Ag Nanoparticles Located on Three-Dimensional Pine Tree-Like Hierarchical TiO2 Nanotube Array Films as High-Efficiency Plasmonic Photocatalysts
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High specific surface area three-dimensional pine tree-like hierarchical TiO2 nanotube array films loaded with Ag nanoparticles were successfully prepared by one-step hydrothermal reaction combining with simple and feasible magnetron sputtering. The composite Ag/TiO2-branched nanotube arrays show outstanding photocatalytic property, which is attributed to the boost of plasmonic enhancement carrier generation and separation, higher specific surface area, higher organic pollutant absorption, faster charge transport, and superior light-harvesting efficiency for efficient charge collection. The work provides a cost-effective and flexible pathway to develop high-performance photocatalyst or optoelectronic devices.
KeywordsThree-dimensional TiO2 Ag nanoparticles Plasmonics Photocatalysts
Fluorine-doped tin oxide
Localized surface plasmon resonance
Potassium titanium oxide oxalate dehydrates
Scanning electron microscopy
X-ray photoelectron spectroscopy
In spite of nearly half a century investigations, since Fujishima and Honda discovered the photocatalytic water splitting on TiO2 electrodes in 1972 , TiO2 still remains to be intensively investigated as semiconductor photocatalyst owing to its important applications in phtocatalysis [2, 3, 4, 5, 6, 7], photoelectrochemical water splitting [8, 9, 10, 11, 12], solar cell [13, 14, 15, 16], and sensors [17, 18], because of its excellent chemical stability, abundance, and low cost. However, the photocatalytic activities of TiO2 are restricted by its low photocatalytic sensitivity in the UV region and rapid recombination of photogenerated electron and hole pairs . Much effort has been dedicated to enhance the photocatalytic efficiency of TiO2 in the aspect of morphology, surface area, and surface defects. Zero-dimensional (D) TiO2 particles fabricated with randomly organized provides a large specific surface area for absorbing sufficient dye molecules. However, it has high charge recombination because of the large grain boundary of nanoparticles (NPs) [20, 21]. Then, well-aligned one-dimensional nanostructure such as nanowires, nanorods, and nanotubes were fabricated to improve charge transport due to a direct transport pathway for photogenerated electrons [22, 23, 24]. Nevertheless, the drawback of the low surface-to-volume ratio of one-dimensional nanostructure results to a low photocatalytic activity. Recently, researchers have been enthusiastically dedicated to develop three-dimensional nanostructures such as nanoflowers and nanotrees for application in photocatalysis [25, 26, 27, 28]. Compared with zero-dimensional NPs and one-dimensional nanowires, the three-dimensional nanostructure offers the advantage of a large surface area that increases dye loading. Additionally, the three-dimensional morphology could offer long optical paths for efficient light absorption and abundant active sites for electrochemical reactions, providing efficient transport pathway for rapid charge transport that leads to improving electron collection and electron-hole separation.
TiO2 decorated with noble metal (Au, Ag, Pt, etc.) NPs named plasmonic photocatalysis is another promising method for enhancing the photocatalytic activity of TiO2 owing to the localized surface plasmon resonance (LSPR) effect of metal NPs [29, 30, 31, 32, 33]. The band gap of TiO2 is about 3.2 eV, and Ag NPs show a very strong LSPR absorption in the near-UV region [34, 35]. So, Ag is an optimal choice due to the Ag NPs’ LSPR position close to the exciton absorption of TiO2; on the other hand, silver is most suitable for industrial applications owing to its easy preparation and low cost. Moreover, the Ag NPs decorated on the surface of TiO2 could act as an electron trap center to effectively prevent electron-hole recombination and enhance the photocatalytic activity.
Results and Discussion
The parameters on spectral distribution and relative intensity of the used mercury lamp in the photocatalytic tests
Relative intensity (%)
The degradation of RhB of TiO2 and Ag/TiO2 composite structure with different Ag deposition times under UV and visible light irradiation
Degradation time (min)
k value (min−1)
9 × 10−3
3.5 × 10−2
3.7 × 10−2
6 × 10−2
1.5 × 10−2
The stability of the photocatalyst is very important for practical applications. Therefore, the stability of Ag(30s)/TiO2 has been further evaluated by recycling the photocatalyst for RhB degradation as shown in Fig. 6b. It can be seen that the degradation rate of RhB solution is more than 90% after 4 cycles with duration of 60 min per cycle, which is to say that the photocatalytic efficiency of Ag(30s)/TiO2 does not exhibit obvious loss after several recycles. The result shows that the Ag/TiO2 composite has high stability during the photocatalytic degradation of RhB.
Large specific surface area and fast charge transport.
Three-dimensional pine tree-like hierarchical TiO2 nanotube arrays composing of a vertically oriented nanotube trunks and grafted with large amounts of short branches that have a large surface and enhance the absorption of dye molecules. Moreover, the photo-induced electrons directly transport through the nanotube , and the nanotube provides efficient transport pathway for rapid charge transport that leads to improving the electron collection and the electron-hole separation. Therefore, the photocatalytic activity of TiO2 could be increased.
LSPR-mediated local field enhancement.It is well known that the LSPR effect of Ag NPs can induce a strong local electric field. Moreover, here, a large number of Ag NPs are uniformly located on the branches of TiO2, so near-field dipolar interactions between adjacent particles were very strong. Therefore, the LSPR of Ag NPs can enhance the local field near the surface of NPs as well as the giant field enhancement between adjacent particles [39, 46, 47]. Hence, there may be induced strong local electric field in the structure. The strong local electric field can increase the light capturing, and therefore boosts the generation of electron-hole pairs in Ag/TiO2 composite, and hence improves the performance of photocatalysis. The typical Raman spectra of TiO2, Ag(10s)/TiO2, Ag(20s)/TiO2, Ag(30s)/TiO2, and Ag(40s)/TiO2 are shown in Fig. 7. It can be observed that the Raman intensity of Ag/TiO2 composites increases compared to that of pure TiO2. The Raman scattering intensity increased with the increase of Ag deposition time and then decreased when the deposition time reaches 40 s, and the sample of Ag(30s)/TiO2 shows the strongest Raman intensity. As is well known that Raman scattering intensity is proportional to the square of the intensity of a local field , Raman peak intensity enhancement is due to LSPR-mediated large near-field enhancement. The scattering and absorption cross-sections are separately proportional to R6 and R3 when the size of NPs is much smaller than the wavelength of light. As a result, for small particles (about <30 nm), the optical response dominates by absorption. Moreover, the absorption increases with the increased size of Ag NPs, it implies that the local electrical field induced by LSPR will increase with the increase of NP size. Therefore, the Ag(30s)/TiO2 shows high enhancement factors of Raman scattering. However, the optical response dominates by scattering for larger particles [48, 49], so, the local field induced by LSPR of the Ag NPs of Ag(40s)/TiO2 is weaker than that of other samples. On the other hand, due to the light scatter increase for Ag(40s)/TiO2, the optical path increases, so the absorption of light can also increase. Therefore the Ag(40s)/TiO2 also shows much higher photocatalytic performance than that of pure TiO2. Based on our experiment results and literature, the boost generation of the electron-hole pairs due to the effect of LSPR is the critical factor to enhance the photocatalytic activity of TiO2.
The electron transfer from Ag to TiO2.
Figure 1c illustrates the possible charge transfer process in Ag/TiO2 system. When TiO2 is irradiated by light with energy higher than its band gap, valence electron is excited into the conduction band and hole still in the valence. Valence holes accumulated to the surface and induced the surface hydroxyl radical · OH , then oxidated the decomposition of RhB. However, the generated electrons would transfer from the TiO2 to Ag NPs because the work function of TiO2 (4.2 eV ) is lower than that of Ag (4.52 to 4.74 eV ), and Ag NPs act as electron trap which effectively facilitate the separation of photogenerated carriers, thus improves the transfer efficiency of electron and hole pairs [53, 54]. The electrons on Ag NPs will be transferred to the absorbed oxygen and form superoxide, the formed superoxide is responsible for the reduction of organic RhB . Thus, the Ag/TiO2 structure can efficiently prevent the recombination of electron and hole, therefore improves the photocatalytic efficiency of TiO2.
In summary, the high-performance plasmonic photocatalyst three-dimensional pine tree-like hierarchical TiO2 nanotube array films loaded with Ag NPs were fabricated by a simple two-step process. A large number of uniform Ag NPs dispersed in the pin tree-like hierachical TiO2, which effectively improved the light harvest, boosted the generation of electron and hole pairs, and notably improved the separation, transport, and electron-hole pairs with large specific surface area, significantly improved the photocatalytic performance under UV-visible light irradiation (seven times than pure TiO2). Therefore, this research supplies an effective synthetic strategy for noble metal NP-modified three-dimensional hierarchical TiO2, which will be of great significance for promising applications in the fields of environment and energy for high-efficiency light-energy conversion.
Synthesis of Three-Dimensional Pine Tree-Like Hierarchical TiO2 Nanotube Array Films
Three-dimensional tree-like TiO2nanotube arrays were synthesized using a hydrothermal method. The details of the synthetic procedure were described by Roh et al. . Briefly, 0.73 g of potassium titanium oxide oxalate dehydrates (PTO) was dissolved in 7-ml deionized water, and then, the mixed solution was added to 33-ml diethylene glycol (DEG) and stirred well. Fluorine-doped tin oxide (FTO) transparent conductive glass substrates were washed by isopropanol, chloroform, and deionized water successively. And then, a cleaned FTO was placed in a teflon-lined stainless steel autoclave filled with mixed solutions. The hydrothermal reactions temperature is 200 °C, and the reaction time is 11 h. After the reaction, the as-synthesized samples were washed with water more than once. After that, the as-prepared samples were annealed in air at 500 °C for 1 h to remove the residuary organic substance.
Preparation of Ag/TiO2 Composites
Ag NPs were deposited on three-dimensional pine tree-like hierarchical TiO2 nanotube array films by a magnetron sputtering system. The deposition rate of Ag is about 20 nm min−1, and deposition time is 10, 20, 30, and 40 s, then obtained Ag NP-decorated three-dimensional pine tree-like hierarchical TiO2 nanotube array films were named as Ag(10s)/TiO2, Ag(20s)/TiO2, Ag(30s)/TiO2, and Ag(40s)/TiO2.
The morphology and microstructure of the sample was examined by SEM (S4800, Hitachi) operated at an acceleration voltage of 5 kV. The crystallinity and phase constitutions of all samples were analyzed with X-ray diffraction (XRD) (D8 Germany, Bruker Axs). The elemental chemical status and compositions were analyzed with X-ray photoelectron spectroscopy (XPS) using Mg Ka1, 2 (1253.6 eV) excitation. The Raman scattering spectra were analyzed by a micro-Raman system (LabRAM HR800, HORIBA JobinYvon, Paris, France). Ar laser (532.0 nm) is the excitation source, and laser power was kept at 2.5 mW. The UV-vis absorption spectra of all samples were determined by UV-visible dual-beam spectrophotometer (Shimadzu UV 2550).
Photocatalytic Activity Measurement
The photocatalytic activity of the prepared sample with an area of about 6 cm2 was evaluated by decolorization of 10-ml rhodamine B (RhB) solution with the concentration of 10 mg/L. Mercury lamplight as a UV and visible light source (spectral distribution and relative intensity of the mercury lamp in Table 1). The temperature was maintained at 18 °C in the process of photocatalytic reaction by equipping with a water circulation facility. Before irradiation, all samples were put into 10-ml RhB solution for 30 min in darkness in order to establish an adsorption/desorption equilibrium of RhB molecules on the surface of the photocatalysts. The degradation of RhB solution was determined by using an UV-vis spectrophotometer (Shimadzu UV 2550) at 554.0 nm. The time interval is 30 min, and the total reaction time was 2 h. As a comparison, the RhB solution was also measured under the same experimental conditions.
The authors are thankful to the NSFC (51371131, 11305056, and 61422407), Jiangsu Provincial Natural Science Foundation (BK20141217) and the Open Foundation of Hubei Collaborative Innovation Center for High-efficiency Utilization of Solar Energy (Grant Nos. HBSKFM2014013).
NSFC (51371131, 11305056, and 61422407), Jiangsu Provincial Natural Science Foundation (BK20141217) and the Open Foundation of Hubei Collaborative Innovation Center for High-efficiency Utilization of Solar Energy (Grant Nos. HBSKFM2014013).
JX participated in the material preparation and data analysis and drafted the manuscript. ZX, WQ, XG, and DH participated in the sample characterization. XX participated in its design and coordination. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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