Two novel hierarchical homogeneous nanoarchitectures of TiO2 nanorods branched and P25-coated TiO2 nanotube arrays and their photocurrent performances
© Hu et al; licensee Springer. 2011
Received: 28 July 2010
Accepted: 18 January 2011
Published: 18 January 2011
We report here for the first time the synthesis of two novel hierarchical homogeneous nanoarchitectures of TiO2 nanorods branched TiO2 nanotube arrays (BTs) and P25-coated TiO2 nanotube arrays (PCTs) using two-step method including electrochemical anodization and hydrothermal modification process. Then the photocurrent densities versus applied potentials of BTs, PCTs, and pure TiO2 nanotube arrays (TNTAs) were investigated as well. Interestingly, at -0.11 V and under the same illumination condition, the photocurrent densities of BTs and PCTs show more than 1.5 and 1 times higher than that of pure TNTAs, respectively, which can be mainly attributed to significant improvement of the light-absorbing and charge-harvesting efficiency resulting from both larger and rougher surface areas of BTs and PCTs. Furthermore, these dramatic improvements suggest that BTs and PCTs will achieve better photoelectric conversion efficiency and become the promising candidates for applications in DSSCs, sensors, and photocatalysis.
In current years, one-dimensional (1D) TiO2 nanostructure materials, especially nanotubular [1–3] and hierarchical [4–7] nanoarchitecture TiO2 nanotube arrays (TNTAs), have initiated increasing research interest owing to their intriguing architectures because they possess very high specific surface areas and a dual-channel for the benefit of the electrons transportation from interfaces to electrodes [7–13]. These nanostructure materials have shown very promising applications in dye-sensitized solar cells (DSSCs) [14–16], photocatalysis [17–19], photosplitting water [20, 21], sensors [22, 23], photoelectrochemical cells , and piezoelectronics . However, as far as we are concerned, tremendous efforts have been conducted to improve the geometrical factors of the nanotube layers [8–13, 26], to convert amorphous TiO2 nanotubes into different crystalline forms (i.e., anatase or rutile phase, or mixture phases of anatase and rutile) through high temperature annealing for high performance applications [27–29], and also many studies have devoted one's mind to change the crystal structure or chemistry composition of the tubes by modifying and doping [30–33]. There still remain many challenges to prepare and discuss the homogeneous modification of TNTAs, although the similar synthesis method of growing branched ZnO nanowires  and the decoration process of growing TiO2 nanoparticles on TiO2 nanotubes by a TiCl4 treatment  have been reported. Therefore, it is particularly valuable to seek some facile and high-efficiency method to synthesize the modification of TNTAs nanostructures for further specific surface area.
In this communication, we report for the first time the synthesis of two novel hierarchical homogeneous modification nanoarchitectures (i.e., P25-coated TNTAs, PCTs; and TiO2 nanorods branched TNTAs, BTs) via two-step method of electrochemical anodization and hydrothermal modification approach. The main precursors of modification are the P25 (Degussa, Germany) and titanium(IV) isopropoxide (TTIP of 95%). Erenow, the optimized nanoarchitecture TNTAs (with bigger pore diameter, longer length, and larger space among tubes) have been prepared by electrochemical anodization method. Interestingly, the as-synthesized BTs and PCTs with beautiful morphologies show both larger and rougher surface area, and these properties result in dramatic improvement of light-absorbing and charge-harvesting efficiency, which has been shown through the UV-Vis diffuse reflectance spectroscopic spectra and photoelectrochemical performances in this article.
Fabrication of optimum nanoarchitecture TNTAs
In this article, TNTAs were prepared using a typical anodization approach . Briefly, the fabrication process of the optimum nanoarchitecture TNTAs with bigger pore diameter, larger space among tubes and longer length was described as follows, Titanium foil samples, about 200 μm × 2 cm × 3.5 cm (Purity≥99.6%, from ShengXin non-ferrous metal Co., LTD, Baoji, Shanxi, China) were cleaned with soap, acetone, and iso-propanol before anodization. A two-electrode configuration was used for anodization, with Ti foil as the anode, and platinum foil as the cathode. A 99.7% pure Ti foil (0.2 mm thickness, 2 × 3 cm2) was immersed in the electrolyte containing 0.35 wt% NH4F (85% Lactic Acid) and 10 vol.% DMSO (dimethyl sulphoxide: purity ≥99.0%) at a 45 V constant potential for 9 h. Thus we obtained the amorphous TNTAs, and then the as-prepared TNTAs were annealed at 400°C for 1.5 h for further use.
Synthesis of hierarchical homogeneous nanoarchitecture BTs
The BTs were obtained via a modification process of growing TiO2 nanorods on the as-prepared TNTAs by conventional hydrothermal growth method. Briefly, the as-prepared TNTAs were immersed in a beaker with growth solution, this solution was consisted of 90 mL of 0.8 M HCl (36-38%) with constant stirring at 25°C for about 15 min. After that, 6 mL of TTIP of 95% as precursor was dropped (0.16 μL/s) in mixture solution, kept stirring for 1 h [7, 32, 33], and then the beaker was sealed and heated at 95°C for 9 h, with slight stirring maintained for the entire heating process to grow TiO2 nanorods on the TNTAs. After the reaction, the reactant was cooled freely to room temperature and washed several times with ethanol and distilled water, and the as-prepared BTs were obtained. The BTs were finally achieved through annealing in a muffle furnace at 400°C for 2 h.
Fabrication of hierarchical homogeneous nanoarchitecture PCTs
We fabricated PCTs via a hydrothermal approach of coating P25 on the as-prepared TNTAs. About 0.4 g P25 (Degussa, Germany) was put into a beaker with 300 mL of distilled water, then they were mixed through vigorous magnetic stirring and ultrasonicating alternately at room temperature more than 5 times (about 10 min per time), After that, the mixed solution was kept state static more than 3 h, and then transferred into a Teflon-lined autoclave (80 mL), in which the as-prepared TNTAs were suspended. The autoclave was sealed and heated at 80-120°C for 12 h to coat P25 on the TNTAs, and then it was cooled freely to room temperature and washed several times with distilled water, thus the as-prepared PCTs were obtained. Finally, the PCTs were fabricated after the as-prepared PCTs were annealed at 400°C for 2 h.
The crystal structures of the as-synthesized samples were firstly determined by using a Bruker D8 advance X-ray diffractometer (XRD, Cu Kα radiation; λ = 1.5418 Å). Then the morphologies were observed by field-emission scanning electron microscopy (FESEM, JOEL, JSM-6700F), and transmission electron microscopy (TEM and HRTEM, JEM-2010FEF; 200 kV). Photoelectrochemical experiments were carried out using a three-electrode configuration (CH instruments, CHI 660C) with a Pt wire counter electrode, a reference saturated calomel electrode and a working electrode. The all samples used as working electrodes were illuminated with a 150~350 W adjustable xenon lamp (from Shanghai Lansheng Electronics Co., LTD., Model, XQ350W). The measured light irradiance was approximately 100 mW/cm2, and the scan rate was 100 mV/s
Results and discussion
Figure 3 is the characterization of another homogeneity nanostructure (the PCTs). Figure 3a is the top view FESEM image of the PCTs. A cross-sectional view in Figure 3b shows that the length of the tubes is the same as that of TNTAs (about 3.5 μm) and the P25 nanoparticles are densely grown on the whole surface (including inside and outside) of the TiO2 tubes. And the top view of the PCTs with many attached P25 particles is clearly shown by the high-magnification FESEM image in Figure 3c. Meanwhile, Figure 3d shows the PCTs' TEM image, and its inset of the HRTEM image shows the (101) crystal facet and the 0.35 nm interplane distance of a typical anatase TiO2 while the another inset of the SAED pattern shows that the PCTs are polycrystalline structure . The growth mechanism of the PCTs is mainly dependent on the special structures and morphology of TNTAs, especially its bigger pore diameter, larger space among tubes, and rough surface. Moreover, annealing plays an important role in the process of transforming the P25 on the TiO2 tube surface from attached state into crystallization state.
On the basis of the above observations and structural analyses, we conclude that both of the BTs and PCTs can provide larger and rougher surface areas than the TNTAs compared with the arrays of same geometrical size and quantity [7, 34, 35]. As a result, this larger and rougher surface areas are favorable to improve light-absorbing and charge-harvesting efficiency and to absorb more dye for better photoelectric conversion efficiency and better applications such as photocatalysis, sensors, etc. Moreover, it is also found that the growth length and density of the TiO2 nanorods of the BTs can be readily controlled by adjusting the growth time and the concentration of growth solution, and that the density of the coated P25 particles can also be controlled through changing the coating time and the concentration of coating solution.
In summary, we have reported here the fabrication of two novel hierarchical homogeneous nanoarchitectures of BTs and PCTs with larger and rougher surface areas via facile hydrothermal modification process. Based on the investigation of the photocurrent densities versus applied potential, the photocurrent density of BTs, at -0.11 V and under the same illumination conditions, shows more than 1.5 times higher than that of TNTAs while PCTs versus TNTAs is more than 1 times higher. On the basis of the results and discussion, we conclude that the dramatically improved photocurrent densities of the BTs and PCTs used as photoanodes are mainly due to their better incident photons and photogenerated charge-harvesting capability compared to TNTAs resulting from their further enhanced and rough surface areas. As a result, our study will also provide a new approach in conformating hierarchical homogeneity nanostructure materials and presenting two kinds of promising candidates for applications in DSSCs, sensors, and photocatalysis.
branched TiO2 nanotube arrays
dye-sensitized solar cells
field-emission scanning electron microscopy
P25-coated TiO2 nanotube arrays
transmission electron microscopy
TiO2 nanotube arrays
The authors would like to acknowledge financial support for this study from the National Natural Science Foundation of China (No. 50872039; 50802032), and the Xiangyang Plans Projects of Scientific and Technological Research and Development (No. 2010GG1B35).
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