- Email: [email protected]
Assembly of binary templates for fabricating arrayed pores of TiO2 films with hierarchical structures
Materials Letters 133 (2014) 163–167
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Assembly of binary templates for fabricating arrayed pores of TiO2 films with hierarchical structures Jianfei Lei a,n, Kai Du a, Ronghui Wei a, Shaofeng Zhang a, Jing Ni a, Weishan Li b a b
School of Physics and Engineering, Henan University of Science and Technology, Luoyang 471023, China School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China
art ic l e i nf o
a b s t r a c t
Article history: Received 26 May 2014 Accepted 28 June 2014 Available online 8 July 2014
A novel assembly of binary templates was reported and TiO2 films with hierarchical structures were fabricated by using the assembled templates. According to the result of simulating the templates assembly, it is found that the ratio of the diameter of the bowl to the sphere (Db:Ds) is vital for the assembly of ordered templates, which contains arrayed TiO2 bowls and ordered polystyrene spheres (PS). As a result, when the Db:Ds is more than 3:1 (Db:Ds 43:1), ideal and perfect secondary templates can be obtained and hence arrayed pores of TiO2 films with hierarchical structures can be fabricated. & 2014 Elsevier B.V. All rights reserved.
Keywords: Adhesion Aerospace materials Films Hierarchical structures
1. Introduction Titanium dioxide (TiO2) is a kind of technological material due to its potential applications in advanced devices and systems, such as solar cell electrodes [1–5], anodes for lithium ion batteries [6,7], gas sensors [8] and photocatalysts [9–11]. Significant progresses have been achieved recently in developing film-based materials composed of TiO2 nanoparticles. However, in TiO2 nanoparticlebased materials, the performances are often limited by the disordered morphology, which gives rise to interfacial interferences for charge transport or ionic migrating. To overcome this limitation, TiO2 films with ordered structures comprising uniform patterns and hierarchical architectures have been proposed as promising solutions. These include arrayed pores, ordered nanotubes and arrayed shells architectures in TiO2 films. Varied techniques have been utilized for fabricating TiO2 films, such as thermal deposition [12], sputtering [13], chemical vapor deposition [14], spray pyrolysis [15] and electrochemical deposition [16]. Colloidal crystals are favorable for using candidates as templates to prepare thin films because of their simple, fast and cheap approach to creating ordered materials. The majority of recently reported ordered materials, however, have been fabricated from uniformly sized colloidal particles. These materials exhibit a limited range of crystal structures and manacled performances for applications, which are not ideal structures for investigating condensed-matter physics or for practical applications [17]. In contrast to the limited crystal structures formed by collections of
n
Corresponding author. Tel.: þ 86 379 65626265; fax: þ 86 379 65626265. E-mail address: [email protected] (J. Lei).
http://dx.doi.org/10.1016/j.matlet.2014.06.175 0167-577X/& 2014 Elsevier B.V. All rights reserved.
uniformly sized colloidal particles, using binary templates could generate rather rich arrays of crystal structures depending on the size ratio of large and small candidates. However, binary templates are hard to grow and characterize, and thus they have not yet been extensively investigated. Here, we present a method that provides a facile approach to the fabrication of ordered TiO2 films with hierarchical structures. The method is referred to which is good for the rapid fabrication of well-ordered TiO2 films with hierarchical structures by using varied diameter-ratio of colloidal particles. On the basis of simulating varied assembly of templates, TiO2 films with arrayed pores, in which a large pore containing ordered seven small pores (or more pores) to form hierarchical structures, can be fabricated by using the binary templates, which are composed of arrayed pores of TiO2 films and ordered polystyrene spheres (PS) templates. Also, TiO2 films with arrayed pores, in which a large pore with an ordered porous cover, were fabricated by using the binary PS templates.
2. Experimental section Monodisperse PS (average diameters 3 μm, 750 nm, 300 μm, 250 nm and 210 nm) were synthesized by an emulsion polymerization method as previously reported [18]. The typical procedure is as follows: the monomer, including styrene (St) and polyvinylpyrrolidone (PVP), and ethanol were added, argon was passed through the ethanol in the flask for 30 min under 300 rpm agitation and then the temperature was elevated. When the temperature reached 70 1C, the potassium persulfate (KPS) in
164
J. Lei et al. / Materials Letters 133 (2014) 163–167
aqueous solution was injected using a syringe to initiate polymerization, and the polymerization was performed at 70 1C for 18 h. The experimental parameters are summarized in Table 1. PS spheres arrayed in a single layer were assembled on a conductive glass (F-SnO2 conductive glass, FTO, Rs ¼14 Ω/cm2, Table 1 The experimental parameters for preparing PS. PSx
St (wt%)
PVP (wt%)
KPS (wt%)
EtOH þH2O (v: v¼ 9:1) (wt%)
x ¼3000 x ¼750 x ¼300 x ¼250 x ¼210
10 30 30 30 30
5 2 1 0.7 0.5
0.5 0.9 1 1.3 1.5
85 67 68 68 68
Nippon Sheet Glass, Japan) by an improved floating-transfer method described as our previous report [19]. Firstly, 100 μml emulsion containing 20 wt% PS particles was flown into water with the help of slides and formed a film of PS on the surface of water. Then, a drop of sodium dodecyl sulfate (SDS, 5 wt%) was added to the above system to make close assembly of PS particles. Finally, the film was lifted by a clean FTO substrate. This improved method provides an easy way to prepare more regular arrayed PS spheres with controllable layer number. After being dried in air at room temperature, the substrate coated with the monolayered PS templates was heated in an oven to 110 1C for 5 min to bond the monolayer to the substrate. Tetrabutyl titanate (Ti(OBu)4) used as titanium source and the TiO2 sol was formed in the mixture Ti (OBu)4:EtOH:HCl:H2O:CH3COOH¼1:10:7:2:0.6 in molar. The procedure of sol–gel technique is as following: the conductive glass covered by monolayer PS was immersed in TiO2 sol
Fig. 1. Simulation of varied arrangements of the secondary templates (a) Db:Ds ¼ 3:1; (b) Db:Ds ¼ 2:1; (c) Db:Ds ¼ 1:1; (d) Db:Ds ¼ 3.5:1; (e) Db:Ds ¼4:1; and (f) Db:Ds ¼ 6.5:1.
J. Lei et al. / Materials Letters 133 (2014) 163–167
165
Fig. 2. SEM images of TiO2 films fabricated by using the secondary templates (a and b) Db:Ds ¼ 3:1; (c and d) Db:Ds ¼3.5:1. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
vertically for 2 min to keep the sol filling the interspaces of the PS templates fully under the capillary force. Then the conductive glass was drawn from the sol vertically with a drawing rate of 4 cm min 1 and exposed to the air fully at room temperature for the solvents evaporated for the hydrolysis of titanium ions and the formation of TiO2 gel. Subsequently, the gel was calcined at 450 1C in the oven for 3 h to remove the PS templates leaving the arrayed porous TiO2 on the conductive glass. Hierarchical structures of TiO2 films with two-size ordered pores were fabricated as the following procedure. Firstly, monodisperse PS were assembled on a substrate (FTO) to form monolayered templates via the floating-transfer method, and a sol–gel approach was employed for filling TiO2 sol into the interspaces of the PS templates and generating bowl-like arrayed pores of TiO2 films after removing the templates. Then, another sized PS templates with monolayered PS were transferred on the surface of the as-prepared arrayed porous TiO2 film. Finally, the same sol–gel technique as above mentioned for filling the interspaces of templates was employed to form hierarchical structures of TiO2 films.
3. Results and discussion Fig. 1 illustrates varied stages of simulating the assembly of secondary templates. In theory, the prerequisite of assembling ideal and perfect arrangement of small PS placed into a large bowl-like pore (bowl) is that the ratio of the diameter of the bowl to the sphere should be of 3:1 (Db:Ds ¼3:1). As shown in Fig 1a1, when the monolayered PS templates covered on the bowls of TiO2 films, seven PS (marked number 1(PS-1) and marked number 2 (PS-2)) will be in the range of one bowl. Under the work of gravity, these PS have the tendency of sliding to the bottom of the bowls. As a result, PS-2 land
to the bottom of the bowl directly and PS-1 are placed at the edge of the bowl orderly, while PS-3 stay on islands among three bowls (Fig. 1a2). Actually, PS-1 will not be placed at the edge of the bowl perfectly as that we expected, but slid down together with PS-2 and finally touch to PS-2 partially. Thus, as shown in Fig. 1a3 (top view and perspective view), PS-1 are not arrayed in the same plane, but misplaced in varied plans. Obviously, this combination between bowls and PS templates is not expected for the preparation of ordered pores of TiO2 films to form hierarchical structures. On the basis of the above conclusion, we infer that when the Db:Ds is less than 3:1, the original arrays of monolayered PS templates will be disordered due to the landing of the central PS-2 to the bottom of the bowl (Fig. 1b, Db:Ds ¼2:1) or will be placed into bowls completely in the way of one sphere to one bowl (Fig. 1c, Db:Ds ¼1:1). In this case, to fabricate hierarchical structures with ordered pores is impossible and samples fabricated by this model are not expected. On the contrary, when the Db:Ds is more than 3:1, the arrangement of PS in the bowls is different from that mentioned above. As shown in Figs. 1d–f, more PS particles intrude the bowls and the arrangement of PS becomes more orderly than that shown in Figs. 1a–c. Monolayered PS templates have the feature of normal templates before being lifted by the TiO2 bowls, which is one sphere surrounded by six spheres (Fig. 1d1). However, when the PS templates are lifted by the TiO2 bowls, PS-2 will land at the bottom of the bowls directly and PS-1 slides down together with PS-2. Simultaneously, PS-3 and PS-4, which are located at the edge of the bowls, have the tendency of sliding to the bottom of the neighboring bowls together with PS-1 and PS-2 (Fig. 1d2). Interestingly, a funny phenomenon occurs, that the sliding of PS-3 leads to a rearrangement of PS-1 in the bowls. As a result, one of PS-3 will join the arrangement of PS-1 and seven spheres (six PS-1 and one PS-3) rearrange to ordered circles
166
J. Lei et al. / Materials Letters 133 (2014) 163–167
Fig. 3. SEM images of binary colloidal templates and TiO2 films fabricated by using this templates.
finally, which are placed in the bowls and does not touch PS-2 entirely due to the equilibrium pressure generated from the remaining of five PS-3 and two PS-4 (Fig. 1d3). This unique arrangement between PS and bowls exhibits superior spatial configuration for fabricating novel structures. When the Db:Ds reaches 4:1 (Fig. 1e), PS templates can form ordered arrays in a whole bowl as that in a plane substrate, and with the increase of the Db:Ds, one bowl can accommodate more spheres (Fig. 1f, Db:Ds ¼6.5:1) to form hierarchical structures of the secondary templates, which is very useful to fabricate functional materials with hierarchical structures. Fig. 2 shows SEM images of TiO2 films fabricated by using the described template technique above. TiO2 films shown in Figs. 2a and b were fabricated by using two-size PS templates (Ds ¼750 nm (PS750) and Ds ¼250 nm (PS250)). Firstly, monodisperse PS750 templates were used to fabricate TiO2 films containing arrayed pores (Db ¼750 nm). Then PS250 templates were lifted by the as-prepared TiO2 films to form the secondary templates, which contain arrayed bowls (Db ¼ 750 nm) and PS250. Finally, the sol–gel approach was employed for filling TiO2 sol into the interspaces of PS250 templates and generating two-size arrayed pores of TiO2 films. It can be seen from Fig. 2b that small pores embedded in bowls are not ordered and uniform as marked with a red circle and blue circle. The reason is that the Db:Ds is of 3:1, which leads to the disordered arrangement of PS250 templates in the TiO2 bowls. Therefore, when we change PS250 templates to PS210 templates to make the secondary templates with Db:Ds ¼3.5:1, arrayed porous TiO2 film with largescale area are formed (Fig. 2c), and to the most arrayed bowls, each of them have seven embedded pores, which are arranged orderly in the bowls as marked with a red circle in Fig. 2d. To make rich morphologies of hierarchical structures, another combination of PS templates was investigated. As shown in Fig. 3,
when we used the monolayered PS templates (Ds ¼3 μm, PS3000) supported by FTO as the secondary substrate, another sized PS templates (Ds ¼300 nm, PS300) can be orderly assembled on the surface of PS templates with large-scale area (Figs. 3a–c). Similarly, the sol–gel approach was employed for filling TiO2 sol in the interspaces of PS3000 and PS300 templates. Novel structures of TiO2 films can be obtained after removing the PS templates. It can be seen from Fig. 3d that the morphology of TiO2 film exhibits arrayed flowerlike patterns. In fact, this film is composed of two layers, which are the bottom layer and the upper layer. The bottom layer is composed of the arrayed TiO2 pores generated from PS3000 templates and the upper layer is composed of hemispherical shells with arrayed small pores, which are formed by PS300 templates.
4. Conclusion In summary, the Db:Ds was investigated in detail for assembling ordered PS into the arrayed bowls to form the secondary templates and TiO2 films with arrayed pores have been fabricated by using these templates. The result of the simulation indicates that assembling ideal and perfect secondary templates depends on the ratio of the Db:Ds. Also, the technique is not only for fabricating hierarchical TiO2 films, but also for preparing other film materials with hierarchical structures.
Acknowledgments This work is supported by the key Project for Education Department of Henan Province (Grant no. 14A150030) and the
J. Lei et al. / Materials Letters 133 (2014) 163–167
Scientific Research Fund of Henan University of Science and Technology (Grant nos. 2013QN033 and 2014ZCX012). References [1] Carnie MJ, Charbonneau C, Barnes PRF, Davies ML, Mabbett I, Watson TM, et al. J Mater Chem A 2013;1:2225–23011. [2] Kim HS, Lee JW, Yantara N, Boix PP, Kulkarni SA, Mhaisalkar S, et al. Nano Lett 2013;13:2412–7. [3] Lee TY, Kim HS, Park NG. ChemPhysChem 2014;15:1098–105. [4] Jiang C, Koh WL, Leung MY, Hong W, Li Y, Zhang J. J Solid State Chem 2013;198:197–202. [5] Gao C, Li X, Wang Y, Chen L, Pan X, Zhang Z, et al. J Power Sour 2013;239:458–65. [6] Lei JF, Li WS, Li XP, Cairns EJ. J Mater Chem 2012;22:22022–7. [7] Lei JF, Li WS, Li XP, Zeng LZ. J Power Sour 2013;242:838–43. [8] Tian WC, Ho YH, Chen CH, Kuo CY. Sensors 2013;13:865–74.
167
[9] Fateh R, Dillert R, Bahnemann D. Langmuir 2013;29:3730–9. [10] Lei JF, Li LB, Shen XH, Du K, Ni J, Liu CJ, et al. Langmuir 2013;29:13975–81. [11] Bian Z, Tachikawa T, Zhang P, Fujitsuka M, Majima T. J Am Chem Soc 2014;136:458–65. [12] Park JJ, Kim DY, Latthe SS, Lee JG, Swihart MT, Yoon SS. ACS Appl Mater Interfaces 2013;5:6155–60. [13] Pandiyan R, Micheli V, Ristic D, Bartali R, Pepponi G, Barozzi M, et al. J Mater Chem 2012;22:22424–32. [14] Han JH, Lee SW, Kim SK, Han S, Lee W, Hwang CS. Chem Mater 2012;24:1407–14. [15] Wobkenberg PH, Ishwara T, Nelson J, Bradley DDC, Haque SA, Anthopoulos TD. Appl Phys Lett 2010;96:082116–6-3. [16] Wu MS, Tsai CH, Wei TC. Chem Commun 2011;47:2871–3. [17] Wang D, Möhwald H. Adv Mater 2004;16:244–7. [18] Lei JF, Li XP, Li WS, Sun FQ, Lu DS, Yi J. Int J Hydrog Energy 2010;36:8167–72. [19] Lei JF, Li XP, Li WS, Sun FQ, Lu DS, Lin YL. J Solid State Electrochem 2012;6:625–32.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Assembly of binary templates for fabricating arrayed pores of TiO2 films with hierarchical structures Jianfei Lei a,n, Kai Du a, Ronghui Wei a, Shaofeng Zhang a, Jing Ni a, Weishan Li b a b
School of Physics and Engineering, Henan University of Science and Technology, Luoyang 471023, China School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China
art ic l e i nf o
a b s t r a c t
Article history: Received 26 May 2014 Accepted 28 June 2014 Available online 8 July 2014
A novel assembly of binary templates was reported and TiO2 films with hierarchical structures were fabricated by using the assembled templates. According to the result of simulating the templates assembly, it is found that the ratio of the diameter of the bowl to the sphere (Db:Ds) is vital for the assembly of ordered templates, which contains arrayed TiO2 bowls and ordered polystyrene spheres (PS). As a result, when the Db:Ds is more than 3:1 (Db:Ds 43:1), ideal and perfect secondary templates can be obtained and hence arrayed pores of TiO2 films with hierarchical structures can be fabricated. & 2014 Elsevier B.V. All rights reserved.
Keywords: Adhesion Aerospace materials Films Hierarchical structures
1. Introduction Titanium dioxide (TiO2) is a kind of technological material due to its potential applications in advanced devices and systems, such as solar cell electrodes [1–5], anodes for lithium ion batteries [6,7], gas sensors [8] and photocatalysts [9–11]. Significant progresses have been achieved recently in developing film-based materials composed of TiO2 nanoparticles. However, in TiO2 nanoparticlebased materials, the performances are often limited by the disordered morphology, which gives rise to interfacial interferences for charge transport or ionic migrating. To overcome this limitation, TiO2 films with ordered structures comprising uniform patterns and hierarchical architectures have been proposed as promising solutions. These include arrayed pores, ordered nanotubes and arrayed shells architectures in TiO2 films. Varied techniques have been utilized for fabricating TiO2 films, such as thermal deposition [12], sputtering [13], chemical vapor deposition [14], spray pyrolysis [15] and electrochemical deposition [16]. Colloidal crystals are favorable for using candidates as templates to prepare thin films because of their simple, fast and cheap approach to creating ordered materials. The majority of recently reported ordered materials, however, have been fabricated from uniformly sized colloidal particles. These materials exhibit a limited range of crystal structures and manacled performances for applications, which are not ideal structures for investigating condensed-matter physics or for practical applications [17]. In contrast to the limited crystal structures formed by collections of
n
Corresponding author. Tel.: þ 86 379 65626265; fax: þ 86 379 65626265. E-mail address: [email protected] (J. Lei).
http://dx.doi.org/10.1016/j.matlet.2014.06.175 0167-577X/& 2014 Elsevier B.V. All rights reserved.
uniformly sized colloidal particles, using binary templates could generate rather rich arrays of crystal structures depending on the size ratio of large and small candidates. However, binary templates are hard to grow and characterize, and thus they have not yet been extensively investigated. Here, we present a method that provides a facile approach to the fabrication of ordered TiO2 films with hierarchical structures. The method is referred to which is good for the rapid fabrication of well-ordered TiO2 films with hierarchical structures by using varied diameter-ratio of colloidal particles. On the basis of simulating varied assembly of templates, TiO2 films with arrayed pores, in which a large pore containing ordered seven small pores (or more pores) to form hierarchical structures, can be fabricated by using the binary templates, which are composed of arrayed pores of TiO2 films and ordered polystyrene spheres (PS) templates. Also, TiO2 films with arrayed pores, in which a large pore with an ordered porous cover, were fabricated by using the binary PS templates.
2. Experimental section Monodisperse PS (average diameters 3 μm, 750 nm, 300 μm, 250 nm and 210 nm) were synthesized by an emulsion polymerization method as previously reported [18]. The typical procedure is as follows: the monomer, including styrene (St) and polyvinylpyrrolidone (PVP), and ethanol were added, argon was passed through the ethanol in the flask for 30 min under 300 rpm agitation and then the temperature was elevated. When the temperature reached 70 1C, the potassium persulfate (KPS) in
164
J. Lei et al. / Materials Letters 133 (2014) 163–167
aqueous solution was injected using a syringe to initiate polymerization, and the polymerization was performed at 70 1C for 18 h. The experimental parameters are summarized in Table 1. PS spheres arrayed in a single layer were assembled on a conductive glass (F-SnO2 conductive glass, FTO, Rs ¼14 Ω/cm2, Table 1 The experimental parameters for preparing PS. PSx
St (wt%)
PVP (wt%)
KPS (wt%)
EtOH þH2O (v: v¼ 9:1) (wt%)
x ¼3000 x ¼750 x ¼300 x ¼250 x ¼210
10 30 30 30 30
5 2 1 0.7 0.5
0.5 0.9 1 1.3 1.5
85 67 68 68 68
Nippon Sheet Glass, Japan) by an improved floating-transfer method described as our previous report [19]. Firstly, 100 μml emulsion containing 20 wt% PS particles was flown into water with the help of slides and formed a film of PS on the surface of water. Then, a drop of sodium dodecyl sulfate (SDS, 5 wt%) was added to the above system to make close assembly of PS particles. Finally, the film was lifted by a clean FTO substrate. This improved method provides an easy way to prepare more regular arrayed PS spheres with controllable layer number. After being dried in air at room temperature, the substrate coated with the monolayered PS templates was heated in an oven to 110 1C for 5 min to bond the monolayer to the substrate. Tetrabutyl titanate (Ti(OBu)4) used as titanium source and the TiO2 sol was formed in the mixture Ti (OBu)4:EtOH:HCl:H2O:CH3COOH¼1:10:7:2:0.6 in molar. The procedure of sol–gel technique is as following: the conductive glass covered by monolayer PS was immersed in TiO2 sol
Fig. 1. Simulation of varied arrangements of the secondary templates (a) Db:Ds ¼ 3:1; (b) Db:Ds ¼ 2:1; (c) Db:Ds ¼ 1:1; (d) Db:Ds ¼ 3.5:1; (e) Db:Ds ¼4:1; and (f) Db:Ds ¼ 6.5:1.
J. Lei et al. / Materials Letters 133 (2014) 163–167
165
Fig. 2. SEM images of TiO2 films fabricated by using the secondary templates (a and b) Db:Ds ¼ 3:1; (c and d) Db:Ds ¼3.5:1. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
vertically for 2 min to keep the sol filling the interspaces of the PS templates fully under the capillary force. Then the conductive glass was drawn from the sol vertically with a drawing rate of 4 cm min 1 and exposed to the air fully at room temperature for the solvents evaporated for the hydrolysis of titanium ions and the formation of TiO2 gel. Subsequently, the gel was calcined at 450 1C in the oven for 3 h to remove the PS templates leaving the arrayed porous TiO2 on the conductive glass. Hierarchical structures of TiO2 films with two-size ordered pores were fabricated as the following procedure. Firstly, monodisperse PS were assembled on a substrate (FTO) to form monolayered templates via the floating-transfer method, and a sol–gel approach was employed for filling TiO2 sol into the interspaces of the PS templates and generating bowl-like arrayed pores of TiO2 films after removing the templates. Then, another sized PS templates with monolayered PS were transferred on the surface of the as-prepared arrayed porous TiO2 film. Finally, the same sol–gel technique as above mentioned for filling the interspaces of templates was employed to form hierarchical structures of TiO2 films.
3. Results and discussion Fig. 1 illustrates varied stages of simulating the assembly of secondary templates. In theory, the prerequisite of assembling ideal and perfect arrangement of small PS placed into a large bowl-like pore (bowl) is that the ratio of the diameter of the bowl to the sphere should be of 3:1 (Db:Ds ¼3:1). As shown in Fig 1a1, when the monolayered PS templates covered on the bowls of TiO2 films, seven PS (marked number 1(PS-1) and marked number 2 (PS-2)) will be in the range of one bowl. Under the work of gravity, these PS have the tendency of sliding to the bottom of the bowls. As a result, PS-2 land
to the bottom of the bowl directly and PS-1 are placed at the edge of the bowl orderly, while PS-3 stay on islands among three bowls (Fig. 1a2). Actually, PS-1 will not be placed at the edge of the bowl perfectly as that we expected, but slid down together with PS-2 and finally touch to PS-2 partially. Thus, as shown in Fig. 1a3 (top view and perspective view), PS-1 are not arrayed in the same plane, but misplaced in varied plans. Obviously, this combination between bowls and PS templates is not expected for the preparation of ordered pores of TiO2 films to form hierarchical structures. On the basis of the above conclusion, we infer that when the Db:Ds is less than 3:1, the original arrays of monolayered PS templates will be disordered due to the landing of the central PS-2 to the bottom of the bowl (Fig. 1b, Db:Ds ¼2:1) or will be placed into bowls completely in the way of one sphere to one bowl (Fig. 1c, Db:Ds ¼1:1). In this case, to fabricate hierarchical structures with ordered pores is impossible and samples fabricated by this model are not expected. On the contrary, when the Db:Ds is more than 3:1, the arrangement of PS in the bowls is different from that mentioned above. As shown in Figs. 1d–f, more PS particles intrude the bowls and the arrangement of PS becomes more orderly than that shown in Figs. 1a–c. Monolayered PS templates have the feature of normal templates before being lifted by the TiO2 bowls, which is one sphere surrounded by six spheres (Fig. 1d1). However, when the PS templates are lifted by the TiO2 bowls, PS-2 will land at the bottom of the bowls directly and PS-1 slides down together with PS-2. Simultaneously, PS-3 and PS-4, which are located at the edge of the bowls, have the tendency of sliding to the bottom of the neighboring bowls together with PS-1 and PS-2 (Fig. 1d2). Interestingly, a funny phenomenon occurs, that the sliding of PS-3 leads to a rearrangement of PS-1 in the bowls. As a result, one of PS-3 will join the arrangement of PS-1 and seven spheres (six PS-1 and one PS-3) rearrange to ordered circles
166
J. Lei et al. / Materials Letters 133 (2014) 163–167
Fig. 3. SEM images of binary colloidal templates and TiO2 films fabricated by using this templates.
finally, which are placed in the bowls and does not touch PS-2 entirely due to the equilibrium pressure generated from the remaining of five PS-3 and two PS-4 (Fig. 1d3). This unique arrangement between PS and bowls exhibits superior spatial configuration for fabricating novel structures. When the Db:Ds reaches 4:1 (Fig. 1e), PS templates can form ordered arrays in a whole bowl as that in a plane substrate, and with the increase of the Db:Ds, one bowl can accommodate more spheres (Fig. 1f, Db:Ds ¼6.5:1) to form hierarchical structures of the secondary templates, which is very useful to fabricate functional materials with hierarchical structures. Fig. 2 shows SEM images of TiO2 films fabricated by using the described template technique above. TiO2 films shown in Figs. 2a and b were fabricated by using two-size PS templates (Ds ¼750 nm (PS750) and Ds ¼250 nm (PS250)). Firstly, monodisperse PS750 templates were used to fabricate TiO2 films containing arrayed pores (Db ¼750 nm). Then PS250 templates were lifted by the as-prepared TiO2 films to form the secondary templates, which contain arrayed bowls (Db ¼ 750 nm) and PS250. Finally, the sol–gel approach was employed for filling TiO2 sol into the interspaces of PS250 templates and generating two-size arrayed pores of TiO2 films. It can be seen from Fig. 2b that small pores embedded in bowls are not ordered and uniform as marked with a red circle and blue circle. The reason is that the Db:Ds is of 3:1, which leads to the disordered arrangement of PS250 templates in the TiO2 bowls. Therefore, when we change PS250 templates to PS210 templates to make the secondary templates with Db:Ds ¼3.5:1, arrayed porous TiO2 film with largescale area are formed (Fig. 2c), and to the most arrayed bowls, each of them have seven embedded pores, which are arranged orderly in the bowls as marked with a red circle in Fig. 2d. To make rich morphologies of hierarchical structures, another combination of PS templates was investigated. As shown in Fig. 3,
when we used the monolayered PS templates (Ds ¼3 μm, PS3000) supported by FTO as the secondary substrate, another sized PS templates (Ds ¼300 nm, PS300) can be orderly assembled on the surface of PS templates with large-scale area (Figs. 3a–c). Similarly, the sol–gel approach was employed for filling TiO2 sol in the interspaces of PS3000 and PS300 templates. Novel structures of TiO2 films can be obtained after removing the PS templates. It can be seen from Fig. 3d that the morphology of TiO2 film exhibits arrayed flowerlike patterns. In fact, this film is composed of two layers, which are the bottom layer and the upper layer. The bottom layer is composed of the arrayed TiO2 pores generated from PS3000 templates and the upper layer is composed of hemispherical shells with arrayed small pores, which are formed by PS300 templates.
4. Conclusion In summary, the Db:Ds was investigated in detail for assembling ordered PS into the arrayed bowls to form the secondary templates and TiO2 films with arrayed pores have been fabricated by using these templates. The result of the simulation indicates that assembling ideal and perfect secondary templates depends on the ratio of the Db:Ds. Also, the technique is not only for fabricating hierarchical TiO2 films, but also for preparing other film materials with hierarchical structures.
Acknowledgments This work is supported by the key Project for Education Department of Henan Province (Grant no. 14A150030) and the
J. Lei et al. / Materials Letters 133 (2014) 163–167
Scientific Research Fund of Henan University of Science and Technology (Grant nos. 2013QN033 and 2014ZCX012). References [1] Carnie MJ, Charbonneau C, Barnes PRF, Davies ML, Mabbett I, Watson TM, et al. J Mater Chem A 2013;1:2225–23011. [2] Kim HS, Lee JW, Yantara N, Boix PP, Kulkarni SA, Mhaisalkar S, et al. Nano Lett 2013;13:2412–7. [3] Lee TY, Kim HS, Park NG. ChemPhysChem 2014;15:1098–105. [4] Jiang C, Koh WL, Leung MY, Hong W, Li Y, Zhang J. J Solid State Chem 2013;198:197–202. [5] Gao C, Li X, Wang Y, Chen L, Pan X, Zhang Z, et al. J Power Sour 2013;239:458–65. [6] Lei JF, Li WS, Li XP, Cairns EJ. J Mater Chem 2012;22:22022–7. [7] Lei JF, Li WS, Li XP, Zeng LZ. J Power Sour 2013;242:838–43. [8] Tian WC, Ho YH, Chen CH, Kuo CY. Sensors 2013;13:865–74.
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