cassiterite and (101) of TiO2 shows a slight shift to a larger angle when the Sn doping content increases. This could be attributed to the substitution of Ti4+ by Sn4+ in the TiO2 crystal lattice, and the fact that the ionic radii of Ti4+ (0.605 A) is slightly smaller than that of Sn4+ (0.690 A), which induces a formation of more compact crystal structure and a decrease of the corresponding lattice parameters [43]. The chemical composition of prepared powder is analyzed by using XPS and the obtained spectra are depicted in Fig. 3. It can be seen from XPS survey in Fig. 3(a) that the prepared powders show the presence of intense peaks corresponding to O1s, Ti2p and Sn3d core levels. The high resolution spectra of Sn3d core levels show two peaks located at 496,6 eV and 488,4 eV corresponding respectively to Sn3d3/2 and
Sn3d5/2. In addition, the high resolution XPS spectra in Fig. 3(b) and Fig. 4 show one peak located at 529.2 eV assigned to O1s core level, which position is shifted toward high binding energy with the addition of Sn doping. Additionally, the two peaks located at 464.1 eV and 458.4 eV assigned respectively to Ti2p1/2 and Ti2p3/2 core levels show slight shift toward high binding energy with the addition of Sn doping. Different shifts observed from XPS spectra can be attributed to chemical environment changes of Ti and O when adding Sn doping, which confirm the formation of Ti-O-Sn bonding in the Sn doping TiO2 through the substitution of Ti by Sn. From the XPS results corresponding to synthesis temperature of 150 °C (Fig. 4), it can be noted that the atomic ratio of Ti and Sn for prepared powder corresponds to a solid solution of Ti0.92 Sn0.08 O2, Ti0.85 Sn0.15 O2, Ti0.89 Sn0.11 O2 and Ti0.89 Sn0.11 O2 for respectively Sn doping content of wt%Sn, wt%Sn, wt%, wt10%Sn. This confirms that Sn doping content increases with the increasing of added amount of Sn to reach a limit from which the incorporated Sn in TiO2 lattice structure 5
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22 20 18 16 14 12 10 8 6 4
specific surface area BET (m^2 g^-1) Mean pore diameter (nm)
90
Crystallite size (nm)
specific surface area BET (m 2 g -1)
specific surface area BET (m 2 g -1 ) specific surface area BET (m 2 g -1)
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Sn content (%)
Sn content (%)
Fig. 7. Plots of specific surface area determined from BET experiments and average crystallite size determined from XRD experiments versus the Sn doping amount by considering the weight of added Sn for the synthesis for indicate temperature.
content. For lower temperature than 100 °C, nanoparticles of more or less spherical shapes are obtained and their size and shape are not affected by the Sn doping content (Fig. 5). At higher temperature than 200 °C, the size and shape of nanoparticles building units depend strongly on the Sn doping content (Fig. 6). It can be observed that the shape of nanoparticles is of more or less spherical shape, and it changes to large hexagonal plat at high Sn doping content (10% Sn). From these results it can be concluded that the temperature favors the
remains constant whatever the added amount of Sn. Furthermore the discrepancy between the atomic ratio of Ti and Sn with the ratio of the respective added weight is ascribed mainly to their loss during the synthesis process and to the synthesis temperature. Furthermore, the morphology of prepared Sn doped TiO2 aggregates is characterized by the FEGSEM experiments and it is depicted in Figs. 5 and 6. It can be observed that the aggregates are made of nanoparticles, which size and shape depend on the temperature and Sn doping 6
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(a) 200
TiO2 1Sn/TiO2
(αhν)2(eV/cm2)
160
3Sn/TiO2 5Sn/TiO2
120
7Sn/TiO2 10Sn/TiO2
80
40
0 2,6 2.6
2,8 2.8
3,0 3.0
3,2 3.2
3,4 3.4
3,6 3.6
3,8 3.8
4,0
Band gap energy (eV)
Fig. 8. Schematics illustration of particles assembly compactness depending on their morphology (a) spherical particles (b) polyhydric faceted particles.
(b) 3.5
nanoparticles size increase and the morphology change. The specific surface area and the average pore size of prepared Sn doped TiO2 aggregates are subsequently estimated using the Brunauer, Emmett and Teller method, through the nitrogen adsorption/desorption isotherms. From hysteresis loop of the nitrogen isotherm curves, the corresponding BET specific surface areas are calculated to be 93.78 m2 g−1, 83.84 m2 g−1, 70.05 m2 g−1, 18.54 m2 g−1 and 41 m2 g−1 for respectively the Sn doping content of about 0%, 1%, 3%, 5% and 7% for the synthesis temperature of 100 °C (Fig. 7c and d). Furthermore, based on the Barrett-Joyner-Halenda (BJH) method and the desorption branch of the nitrogen isotherms, the average calculated pore-size is about 9.8 nm, 10.1 nm, 11.6 nm, 11.1 nm and 11.4 nm for respectively the same mentioned Sn doping contents and synthesis temperature (Fig. 7d). Similar results are obtained for Sn doped TiO2 aggregates powder prepared at temperatures of 150 °C and higher (Fig. 7e and f). The BET and BJH characteristics of porous Sn doped TiO2 aggregates are in opposition to the results obtained for TiO2 powder with 0% Sn doping content prepared at different temperatures. This is due to the fact that, when the TiO2 nanoparticles size increases, the pore size of TiO2 aggregates increases too, and the specific surface area decreases (Fig. 7a and b). When the Sn doping content increases, TiO2 nanoparticles size increases and their morphology changes from more or less spherical nanoparticles to hexagonal plat nanoparticles (Fig. 6). The sticking of these plate nanoparticles reduces drastically the specific surface area and the pore size. In fact, the assembly process of nanoparticles without any energy barriers and within a certain facet is strongly dependent on the area and frequency of this facet. Furthermore, particles follow different mechanisms to assemble, based mainly on the properties of their size and morphology [44]. These mechanisms induce different compactnesses of nanoparticles assembly as it is illustrated in the schematics of Fig. 8(a) and (b). It can be observed that the more the particles is faceted, the more the structure of their assembly is compact with less structure void. These results are opposite to those reported recently [45] on the Sn doped TiO2 particles prepared by the sol gel technique. Those authors reported that the specific surface area and the TiO2 nanoparticles size increase with the added Sn doping amount. The comparison of these results point out clearly that the effect of added Sn doping depends strongly on the synthesis technique. To study the optical properties of prepared Sn doped TiO2 aggregates, a qualitative evaluation of the band gap was performed with the Tauc plot [46] (Fig. 9a) for different synthesis conditions of TiO2 powder. The optical band gap of TiO2 film can be determined from the
80°C 100°C
3.4
150°C
Eg (eV)
200°C
3.3
220°C
3.2
3.1
3.0
0
2
4
6
8
10
Sn (mol%) Fig. 9. (a) The (αhν)2 versus photon energy curve of representative pure TiO2, 1%Sn/ TiO2, 3%Sn/TiO2, 5%Sn/TiO2, 7%Sn/TiO2 and, 10%Sn/TiO2 samples hydrothermal at 150 °C. (b) Plot of band gap energy obtained from Tauc plot versus the Sn doping content of Sn doped TiO2 aggregates prepared at different synthesis temperatures as indicated.
sharply falling transmission region. According to Tauc equation below [46], the absorption coefficient has the following energy dependence: 1
α=
B (hν − Eg) r hν
where B is a constant which does not depend on hυ, and for most r−1 semiconductors it is in the range of 105–106 cm−1 eV r [47]; r is the power coefficient whose value indicates the type of predominant electronic transitions, it takes the following values of 1/2, 2, 3/2 or 3 respectively for allowed direct, allowed indirect, forbidden direct and forbidden indirect electronic transitions [48,49]. The best fit of (αhυ)r versus hυ gives the value of r = 1/2 in the case of the present Sn doped TiO2 aggregates. The extrapolation of the linear part of the Tauc curve as shown in Fig. 9a allows the determination of the band gap energy. In Fig. 9b, the obtained band gap energy is plotted for prepared powders at different conditions of temperature and Sn doping content. It can be observed that the behavior of the band gap energy versus the Sn doping content depends on the synthesis temperature (Fig. 9b). 7
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At temperatures of 100 °C and lower, the addition of Sn doping at a lower amount of 1% Sn induces the reduction of the band gap energy and the phase transition from anatase to rutile but after, the band gap energy remains constant for higher Sn doping content (Fig. 9b). For temperature of 150 °C and higher, the band gap energy depends on the effective Sn doping content and the obtained results are in accordance with those of XPS and XRD, which confirms the incorporation of Sn doping in TiO2 structure lattice at certain threshold of Sn doping added weight. From Fig. 9b, it can be observed than this threshold depends on the synthesis temperature as it is confirmed by XRD experiment in terms of the appearance of the peak corresponding to crystallographic plane (110) of SnO2-cassiterite (Fig. 1) and by XPS in terms of the shift of O1s core level toward high binding energy (Fig. 4). 4. Conclusion A large scale production, of Sn doped TiO2 aggregates was achieved using the hydrothermal method. Furthermore, it was demonstrated that the prepared Sn doped TiO2 aggregates are made of TiO2 nanoparticles as a building unit. In addition, it has been demonstrated that TiO2 nanoparticles size and shape depend strongly on the synthesis temperature and the Sn doping content. It was illustrated that the addition of Sn doping induces strongly the reduction of specific surface area as a consequence of TiO2 nanoparticles size increasing and shape change. At low synthesis temperature of 80 °C, it was demonstrated that the addition of Sn doping promotes the phase transition of TiO2 from anatase to rutile. At high temperatures it was shown that the Sn doping addition induces the TiO2 nanoparticle shape change from more or less spherical shape to hexagonal plat shape. Furthermore, the effective Sn doping incorporated in TiO2 structure lattice, was shown to depend on the synthesis temperature and the weight of added Sn doping amount. From Tauc curves, it was shown that a significant reduction of the TiO2 band gap energy takes effect from a certain Sn doping amount threshold which depend on the synthesis temperature. Acknowledgement The authors would like to thank the Thailand Research Fund (TRF) and the Algerian Ministry of High Education and Scientific Research (PNE 451, 2016–2017) for supporting this work. References [1] A. Taleb, F. Mesguich, X. Yanpeng, C. Colbeau-Justin, P. Dubot, Optimized TiO2 nanoparticles packing for photovoltaic applications, J. Solmat 148 (2016) 52. [2] Y. Cai, H.E. Wang, S.Z. Huang, M.F. Yuen, H.H. Cai, C. Wang, Y. Yu, Y. Li, W.J. Zhang, B.L. Su, Porous TiO2 urchins for high performance Li-ion battery electrode: facile synthesis, characterization and structural evolution, Electrochem. Acta 210 (2016) 206–214. [3] Y.T. He, J. Wan, T. Tokunaga, Kinetic stability hematic nanoparticles the effect of particle size, J. Nanopart. Res. 10 (2007) 321–332. [4] D. Zhou, Z. Ji, X. Jiang, D.R. Dunphy, J. Brinker, A.A. Keller, Influence of material properties on TiO2 Nanoparticle Agglomeration, PLoS ONE 8 (2013) e81239. [5] Q. Zhang, T.P. Chou, B. Russo, S.A. Jenekhe, G. Cao, Polydisperse aggregates of ZnO nanocystallites: a method for energy conversion efficiency enhancement in dye sensitized solar cells, Adv. Funct. Mater. 16 (2008) 1654–1660. [6] J. Xi, Q. Zhang, K. Park, Y. Sun, G. Cao, Enhanced power conversion efficiency in dye sensitized solar cells with TiO2 aggregates/nanocrystallites mixed photoelectrodes, Electrochim. Acta 56 (2011) 1960–1966. [7] S. Mukhopadhyay, D. Maiti, S. Chatterjee, S. Devi, G. Suresh Kumar, Design and application of Au decorated ZnO/TiO2 as a stable photocatalyst for wide spectral coverage, Phys. Chem. Chem. Phys. 18 (2016) 31622–31633. [8] V. Dhas, S. Muduli, S. Agarkar, A. Rana, B. Hannoyer, R. Banerjee, S. Ogale, Enhanced DSSC performance with high surface area thin anatase TiO2 nanoleaves, Sol. Energy 85 (2011) 1213–1219. [9] O. Carp, C.I. huisman, A. rerller, photoinduced reactivity of titanium dioxide, Progress. Solid State Chem. 32 (2004) 33–177. [10] M. Durr, S. Rosselli, A. Yasuda, G. Nelles, band-gap engineering of metal oxides for dye sensitized solar cells, J. Phys. Chem. B 110 (2006) 21899–21902. [11] C. Petit, A. Taleb, M.P. Pileni, 3D self-organization of cobalt nanoparticles: magnetic properties, J. Phys. Chem. B 103 (1999) 1805. [12] A. Taleb, V. Russier, A. Courty, M.P. Pileni, Collective optical properties of silver
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