DOI: 10.1002/chem.201403428
Communication
& Nanoparticles
Au Photosensitized TiO2 Ultrathin Nanosheets with {001} Exposed Facets Chao Hu, Xuan Zhang, Xinshi Li, Yan Yan, Guangcheng Xi,* Haifeng Yang, and Hua Bai*[a] Abstract: We report a general route for the direct growth of metal particles on TiO2 nanosheets with (001) exposed facets by an oxygen-vacancy-driven self-redox reaction. As there is no need for thermal treatment to remove stabilizing agents, the structure of the nanoparticles can be retained, preserving the active sites associated with high activity.
As a wide-band-gap semiconductor, TiO2 exhibits excellent photocatalytic activity because of its suitable energy levels. It has been extensively investigated and its activity has been found to be influenced by a variety of factors including crystalline phases, exposed surfaces, crystallinity, morphology, and preparation conditions.[1] Among the common crystalline phases of TiO2, anatase is generally considered to be more catalytically active than rutile.[2] Normally, a natural anatase TiO2 crystal exhibits (101), (100), (010) and (001) surfaces. Among them, the (001) surface has been suggested to be the most reactive one from theoretical calculations and experimental certification.[3] However, there is an inherent drawback for TiO2 : the wide band gap limits its light absorption to the UV region only, which accounts for about 4–5 % of total sunlight. The recent and rapid development of localized surface plasmon resonance (LSPR) photosensitization has offered a strategy to overcome the unsatisfactory efficiency of photocatalysts.[4] That is, semiconductors loaded with noble-metal particles, such as Au and Ag, exhibit visible-light activity based on LSPR.[5] These improved performances might be caused by the charge transfer from photoexcited metal to the semiconductor and/or LSPR-induced electromagnetic fields in the surrounding metal/semiconductor nanostructure.[6] In this respect, a closecontacted interface between metal and semiconductor is important to improve the activity of photocatalysts. The presence of an obstacle on the interface, such as a surfactant, which are often used in the fabrication of metal nanoparticles, will inevi-
[a] C. Hu, X. Zhang, X. Li, Dr. Y. Yan, Prof. G. Xi, Dr. H. Yang, Prof. H. Bai Nanomaterials and Nanoproducts Research Center Chinese Academy of Inspection and Quarantine No.3, Gaobeidian North Road Chaoyang Distract, Beijing, 100123 (P.R. China) Fax: (+ 86) 10-8577-2625 E-mail: [email protected] [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201403428. Chem. Eur. J. 2014, 20, 13557 – 13560
tably hinder the direct charge transfer or dramatically decrease the intensity of the LSPR-induced electromagnetic field, as this field decays with distance.[5a, 7] Calcination is typically the most simple and efficient route to remove surfactants, but it is difficult to apply to nanostructures with special morphology, such as rods, wires, or sheets, all of which cannot survive in high temperatures.[8] Such high-temperature methods lead to significant changes in the size and morphology of the nanoparticles that often extensively negate their catalytic efficacy. For most applications, once the metal nanoparticles have been immobilized on a solid support, the stabilizing molecules are no longer required. Therefore, if metal nanoparticles could be grown directly over semiconductor surfaces without any stabilizing agents, their catalytic activity would be significantly enhanced. Here, we report a free-surfactant self-redox process for the direct growth of noble-metal nanoparticles on TiO2 ultrathin nanosheets with (001) exposed active facets through a spontaneous oxygen-defect-driven reaction between weakly reductive TiO2 x and oxidative metal salt precursors in aqueous solution. Figure S1 in the Supporting Information shows the synthetic route to the Au/TiO2 nanosheets. It is well known that metal oxides with low valence states often have weak reducing power. Therefore, we could first synthesize TiO2 nanoparticles with a desired morphology; then low valence state or nonstoichiometric TiO2 x could be synthesized by treating with NaBH4. The obtained oxygen-vacancy-rich and reductive TiO2 x could directly react with the oxidative HAuCl4 in solution. By this process, HAuCl4 was reduced to elemental Au on the surface of TiO2 x, while TiO2 x was re-oxidized to TiO2. Because no foreign reducing agents or surfactants were used in the formation of the Au/TiO2, we call this process a self-redox process. The TiO2 nanosheets with (001) exposed facets were prepared by an improved method.[3d] Simply, tetrabutyl titanate and hydrofluoric acid (HF) was dissolved in a mixture of propanol and isopropanol, and the obtained white suspension was transferred to a teflon-lined stainless-steel autoclave and heated at 200 8C for 18 h. The X-ray diffraction (XRD) pattern (Figure S2 in the Supporting Information) of the prepared TiO2 product is in good agreement with the standard data for anatase TiO2 (JCPDS No.: 21-1272). Compared with the relative peak width shown in the XRD standard pattern, the apparently broadened (004) diffraction peak suggests that the as-synthesized products have a very small size along the [001] direction, whereas the very sharp (200) diffraction peak strongly suggests a large crystal size along the [100]/[010] direction. Therefore, the results suggest that the product possesses a sheetlike
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Communication nanostructure, which was further demonstrated by high-resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED). The micro-Raman spectrum also confirms that the product possesses the typical six Raman modes of anatase phase TiO2 (Figure S3 in the Supporting Information). The morphology of the anatase TiO2 was first characterized by field-emission scanning electron microscopy (FE-SEM). Figure S4a (see the Supporting Information) presents a typical FESEM image of the sample, showing that the sample is composed of a large number of nanosheets that are 70–200 nm in width and 1.5–3 nm in thickness. Transmission electron microscopy (TEM) image (Figure S4b) analysis further showed that the sample consisted of well-defined nanosheets having a rectangular outline. Figure S4c shows a HR-TEM image recorded from the white-framed area indicated in Figure S4b. The fringe spacing of 0.19 nm agrees well with the spacing of the (200) and (020) lattice planes of anatase TiO2. The diffraction spots of the corresponding SAED pattern (indexed as the [001] zone) can be indexed as the 200 and 020 reflections (inset in Figure S4c), demonstrating that the rectangular facets are characterized by (001) planes, in agreement with the HRTEM image. Figure S4d gives a schematic illustration of the crystal orientation of the nanosheets. Energy-dispersive X-ray spectroscopy (EDS) of the anatase TiO2 nanosheets confirms that the sheets were titanium dioxide (Figure S5 in the Supporting Information). It is should be noted that the width and thickness of the nanosheets could be conveniently controlled by adjusting the precursor concentrations (Figure S6 in the Supporting Information). Moreover, the synthesis can be easily scaled up by using a large autoclave to prepare the TiO2 nanosheets on a multigram scale (Figure S7 in the Supporting Information), making our finding, together with the dimension control, very relevant for applications. The anatase TiO2 nanosheets were further characterized by nitrogen adsorption and desorption isotherms at 77 K (Figure S8 in the Supporting Information). It was found that the TiO2 nanosheets have a high Brunauer–Emmett–Teller (BET) surface area of 176 m2g 1, a property that is a favorable factor for catalysis. In addition, the band gap of the TiO2 nanosheets is estimated to be 3.26 eV from the UV/Vis absorption spectrum (Figure S9 in the Supporting Information). The surfaces of the TiO2 nanosheets were reduced by treating with NaBH4. Figure 1 a shows a typical FE-SEM image of the TiO2 x nanosheets obtained by treating with NaBH4 aqueous solution. The morphology of the sample is still sheetlike (Figure S10 in the Supporting Information). We have confirmed that the pure TiO2 nanosheets were highly crystalline, as seen from the well-resolved lattice features shown in Figure S4c. After the treating with NaBH4, however, the surface of the TiO2 x nanosheets became disordered; the disordered outer layer surrounding the crystalline core was ~ 0.7 nm thick (Figure 1 a). The TiO2 x nanosheets show unusual photophysical properties as indicated by UV/Vis absorption (Figure 1 b); a very large absorption tail is present in the visible region and extends to the near infrared (NIR) region. Despite this, the XRD Chem. Eur. J. 2014, 20, 13557 – 13560
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Figure 1. a) HR-TEM image of the TiO2 x nanosheets. The dashed curve is shown to outline a portion of the interface between the crystalline core and the disordered outer layer (marked with white arrows) of the TiO2 x nanosheets. b–d) UV/Vis absorption spectra (b), XRD patterns (c), and Raman spectra (d) of the TiO2 x and TiO2 nanosheets.
patterns of TiO2 x and TiO2 nanosheets are very similar (Figure 1 c). This result demonstrates that the reduction only took place on the surface of the nanosheets, and that the inside of the TiO2 x nanosheets is still composed of TiO2. Raman spectroscopy was used to examine surface structural changes in the TiO2 nanosheets after the introduction of disorder with NaBH4 treatment (Figure 1 d). The original TiO2 nanosheets display the typical anatase Raman bands, but a very wide Raman scattering band from 800–3000 cm 1 emerges for the TiO2 x nanosheets, an observation that indicates that surface structural changes occur after NaBH4 treatment. The results demonstrate that the surface of the nanosheets consist of a large number of oxygen vacancy defects, which indicates that the TiO2 x nanosheets possess reducing power.[9] To obtained Au-nanoparticle-loaded TiO2 nanosheets, the prepared TiO2 x nanosheets were reacted with HAuCl4 aqueous solution at room temperature. Figure 2 a and b show the FESEM and TEM images of the materials. Compared with the morphology of TiO2 x nanosheets, the surface of the Au/TiO2 nanosheets is coarse and many tiny Au nanoparticles can be seen on the nanosheets. At the same time, the well-resolved lattice features of the Au nanoparticles shown in the HR-TEM image (Figure 2 c) confirm that the Au nanoparticles are highly crystalline. Furthermore, we found Au nanorods could formed on the (001) facets of the thicker TiO2 nanosheets (50–80 nm wide, 2–5 nm thick) by adjusting the HAuCl4 concentration (see the Supporting Information), as shown in Figure 2 d. The HR-TEM image confirms the Au nanorods are also highly crystalline (Figure 2 e). Figure 2 f gives the UV/Vis absorption spectra of the Au-nanoparticle- and nanorod-loaded TiO2 nanosheets. Owing to the LSPR effect, the light absorption regions of the two samples were extended to the entire visible-light region (400–700 nm). Compared with the single peak of the Au-nanoparticle-loaded TiO2 nanosheets, the plasmon resonance absorption is split into two modes for the Au-nanorod-
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Figure 4. FE-SEM images of Au/P25 (a), Au/WO3 (b), Au/MoO3 (c), and Au/ SnO2 (d) obtained by the self-redox method.
Figure 2. FE-SEM (a), TEM (b), and HR-TEM (c) images of the Au-nanoparticle-loaded TiO2 nanosheets. TEM (d) and HR-TEM (e) image of the Au-nanorod-loaded TiO2 nanosheets. UV/Vis absorption spectra (f) of the Au-nanocrystal-loaded TiO2 nanosheets.
loaded TiO2 nanosheets, an observation corresponding to the oscillation of the free electrons along and perpendicular to the long axis of the nanorods. We also prepared Pt/TiO2 and Pd/TiO2 hybrid nanosheets by using the self-redox method described above. TEM images of these samples (Figure 3) show that the surfaces of the TiO2 nanosheets are decorated with numerous nanoparticles having an ultrathin size and a very narrow size distribution. To assess the universality and utility of this synthetic method for potential industrial processes of loading of metal particles, 100 g of commercial P25, WO3, SnO2, and MoO3 powders were used as precursors to synthesize Au/semiconductors by this self-redox method. FE-SEM images of these samples (Figure 4) show that the surfaces of commercial semiconductor particles are decorated with numerous Au nanoparticles with narrow size distribution.
Figure 3. TEM images of the Pt/TiO2 nanosheets (a) and Pd/TiO2 nanosheets (b) obtained by the self-redox method. Chem. Eur. J. 2014, 20, 13557 – 13560
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The photocatalytic degradation of toxic pollutants is of great significance in environmental pollutant treatment and represents a commonly used approach to characterize the activity of photocatalysts. The solar-driven photocatalytic activity of the Au-nanocrystal-loaded TiO2 nanosheets was measured by monitoring the change in optical absorption of a rhodamine B (RhB) solution at approximately 554 nm during its photocatalytic decomposition process. The photocatalytic results were compared with RhB photolysis (without photocatalyst) and with those obtained over bare TiO2 nanosheets and P25 powders (Figure 5 a). Photodegradation of RhB was complete after 7 min and 9 min for the Au-nanorod- and Au-nanoparticlemodified TiO2 nanosheets, respectively. For the unmodified TiO2 nanosheets under the same experimental conditions, it
Figure 5. Rate of photodegradation of RhB (a) and MB (b) in the presence of the photocatalysts. NR: nanorods; NP: nanoparticles. c) Rate of photodegradation of RhB in the presence of Au/TiO2 obtained by different synthetic methods. d) Cycling tests of the photocatalytic activity of the Au-nanorodloaded TiO2 nanosheets. Data in the figure represent the first 8 min of measurements in each of the nine consecutive testing cycles.
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication took almost half an hour for complete degradation to occur. When using the P25 powder as the photocatalyst, the photodegradation of the RhB solution was complete after nearly 1 h. It should be noted that the degradation rate achieved with the Au-nanorod-loaded TiO2 nanosheets is faster than that of the Au-nanoparticle-loaded TiO2 nanosheets, a difference that can be explained by greater light harvesting ability of the Au-nanorod-loaded TiO2 nanosheets, as shown in Figure 2 e. For the degradation of methylene blue (MB), another common azo dye, the Au/TiO2 nanosheets also showed significantly enhanced photocatalytic activities (Figure 5 b). We also compared the activities of the Au/TiO2 nanosheets obtained by the self-redox method and Au/TiO2 nanosheets obtained by the traditional surfactant-assisted method (see the Supporting Information for details on the synthetic method). The Au/TiO2 nanosheets prepared by the self-redox method show the faster degradation rate (Figure 5 c). The results of cycling tests of the photocatalytic activity of Au-nanorod-loaded TiO2 nanosheets in decomposing RhB are shown in Figure 5 d. Once a cycle was complete, an amount of concentrated RhB was added and the subsequent cycle began. The Au-nanorodloaded TiO2 nanosheets did not exhibit any reduction in their activity under light irradiation after nine cycles. The enhanced photocatalytic activity of the Au/TiO2 nanosheets probably results from the synergistic effects of the Au nanoparticles and the active (001) facets of TiO2 nanosheets. First, owing to the very clean interface between the Au nanoparticles and the TiO2 nanosheets, high intensity of LSPR is manifested as an amplification effect for the electromagnetic field in the vicinity of Au nanoparticles. Our experiments show that the Au-nanoparticle-loaded TiO2 nanosheets display photocatalytic activity even under visible-light irradiation, which clearly demonstrates the LSPR effect of the photocatalysts. Second, surface scientists have recently demonstrated that the order of the average surface energies of anatase TiO2 is 0.90 J m 2 for {001} > 0.53 J m 2 for {100} > 0.44 J m 2 for {101}.[10] The TiO2 nanosheets with a higher percentage of (001) facets exhibited a more effective photocatalytic performance, demonstrating the high catalytic activity of the (001) facets.[3b, c] In the present work, as a photocatalyst, the Au-nanocrystalmodified TiO2 nanosheets have a high percentage of (001) facets (above 95 %), resulting their high photocatalytic efficiency. In addition, the high electron conductivity of the Au nanoparticles also helps to improve the photogenerated electron/ hole separation process.[11] Despite this analysis, the underlying physical mechanism is still obscure. Therefore, further study on this issue is required, as it is important for developing effective plasmonic composite photocatalysts. In summary, through an in situ self-redox reaction between oxygen-defect-rich metal oxides and oxidative metal salt precursors, a series of noble metal/semiconductor nanocomposites with uniform metal dispersion and tunable metal particle morphology were obtained. We believe this general and gramscale method can be used as a new strategy to prepare high-
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quality metal-particle-loaded semiconductor hybrid composites. The as-synthesized Au/TiO2 nanosheets are able to harvest wide-range visible and NIR light, and have greatly enhanced photocatalytic performance.
Acknowledgements This work received financial support from the Natural Science Foundation of China (51102220) and the Dean Fund of CAIQ (2014JK006). Keywords: active facets · nanosheets · photocatalysis · surface plasmon resonance · visible light
[1] X. B. Chen, L. Liu, P. Y. Yu, S. S. Mao, Science 2011, 331, 746 – 749. [2] M. A. Henderson, Surf. Sci. Rep. 2011, 66, 185 – 297. [3] a) H. G. Yang, C. H. Sun, S. Z. Qiao, J. Zou, G. Liu, S. C. Smith, H. M. Cheng, G. Q. Lu, Nature 2008, 453, 638 – 642; b) J. S. Chen, Y. L. Tan, C. M. Li, Y. L. Cheah, D. Y. Luan, S. Madhavi, F. Y. C. Boey, L. A. Archer, X. W. Lou, J. Am. Chem. Soc. 2010, 132, 6124; c) X. G. Han, Q. Kuang, M. S. Jin, Z. X. Xie, L.S Zheng, J. Am. Chem. Soc. 2009, 131, 3152 – 3153; d) S. W. Liu, J. G. Yu, M. Jaroniec, J. Am. Chem. Soc. 2010, 132, 11914 – 11916; e) C. Hu, X. Zhang, W. T. Li„ Y. Yan, G. G. Xi, H. F. Yang, J. F. Li, H. Bai, J. Mater. Chem. A 2014, 2, 2040 – 2043. [4] a) Z. W. Liu, W. B. Hou, P. Pavaskar, M. Aykol, S. B. Cronin, Nano Lett. 2011, 11, 1111 – 1116; b) S. Linic, P. Christopher, D. B. Ingram, Nat. Mater. 2011, 10, 911 – 921; c) E. Kowalska, O. O. P. Mahaney, R. Abea, B. Ohtani, Phys. Chem. Chem. Phys. 2010, 12, 2344 – 2355; d) Z. K. Zheng, B. B. Huang, X. Y. Qin, X. Y. Zhang, Y. Dai, M. H. Whangbo, J. Mater. Chem. 2011, 21, 9079 – 9087. [5] a) L. Q. Liu, S. X. Ouyang, J. H. Ye, Angew. Chem. 2013, 125, 6821 – 6825; Angew. Chem. Int. Ed. 2013, 52, 6689 – 6693; b) Q. Zhang, D. Q. Lima, I. Lee, F. Zaera, M. F. Chi, Y. D. Yin, Angew. Chem. 2011, 123, 7226 – 7230; Angew. Chem. Int. Ed. 2011, 50, 7088 – 7092; c) H. M. Chen, C. K. Chen, C. J. Chen, L. C. Cheng, P. C. Wu, B. H. Cheng, Y. Z. Ho, M. L. Tseng, Y. Y. Hsu, T. S. Chan, J. F. Lee, R. S. Liu, D. P. Tsai, ACS Nano 2012, 6, 7362 – 7372; d) W. B. Hou, W. H. Hung, P. Pavaskar, A. Goeppert, M. Aykol, S. B. Cronin, ACS Catal. 2011, 1, 929 – 936; e) P. Christopher, H. Xin, A. Marimuthu, S. Linic, Nat. Mater. 2012, 11, 1044 – 1050. [6] S. Linic, P. Christopher, D. B. Ingram, Nat. Mater. 2011, 10, 911 – 921. [7] E. A. Coronado, E. R. Encina, F. D. Stefani, Nanoscale 2011, 3, 4042 – 4059. [8] a) J. A. Lopez-Sanchez, N. Dimitratos, C. Hammond, G. L. Brett, L. Kesavan, S. White, P. Miedziak, R. Tiruvalam, R. L. Jenkins, A. F. Carley, D. Knight, C. J. Kiely, G. J. Hutchings, Nat. Chem. 2011, 3, 551 – 556; b) L. D. Menard, F. Xu, R. G. Nuzzo, J. C. Yang, J. Catal. 2006, 243, 64 – 73; c) H. Yin, Z. Ma, M. Chi, S. Dai, Cata. Lett. 2010, 136, 209 – 221. [9] a) T. Ntz, M. Haase, J. Phys. Chem. B 2000, 104, 8430 – 8437; b) G. C. Xi, J. H. Ye, Q. Ma, N. Su, H. Bai, C. Wang, J. Am. Chem. Soc. 2012, 134, 6508 – 6511; c) T. R. Gordon, M. Cargnello, T. Paik, F. Mangolini, R. T. Weber, P. Fornasiero, C. B. Murray, J. Am. Chem. Soc. 2012, 134, 6751 – 6761. [10] a) M. Lazzeri, A. Vittadini, A. Selloni, Phys. Rev. B 2002, 65, 119901; b) U. Diebold, Surf. Sci. Rep. 2003, 48, 53. [11] P. Wang, B. B. Huang, X. Y. Qin, X. Y. Zhang, Y. Dai, J. Y. Wei, M. H. Whangbo, Angew. Chem. 2008, 120, 8049 – 8051; Angew. Chem. Int. Ed. 2008, 47, 7931 – 7933. Received: May 7, 2014 Published online on August 29, 2014
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& Nanoparticles
Au Photosensitized TiO2 Ultrathin Nanosheets with {001} Exposed Facets Chao Hu, Xuan Zhang, Xinshi Li, Yan Yan, Guangcheng Xi,* Haifeng Yang, and Hua Bai*[a] Abstract: We report a general route for the direct growth of metal particles on TiO2 nanosheets with (001) exposed facets by an oxygen-vacancy-driven self-redox reaction. As there is no need for thermal treatment to remove stabilizing agents, the structure of the nanoparticles can be retained, preserving the active sites associated with high activity.
As a wide-band-gap semiconductor, TiO2 exhibits excellent photocatalytic activity because of its suitable energy levels. It has been extensively investigated and its activity has been found to be influenced by a variety of factors including crystalline phases, exposed surfaces, crystallinity, morphology, and preparation conditions.[1] Among the common crystalline phases of TiO2, anatase is generally considered to be more catalytically active than rutile.[2] Normally, a natural anatase TiO2 crystal exhibits (101), (100), (010) and (001) surfaces. Among them, the (001) surface has been suggested to be the most reactive one from theoretical calculations and experimental certification.[3] However, there is an inherent drawback for TiO2 : the wide band gap limits its light absorption to the UV region only, which accounts for about 4–5 % of total sunlight. The recent and rapid development of localized surface plasmon resonance (LSPR) photosensitization has offered a strategy to overcome the unsatisfactory efficiency of photocatalysts.[4] That is, semiconductors loaded with noble-metal particles, such as Au and Ag, exhibit visible-light activity based on LSPR.[5] These improved performances might be caused by the charge transfer from photoexcited metal to the semiconductor and/or LSPR-induced electromagnetic fields in the surrounding metal/semiconductor nanostructure.[6] In this respect, a closecontacted interface between metal and semiconductor is important to improve the activity of photocatalysts. The presence of an obstacle on the interface, such as a surfactant, which are often used in the fabrication of metal nanoparticles, will inevi-
[a] C. Hu, X. Zhang, X. Li, Dr. Y. Yan, Prof. G. Xi, Dr. H. Yang, Prof. H. Bai Nanomaterials and Nanoproducts Research Center Chinese Academy of Inspection and Quarantine No.3, Gaobeidian North Road Chaoyang Distract, Beijing, 100123 (P.R. China) Fax: (+ 86) 10-8577-2625 E-mail: [email protected] [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201403428. Chem. Eur. J. 2014, 20, 13557 – 13560
tably hinder the direct charge transfer or dramatically decrease the intensity of the LSPR-induced electromagnetic field, as this field decays with distance.[5a, 7] Calcination is typically the most simple and efficient route to remove surfactants, but it is difficult to apply to nanostructures with special morphology, such as rods, wires, or sheets, all of which cannot survive in high temperatures.[8] Such high-temperature methods lead to significant changes in the size and morphology of the nanoparticles that often extensively negate their catalytic efficacy. For most applications, once the metal nanoparticles have been immobilized on a solid support, the stabilizing molecules are no longer required. Therefore, if metal nanoparticles could be grown directly over semiconductor surfaces without any stabilizing agents, their catalytic activity would be significantly enhanced. Here, we report a free-surfactant self-redox process for the direct growth of noble-metal nanoparticles on TiO2 ultrathin nanosheets with (001) exposed active facets through a spontaneous oxygen-defect-driven reaction between weakly reductive TiO2 x and oxidative metal salt precursors in aqueous solution. Figure S1 in the Supporting Information shows the synthetic route to the Au/TiO2 nanosheets. It is well known that metal oxides with low valence states often have weak reducing power. Therefore, we could first synthesize TiO2 nanoparticles with a desired morphology; then low valence state or nonstoichiometric TiO2 x could be synthesized by treating with NaBH4. The obtained oxygen-vacancy-rich and reductive TiO2 x could directly react with the oxidative HAuCl4 in solution. By this process, HAuCl4 was reduced to elemental Au on the surface of TiO2 x, while TiO2 x was re-oxidized to TiO2. Because no foreign reducing agents or surfactants were used in the formation of the Au/TiO2, we call this process a self-redox process. The TiO2 nanosheets with (001) exposed facets were prepared by an improved method.[3d] Simply, tetrabutyl titanate and hydrofluoric acid (HF) was dissolved in a mixture of propanol and isopropanol, and the obtained white suspension was transferred to a teflon-lined stainless-steel autoclave and heated at 200 8C for 18 h. The X-ray diffraction (XRD) pattern (Figure S2 in the Supporting Information) of the prepared TiO2 product is in good agreement with the standard data for anatase TiO2 (JCPDS No.: 21-1272). Compared with the relative peak width shown in the XRD standard pattern, the apparently broadened (004) diffraction peak suggests that the as-synthesized products have a very small size along the [001] direction, whereas the very sharp (200) diffraction peak strongly suggests a large crystal size along the [100]/[010] direction. Therefore, the results suggest that the product possesses a sheetlike
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Communication nanostructure, which was further demonstrated by high-resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED). The micro-Raman spectrum also confirms that the product possesses the typical six Raman modes of anatase phase TiO2 (Figure S3 in the Supporting Information). The morphology of the anatase TiO2 was first characterized by field-emission scanning electron microscopy (FE-SEM). Figure S4a (see the Supporting Information) presents a typical FESEM image of the sample, showing that the sample is composed of a large number of nanosheets that are 70–200 nm in width and 1.5–3 nm in thickness. Transmission electron microscopy (TEM) image (Figure S4b) analysis further showed that the sample consisted of well-defined nanosheets having a rectangular outline. Figure S4c shows a HR-TEM image recorded from the white-framed area indicated in Figure S4b. The fringe spacing of 0.19 nm agrees well with the spacing of the (200) and (020) lattice planes of anatase TiO2. The diffraction spots of the corresponding SAED pattern (indexed as the [001] zone) can be indexed as the 200 and 020 reflections (inset in Figure S4c), demonstrating that the rectangular facets are characterized by (001) planes, in agreement with the HRTEM image. Figure S4d gives a schematic illustration of the crystal orientation of the nanosheets. Energy-dispersive X-ray spectroscopy (EDS) of the anatase TiO2 nanosheets confirms that the sheets were titanium dioxide (Figure S5 in the Supporting Information). It is should be noted that the width and thickness of the nanosheets could be conveniently controlled by adjusting the precursor concentrations (Figure S6 in the Supporting Information). Moreover, the synthesis can be easily scaled up by using a large autoclave to prepare the TiO2 nanosheets on a multigram scale (Figure S7 in the Supporting Information), making our finding, together with the dimension control, very relevant for applications. The anatase TiO2 nanosheets were further characterized by nitrogen adsorption and desorption isotherms at 77 K (Figure S8 in the Supporting Information). It was found that the TiO2 nanosheets have a high Brunauer–Emmett–Teller (BET) surface area of 176 m2g 1, a property that is a favorable factor for catalysis. In addition, the band gap of the TiO2 nanosheets is estimated to be 3.26 eV from the UV/Vis absorption spectrum (Figure S9 in the Supporting Information). The surfaces of the TiO2 nanosheets were reduced by treating with NaBH4. Figure 1 a shows a typical FE-SEM image of the TiO2 x nanosheets obtained by treating with NaBH4 aqueous solution. The morphology of the sample is still sheetlike (Figure S10 in the Supporting Information). We have confirmed that the pure TiO2 nanosheets were highly crystalline, as seen from the well-resolved lattice features shown in Figure S4c. After the treating with NaBH4, however, the surface of the TiO2 x nanosheets became disordered; the disordered outer layer surrounding the crystalline core was ~ 0.7 nm thick (Figure 1 a). The TiO2 x nanosheets show unusual photophysical properties as indicated by UV/Vis absorption (Figure 1 b); a very large absorption tail is present in the visible region and extends to the near infrared (NIR) region. Despite this, the XRD Chem. Eur. J. 2014, 20, 13557 – 13560
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Figure 1. a) HR-TEM image of the TiO2 x nanosheets. The dashed curve is shown to outline a portion of the interface between the crystalline core and the disordered outer layer (marked with white arrows) of the TiO2 x nanosheets. b–d) UV/Vis absorption spectra (b), XRD patterns (c), and Raman spectra (d) of the TiO2 x and TiO2 nanosheets.
patterns of TiO2 x and TiO2 nanosheets are very similar (Figure 1 c). This result demonstrates that the reduction only took place on the surface of the nanosheets, and that the inside of the TiO2 x nanosheets is still composed of TiO2. Raman spectroscopy was used to examine surface structural changes in the TiO2 nanosheets after the introduction of disorder with NaBH4 treatment (Figure 1 d). The original TiO2 nanosheets display the typical anatase Raman bands, but a very wide Raman scattering band from 800–3000 cm 1 emerges for the TiO2 x nanosheets, an observation that indicates that surface structural changes occur after NaBH4 treatment. The results demonstrate that the surface of the nanosheets consist of a large number of oxygen vacancy defects, which indicates that the TiO2 x nanosheets possess reducing power.[9] To obtained Au-nanoparticle-loaded TiO2 nanosheets, the prepared TiO2 x nanosheets were reacted with HAuCl4 aqueous solution at room temperature. Figure 2 a and b show the FESEM and TEM images of the materials. Compared with the morphology of TiO2 x nanosheets, the surface of the Au/TiO2 nanosheets is coarse and many tiny Au nanoparticles can be seen on the nanosheets. At the same time, the well-resolved lattice features of the Au nanoparticles shown in the HR-TEM image (Figure 2 c) confirm that the Au nanoparticles are highly crystalline. Furthermore, we found Au nanorods could formed on the (001) facets of the thicker TiO2 nanosheets (50–80 nm wide, 2–5 nm thick) by adjusting the HAuCl4 concentration (see the Supporting Information), as shown in Figure 2 d. The HR-TEM image confirms the Au nanorods are also highly crystalline (Figure 2 e). Figure 2 f gives the UV/Vis absorption spectra of the Au-nanoparticle- and nanorod-loaded TiO2 nanosheets. Owing to the LSPR effect, the light absorption regions of the two samples were extended to the entire visible-light region (400–700 nm). Compared with the single peak of the Au-nanoparticle-loaded TiO2 nanosheets, the plasmon resonance absorption is split into two modes for the Au-nanorod-
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Figure 4. FE-SEM images of Au/P25 (a), Au/WO3 (b), Au/MoO3 (c), and Au/ SnO2 (d) obtained by the self-redox method.
Figure 2. FE-SEM (a), TEM (b), and HR-TEM (c) images of the Au-nanoparticle-loaded TiO2 nanosheets. TEM (d) and HR-TEM (e) image of the Au-nanorod-loaded TiO2 nanosheets. UV/Vis absorption spectra (f) of the Au-nanocrystal-loaded TiO2 nanosheets.
loaded TiO2 nanosheets, an observation corresponding to the oscillation of the free electrons along and perpendicular to the long axis of the nanorods. We also prepared Pt/TiO2 and Pd/TiO2 hybrid nanosheets by using the self-redox method described above. TEM images of these samples (Figure 3) show that the surfaces of the TiO2 nanosheets are decorated with numerous nanoparticles having an ultrathin size and a very narrow size distribution. To assess the universality and utility of this synthetic method for potential industrial processes of loading of metal particles, 100 g of commercial P25, WO3, SnO2, and MoO3 powders were used as precursors to synthesize Au/semiconductors by this self-redox method. FE-SEM images of these samples (Figure 4) show that the surfaces of commercial semiconductor particles are decorated with numerous Au nanoparticles with narrow size distribution.
Figure 3. TEM images of the Pt/TiO2 nanosheets (a) and Pd/TiO2 nanosheets (b) obtained by the self-redox method. Chem. Eur. J. 2014, 20, 13557 – 13560
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The photocatalytic degradation of toxic pollutants is of great significance in environmental pollutant treatment and represents a commonly used approach to characterize the activity of photocatalysts. The solar-driven photocatalytic activity of the Au-nanocrystal-loaded TiO2 nanosheets was measured by monitoring the change in optical absorption of a rhodamine B (RhB) solution at approximately 554 nm during its photocatalytic decomposition process. The photocatalytic results were compared with RhB photolysis (without photocatalyst) and with those obtained over bare TiO2 nanosheets and P25 powders (Figure 5 a). Photodegradation of RhB was complete after 7 min and 9 min for the Au-nanorod- and Au-nanoparticlemodified TiO2 nanosheets, respectively. For the unmodified TiO2 nanosheets under the same experimental conditions, it
Figure 5. Rate of photodegradation of RhB (a) and MB (b) in the presence of the photocatalysts. NR: nanorods; NP: nanoparticles. c) Rate of photodegradation of RhB in the presence of Au/TiO2 obtained by different synthetic methods. d) Cycling tests of the photocatalytic activity of the Au-nanorodloaded TiO2 nanosheets. Data in the figure represent the first 8 min of measurements in each of the nine consecutive testing cycles.
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Communication took almost half an hour for complete degradation to occur. When using the P25 powder as the photocatalyst, the photodegradation of the RhB solution was complete after nearly 1 h. It should be noted that the degradation rate achieved with the Au-nanorod-loaded TiO2 nanosheets is faster than that of the Au-nanoparticle-loaded TiO2 nanosheets, a difference that can be explained by greater light harvesting ability of the Au-nanorod-loaded TiO2 nanosheets, as shown in Figure 2 e. For the degradation of methylene blue (MB), another common azo dye, the Au/TiO2 nanosheets also showed significantly enhanced photocatalytic activities (Figure 5 b). We also compared the activities of the Au/TiO2 nanosheets obtained by the self-redox method and Au/TiO2 nanosheets obtained by the traditional surfactant-assisted method (see the Supporting Information for details on the synthetic method). The Au/TiO2 nanosheets prepared by the self-redox method show the faster degradation rate (Figure 5 c). The results of cycling tests of the photocatalytic activity of Au-nanorod-loaded TiO2 nanosheets in decomposing RhB are shown in Figure 5 d. Once a cycle was complete, an amount of concentrated RhB was added and the subsequent cycle began. The Au-nanorodloaded TiO2 nanosheets did not exhibit any reduction in their activity under light irradiation after nine cycles. The enhanced photocatalytic activity of the Au/TiO2 nanosheets probably results from the synergistic effects of the Au nanoparticles and the active (001) facets of TiO2 nanosheets. First, owing to the very clean interface between the Au nanoparticles and the TiO2 nanosheets, high intensity of LSPR is manifested as an amplification effect for the electromagnetic field in the vicinity of Au nanoparticles. Our experiments show that the Au-nanoparticle-loaded TiO2 nanosheets display photocatalytic activity even under visible-light irradiation, which clearly demonstrates the LSPR effect of the photocatalysts. Second, surface scientists have recently demonstrated that the order of the average surface energies of anatase TiO2 is 0.90 J m 2 for {001} > 0.53 J m 2 for {100} > 0.44 J m 2 for {101}.[10] The TiO2 nanosheets with a higher percentage of (001) facets exhibited a more effective photocatalytic performance, demonstrating the high catalytic activity of the (001) facets.[3b, c] In the present work, as a photocatalyst, the Au-nanocrystalmodified TiO2 nanosheets have a high percentage of (001) facets (above 95 %), resulting their high photocatalytic efficiency. In addition, the high electron conductivity of the Au nanoparticles also helps to improve the photogenerated electron/ hole separation process.[11] Despite this analysis, the underlying physical mechanism is still obscure. Therefore, further study on this issue is required, as it is important for developing effective plasmonic composite photocatalysts. In summary, through an in situ self-redox reaction between oxygen-defect-rich metal oxides and oxidative metal salt precursors, a series of noble metal/semiconductor nanocomposites with uniform metal dispersion and tunable metal particle morphology were obtained. We believe this general and gramscale method can be used as a new strategy to prepare high-
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quality metal-particle-loaded semiconductor hybrid composites. The as-synthesized Au/TiO2 nanosheets are able to harvest wide-range visible and NIR light, and have greatly enhanced photocatalytic performance.
Acknowledgements This work received financial support from the Natural Science Foundation of China (51102220) and the Dean Fund of CAIQ (2014JK006). Keywords: active facets · nanosheets · photocatalysis · surface plasmon resonance · visible light
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