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Surfactant effect on controllable phase transformation and UV-shielding performance of titanium dioxide
Materials Chemistry and Physics 240 (2020) 122079
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Surfactant effect on controllable phase transformation and UV-shielding performance of titanium dioxide Zuohua Liu *, Liang Wang, Li Li, Facheng Qiu, Xia Xiong, Changyuan Tao, Jun Du School of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 400044, China
H I G H L I G H T S
� TiO2 crystalline structure is tunable by surfactant-assisted method. � The UV shielding performance of the surfactant-aided TiO2 product increased. � Mcrospherical Cationic surfactant promoted anatase-rutile transformation of titania. A R T I C L E I N F O
A B S T R A C T
Keywords: Surfactant Titanium dioxide Rutile Phase control UV-Shielding
The crystalline structure, size and morphology of TiO2 have crucial influence on its physicochemical perfor mance. The crystal growth process includes crystal transformation and morphology evolution, which is closely related to the solution environment. In this work, rutile TiO2 was synthesized via surfactant-assisted method by using titanium sulfate as titanium source. Factor associated with crystalline structure of TiO2 were investigated including surfactant type and concentration. Results showed that when the concentration of cetyl triethylammonium bromide (CTMAB) was greater than or equal to 3 mmol/L, or the concentration of dodecyl triethylammonium bromide (1231) was greater than or equal to 4 mmol/L, surfactants promoted transformation from anatase to rutile titania. The UV-shielding performance of the surfactant-aided titania increased, and the band gap width decreased by 0.2–0.3 eV. More importantly, we found that the cationic surfactant played a decisive role in anatase-rutile transformation of titania. The microspherical cationic surfactant micelles were chelated by electrostatic adsorption and [TiO6] octahedron, and the [TiO6] octahedron tended to grow to the opposite side, which was formed to form rutile titania.
1. Introduction TiO2 is widely used in energy and environmental protection because of its superior photocatalytic and UV-shielding performance [1–7]. Rutile TiO2 is the most widely used UV-screen agent in practice because of its UV absorption capability and stability. Actually performance of TiO2 is depended on its structure, particle size and morphology [8–11]. TiO2 exists in three main crystallographic forms, i.e, anatase, rutile, and brookite phases. Each crystalline structure has a different crystal growth mode. During the growth of TiO2 crystals, adjacent [TiO6] octahedrons share points, and then share edge by the Ti-O-Ti oxygen bridge bond. There are two connecting possibilities among the [TiO6] octahedrons. One is the formation of linear chains of octahedrons by continuously sharing the opposite edge, which leads to the formation of rutile titania. Another is the formation of spiral chains of octahedrons, which results in
the formation of anatase [12]. Promoting the formation of two con necting possibilities of [TiO6] octahedron relies on the kinetic and thermodynamic properties during the reaction. Thus, TiO2 crystallo graphic form is tunable by changing the hydrolysis condition. Many methods were developed to promote the transformation from anatase to rutile titania such as roasting [13,14], adding dopant ions �pe et al. [15] pre [15,16] and modifying solvent [17–20]. G� alvez-Lo pared binary Sn-Ti samples with variation of atomic ratios Sn:Ti from zero to one through a hydrothermal method. Pure rutile phase TiO2 was synthesized at atomic ratio Sn: Ti 0.3. Zheng et al. [17] synthesized pure rutile and rutile-anatase composite TiO2 nanoparticles by an ionic liquid-assisted method by hydrolysis of TiCl4. Yuenyongsuwan et al. [20] investigated the influence of surfactants type on phase composition of TiO2 via hydrothermal method, and found that anionic surfactant sodium dodecylbenzene sulfonate promoted the formation of anatase
* Corresponding author. E-mail address: [email protected] (Z. Liu). https://doi.org/10.1016/j.matchemphys.2019.122079 Received 22 May 2019; Received in revised form 24 August 2019; Accepted 27 August 2019 Available online 28 August 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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Materials Chemistry and Physics 240 (2020) 122079
Table 1 Quality indicators of titanium oxysulfate solution. project
Ti4þ concentration g/L
Ti3þ concentration g/L
Effective acid concentration g/L
F value
Clarity
index
195.4
1.3
386.7
2.0
qualified
TiO2. However, in spite of the above research fruits, it should be mentioned that the titanium sources used to synthesize rutile TiO2 are mostly esterified, alcoholic and chloride, which are expensive, low in productivity and poor environmental-friendship. Therefore, more and more attentions have been paid to develop rutile TiO2 that possess su perior shielding efficiency by a facile, low cost and economical method. Surfactants are widely used in the chemical synthesis of colloidal nanocrystals, and they are proven to effectively controlling the crys tallization of inorganic materials by interactions between the head groups of the surfactants and the cations [21]. Casino et al. [22] found that hydrolysis of titanium sulfate would form an intermediate com pound strictly bonded to the hydrophilic group of the surfactant. Our focus is to obtain rutile TiO2 for high UV- Shielding performance from cheap and readily available titanium source by surfactant-assisted method. In this work, titanium sulfate was used as hydrolysis raw material to synthesize rutile TiO2 with surfactants (including CTMAB, 1231, TEBAC, SDBS and sodium citrate). The effects of surfactant type on the crystal structure of TiO2 were investigated. The influences of surfactant content on the TiO2 phase composition were investigated. UV-shielding ability was measured with different TiO2 nanoparticles. Additionally, mechanism of crystal growth with surfactant was analyzed based on above results. 2. Material and methods 2.1. Material Aqueous titanium sulfate solution is from Chongqing Pangang Group Titanium Industry Co., Ltd as the raw material, and the quality indexes were shown in Table 1. Cetyltrimethyl ammonium bromide (CTMA), dodecyltrimethyl ammonium chloride (1231), sodium citrate and benzyltriethyl ammo nium chloride (TEBAC) were purchased from Chengdu Kelong Chemical Reagent Co. Ltd., China. Sodiumdodecylbenzene sulfonate (SDBS) was from Tianjin Guangfu Fine Chemical Research Institute, China.
Fig. 1. XRD patterns of TiO2 samples with different surfactants.
2.3. Determination of surfactant critical micelles Weighing CTAB to prepare 0.5–8 mmol/L CTMAB solution, a total of 12 groups. The pH of the solution was adjusted to 3.86 (pH of the experimental environment after hydrolysis). The UV absorption spectra of CTAB solutions at different concentrations were measured at 90 � C, and the maximum absorbance of solution was read. The concentration logarithm-absorbance logarithm was plotted, and the inflection point of the line was the critical micelle concentration.
2.2. Sample preparation TiO2 samples were synthesized by self-generated seed hydrolysis method to investigate the effects of surfactant type and content on TiO2 crystalline structure. CTMAB, 1231, TEBAC, SDBS and sodium citrate were used to study the effect of surfactant type on phase-controlled TiO2 synthesis. SDBS(0.209 g), sodium citrate(0.176 g), CTMAB(0.219 g), 1231 (0.158 g) and TEBAC(0.137 g) were dissolved in 50 mL deionized water respectively, and stirred until the surfactant thoroughly dissolved. 50 mL titanium sulfate solution and surfactant solution were heated to 96 � C respectively, then the heated titanium sulfate solution was slowly dropped into the pre-adding surfactant solution at feed rate of 5 mL/ min. After feeding off, the reaction solution was heated to 102 � C and continuously stirred for 2 h. After cooling to room temperature, syn thesized TiO2 was filtered and washed with distilled water several times, then dried at 60 � C for 24 h. Lastly, the synthesized powder was calcined for 3 h in muffle furnace at heating rate of 10 � C/min to prepare TiO2. The obtain samples were labeled as TiO2-SDBS, TiO2-CA, TiO2-C16, TiO2-C12 and TiO2-BC in turn. One sample was also prepared without surfactant.
2.4. Sample characterization The crystal structure and phase composition of the sample were analyzed by X-ray diffraction analyzer (XRD-6100/7000). The voltage and anode current were 40 KV and 40 mA, respectively. The ionic bonds in TiO2⋅H2O was analyzed by Fourier transform infrared spectrometer (IR Prestige-21). Sample powder was mixed with KBr and pressured into thin disks. The surface morphology of the sample was observed by electron field emission scanning electron microscope (JSM-7800F). The binding energy of the samples was analyzed using Thermo Fisher Sci entific X-ray Photoelectron Spectrometer (ESCALAB 250Xi). UVshielding properties of the samples were measured in the wavelength range from 200 nm to 800 nm by a dual-speed UV–Vis spectrophotom eter (TU-1901). 2
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Materials Chemistry and Physics 240 (2020) 122079
Fig. 3. The comparison of FTIR spectra of TiO2⋅H2O.
Fig. 2. SEM microphotographs of TiO2.
3. Results and discussion 3.1. Effect of surfactant on phase-controlled TiO2 As shown in Fig. 1, the X-ray powder diffraction (XRD) patterns illustrated the crystalline phase evolution of TiO2 synthesized in the presence of surfactant at different temperatures. In the absence of sur factant, anatase TiO2 was formed at 750 � C and 780 � C. In 12 mmol/L surfactant, the crystalline structures of TiO2-SDBS and TiO2-CA were both anatase, indicating that SDBS and sodium citrate did not promote anatase-rutile transformation of titania. On the contrary, by analyzing the patterns of TiO2-C16, TiO2-C12 and TiO2-BC sample, TiO2 nano particles were consist of anatase and rutile phases, indicating that CTMAB, 1231 and benzyltriethylammonium chloride promoted trans formation from anatase into rutile titania at 750 � C and 780 � C. Furthermore, different kinds of surfactants had different effects in the transformation from anatase to rutile titania. Anionic surfactants were unfavorable to the formation of rutile. Cationic surfactants promoted anatase-rutile transformation of titania. Based on the above results, it found that the cationic surfactant played an important role in the transformation of TiO2 crystalline structure. As demonstrated by Bah nemann [23], a positive charge can adsorb on the surface of TiO2 at pH < 3.5. The positive charge cationic surface active polar group may have more preference for adsorbing on the [TiO6] octahedron, which contributes to cationic surfactant transformation from anatase to rutile titania. Fig. 2 displayed SEM micrograph of TiO2 added with different sur factants. TiO2 nanoparticles were uniform and monodisperse hierar chical microsphere, and the diameters of these isolated microspheres were distributed in a narrow range. Without surfactant, the particle shape of the TiO2 sample was unobvious, and particles agglomeration was severe, because the surface energy of the particles was high and agglomeration was prone to occur. With the addition of surfactant, the TiO2-C16, TiO2-C12 and TiO2-BC samples had obvious particle shapes and the sample particle diameter decreases. Surfactant may be adsorbed onto the surfaces of TiO2 nanocrystal by coordinating with both nitrogen and oxygen atoms [24,25], thereby affecting the final shape and struc ture of the TiO2. Figs. S1a–c showed the XPS spectrum of the CTMAB sample with a concentration of 4 mmol/L (see the Supporting Informa tion). The characteristic peaks of Ti, O and C can be analyzed from the spectrum, and there were no other elements. The peaks confirmed that the sample was pure TiO2. The Cls of the spectrum is the pollution of the test instrument. It can be found from the spectrum o that Ti2p had two absorption peaks: Ti4þ2p3/2 and Ti4þ2p1/2, and their corresponding peaks were 458.4 eV and 464.1 eV, indicating that the Ti element in the
Fig. 4. XRD patterns of TiO2 synthesized with different CTMAB concentration.
material existed in the form of Ti4þ. It could be seen from the spectrum that O1s had two absorption peaks with peaks of 529.58 eV and 531.45 eV, respectively, indicating that there were at least two forms of Oxygen element in the sample, where in the characteristic peak of 529.58 eV was oxygen in the TiO2 lattice; The peak at the 531.45 eV position was formed by the interaction of adsorbed oxygen and surface hydroxyl groups. To further determine surfactant with TiO2 via atomic bond on the surface, FTIR spectra was carried out. Fig. 3 showed an infrared ab sorption spectrum of TiO2⋅H2O with different surfactants. It can be observed that a broad peak between 3600 cm 1 and 3000 cm 1 appeared in the sample, which were corresponding to the stretching vibration and the bending vibration of the –OH group caused by the crystallization water. The infrared absorption peak of 1617 cm 1 were associated to the stretching vibration of the Ti-O-Ti oxygen bridge of TiO2⋅H2O, which was characteristic vibration of the [TiO6] octahedron. The peaks of 1223 cm 1, 1126 cm 1 and 1046 cm 1 corresponded to SO24 inorganic chelate bidentate coordination. The peaks of 746 cm 1 corresponded to the bending vibration peak of the N-O bond, which proved that the cationic group of the surfactant and TiO2⋅H2O formed a chemical bond and stably presented on the surface. It was also confirmed by S. Casino [22] that during the hydrolysis process, titanium sulfate formed a metatitanic acid, which was tightly bonded to the hydrophilic group of the surfactant. 3.2. Effect of CTMAB concentration on phase-controlled TiO2 XRD patterns in Fig. 4 showed that TiO2 rutile phase content changed 3
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Materials Chemistry and Physics 240 (2020) 122079
Table 2 Phase content and grain size of different CTMAB feed concentration samples. Number
Sample
CTMAB mmol/L
Anatase %
Rutile %
Crystalline size of anatasea(nm)
Crystalline size of rutileb(nm)
1 2 3 4 5 6 7 8
TiO2-C16(0) TiO2-C16(1) TiO2-C16(2) TiO2-C16(3) TiO2-C16(4) TiO2-C16(5) TiO2-C16(9) TiO2-C16(12)
0 1 2 3 4 5 9 12
100 100 100 13 6 10.6 28.2 59.8
0 0 0 87 94 89.4 71.8 40.8
46.1 44.5 44.5 36.1 44.5 63.1 44.3 44.3
/ / / 61.2 58.9 74.0 61.5 58.1
a b
Determined from (101) reflection using Scherrer equation. Determined from (110) reflection using Scherrer equation.
CTMAB feed concentration hampered the transformation from anatase to rutile titania. Fig. 5 showed UV–visible spectra of TiO2 at different CTMAB con centrations. Since UVC is absorbed by the ozone layer, here we mainly discuss the absorption capacity of TiO2 for UVB and UVA. In the wavelength range of 280–400 nm, UV-shielding ability of TiO2 added with CTMAB was significantly enhanced compared to TiO2-C16(0). UVshielding ability of TiO2-C16(4) was increased by 73%. TiO2-C16(4) sample had the best UV-shielding performance because of its 100% rutile phase composition. UV absorption curves of TiO2-C16(3), TiO2C16(4), TiO2-C16(5) and TiO2-C16(9) showed a significant red shift, TiO2-C16(0) ultraviolet absorption platform region was 260–360 nm, and the TiO2 added to CTMAB ultraviolet absorption platform region appeared at 260–390 nm. Compared with TiO2-C16(0), TiO2 added with CTMAB had more rutile phase content, stronger UV-shielding ability and larger UV absorption range. According to the experimental data obtained by UV–visible diffuse reflection, the Kubelka-Munk transform was used to calculate the band gap width (Eg) for TiO2 [28]. It was carried out following standard steps: first plotting Kubelka-Munk transform [F(R)*E]^n (n ¼ 1/2 or 2 for indirect or direct semiconductor) compared vs. energy E, then obtaining the corresponding intersection of the linear fit with the baseline [29]. In agreement with previous studies [20], the band gap of anatase TiO2( TiO2-C16(0)) was 3.25 eV(Fig. 6(a)). The band gap of the sample from TiO2-C16(3) to TiO2-C16(12) was 2.94 eV, 2.96 eV, 3.02 eV, 2.99 eV and 3.22 eV, respectively. The band gap of the samples with CTMAB added decreased, TiO2-C16(3) and TiO2-C16(4) had the lowest band gap, which were 2.94 eV and 2.96 eV, respectively. The lower band gap width indicated that rutile TiO2 had larger range of light absorption, which was consistent with the results of UV–visible spectra analysis.
Fig. 5. UV–visible spectra of TiO2 synthesized with different CTMAB concentrations.
with the concentration of CTMAB in the range of 0–12 mmol/L. At a small dosage of surfactant, the crystalline structures of the TiO2-C16(1), and TiO2-C16(2) samples were pure anatase. The rutile phase appeared in the samples of TiO2-C16(3), TiO2-C16(4), TiO2-C16(5), TiO2-C16(9) and TiO2-C16(12). Increasing the concentration of CTMAB led to growth of rutile phase. When the concentration of CTMAB was greater than or equal to 3 mmol/L, the TiO2 sample contains rutile phase, which further confirmed that CTMAB promoted antase-rutile phase transformation of titania. The anatase and rutile content in the TiO2 sample were calcu lated by the following equations [26]: WA¼ KA IA/(KA IA þ IR)
(1)
WR¼ IR/(KA IA þ IR)
(2)
3.3. Effect of 1231 concentration on phase-controlled TiO2 X-ray diffraction (XRD) analysis of the synthesized samples at different concentration of 1231 were shown in Fig. 7, and the phase content and average grain size of the samples were listed in Table S1(see the Supporting Information). It can be seen from Fig. 7 and Table S1 that crystalline structures of TiO2-C12(2) and TiO2-C12(4) were anatase at a small dosage of surfactant, which indicated that the low 1231 concen tration couldn’t promote transformation of anatase into rutile phase. When the concentration of 1231 was greater or equal to 4 mmol/L, the rutile phase appeared in the samples from TiO2-C12(4) to TiO2-C12(12), and TiO2-C12(6) had the highest rutile content of 70.9%. It was confirmed that 1231 promoted transformation of anatase into rutile phase. Content of rutile in the samples TiO2-C12(10) and TiO2-C12(12) decreased, indicating that excessive 1231 hampered the promotion of transformation of anatase into rutile phase. As shown in Fig. 8, UV–visible spectra illustrated UV-shielding ability of TiO2 with different 1231 concentrations. In the wavelength range of 280–400 nm, the addition of 1231 TiO2 was significantly enhanced by UV absorption compared to TiO2-C12(0). The TiO2-C12(6),
KA ¼ 0.886 Where WA and WR are the relative contents of anatase and rutile, respectively, and IA and IR are the peak areas of all the diffraction peaks of anatase and rutile, respectively. The average grain size of the TiO2 sample was estimated by the Debye-Scherrer equation [27]: D ¼ kλ/βcosθ
(3)
Where D is the average crystallite size (nm), λ is the wavelength of X-ray diffraction, k is a constant, usually 0.89, θ is the diffraction angle, and β is the anatase (101) plane or rutile (110) plane half width. The phase content and average grain size of the samples were calculated and listed in Table 2. TiO2 crystalline size increased in the presence of surfactant. In 4 mmol/L CTMAB, TiO2-C16(4) had 94% rutile phase. As the CTMAB concentration was further increased, the rutile content of TiO2-C16(9) and the TiO2-C16(12) decreased, which indicated that the excessive 4
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Materials Chemistry and Physics 240 (2020) 122079
Fig. 6. Effect of CTMAB concentrations on TiO2 band gap width.
Fig. 7. XRD patterns of TiO2 synthesized with different 1231 concentration. Fig. 8. UV–visible spectra of TiO2 synthesized with 1231 concentrations.
TiO2-C12(8) and TiO2-C12(10) samples had the highest UV-shielding performance and UV absorption value increased by 60%, Because the rutile of these samples was above 60%. At the same time, it was found that UV absorption curve of the sample added with 1231 showed a significant red shift. The TiO2-C16(0) ultraviolet absorption plateau appeared at 260–360 nm, and TiO2 added with 1231 ultraviolet ab sorption plateau region appeared at 260–390 nm TiO2 added with 1231 had a wider UV absorption range and could absorb longer wavelength ultraviolet rays. Kubelka-Munk transform was used to calculate the band gap of TiO2. As can be seen from Fig. S2(see the information), the band gap of the sample from TiO2-C12(4) to TiO2-C12(12) was 3.04 eV, 2.91 eV, 3.03 eV, 3.04 eV, and 2.92 eV, respectively. It was found that the rutile content of the samples with different concentrations of 1231 increased, and the band gap decreased by 0.2–0.3 eV. Among them, TiO2-C12(6) with a feed concentration of 6 mmol/L had a minimum
band gap of 2.91 eV, because TiO2-C12(6) had the highest rutile content of 70.9%. The lower band gap width of TiO2-C12(6) indicated that TiO2C16(6) absorbs more light and has higher UV absorption capacity. 3.4. Mechanism analysis The first critical micelle concentration of cetyltrimethylammonium bromide (CTMAB) in the water system is 1.148 mmol/L [30]. Consid ering the electrolyte in the experimental system, the data of first critical micelle concentration is unreliable. Therefore, the probe-free ultraviolet spectroscopy method was used to measure the critical micelle concen tration in the experimental system [31]. Fig. 9 showed the relationship between CTMAB concentration and absorbance in the reaction system. 5
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Materials Chemistry and Physics 240 (2020) 122079
The first critical micelle concentration of CTMAB was 2 mmol/L in the experimental system. When the concentration of CTMAB was less than or equal to the critical micelle concentration, CTMAB was a mono molecular structure and had no effect on anatase-rutile transformation of titania. When the concentration was greater than the critical micelle concentration, the CTMAB structure became microsphere, and TiO2 was adsorbed to the surface of the micelle, promoting anatase-rutile trans formation of titania. Based on the above findings, it found that the cationic surfactant promoted anatase-rutile transformation of titania, and the surface of the micelle formed by the cationic surfactant had a positive charge, so that the cationic surfactant played a good role in the formation of TiO2. When the concentration was greater than the critical micelle concentration, the microspherical surfactant promoted anatase-rutile transformation of titania. During the hydrolysis process of titanium sulfate, the discrete [TiO6] octahedron(Fig. 10-a) was extremely easy to combine with the –OH groups of other [TiO6] octahedrons to construct Ti-O-Ti oxygen bridge (Fig. 10-b) by eliminating H2O molecule [32,33]. The S-O bond length of the SO24 was equal to the length of the O-O bond of the [TiO6]
Fig. 9. CTMAB critical micelle concentration in the reaction system.
Fig. 10. Schematic diagram of the conversion mechanism of TiO2 crystalline structure. 6
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Materials Chemistry and Physics 240 (2020) 122079
octahedron, the SO24 was easily chelated with the two ends of the opposite side of the [TiO6] octahedron(Fig. 10-d), and the SO24 was adsorbed on the surface of the TiO2⋅H2O in the form of an inorganic chelate bidentate coordination [34]. Due to the hindrance of SO24 , the [TiO6] octahedron couldn’t grow along the opposite side [27], but it grew along the oblique side to form anatase nucleus.(Fig. 10-f). When the concentration of the surfactant was greater than the critical micelle concentration, the microspherical cationic surfactant micelles were chelated by electrostatic adsorption and [TiO6] octahedron due to the electrostatic repulsion and steric hindrance effect of the cationic group of the surfactant. The [TiO6] octahedron’s spiral growth was prevented, so that the [TiO6] octahedron was more inclined to grow on the opposite side, forming a chain structure and forming rutile titania (Fig. 10-e).
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4. Conclusions The micelle formed by the cationic surfactant had electrostatic interaction with the [TiO6] octahedron, so that the cationic surfactant played a good role in the formation of TiO2 to promote transformation from anatase to rutile titania. When the concentration of CTMAB was greater than or equal to 3 mmol/L, the rutile phase appeared in the sample. The sample added with surfactant increased UV absorption platform and enhanced the absorption capacity; the band gap of sample decreases by 0.2–0.3 eV, and the absorption wavelength range increased. Cationic surfactants were chelated by the electrostatic adsorption and the oblique sides of the [TiO6] octahedron, making the [TiO6] octahedron more prone to co-edge growth, and the steric hin drance effect of the cationic group of the surfactant prevented the [TiO6] octahedron to spiral growth, which promoting the transformation from anatase to rutile titania. Acknowledgements This work was supported by National Key Research and Develop ment Plan of China (2017YFB0603105), graduate research and inno vation foundation of Chongqing China(Grant No. CYS18031), Development and application of key technologies for green production of polytetrahydrofura(cstc2018jszx-cyzdX0085). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2019.122079. References [1] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Visible-light photocatalysis in nitrogen-doped titanium oxides, Science 293 (2001) 269–271. [2] L. Qu, N. Wang, Y. Li, D. Bao, G. Yang, H. Li, Novel titanium dioxide–graphene–activated carbon ternary nanocomposites with enhanced photocatalytic performance in rhodamine B and tetracycline hydrochloride degradation, J. Mater. Sci. 52 (2017) 8311–8320. [3] M. Grandcolas, K.L. Du, F. Bosc, A. Louvet, N. Keller, V. Keller, Porogen template Assisted TiO 2 rutile coupled nanomaterials for improved visible and solar light photocatalytic applications, Catal. Lett. 123 (2008) 65–71. [4] D. Dolat, S. Mozia, R.J. Wr� obel, D. Moszy� nski, B. Ohtani, N. Guskos, et al., Nitrogen-doped, metal-modified rutile titanium dioxide as photocatalysts for water remediation, Appl. Catal. B Environ. 162 (2015) 310–318. [5] Z. He, W. Que, Y. He, J. Chen, H. Xie, G. Wang, Nanosphere assembled mesoporous titanium dioxide with advanced photocatalystic activity using absorbent cotton as template, J. Mater. Sci. 47 (2012) 7210–7216. [6] A. Molea, V. Popescu, N.A. Rowson, A.M. Dinescu, Influence of pH on the formulation of TiO2 nano-crystalline powders with high photocatalytic activity, Powder Technol. 253 (2014) 22–28. [7] S. Pareek, J.K. Quamara, Dielectric and optical properties of graphitic carbon nitride–titanium dioxide nanocomposite with enhanced charge separation, J. Mater. Sci. 53 (2018) 604–612.
7
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Surfactant effect on controllable phase transformation and UV-shielding performance of titanium dioxide Zuohua Liu *, Liang Wang, Li Li, Facheng Qiu, Xia Xiong, Changyuan Tao, Jun Du School of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 400044, China
H I G H L I G H T S
� TiO2 crystalline structure is tunable by surfactant-assisted method. � The UV shielding performance of the surfactant-aided TiO2 product increased. � Mcrospherical Cationic surfactant promoted anatase-rutile transformation of titania. A R T I C L E I N F O
A B S T R A C T
Keywords: Surfactant Titanium dioxide Rutile Phase control UV-Shielding
The crystalline structure, size and morphology of TiO2 have crucial influence on its physicochemical perfor mance. The crystal growth process includes crystal transformation and morphology evolution, which is closely related to the solution environment. In this work, rutile TiO2 was synthesized via surfactant-assisted method by using titanium sulfate as titanium source. Factor associated with crystalline structure of TiO2 were investigated including surfactant type and concentration. Results showed that when the concentration of cetyl triethylammonium bromide (CTMAB) was greater than or equal to 3 mmol/L, or the concentration of dodecyl triethylammonium bromide (1231) was greater than or equal to 4 mmol/L, surfactants promoted transformation from anatase to rutile titania. The UV-shielding performance of the surfactant-aided titania increased, and the band gap width decreased by 0.2–0.3 eV. More importantly, we found that the cationic surfactant played a decisive role in anatase-rutile transformation of titania. The microspherical cationic surfactant micelles were chelated by electrostatic adsorption and [TiO6] octahedron, and the [TiO6] octahedron tended to grow to the opposite side, which was formed to form rutile titania.
1. Introduction TiO2 is widely used in energy and environmental protection because of its superior photocatalytic and UV-shielding performance [1–7]. Rutile TiO2 is the most widely used UV-screen agent in practice because of its UV absorption capability and stability. Actually performance of TiO2 is depended on its structure, particle size and morphology [8–11]. TiO2 exists in three main crystallographic forms, i.e, anatase, rutile, and brookite phases. Each crystalline structure has a different crystal growth mode. During the growth of TiO2 crystals, adjacent [TiO6] octahedrons share points, and then share edge by the Ti-O-Ti oxygen bridge bond. There are two connecting possibilities among the [TiO6] octahedrons. One is the formation of linear chains of octahedrons by continuously sharing the opposite edge, which leads to the formation of rutile titania. Another is the formation of spiral chains of octahedrons, which results in
the formation of anatase [12]. Promoting the formation of two con necting possibilities of [TiO6] octahedron relies on the kinetic and thermodynamic properties during the reaction. Thus, TiO2 crystallo graphic form is tunable by changing the hydrolysis condition. Many methods were developed to promote the transformation from anatase to rutile titania such as roasting [13,14], adding dopant ions �pe et al. [15] pre [15,16] and modifying solvent [17–20]. G� alvez-Lo pared binary Sn-Ti samples with variation of atomic ratios Sn:Ti from zero to one through a hydrothermal method. Pure rutile phase TiO2 was synthesized at atomic ratio Sn: Ti 0.3. Zheng et al. [17] synthesized pure rutile and rutile-anatase composite TiO2 nanoparticles by an ionic liquid-assisted method by hydrolysis of TiCl4. Yuenyongsuwan et al. [20] investigated the influence of surfactants type on phase composition of TiO2 via hydrothermal method, and found that anionic surfactant sodium dodecylbenzene sulfonate promoted the formation of anatase
* Corresponding author. E-mail address: [email protected] (Z. Liu). https://doi.org/10.1016/j.matchemphys.2019.122079 Received 22 May 2019; Received in revised form 24 August 2019; Accepted 27 August 2019 Available online 28 August 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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Table 1 Quality indicators of titanium oxysulfate solution. project
Ti4þ concentration g/L
Ti3þ concentration g/L
Effective acid concentration g/L
F value
Clarity
index
195.4
1.3
386.7
2.0
qualified
TiO2. However, in spite of the above research fruits, it should be mentioned that the titanium sources used to synthesize rutile TiO2 are mostly esterified, alcoholic and chloride, which are expensive, low in productivity and poor environmental-friendship. Therefore, more and more attentions have been paid to develop rutile TiO2 that possess su perior shielding efficiency by a facile, low cost and economical method. Surfactants are widely used in the chemical synthesis of colloidal nanocrystals, and they are proven to effectively controlling the crys tallization of inorganic materials by interactions between the head groups of the surfactants and the cations [21]. Casino et al. [22] found that hydrolysis of titanium sulfate would form an intermediate com pound strictly bonded to the hydrophilic group of the surfactant. Our focus is to obtain rutile TiO2 for high UV- Shielding performance from cheap and readily available titanium source by surfactant-assisted method. In this work, titanium sulfate was used as hydrolysis raw material to synthesize rutile TiO2 with surfactants (including CTMAB, 1231, TEBAC, SDBS and sodium citrate). The effects of surfactant type on the crystal structure of TiO2 were investigated. The influences of surfactant content on the TiO2 phase composition were investigated. UV-shielding ability was measured with different TiO2 nanoparticles. Additionally, mechanism of crystal growth with surfactant was analyzed based on above results. 2. Material and methods 2.1. Material Aqueous titanium sulfate solution is from Chongqing Pangang Group Titanium Industry Co., Ltd as the raw material, and the quality indexes were shown in Table 1. Cetyltrimethyl ammonium bromide (CTMA), dodecyltrimethyl ammonium chloride (1231), sodium citrate and benzyltriethyl ammo nium chloride (TEBAC) were purchased from Chengdu Kelong Chemical Reagent Co. Ltd., China. Sodiumdodecylbenzene sulfonate (SDBS) was from Tianjin Guangfu Fine Chemical Research Institute, China.
Fig. 1. XRD patterns of TiO2 samples with different surfactants.
2.3. Determination of surfactant critical micelles Weighing CTAB to prepare 0.5–8 mmol/L CTMAB solution, a total of 12 groups. The pH of the solution was adjusted to 3.86 (pH of the experimental environment after hydrolysis). The UV absorption spectra of CTAB solutions at different concentrations were measured at 90 � C, and the maximum absorbance of solution was read. The concentration logarithm-absorbance logarithm was plotted, and the inflection point of the line was the critical micelle concentration.
2.2. Sample preparation TiO2 samples were synthesized by self-generated seed hydrolysis method to investigate the effects of surfactant type and content on TiO2 crystalline structure. CTMAB, 1231, TEBAC, SDBS and sodium citrate were used to study the effect of surfactant type on phase-controlled TiO2 synthesis. SDBS(0.209 g), sodium citrate(0.176 g), CTMAB(0.219 g), 1231 (0.158 g) and TEBAC(0.137 g) were dissolved in 50 mL deionized water respectively, and stirred until the surfactant thoroughly dissolved. 50 mL titanium sulfate solution and surfactant solution were heated to 96 � C respectively, then the heated titanium sulfate solution was slowly dropped into the pre-adding surfactant solution at feed rate of 5 mL/ min. After feeding off, the reaction solution was heated to 102 � C and continuously stirred for 2 h. After cooling to room temperature, syn thesized TiO2 was filtered and washed with distilled water several times, then dried at 60 � C for 24 h. Lastly, the synthesized powder was calcined for 3 h in muffle furnace at heating rate of 10 � C/min to prepare TiO2. The obtain samples were labeled as TiO2-SDBS, TiO2-CA, TiO2-C16, TiO2-C12 and TiO2-BC in turn. One sample was also prepared without surfactant.
2.4. Sample characterization The crystal structure and phase composition of the sample were analyzed by X-ray diffraction analyzer (XRD-6100/7000). The voltage and anode current were 40 KV and 40 mA, respectively. The ionic bonds in TiO2⋅H2O was analyzed by Fourier transform infrared spectrometer (IR Prestige-21). Sample powder was mixed with KBr and pressured into thin disks. The surface morphology of the sample was observed by electron field emission scanning electron microscope (JSM-7800F). The binding energy of the samples was analyzed using Thermo Fisher Sci entific X-ray Photoelectron Spectrometer (ESCALAB 250Xi). UVshielding properties of the samples were measured in the wavelength range from 200 nm to 800 nm by a dual-speed UV–Vis spectrophotom eter (TU-1901). 2
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Fig. 3. The comparison of FTIR spectra of TiO2⋅H2O.
Fig. 2. SEM microphotographs of TiO2.
3. Results and discussion 3.1. Effect of surfactant on phase-controlled TiO2 As shown in Fig. 1, the X-ray powder diffraction (XRD) patterns illustrated the crystalline phase evolution of TiO2 synthesized in the presence of surfactant at different temperatures. In the absence of sur factant, anatase TiO2 was formed at 750 � C and 780 � C. In 12 mmol/L surfactant, the crystalline structures of TiO2-SDBS and TiO2-CA were both anatase, indicating that SDBS and sodium citrate did not promote anatase-rutile transformation of titania. On the contrary, by analyzing the patterns of TiO2-C16, TiO2-C12 and TiO2-BC sample, TiO2 nano particles were consist of anatase and rutile phases, indicating that CTMAB, 1231 and benzyltriethylammonium chloride promoted trans formation from anatase into rutile titania at 750 � C and 780 � C. Furthermore, different kinds of surfactants had different effects in the transformation from anatase to rutile titania. Anionic surfactants were unfavorable to the formation of rutile. Cationic surfactants promoted anatase-rutile transformation of titania. Based on the above results, it found that the cationic surfactant played an important role in the transformation of TiO2 crystalline structure. As demonstrated by Bah nemann [23], a positive charge can adsorb on the surface of TiO2 at pH < 3.5. The positive charge cationic surface active polar group may have more preference for adsorbing on the [TiO6] octahedron, which contributes to cationic surfactant transformation from anatase to rutile titania. Fig. 2 displayed SEM micrograph of TiO2 added with different sur factants. TiO2 nanoparticles were uniform and monodisperse hierar chical microsphere, and the diameters of these isolated microspheres were distributed in a narrow range. Without surfactant, the particle shape of the TiO2 sample was unobvious, and particles agglomeration was severe, because the surface energy of the particles was high and agglomeration was prone to occur. With the addition of surfactant, the TiO2-C16, TiO2-C12 and TiO2-BC samples had obvious particle shapes and the sample particle diameter decreases. Surfactant may be adsorbed onto the surfaces of TiO2 nanocrystal by coordinating with both nitrogen and oxygen atoms [24,25], thereby affecting the final shape and struc ture of the TiO2. Figs. S1a–c showed the XPS spectrum of the CTMAB sample with a concentration of 4 mmol/L (see the Supporting Informa tion). The characteristic peaks of Ti, O and C can be analyzed from the spectrum, and there were no other elements. The peaks confirmed that the sample was pure TiO2. The Cls of the spectrum is the pollution of the test instrument. It can be found from the spectrum o that Ti2p had two absorption peaks: Ti4þ2p3/2 and Ti4þ2p1/2, and their corresponding peaks were 458.4 eV and 464.1 eV, indicating that the Ti element in the
Fig. 4. XRD patterns of TiO2 synthesized with different CTMAB concentration.
material existed in the form of Ti4þ. It could be seen from the spectrum that O1s had two absorption peaks with peaks of 529.58 eV and 531.45 eV, respectively, indicating that there were at least two forms of Oxygen element in the sample, where in the characteristic peak of 529.58 eV was oxygen in the TiO2 lattice; The peak at the 531.45 eV position was formed by the interaction of adsorbed oxygen and surface hydroxyl groups. To further determine surfactant with TiO2 via atomic bond on the surface, FTIR spectra was carried out. Fig. 3 showed an infrared ab sorption spectrum of TiO2⋅H2O with different surfactants. It can be observed that a broad peak between 3600 cm 1 and 3000 cm 1 appeared in the sample, which were corresponding to the stretching vibration and the bending vibration of the –OH group caused by the crystallization water. The infrared absorption peak of 1617 cm 1 were associated to the stretching vibration of the Ti-O-Ti oxygen bridge of TiO2⋅H2O, which was characteristic vibration of the [TiO6] octahedron. The peaks of 1223 cm 1, 1126 cm 1 and 1046 cm 1 corresponded to SO24 inorganic chelate bidentate coordination. The peaks of 746 cm 1 corresponded to the bending vibration peak of the N-O bond, which proved that the cationic group of the surfactant and TiO2⋅H2O formed a chemical bond and stably presented on the surface. It was also confirmed by S. Casino [22] that during the hydrolysis process, titanium sulfate formed a metatitanic acid, which was tightly bonded to the hydrophilic group of the surfactant. 3.2. Effect of CTMAB concentration on phase-controlled TiO2 XRD patterns in Fig. 4 showed that TiO2 rutile phase content changed 3
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Table 2 Phase content and grain size of different CTMAB feed concentration samples. Number
Sample
CTMAB mmol/L
Anatase %
Rutile %
Crystalline size of anatasea(nm)
Crystalline size of rutileb(nm)
1 2 3 4 5 6 7 8
TiO2-C16(0) TiO2-C16(1) TiO2-C16(2) TiO2-C16(3) TiO2-C16(4) TiO2-C16(5) TiO2-C16(9) TiO2-C16(12)
0 1 2 3 4 5 9 12
100 100 100 13 6 10.6 28.2 59.8
0 0 0 87 94 89.4 71.8 40.8
46.1 44.5 44.5 36.1 44.5 63.1 44.3 44.3
/ / / 61.2 58.9 74.0 61.5 58.1
a b
Determined from (101) reflection using Scherrer equation. Determined from (110) reflection using Scherrer equation.
CTMAB feed concentration hampered the transformation from anatase to rutile titania. Fig. 5 showed UV–visible spectra of TiO2 at different CTMAB con centrations. Since UVC is absorbed by the ozone layer, here we mainly discuss the absorption capacity of TiO2 for UVB and UVA. In the wavelength range of 280–400 nm, UV-shielding ability of TiO2 added with CTMAB was significantly enhanced compared to TiO2-C16(0). UVshielding ability of TiO2-C16(4) was increased by 73%. TiO2-C16(4) sample had the best UV-shielding performance because of its 100% rutile phase composition. UV absorption curves of TiO2-C16(3), TiO2C16(4), TiO2-C16(5) and TiO2-C16(9) showed a significant red shift, TiO2-C16(0) ultraviolet absorption platform region was 260–360 nm, and the TiO2 added to CTMAB ultraviolet absorption platform region appeared at 260–390 nm. Compared with TiO2-C16(0), TiO2 added with CTMAB had more rutile phase content, stronger UV-shielding ability and larger UV absorption range. According to the experimental data obtained by UV–visible diffuse reflection, the Kubelka-Munk transform was used to calculate the band gap width (Eg) for TiO2 [28]. It was carried out following standard steps: first plotting Kubelka-Munk transform [F(R)*E]^n (n ¼ 1/2 or 2 for indirect or direct semiconductor) compared vs. energy E, then obtaining the corresponding intersection of the linear fit with the baseline [29]. In agreement with previous studies [20], the band gap of anatase TiO2( TiO2-C16(0)) was 3.25 eV(Fig. 6(a)). The band gap of the sample from TiO2-C16(3) to TiO2-C16(12) was 2.94 eV, 2.96 eV, 3.02 eV, 2.99 eV and 3.22 eV, respectively. The band gap of the samples with CTMAB added decreased, TiO2-C16(3) and TiO2-C16(4) had the lowest band gap, which were 2.94 eV and 2.96 eV, respectively. The lower band gap width indicated that rutile TiO2 had larger range of light absorption, which was consistent with the results of UV–visible spectra analysis.
Fig. 5. UV–visible spectra of TiO2 synthesized with different CTMAB concentrations.
with the concentration of CTMAB in the range of 0–12 mmol/L. At a small dosage of surfactant, the crystalline structures of the TiO2-C16(1), and TiO2-C16(2) samples were pure anatase. The rutile phase appeared in the samples of TiO2-C16(3), TiO2-C16(4), TiO2-C16(5), TiO2-C16(9) and TiO2-C16(12). Increasing the concentration of CTMAB led to growth of rutile phase. When the concentration of CTMAB was greater than or equal to 3 mmol/L, the TiO2 sample contains rutile phase, which further confirmed that CTMAB promoted antase-rutile phase transformation of titania. The anatase and rutile content in the TiO2 sample were calcu lated by the following equations [26]: WA¼ KA IA/(KA IA þ IR)
(1)
WR¼ IR/(KA IA þ IR)
(2)
3.3. Effect of 1231 concentration on phase-controlled TiO2 X-ray diffraction (XRD) analysis of the synthesized samples at different concentration of 1231 were shown in Fig. 7, and the phase content and average grain size of the samples were listed in Table S1(see the Supporting Information). It can be seen from Fig. 7 and Table S1 that crystalline structures of TiO2-C12(2) and TiO2-C12(4) were anatase at a small dosage of surfactant, which indicated that the low 1231 concen tration couldn’t promote transformation of anatase into rutile phase. When the concentration of 1231 was greater or equal to 4 mmol/L, the rutile phase appeared in the samples from TiO2-C12(4) to TiO2-C12(12), and TiO2-C12(6) had the highest rutile content of 70.9%. It was confirmed that 1231 promoted transformation of anatase into rutile phase. Content of rutile in the samples TiO2-C12(10) and TiO2-C12(12) decreased, indicating that excessive 1231 hampered the promotion of transformation of anatase into rutile phase. As shown in Fig. 8, UV–visible spectra illustrated UV-shielding ability of TiO2 with different 1231 concentrations. In the wavelength range of 280–400 nm, the addition of 1231 TiO2 was significantly enhanced by UV absorption compared to TiO2-C12(0). The TiO2-C12(6),
KA ¼ 0.886 Where WA and WR are the relative contents of anatase and rutile, respectively, and IA and IR are the peak areas of all the diffraction peaks of anatase and rutile, respectively. The average grain size of the TiO2 sample was estimated by the Debye-Scherrer equation [27]: D ¼ kλ/βcosθ
(3)
Where D is the average crystallite size (nm), λ is the wavelength of X-ray diffraction, k is a constant, usually 0.89, θ is the diffraction angle, and β is the anatase (101) plane or rutile (110) plane half width. The phase content and average grain size of the samples were calculated and listed in Table 2. TiO2 crystalline size increased in the presence of surfactant. In 4 mmol/L CTMAB, TiO2-C16(4) had 94% rutile phase. As the CTMAB concentration was further increased, the rutile content of TiO2-C16(9) and the TiO2-C16(12) decreased, which indicated that the excessive 4
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Materials Chemistry and Physics 240 (2020) 122079
Fig. 6. Effect of CTMAB concentrations on TiO2 band gap width.
Fig. 7. XRD patterns of TiO2 synthesized with different 1231 concentration. Fig. 8. UV–visible spectra of TiO2 synthesized with 1231 concentrations.
TiO2-C12(8) and TiO2-C12(10) samples had the highest UV-shielding performance and UV absorption value increased by 60%, Because the rutile of these samples was above 60%. At the same time, it was found that UV absorption curve of the sample added with 1231 showed a significant red shift. The TiO2-C16(0) ultraviolet absorption plateau appeared at 260–360 nm, and TiO2 added with 1231 ultraviolet ab sorption plateau region appeared at 260–390 nm TiO2 added with 1231 had a wider UV absorption range and could absorb longer wavelength ultraviolet rays. Kubelka-Munk transform was used to calculate the band gap of TiO2. As can be seen from Fig. S2(see the information), the band gap of the sample from TiO2-C12(4) to TiO2-C12(12) was 3.04 eV, 2.91 eV, 3.03 eV, 3.04 eV, and 2.92 eV, respectively. It was found that the rutile content of the samples with different concentrations of 1231 increased, and the band gap decreased by 0.2–0.3 eV. Among them, TiO2-C12(6) with a feed concentration of 6 mmol/L had a minimum
band gap of 2.91 eV, because TiO2-C12(6) had the highest rutile content of 70.9%. The lower band gap width of TiO2-C12(6) indicated that TiO2C16(6) absorbs more light and has higher UV absorption capacity. 3.4. Mechanism analysis The first critical micelle concentration of cetyltrimethylammonium bromide (CTMAB) in the water system is 1.148 mmol/L [30]. Consid ering the electrolyte in the experimental system, the data of first critical micelle concentration is unreliable. Therefore, the probe-free ultraviolet spectroscopy method was used to measure the critical micelle concen tration in the experimental system [31]. Fig. 9 showed the relationship between CTMAB concentration and absorbance in the reaction system. 5
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Materials Chemistry and Physics 240 (2020) 122079
The first critical micelle concentration of CTMAB was 2 mmol/L in the experimental system. When the concentration of CTMAB was less than or equal to the critical micelle concentration, CTMAB was a mono molecular structure and had no effect on anatase-rutile transformation of titania. When the concentration was greater than the critical micelle concentration, the CTMAB structure became microsphere, and TiO2 was adsorbed to the surface of the micelle, promoting anatase-rutile trans formation of titania. Based on the above findings, it found that the cationic surfactant promoted anatase-rutile transformation of titania, and the surface of the micelle formed by the cationic surfactant had a positive charge, so that the cationic surfactant played a good role in the formation of TiO2. When the concentration was greater than the critical micelle concentration, the microspherical surfactant promoted anatase-rutile transformation of titania. During the hydrolysis process of titanium sulfate, the discrete [TiO6] octahedron(Fig. 10-a) was extremely easy to combine with the –OH groups of other [TiO6] octahedrons to construct Ti-O-Ti oxygen bridge (Fig. 10-b) by eliminating H2O molecule [32,33]. The S-O bond length of the SO24 was equal to the length of the O-O bond of the [TiO6]
Fig. 9. CTMAB critical micelle concentration in the reaction system.
Fig. 10. Schematic diagram of the conversion mechanism of TiO2 crystalline structure. 6
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Materials Chemistry and Physics 240 (2020) 122079
octahedron, the SO24 was easily chelated with the two ends of the opposite side of the [TiO6] octahedron(Fig. 10-d), and the SO24 was adsorbed on the surface of the TiO2⋅H2O in the form of an inorganic chelate bidentate coordination [34]. Due to the hindrance of SO24 , the [TiO6] octahedron couldn’t grow along the opposite side [27], but it grew along the oblique side to form anatase nucleus.(Fig. 10-f). When the concentration of the surfactant was greater than the critical micelle concentration, the microspherical cationic surfactant micelles were chelated by electrostatic adsorption and [TiO6] octahedron due to the electrostatic repulsion and steric hindrance effect of the cationic group of the surfactant. The [TiO6] octahedron’s spiral growth was prevented, so that the [TiO6] octahedron was more inclined to grow on the opposite side, forming a chain structure and forming rutile titania (Fig. 10-e).
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4. Conclusions The micelle formed by the cationic surfactant had electrostatic interaction with the [TiO6] octahedron, so that the cationic surfactant played a good role in the formation of TiO2 to promote transformation from anatase to rutile titania. When the concentration of CTMAB was greater than or equal to 3 mmol/L, the rutile phase appeared in the sample. The sample added with surfactant increased UV absorption platform and enhanced the absorption capacity; the band gap of sample decreases by 0.2–0.3 eV, and the absorption wavelength range increased. Cationic surfactants were chelated by the electrostatic adsorption and the oblique sides of the [TiO6] octahedron, making the [TiO6] octahedron more prone to co-edge growth, and the steric hin drance effect of the cationic group of the surfactant prevented the [TiO6] octahedron to spiral growth, which promoting the transformation from anatase to rutile titania. Acknowledgements This work was supported by National Key Research and Develop ment Plan of China (2017YFB0603105), graduate research and inno vation foundation of Chongqing China(Grant No. CYS18031), Development and application of key technologies for green production of polytetrahydrofura(cstc2018jszx-cyzdX0085). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2019.122079. References [1] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Visible-light photocatalysis in nitrogen-doped titanium oxides, Science 293 (2001) 269–271. [2] L. Qu, N. Wang, Y. Li, D. Bao, G. Yang, H. Li, Novel titanium dioxide–graphene–activated carbon ternary nanocomposites with enhanced photocatalytic performance in rhodamine B and tetracycline hydrochloride degradation, J. Mater. Sci. 52 (2017) 8311–8320. [3] M. Grandcolas, K.L. Du, F. Bosc, A. Louvet, N. Keller, V. Keller, Porogen template Assisted TiO 2 rutile coupled nanomaterials for improved visible and solar light photocatalytic applications, Catal. Lett. 123 (2008) 65–71. [4] D. Dolat, S. Mozia, R.J. Wr� obel, D. Moszy� nski, B. Ohtani, N. Guskos, et al., Nitrogen-doped, metal-modified rutile titanium dioxide as photocatalysts for water remediation, Appl. Catal. B Environ. 162 (2015) 310–318. [5] Z. He, W. Que, Y. He, J. Chen, H. Xie, G. Wang, Nanosphere assembled mesoporous titanium dioxide with advanced photocatalystic activity using absorbent cotton as template, J. Mater. Sci. 47 (2012) 7210–7216. [6] A. Molea, V. Popescu, N.A. Rowson, A.M. Dinescu, Influence of pH on the formulation of TiO2 nano-crystalline powders with high photocatalytic activity, Powder Technol. 253 (2014) 22–28. [7] S. Pareek, J.K. Quamara, Dielectric and optical properties of graphitic carbon nitride–titanium dioxide nanocomposite with enhanced charge separation, J. Mater. Sci. 53 (2018) 604–612.
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