Nanocrystalline NaTaO₃ thin film materials with ordered 3D mesoporous and nanopillar-like structures through PIB-b-PEO polymer templating: towards high-performance UV-light photocatalysts

Abstract

Ordered mesoporous NaTaO₃ thin films with both 3D honeycomb and nanopillar-like structures were successfully produced through sol–gel templating using a novel amphiphilic diblock copolymer, PIB₅₃-b-PEO₄₅, as the structure-directing agent. These nanocrystalline materials not only exhibit enhanced UV-light photocatalytic activity but are also able to maintain stable performance.

---

Perovskite-type sodium tantalate (NaTaO₃) is known to be a highly active UV-light-responsive photocatalyst and is therefore a material of great scientific interest.¹ While several methods, including hydrothermal and microwave syntheses, and so forth, have been utilized to make NaTaO₃ in forms other than bulk,² to our knowledge, the use of evaporation-induced self-assembly (EISA) to produce ordered mesoporous versions of this oxide material has not yet been reported in the literature. The EISA process³ was introduced in the late 1990s and is based on the solution-phase coassembly of either sol–gel precursors or preformed building blocks with a structure-directing agent. Breakthroughs over the past several years have made EISA the method of choice in the synthesis of mesoporous metal oxide thin films with periodicities in the sub-50 nm size range.^4,5 The preparation of ternary oxides still remains challenging, however.

The NaTaO₃ thin films employed in this work were produced through coassembly strategies using an EISA process. Briefly, an isotropic solution containing ethanol, 2-methoxyethanol, polyisobutylene-block-poly(ethylene oxide), referred to as PIB₅₃-b-PEO₄₅, tantalum(V) n-butoxide, sodium acetate, and glacial acetic acid is dip-coated onto a polar substrate (see also the experimental procedure in the ESI†). On evaporation of the volatile constituents, the system coassembles to form an inorganic–organic composite with long-range periodicity. Thermal annealing can then be used to fully crosslink the material, combust the polymer template and crystallize the amorphous inorganic framework. During the course of this work, we found that the use of 2-methoxyethanol as a cosolvent is beneficial in that NaTaO₃ materials with more ordered structures can be achieved. We believe that this is due in part to the fact that 2-methoxyethanol helps slow down the drying process of the PIB₅₃-b-PEO₄₅-templated thin films, which ultimately leads to greater structural flexibility of the inorganic–organic composite that is formed during the EISA process.⁵

As mentioned above, in this work we incorporated a novel polyisobutylene-block-poly(ethylene oxide) diblock copolymer as the structure-directing agent. This amphiphilic polymer possesses many desirable templating properties, including a strong tendency to form lyotropic liquid crystal phases with up to 30 nm repeat distances in a broad range of solvents. Preliminary results have already shown that powder and thin film materials templated using PIB₅₃-b-PEO₄₅ accommodate walls sufficiently thick to allow for both uniform nucleation and for uniform growth of the crystalline phase while retaining nanoscale periodicity.⁶ These features make it particularly attractive for the fabrication of nanocrystalline solids with ordered mesoporous morphologies.

To probe the nanocrystalline NaTaO₃ materials, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used. Fig. 1a–d show SEM data of ∼250 nm thick PIB₅₃-b-PEO₄₅-templated films heated to different annealing temperatures of 650 °C and 700 °C. The top view SEM image in panel (a) reveals a honeycomb structure of pores averaging 12 nm in diameter. From this image it is apparent that the mesopore cavities at the hexagonal top surface are open and that the wall thickness is of the same dimension as the in-plane pore size.⁷ We note that commercially available polymers typically produce materials with wall thicknesses closer to only 5 nm. This is often the reason why the nanostructure is not well preserved after crystallization.⁸ It also provides an explanation for the fact that the existence of ternary oxide thin films with ordered mesoporous morphologies is still scarce. From the cross-sectional SEM image in panel (d), we are able to establish that the cubic pore network observed at the top surface persists throughout the bulk of the films. Panels (b) and (c) are high- and low-magnification top view SEM images, which show that the film morphology is different after thermal annealing at 700 °C. The reason for this is not yet fully understood, but might be associated with sintering. However, it can be clearly seen that the process of restructuring the pore–solid architecture is not accompanied by the loss of nanoscale periodicity but rather produces films with a unique morphology (reminiscent of a nanopillar-like structure). A similar observation was made recently by Oveisi et al.⁹ They reported the conversion of mesoporous Al₂O₃ thin films with a body-centered-cubic Im3̄m-derived structure to γ-Al₂O₃ with vertical pore orientation during the course of crystallization.

Fig. 1 Morphology of ∼250 nm thick PIB₅₃-b-PEO₄₅-templated NaTaO₃ films heated to 650 °C (a and d) and 700 °C (b and c). (a–c) High- and low-magnification top view SEM images. (d) Cross-sectional SEM image showing that the honeycomb pore structure persists throughout the films.

Fig. 2a, d and e show TEM images of PIB₅₃-b-PEO₄₅-templated NaTaO₃ thin films with nanocrystalline walls. These data are consistent with the results from the SEM imaging in that they show an interconnected pore structure for samples heated to 650 °C, while those at 700 °C seem to consist of particulate nanodomains arranged on a hexagonal lattice. Panel (c) is a selected-area electron diffraction (SAED) pattern. Calculated lattice spacings match best with the JCPDS reference card no. 74-2479 for monoclinic NaTaO₃. The crystalline nature of the mesoporous thin films is also confirmed by high-resolution TEM (HRTEM), as can be seen in panel (b).

Fig. 2 Nanoscale structure of PIB₅₃-b-PEO₄₅-templated NaTaO₃ thin films heated to 650 °C (a–c) and 700 °C (d and e). (a, d, e) Low- and high-magnification bright-field TEM images. (b) HRTEM image showing the (100) lattice planes of monoclinic NaTaO₃. (c) SAED pattern of the same film in panel (a). (f–h) GISAXS patterns at an angle of incidence β = 0.2° obtained on thin films heated at 300 °C for 12 h (f), and 650 °C (g) and 700 °C (h) for 5 s, respectively. Scattering vector, q, components are given in nm⁻¹.

Overall, the electron microscopy images in Fig. 1 and 2 collectively verify that NaTaO₃ can be templated using the amphiphilic diblock copolymer PIB₅₃-b-PEO₄₅ to produce high-quality mesoporous thin films.⁶ In addition, they provide ample evidence that the sol–gel derived materials can accommodate the crystallites that form at the onset of crystallization while retaining both nanoscale porosity and periodicity.

More quantitative information on the nanoscale structure was obtained by grazing incidence small-angle X-ray scattering (GISAXS). Fig. 2f–h show GISAXS patterns collected on thin films with both amorphous and nanocrystalline pore walls. Amorphous NaTaO₃ produces patterns with distinct maxima that can be indexed to a face-centered-cubic close-packed pore structure with (111) orientation relative to the plane of the substrate, in accordance with the hexagonal symmetry of the top surface. The elliptical shape of the GISAXS pattern in panel (f) indicates a comparatively large unidirectional lattice contraction. On the basis of the relative position of the scattering maxima in the z-direction, a decrease in film volume of ∼70% is determined for samples aged at 300 °C for 12 h (see also Fig. S1 in the ESI†). Upon heating the PIB₅₃-b-PEO₄₅-templated NaTaO₃ to 650 °C to fully crystallize the inorganic framework, a loss of out-of-plane scattering is observed, as shown in panel (g). This loss is due both to the small number of repeat units in the direction normal to the plane of the substrate and to the process of crystallizing the wall structure itself. We note that the thickness of the films used for GISAXS experiments was approximately 250 nm and therefore only ∼17 repeat units contributed to the overall scattering in the z-direction. However, the results with GISAXS clearly show that the distorted honeycomb network of 12 nm diameter pores is retained when the crystalline phase is achieved. The pattern in panel (g) further indicates the appearance of new in-plane maxima at lower q-values. These diffuse maxima result from the restructuring of the pore–solid architecture, and thus they become more intense as the annealing temperature is increased from 650 °C to 700 °C.

The porous properties were also analyzed by N₂-physisorption. Typical type-IV adsorption–desorption isotherms for PIB₅₃-b-PEO₄₅-templated NaTaO₃ thin films heated to 650 °C are shown in panel (a) of Fig. 3. The 650 °C material exhibits a Brunauer–Emmett–Teller (BET) surface area of ∼270 m² cm⁻³, while samples heated to 700 °C produce smaller values of ∼210 m² cm⁻³. These results confirm the accessibility of the mesopore cavities, which are interconnected through smaller necks. In addition they show that the change in film morphology is only accompanied by a slight reduction in the BET surface area; the total porosity remains virtually unaltered.

Fig. 3 (a) N₂-adsorption–desorption isotherms of ∼320 nm thick PIB₅₃-b-PEO₄₅-templated NaTaO₃ films with a total area of ∼55 cm² heated to 650 °C. (b) WAXD patterns collected after thermal annealing at 650 °C (A) and 700 °C (B). (c and d) XPS spectra of the Na1s and Ta4f core level regions. Solid lines in red are fits to the data assuming a linear background in panel (c) and Shirley background in panel (d).

Fig. 3b shows wide-angle X-ray diffraction (WAXD) patterns obtained on PIB₅₃-b-PEO₄₅-templated thin films heated to 650 °C and 700 °C. The crystallization occurs within a narrow temperature interval of 635 ± 5 °C and leads to the formation of monoclinic NaTaO₃ (see also Table S1 and Fig. S2 in the ESI†). This phase is characteristic of sol–gel derived NaTaO₃ materials, as shown recently by Hu and Teng.¹⁰ From the data in Fig. 3b it can also be seen that neither of the two patterns shows the presence of impurity phases, such as L-Ta₂O₅,¹¹ which could potentially be formed due to phase separation. Applying the Scherrer equation to the most intense (100) peak indicates that the full width at half maximum intensity slightly changes with increasing annealing temperature. The average crystalline domain sizes start at 21 nm and reach 25 nm at 700 °C. Considering the dimensions of the pore–solid architecture, this implies that a single crystallite forms the wall over a distance encompassing at least two pores. Lastly, we note that annealing temperatures higher than 700 °C lead to the loss of nanoscale periodicity both in the off- and in-plane directions due to the sintering of NaTaO₃ grains.

Fig. 3c and d show X-ray photoelectron spectroscopy (XPS) data of the Na1s and Ta4f core level regions, which were curve fit using mixed Gaussian–Lorentzian functions. A typical XPS survey spectrum for PIB₅₃-b-PEO₄₅-templated NaTaO₃ thin films heated to 650 °C in air is given in Fig. S3 (see ESI†). The Na1s region reveals a single peak located at 1071.49 ± 0.05 eV. In contrast, the Ta4f spectrum contains a doublet due to spin–orbit splitting with binding energies of 25.81 ± 0.05 eV and 27.68 ± 0.05 eV for the 4f_7/2 and 4f_5/2 lines, respectively. These peak positions are in agreement with reported measured values for other oxide materials containing Na⁺ and/or Ta⁵⁺.¹² Elemental analyses carried out by comparing the peak areas provide atomic sodium-to-tantalum ratios close to the expected value of 1.0 for stoichiometric NaTaO₃. The results with WAXD and XPS therefore lead us to propose that larger fluctuations in composition can be ruled out and that all characteristics of the mesoporous thin films employed in this work can be associated with the monoclinic NaTaO₃ phase.

For photocatalytic applications, knowledge of the optical characteristics is crucial. Fig. 4a shows plots for direct and indirect optical transitions in PIB₅₃-b-PEO₄₅-templated NaTaO₃ thin films heated to 650 °C. Extrapolation of the linear part of the curves to zero indicates a direct band gap at ∼4.5 eV (equivalent to ∼282 nm light) and an indirect one at ∼3.9 eV (equivalent to ∼318 nm light).¹⁰ The fact that the indirect transition is smaller than the direct one is beneficial for photocatalytic applications; phonon absorption or emission is involved in indirect band transitions. This is due in part to the fact that phonon-assisted mechanisms may enhance the lifetime of generated electron–hole pairs by reducing the recombination rates and, thus, also the activity of the photocatalyst.

Fig. 4 (a) Plots for both direct (○) and indirect (□) optical transitions in PIB₅₃-b-PEO₄₅-templated NaTaO₃ thin films heated to 650 °C. (b) Photobleaching of MB over an exposure period of 120 min (15 min steps) achieved using PIB₅₃-b-PEO₄₅-templated NaTaO₃ heated to 650 °C. (c) Semilogarithmic plots of MB photobleaching: no catalyst (△), nontemplated NaTaO₃ heated to 650 °C (▽), PIB₅₃-b-PEO₄₅-templated NaTaO₃ heated to 650 °C (○) and 700 °C (□). Solid lines in red are linear fits to the data. (d) Bright-field TEM image of PIB₅₃-b-PEO₄₅-templated NaTaO₃ heated to 650 °C after repeated photobleaching experiments.

To probe the photocatalytic activity, bleaching experiments were conducted on templated and nontemplated NaTaO₃ thin films using methylene blue (MB) as the model pollutant. Panels (b) and (c) of Fig. 4 show both typical absorbance data and semilogarithmic plots, indicating that the macro-kinetics are pseudo-first-order. It can also be clearly seen that the photobleaching of MB in the absence of any catalyst is negligible. The degradation rate achieved using PIB₅₃-b-PEO₄₅-templated NaTaO₃ heated to 650 °C is about seven times higher than that of thin films prepared with no polymer template but under otherwise identical conditions. This result shows that the introduction of mesopore cavities provides a beneficial microstructure for photocatalytic applications. Because the density of the mesoporous NaTaO₃ is significantly lower than that of nontemplated material, this value underestimates the improvements on a mass normalized basis though.

To better understand the enhanced activity, the active surface area of the various NaTaO₃ materials has to be taken into account. The calculated areas for PIB₅₃-b-PEO₄₅-templated thin films heated to 650 °C and 700 °C are ∼180 cm² and 140 cm², respectively. For nontemplated NaTaO₃, the active surface area corresponds roughly to the geometric area of ∼4 cm², which is 45 times less compared to PIB₅₃-b-PEO₄₅-templated NaTaO₃ heated to 650 °C. This result, therefore, indicates that the actual surface site density must be lower, i.e., not all sites upon the mesoporous catalysts are available for MB, presumably due to its bulky nature. Similar observations have been made recently for the diffusion of other bulky molecules through nanoporous materials.¹³ However, the present work clearly shows that the photocatalytic activity of NaTaO₃ can be significantly enhanced by structuring at the nanoscale. The fact that the 700 °C material exhibits a lower activity than PIB₅₃-b-PEO₄₅-templated thin films heated to 650 °C is likely due to the reduced BET surface area.

From the data in Fig. 4c it is also apparent that, especially for the PIB₅₃-b-PEO₄₅-templated materials, the first data point does not fit into the overall trend of the photobleaching data. The reason for this is that adsorption of MB onto the NaTaO₃ surface is a fast reaction that occurs within the first few minutes. However, after an exposure time of 15 min we find a steady degradation of MB over time. Lastly, we note that the mesoporous NaTaO₃ thin films are able to maintain a stable performance over several cycles as there is no obvious decline in activity. We associate this excellent stability with the fact that the nanoscale structure is unaffected by photobleaching experiments in aqueous solution, as can be seen in Fig. 4d.

Conclusions

In summary, ordered mesoporous NaTaO₃ thin films with 3D honeycomb and nanopillar-like structures have been successfully prepared through sol–gel templating using a novel polyisobutylene-block-poly(ethylene oxide) diblock copolymer as the structure-directing agent. Electron microscopy, GISAXS, and N₂-physisorption studies collectively verify that both the porosity and the periodicity are retained when the monoclinic phase is achieved, despite restructuring of the pore network. The results with WAXD and XPS further show that the materials studied here are well-defined at the atomic level after annealing in air at temperatures as high as 700 °C.

The present work further establishes the benefits of combining a mesoporous morphology with nanocrystalline thin films to achieve an enhancement in photocatalytic properties. PIB₅₃-b-PEO₄₅-templated NaTaO₃ exhibits a UV-light photocatalytic activity highly superior to that of non-templated thin films of the same initial composition. Part of the reason for this is that the introduction of interconnected porosity has a profound effect on the surface site density, and thus also on the macro-kinetics. Overall, this research shows that mesoporous versions of nanocrystalline tantalates are promising candidates for photocatalytic applications.¹⁴

Future work will be dedicated to extending our synthesis method both to composites by loading the thin films with other materials, such as NiO, and so forth, and to hierarchically porous structures by dual templating in order to tailor the catalytic activity for certain reactions.

Acknowledgements

The authors thank HASYLAB at DESY for beamtime and BASF SE for the supply of diblock copolymer.

References

1. (a) H. Kato, K. Asakura and A. Kudo, J. Am. Chem. Soc., 2003, 125, 3082; (b) A. Kudo, Catal. Surv. Asia, 2003, 7, 31.
2. (a) J. A. Nelson and M. J. Wagner, J. Am. Chem. Soc., 2003, 125, 332; (b) Y. He, Y. Zhu and N. Wu, J. Solid State Chem., 2004, 177, 3868; (c) C. Yan, Z. Q. Wang, Z. S. Li and Z. G. Zou, Solid State Ionics, 2009, 180, 1539.
3. (a) C. J. Brinker, Y. F. Lu, A. Sellinger and H. Y. Fan, Adv. Mater., 1999, 11, 579; (b) Y. F. Lu, R. Ganguli, C. A. Drewien, M. T. Anderson, C. J. Brinker, W. L. Gong, Y. X. Guo, H. Soyez, B. Dunn, M. H. Huang and J. I. Zink, Nature, 1997, 389, 364.
4. (a) P. D. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka and G. D. Stucky, Chem. Mater., 1999, 11, 2813; (b) S. Y. Choi, M. Mamak, N. Coombs, N. Chopra and G. A. Ozin, Adv. Funct. Mater., 2004, 14, 335; (c) B. Smarsly, D. Grosso, T. Brezesinski, N. Pinna, C. Boissiere, M. Antonietti and C. Sanchez, Chem. Mater., 2004, 16, 2948; (d) C. Sanchez, B. Boissiere, D. Grosso, C. Laberty and L. Nicole, Chem. Mater., 2008, 20, 682.
5. J. Haetge, P. Hartmann, K. Brezesinski, J. Janek and T. Brezesinski, Chem. Mater., 2011, 23, 4384.
6. (a) T. von Graberg, P. Hartmann, A. Rein, S. Gross, B. Seelandt, C. Roger, R. Zieba, A. Traut, M. Wark, J. Janek and B. M. Smarsly, Sci. Technol. Adv. Mater., 2011, 12, 025005; (b) C. Weidmann, K. Brezesinski, C. Suchomski, K. Tropp, N. Grosser, J. Haetge, B. M. Smarsly and T. Brezesinski, Chem. Mater., 2012, 24, 486.
7. Y.-J. Cheng, P. Muller-Buschbaum and J. S. Gutmann, Small, 2007, 3, 1379.
8. D. L. Li, H. S. Zhou and I. Honma, Nat. Mater., 2004, 3, 65.
9. H. Oveisi, X. F. Jiang, M. Imura, Y. Nemoto, Y. Sakamoto and Y. Yamauchi, Angew. Chem., Int. Ed., 2011, 50, 7410.
10. C.-C. Hu and H. Teng, Appl. Catal., A, 2007, 331, 44.
11. K. Brezesinski, J. Wang, J. Haetge, C. Reitz, S. O. Steinmueller, S. H. Tolbert, B. M. Smarsly, B. Dunn and T. Brezesinski, J. Am. Chem. Soc., 2010, 132, 6982.
12. S. F. Ho, S. Contarini and J. W. Rabalais, J. Phys. Chem., 1987, 91, 4779.
13. (a) H. T. Chang, N.-M. Wu and F. Zhu, Water Res., 2000, 34, 407; (b) P. Z. Araujo, V. Luca, P. B. Bozzano, H. L. Bianchi, G. J. D. A. Soler-Illia and M. A. Blesa, ACS Appl. Mater. Interfaces, 2010, 2, 1663.
14. J. M. Wu, I. Djerdj, T. von Graberg and B. M. Smarsly, Beilstein J. Nanotechnol., 2012, 3, 123.

---

Footnote

† Electronic Supplementary Information (ESI) available: experimental procedures, SAXS patterns, crystallographic data, XPS survey spectrum. See DOI: 10.1039/c2ra20203d/

---