TiO2 Heterostructures via a Templating

Oct 16, 2013 - College of Life Science, Tarim University, Alar 843300, China. §. School of Chemistry and Environment, South China Normal University, ...
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Fabrication of Ordered ZnO/TiO2 Heterostructures via a Templating Technique Jian F. Lei,*,† Li B. Li,† Xue H. Shen,‡ Kai Du,† Jing Ni,† Chao J. Liu,† and Wei S. Li§ †

School of Physics and Engineering, Henan University of Science and Technology, Luoyang 471023, China College of Life Science, Tarim University, Alar 843300, China § School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China ‡

S Supporting Information *

ABSTRACT: Two kinds of ordered ZnO/TiO2 heterostructures were fabricated via a facile approach. The architecture of the TiO2 substrate could be controlled by alternating the filling forms of the template, and the morphology of the secondary ZnO nanostructure could be further tuned by adjusting the parameters of the hydrothermal reaction. Then two different morphologies of ZnO/ TiO2 heteroarchitectures with ZnO nanorods and nanoplates growing on TiO2 shells and bowls were successfully achieved, respectively.

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INTRODUCTION Titanium dioxide (TiO2) and zinc oxide (ZnO) are two kinds of technologically important semiconductors because of their potential applications in advanced devices and systems.1−3 When used as photocatalyts and illuminated with an appropriate light source, TiO2 and ZnO can generate photoinduced electron/hole (e−/h+) pairs to initiate a series of chemical reactions that eventually decompose the pollutants.4 However, they can absorb only a small portion of the solar spectrum in the ultraviolet (UV) region because of the wide band gap energy (TiO2, 3.2 eV; ZnO, 3.37 eV) and the high recombination rate of photoinduced e−/h+ pairs at or near their surface (as shown in Scheme S1 of the Supporting Information). To make them suitable for receiving and utilizing solar energy with good efficiency, many methods such as dye sensitization5,6 and metal- or nonmetal-doped TiO2- or ZnObased techniques7−10 have been investigated. Because the photocatalytic process is based on the generation of e−/h+ pairs by means of band gap radiation, the coupling of different semiconductor oxides seems useful for achieving a more efficient e−/h+ pair separation under irradiation and, consequently, a higher photocatalytic activity. As for the synthesis of heterogeneous structures, many novel structures have also been achieved, such as ZnO/SnO2 hierarchical structures with one-dimensional ZnO nanostructures grown on SnO2 nanobelts,11 a SnO2/TiO2 core−shell structure with SnO2 nanowires coated by TiO2 nanoparticles,12 V2O5/TiO2 composite nanofibers with single-crystal V2O5 grown on rutile TiO2 nanofibers,13 ZnO/TiO2 hierarchical structures with high densities of secondary ZnO nanostructures grown on primary TiO2 fibers,14 etc. The coupling of two semiconductors possessing different redox energy levels for their corresponding conduction bands © 2013 American Chemical Society

(CB) and valence bands (VB) provides an attractive approach to achieving more efficient charge separation, increasing the lifetime of the charge carriers, and enhancing the efficiency of the interfacial charge transfer to adsorbed substrates. As shown in Scheme S2 of the Supporting Information, in the case of ZnO/TiO2 heterostructure, ZnO has more negative conduction band potential and has a higher exciton binding energy and a higher electron mobility than TiO2; thus, photoelectrons generated on ZnO can be easily injected into the CB of TiO2.15 Simultaneously, photoholes will transfer from the VB of TiO2 to ZnO because of the higher VB potential of ZnO.16 This interparticle charge transfer pathway therefore affords vectorial charge transfer, which is irreversible under certain conditions such as being consumed in a timely fashion by another species. Generally, the photoelectron can be easily trapped by electronic acceptors like adsorbed O2 on the surface and interface of particles, to further produce a superoxide radical anion (•O2−), and the photoholes can be easily trapped by OH− to further produce a hydroxyl radical species (•OH). The generated •O2− and •OH almost determine the overall photocatalytic reaction. For example, •OH and •O2− are extremely strong oxidants for the partial or complete mineralization of organic chemicals. Therefore, the charge transport in the ZnO/TiO2 heterostructure is no doubt a key step for further increasing the photocatalytic efficiency. Obviously, the energy level for electron injection is decreased after ZnO nanostructure covers the surface of TiO2 substrate, which increases the driving force for electron injection and hence reduces the level of recombination between electrons Received: May 5, 2013 Revised: September 19, 2013 Published: October 16, 2013 13975

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Scheme 1. Illustration of the Formation of ZnO/TiO2 Heterostructures

containing a Zn(Ac)2/(CH2)6N4/H2O solution to form ZnO nanorods/TiO2 shells (ZnO/TiO2-RS) and ZnO nanoplates/ TiO2 bowls (ZnO/TiO2-PB), respectively. The hydrothermal reaction was conducted at 95 °C for 8 h. The as-prepared products could be easily collected and then washed with deionized water. In comparison to those of previously reported studies,20 our approach offers an easy, facile, and versatile way to the mixed metal oxide nanostructures. Also, the as-prepared ZnO/TiO2 heterostructures are expected to enhance activity for photocatalysis.

and holes. On the other hand, ZnO can increase the concentration of free electrons in the CB of TiO2; this result implies that the charge recombination is reduced in the process of electron transport. Semiconductor particulates have proven to be particularly attractive photocatalysts in heterogeneous photocatalysis and in advanced oxidation processes in general as they provide an interface with the aqueous medium. Redox species (•O2− and • OH) are eventually poised at the interface following degradation of organic substrates.17 On the basis of this knowledge, the design of heterostructure with a high surface area and the manner in which two kinds of particles touch are very important for maximizing the usage of the redox species. ZnO/TiO2 hierarchical structures with high densities of nanosized ZnO particles growing on the hierarchical and mesoporous TiO2 arrays are interesting composites, which may possess improved physical and chemical properties, but how to prepare such heterostructured materials is still a challenge because of the structural complexity and difficulty in controlling the crystal growth of two different materials. In this paper, we report an effective two-step route for synthesizing two kinds of ZnO/TiO2 hierarchical structures. Constructing tunable TiO2 substrates is vital for the following growth of secondary ZnO nanostructures. In our procedure, TiO2 nanoparticles were assembled into spherical shells or ordered bowls first by using PS templates, in which the size of the substrates and the morphologies of TiO2 can be easily controlled by adjusting the size of the PS templates and the reaction precursors, respectively. Then, ZnO nanostructures were self-assembling on the surface of the TiO2 substrates in the hydrothermal system, and the morphology of ZnO can be tuned via addition of trisodium citrate. As shown in Scheme 1, in the first step, we employed the templating technique [polystyrene (PS) spheres] to generate ordered TiO2 shells18 or bowls,19 which served as substrates to guide the growth of secondary ZnO nanostructures. Subsequently, ZnO nanostructures were formed on the TiO2 through hydrothermal treatment. The experimental process included two typical steps. First, ordered TiO2 shells were assembled on a substrate from PS templates in a liquid-phase deposition (LPD) solution containing (NH4)2[TiF6], and ordered TiO2 bowls were fabricated from implanting Ti(OBu)4 sol into interspaces of arrayed PS templates. Then the obtained films were dispersed in toluene to remove the PS template to form a TiO2 spherical film and a bowl-shaped film, respectively. For the second step, the TiO2 films (shells and bowls) were put into an autoclave

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RESULTS AND DISCUSSION Figure 1 presents the FE-SEM image of PS templates supported by a conducting substrate [FTO, □ = 14 Ω/sq (Nippon Sheet

Figure 1. SEM image of the PS template.

Glass)]. It can be seen from Figure 1 that monolayer PS spheres were easily assembled via a floating transfer method over a large area, and the ∼1.5 μm diameter PS spheres were assembled in arrays. The formed TiO2 shells and bowls with ordered structure mentioned below are based on this templating technique. Panels a and b of Figure 2 show the FE-SEM images of the as-prepared TiO2 shells. The top view clearly displays welldefined and uniform TiO2 layers achieved successfully based on PS templates. The wall size of the shell is ∼450 nm, measured from the side view image of Figure 2a. An interesting result from the side view and the bottom view (Figure 2a) is that macroporous channels (red arrows shown, ∼500 nm) were formed because of the close contact among the PS spheres and between spheres and the substrate, where TiO2 growth is restricted and the channels emerged after the removal of the PS templates. We can expect that those channels help to improve 13976

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Figure 2. (a) Top view image of ordered TiO2 shells with side and bottom views and (b) a high-magnification image. (c) Low-magnification image of ZnO/TiO2 heterojunctions with ZnO nanorods grown on TiO2 shells and high-magnification image of an individual ZnO rod (inset).

Figure 3. (a) SEM images of ordered TiO2 bowls and (b) ZnO/TiO2-PB heterostructures with single-pore images (insets). (c and d) Obtained ZnO/TiO2-PB at various precursor concentrations: (c) 0.01 M Zn(AC)2 and 0.01 M (CH2)6N4 and (d) 0.03 M Zn(AC)2 and 0.03 M (CH2)6N4.

lengths ranging from 2 to 4 μm and diameters ranging from 100 to 250 nm. From the high-magnification SEM image (Figure 2c, inset), the hydrothermally generated ZnO nanorods have a hexagonal morphology with exciting mesopores, which may be formed during the growth of ZnO rods and possibly help to improve the performance of catalysis. As we know, the properties of ZnO or TiO2 depend strongly on their structures, including crystal sizes, orientations, and morphologies. Therefore, the ZnO/TiO2 heterostructure with different ZnO and TiO2 structures will exhibit enhanced performance and afford more opportunities for its application.22

the solution transfer in materials and enhance the activity of catalysis.21 In Figure 2b, the image reveals that the shell with a 1.88 μm diameter is constructed of orderly nanocones that are perpendicular to the surface of the PS and grows layer by layer into the network structure. This feature was confirmed previously.18 On the basis of TiO2 shells, one kind of ZnO/TiO2 heterostructure, the ZnO/TiO2-RS, was fabricated via the hydrothermal reaction at 95 °C. The SEM image (Figure 2c) presents the high density of secondary ZnO nanorods grown on the primary TiO2 substrates, and ZnO nanorods with 13977

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Figure 4. XRD patterns of ZnO/TiO2-RS (a) and ZnO/TiO2-PB (b) heterostructures calcined at 500 °C.

the pore distribution function calculated using the Barret− Joyner−Halenda (BJH) method based on the desorption isotherms could be found for Dpore values of 10−150 nm for two samples; thus, they belong to the class of meso- and macroporous materials, which agrees well with the FE-SEM results. Moreover, the determined BET surface area value was found to be 97.0 m2/g for the PB and 68.4 m2/g for the RS. The main causes of this phenomenon may be the size of the ZnO crystal and the various types of TiO2. To the PB, smaller ZnO plates and mesoporous TiO2 bowl-like skeletons lead to the material having a larger surface.19 Another notable result shown in Figure S1a of the Supporting Information is that a narrow maximum could be found for Dpore values of 2−5 nm to the PB, which can also be attributed to the mesopores existing in TiO2 bowls. Figure 4a reveals that the crystal structures of the ZnO/ TiO2-RS are of wurtzite and anatase mainly, but rutile TiO2 also can be observed because of the rutile formed at a higher calcining temperature (500 °C). Moreover, X-ray diffraction (XRD) studies also indicate that ZnO rods strongly prefer the orientation along the c-axis, because the (002) reflection is greatly enhanced relative to the usual (101) maximal reflection.14 It is known that the (002) reflection of the ZnO crystal associates with the ⟨001⟩ direction, which reflects the growth of the ZnO crystal along the c-axis. The crystals are mostly randomly oriented at the beginning but became ⟨001⟩ oriented after a long period of growth in the absence of surfactants or polyelectrolytes.25 Figure 4b presents the XRD pattern of the ZnO/TiO2-PB heterostructure calcined at 500 °C. Obviously, the intensity of diffraction peaks for the ZnO/ TiO2-PB is different from that of the ZnO/TiO2-RS because of the different proportion of ZnO and TiO2 and the different morphology of two kinds of heterostructures. It is notable that, compared to Figure 4a, the (002) reflection is greatly weakened relative to the usual (101) maximal reflection in Figure 4b because of the addition of trisodium citrate to the hydrothermal solution, which adsorbs preferentially on the (001) surface and slows crystal growth along the ⟨001⟩ orientation.26 Moreover, as shown in Figure S2 of the Supporting Information, the conelike TiO2 samples prepared in the LPD system exhibit a weak feature before being calcined,27 but a typical anatase feature can be observed after the samples are calcined at 500 °C. However, in the sol−gel system, the bowl-like TiO2 samples are amorphous before being calcined and change to anatase and rutile phases after being calcined at 500 °C.19 Figures S3 and S4 of the Supporting Information show the XRD patterns of ZnO/TiO2-RS and ZnO/TiO2-PB hetero-

As mentioned in the Introduction, the ZnO/TiO2 heterostructure can make photoinduced e−/h+ pairs that are separated efficiently and improve the catalytic efficiency of the photocatalyst. Moreover, ZnO/TiO2 heterostructure has excellent performance in energy storage and sensing because of its unique chemical and physical properties.23 By changing the formation process of the TiO2 substrate and tuning the synthetic parameters (route B described in Scheme 1), we can further control the morphology of the ZnO/TiO2 heterostructure (for detailed information, see the Supporting Information) and create the other kind of ZnO/TiO 2 heterostructure, the ZnO/TiO2-PB. Panels a and b of Figure 3 show the SEM images of ordered TiO2 bowls and the ZnO/ TiO2-PB heterostructure with single-pore images (inset). On the basis of the PS template, a large area of ordered TiO2 pores also can be fabricated in this approach. In the inset of Figure 3a, the pore has nearly the same size as a PS sphere, which indicates that the structure of ordered TiO2 bowls can be easily controlled by PS templates. Figure 3b shows that the ZnO/ TiO2-PB with the same large area as the bowls was fabricated via the hydrothermal reaction. Trisodium citrate is a polyelectrolyte and can be adsorbed on the (0001) polar surface of the ZnO crystal to form a chelating ligand with Zn2+, which would preferentially bind to the energetically unfavorable (0001) polar surface. Such absorption could reduce the surface energy of the (0001) planes. Therefore, the axial growth of ZnO nanocrystals was drastically suppressed compared to equatorial growth. As a result, ZnO nanosheets were generated. As shown in Figure 3, via addition of trisodium citrate to the hydrothermal solution at a concentration of 0.0001 mol/L, a layer of uniform ZnO nanoplates grown on the edge of TiO2 bowls was generated. The magnified SEM images in panels c and d of Figure 3 reveal that almost all the ZnO nanoplates show a similar hexagonal morphology. The nanoplates are approximately 10−20 nm in thickness and 500−750 nm in diameter. Moreover, panels c and d of Figure 3 present an interesting result; that is, the size and morphology of ZnO nanoplates are almost unchanged at various precursor concentrations. However, the amount of plates increased, and plates are arranged in a more concentrated manner at a higher concentration of precursor, and spread from the edge to the whole surface of the bowls. N2 adsorption−desorption experiments were conducted to determine the Brunauer−Emmett−Teller (BET) surface area of the as-prepared samples. As shown in panels a and b of Figure S1 of the Supporting Information, hysteresis loops (type IV, H3 loops24) can be observed at p/p0 values of 0.4−1.0 and 13978

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Figure 5. Diffuse reflectance spectra (DRS) and plot of transferred Kubelka−Munk vs energy of the light absorbed of the samples.

Figure 6. Degradation efficiency of methyl orange on the samples.

Kubelka−Munk algorithm.29 As shown in Figure 5b, the estimated band gaps of the ZnO/TiO2-PB and the ZnO/TiO2RS were 2.97 and 3.09 eV, respectively, which are smaller than that of TiO2 or ZnO. This may be due to the formation of a ZnO/TiO2 heterojuntion slowing the electron−hole recombination and hence reducing the level of emission. As described in the Introduction, the photogenerated electron in the CB of ZnO may slip to the CB of TiO2 due to the fact that the CB of ZnO is more cathodic than that of TiO2. Similarly, the valence band of TiO2 is more anodic than that of ZnO, which may lead to transfer of the hole from the VB of TiO2 to the VB of ZnO. This transfer mode can greatly decrease the level of combination of photogenerated e−/h+, hence improving the photocatalytic ability of the ZnO/TiO2 heterostructures, which was demonstrated by the following photocatalytic experiments. Figure 6 displays the degradation efficiency of methyl orange on the ZnO/TiO2 heterostructures together with pure ZnO nanoplates, pure ordered TiO2 bowls, and commercial TiO2 (Degussa-P25) samples. Obviously, the ZnO/TiO2 heterostructures exhibit excellent performance in terms of photocatalytic degradation, and the ZnO/TiO2-PB shows the best efficiency. This result indicates that coupling of two semiconductors possesses properties superior to those of a simple conductor.

structures before they had been calcined together with those after they had been calcined. Obviously, only typical wurtzite can be observed in the two products before they are calcined, but anatase TiO2, rutile TiO2, and wurtzite ZnO can be observed simultaneously after being calcined at 500 °C, indicating TiO2 is poor crystal or amorphous in the heterostructures before being calcined and changed to anatase and rutile phases after being calcined at 500 °C. It is well-known that ZnO is a polar crystal consisting of a polar top surface (0001), a polar basal oxygen plane (0001)̅ , and a nonpolar plane (011̅0). Generally, in the absence of structure modifiers, the (0001) polar plane is energetically unfavorable and thus has a faster growth rate than other planes under solution conditions.28 Consequently, aqueous growth of the ZnO crystal leads to elongated, rodlike hexagonal crystals. By addition of surfactants or polyelectrolytes such as citrate ions, sodium malate, polyvinylpyrrolidone, and cetyltrimethylammonium bromide, the aspect ratio of the ZnO nanorods can be adjusted; hence, nanostructures with different morphologies could be created. In this work, we chose trisodium citrate as a structure-modifying agent, and the experimental results clearly demonstrate that the citrate ion can exert a significant influence on the nucleation and growth process of ZnO crystals. When citrate was introduced into the hydrothermal solution, it formed a chelating ligand with Zn2+, which would preferentially bind to the energetically unfavorable (0001) polar surface. Such absorption could reduce the surface energy of the (0001) plane; therefore, the axial growth of ZnO nanocrystals was drastically suppressed compared to equatorial growth. As a result, hexagonal ZnO nanoplates were generated. To determine the band gaps of the ZnO/TiO2 hetrostructures, we replotted the spectra shown in Figure 5a in the form shown in Figure 5b obtained by the application of the

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CONCLUSION In summary, we have successfully synthesized two kinds of ZnO/TiO2 heterostructures with novel hierarchical architectures by a combination of templating and a hydrothermal method. Moreover, the morphologies of secondary ZnO products could be facilely tuned from nanorods to nanoplates by adjusting the experimental parameters. We believe this 13979

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methodology provides a new avenue that offers a relatively mild and environmentally benign approach for large-scale preparation of various heterostructures with structural complexity and thus allows various functions. These special heterostructures possess great potential for applications in photocatalysts, photovoltaics, sensors, and supercapacitors.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the experimental procedures, scheme of the recombination of the photoinduced e−/h+ pair diagram, illustration of the coupling of semiconductors in which vectorial displacement of electrons and holes is possible, N2 adsorption− desorption isotherms, degradation efficiency of methyl orange on the samples, and XRD patterns of the ZnO/TiO2-RS and ZnO/TiO2-PB before they had been calcined. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone and fax: 86-37965626265. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the joint project of the National Natural Science Foundation of China (Grant 21273084), the key project for education department of Henan Province (Grant 13B140988), and the Scientific Research Fund of the Henan University of Science and Technology (Grant 2013QN033).

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