CdSe Heterostructured

Enhanced Photochemical Response of TiO2/CdSe Heterostructured Nanowires Jung-Chul Lee,† Tae Geun Kim,‡ Heon-Jin Choi,§ and Yun-Mo Sung*,† Department of Materials Science & Engineering, Korea UniVersity, Seoul 136-713, South Korea, Department of Electronics Engineering, Korea UniVersity, Seoul 136-713, South Korea, and School of AdVanced Materials Science & Engineering, Yonsei UniVersity, Seoul 120-749, South Korea

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ReceiVed June 26, 2007; ReVised Manuscript ReceiVed September 20, 2007

ABSTRACT: High-density single-crystalline TiO2 nanowires (∼50 nm diam) were successfully grown on Ti substrates by chemical vapor deposition at a low temperature of 700 °C and within a remarkably short time period of 5 min. They were combined with CdSe nanocrystals (∼5 nm diam) to form TiO2/CdSe heterotructured nanowires by overcoating the nanowires with the CdSecontaining solution and subsequent annealing at 600 °C. The TiO2/CdSe nanowires showed uniformly distributed CdSe nanocrystals, and high crystallinity of rutile and wurtzite from the TiO2 and the CdSe, respectively. Owing to the heterostructure of the TiO2/ CdSe, they demonstrate almost full visible-range light absorption and thus enhanced photocatalytic activity by charge separation via electron and hole transfer between the CdSe and the TiO2. Introduction TiO2 is one of the most important oxide semiconductors showing distinct photochemical activities due to its unique energy band gap characteristics.1,2 Thus, for many years, it has been intensively exploited for the purification of contaminated water and air3–6 and for the generation of renewable energy.7–11 Recently, TiO2 nanostructures, including nanoparticles, nanotubes, and organic–inorganic nanohybrids, have attracted a great deal of interest due to the large surface-to-volume ratio that is beneficial to most of the TiO2-based devices. As the surface absorbing the light and reacting with surrounding substances increases, the photochemical activity can be considerably enhanced.12,13 However, the poor natural light absorption capability due to its intrinsically large energy band gap (∼3.2 eV) has been significantly limiting the broad applications of the TiO2 nanostructure for photovoltaics and photocatalysts. To overcome this drawback, organic or inorganic dye-sensitized TiO2 heterostructures have been developed, and the improvement of their natural light absorption capability has been reported.14,15 Also, the dye-sensitized TiO2 can show active photochemical reactions by charge separation.16 The free electrons generated in the dye by the visible light excitation can be injected to the TiO2, and the holes from the TiO2 can transfer to the dye, which can prevent the electron–hole recombination effectively and provide high photovoltaic and photocatalytic efficiency. Semiconductors, such as CdSe, CdS, and PbS, have energy band gaps corresponding to the energy of visible-range light and can serve as the inorganic dye for the TiO2.17–21 In contrast to the aforementioned nanostructures, onedimensional TiO2 nanowires have been relatively less exploited probably due to the difficulty in synthesis, although they also possess a very bright future in various applications. Furthermore, to date, there have been only scattered reports on the wellcontrolled vapor-phase synthesis of TiO2 nanowires,22–24 contrary to the wet-chemical methods such as sol–gel process using anodic aluminum oxide (AAO) templates25–28 and anodizing of titanium.29–32 The vapor-phase growth is highly desirable * Corresponding author. Tel: +82-2-3298-3284. Fax: +82-2-928-3584. E-mail address: [email protected]. † Department of Materials Science & Engineering, Korea University. ‡ Department of Electronics Engineering, Korea University. § Yonsei University.

not only for the synthesis of high-purity and high-crystallinity nanowires but also for the achievement of high-density nanowire arrays on the limited area of a substrate, which is crucial for their various electrical, optical, and chemical device applications. However, despite the presence of reports on the vapor-phase growth of TiO2 nanowires,33,34 there still exist drawbacks to be overcome for their wide-range applications, such as highprocessing temperature (∼1000 °C), low growth density, and slow growth rate of the nanowires, all of which significantly limit the broad applications of the TiO2 nanowires to many devices. These critical problems could come most probably from the insufficient supply of Ti vapor source and the slow nucleation and growth kinetics of TiO2 crystals, respectively. In this paper, we report a successful approach to obtain highdensity arrays of single-crystalline TiO2 nanowires at a low temperature for a short time period. Single-crystalline TiO2 nanowires were prepared on titanium substrates using the vapor–liquid–solid (VLS) mechanism by chemical vapor transport of TiCl4. It has been well-known that due to the intrinsic energy band gap structure, CdSe-TiO2 is a good combination to obtain high photoconversion and high photodecomposition efficiency. In the TiO2/CdSe heterostructure, prior to the electron–hole recombination, photogenerated electrons can transfer from CdSe to TiO2, while the holes transfer from the TiO2 to the CdSe. In this paper, we report the successful synthesis of TiO2/CdSe nanowires through combining the CdSe nanocrystals with the surface of TiO2 nanowires grown on Ti substrates. The light absorption and photocatalytic characteristics of the heterostructure were investigated and compared with those of the bare TiO2 nanowires. Experimental Procedures For the TiO2 nanowire growth, TiCl4 was used as a Ti source gas, and sapphire, quartz, and titanium were used as substrates. Ti buffer layer (∼250 nm thick) and Au catalyst layer (∼25 nm thick) were subsequently deposited onto the substrates by radio frequency (RF)magnetron sputtering. H2 (10 sccm)/TiCl4 (Aldrich, 99.9%) and O2 (0.5 sccm) were used as reactive gases, and H2 (20 sccm)/Ar (100 sccm) was used as a carrier gas. The substrate temperature was ∼700 °C, and the time period for the growth of TiO2 nanowires was 5 min. Cadmium oxide (99.99%) and selenium shot (99.999%) (Aldrich Chemical, Milwaukee, WI) were used as precursors for the synthesis

10.1021/cg070588m CCC: $37.00  2007 American Chemical Society Published on Web 11/15/2007

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Figure 1. Field emission scanning electron microscopy (FESEM) images of TiO2 nanostructures grown on (a) sapphire, (b) quartz, and (c) titanium and (d) an enlarged image of a portion of panel c.

Figure 2. High-resolution transmission electron microscopy (HRTEM) image (a), selected area electron diffraction (SAED) patterns (b), and X-ray diffraction (XRD) patterns (c) of TiO2 nanowires grown on titanium substrates at 700 °C. Here, A, R, and T denote anatase, rutile, and Ti, respectively. of CdSe nanocrystals. Paraffin oil and oleic acid (Aldrich Chemical, Milwaukee, WI) were used as a solvent and a surfactant, respectively. Most details of the synthesis were similar to the CdSe nanocrystal preparation previously reported in the literature.11 In this study, CdO

was added into a mixture of paraffin oil and oleic acid (45:5) to 5 and 1 mM, respectively, in a three-neck flask. The solution was heated to 160 °C under Ar flow and then distilled in vacuum to remove the remaining acetone. The Se metal was dissolved in paraffin oil at 2 mM

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Figure 3. A suggested mechanism for the enhanced growth of the TiO2 nanowires on Ti substrates: (a) preheating of Au film to break up into catalysts, (b) Ti vapor penetration into Au catalysts and TiO2 seed layer foramtion, (c) TiO2 nucleation at the interface between catalysts and TiO2 seed layer, and (d) TiO2 nanowire growth. The light absorption characteristics of the TiO2 and TiO2/CdSe nanowires were investigated using the UV–visible spectrometer (JASCO UV–visible spectrophotometer: V530, Tokyo, Japan) with a full visible and UV range light source. Photocatalytic efficiency of the TiO2 and TiO2/CdSe nanowires was evaluated using the degradation of a methylene blue solution. The solution containing the nanowires was irradiated by an ultraviolet and visible (UV–visible) light source (mercury-xenon lamp, Newport) having light irradiation range from ∼200 to 900 nm for photocatalytic reactions. The decrease in the intensity of UV–visible light absorbance peaks at 664 nm, corresponding to the decomposition of methylene blue, was monitored according to UV-light exposure time period.

Results and Discussion

Figure 4. Field emission scanning electron microscopy (FESEM) images of TiO2/CdSe nanowires. at 220 °C, and 20 mL of Cd solution was rapidly injected into the Se-paraffin oil solution, which allowed fast nucleation and slow growth of the CdSe nanocrystals. The TiO2 nanowires were overcoated by the solution bearing the CdSe nanocrystals in which the surface was passivated by oleic acid molecules at room temperature. Then, they were heat treated at 600 °C for 30 min in Ar gas for strong bonding of the nanocrystals to the nanowires through the sintering process and removal of remaining organic material. The morphological features and crystallinity of the TiO2/CdSe nanowires were investigated using field-emission scanning electron microscopy (FESEM; Hitachi S4300, Tokyo, Japan), high-resolution transmission electron microscopy (HRTEM; JEOL JEM 4010, Tokyo, Japan), and X-ray diffraction (XRD; Rigaku Ultima-2000, Tokyo, Japan). The chemical composition was examined using energydispersive X-ray spectroscopy (EDS; Oxford, Inca, Oxon, UK) analyses.

Distinct TiO2 nanostructures were formed on the three different substrates by chemical vapor transport of TiCl4 gas at 700 °C for 5 to 30 min. Figure 1 shows FESEM images of the TiO2 nanostructures prepared on the (a) sapphire, (b) quartz, and (c, d) titanium substrates. Nanorods with a diameter of ∼30–50 nm and a length of ∼1 µm were formed together with nanodisks at the surface of the sapphire after 30 min, and TiO2 nanoparticles with a size of ∼300–400 nm were formed at the surface of the quartz after 30 min. On the other hand, straight nanowires with a diameter of ∼30–70 nm and a length of ∼5–10 µm were grown on the surface of the titanium after 5 min. XRD and HRTEM analysis results presented in Figure 2 show that the straight nanowire grown on the titanium has high crystallinity of rutile phase. Also, the existence of a trace of anatase phase was identified in the XRD patterns probably due to the relatively low nanowire growth temperature of 700 °C. The nanowires show a strong tendency to grow in the [110] direction, and they were defect-free and single-crystalline. The possible mechanism for the enhanced TiO2 nanowire growth on the Ti substrates was considered as two aspects, as depicted in Figure 3. One is the increased Ti vapor pressure due to the additional vapor supplied from the Ti substrates. The

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Figure 5. High-resolution transmission electron microscopy (HRTEM) image (a), its enlarged image (b), selected area electron diffraction (SAED) patterns (c), and X-ray diffraction (XRD) patterns (d) of TiO2/CdSe nanowires. The white circles indicate the locations of the CdSe nanocrystals. Here, A, C, R, and T denote anatase, CdSe, rutile, and Ti, respectively.

Figure 6. UV–visible light absorption spectra of TiO2 and TiO2/CdSe naowires.

other, believed to be more effective than the first one, is the TiO2 seed layer formation at the interfacial regions between Au catalysts and Ti substrates (b). The TiO2 seed layer can be crystalline rutile since the processing temperature was high enough (700 °C), and this layer can enhance the formation

kinetics of rutile nuclei (c). The possible epitaxy in the rutile nucleation can lower the activation energy barrier for it and thus significantly enhance its kinetics. The considerably lowered synthesis temperature and increased growth rate of the nanwores is the evidence of the lowered activation energy barrier for the nucleation of rutile TiO2. Further, supplying Ti vapor to the Au catalysts causes the growth of the nuclei into the straight rutile nanowires (d). Due to the additional Ti vapor from the substrate, the growth rate of the nanowires can also be substantially increased. Overall nanowire growth kinetics is much enhanced using the titanium substrates. Next, the TiO2 nanowires were overcoated with the prepared CdSe nanocrystals and heat treated. Figure 4 shows a FESEM image of the heterostructured TiO2/CdSe nanowires showing avery rough surface (see the insert) due to the attachment of the CdSe nanocrystals onto the TiO2 nanowires grown on the Ti substrates. Quantitative chemical analysis of the heterosturctured TiO2/CdSe nanowires was performed using EDS, and the results indicated that the average content of Ti, O, Cd, and Se was 32.81, 64.42, 1.58, and 1.18 atom %, respectively. Detailed morphological features and crystallinity of the TiO2/ CdSe nanostructues were investigated using HRTEM and SAED analyses as shown in Figure 5. The HRTEM image of the TiO2/ CdSe nanostructure shows that the high-density CdSe nanocrystals are uniformly bonded to the surface of the TiO2 nanowire.

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Figure 7. Comparison of the photocatalytic efficiency of bare TiO2 and TiO2/CdSe nanowires (a) and schematic diagram showing the energy band structure and electron–hole separation in the nanowires (b).

The oleic acid must have played a major role in uniformly dispersing the CdSe nanocrystals. This is critically important not only to maintain their particle size to obtain the quantum confinement but also to secure the surface area of the nanocrystals for maximum light absorption. The HRTEM images show the strong bonding between the TiO2 nanowires and the CdSe nanocrystals via the sintering process during annealing, which can provide the excited electrons in the CdSe with a path to be injected into the TiO2. Selected area electron diffraction (SAED) on the TiO2/CdSe nanostructure shows the apparent ring diffraction patterns from the crystallinity of the wurtzite CdSe nanocrystals, as well as the spot diffraction patterns from the rutile TiO2 nanowires. Also, the crystallinity of the TiO2/ CdSe nanowires was investigated in Figure 5 using XRD, and the rutile and wurtzite phases were identified. Other secondary phases such as CdO that may form during the annealing step for the CdSe nanocrystal attachment were not identified in the XRD patterns. Since the annealing was performed in Ar gas atmosphere at 600 °C and the time period was 30 min, oxidation or interdiffusion between the TiO2 and CdSe did not occur seriously. Figure 6 shows the comparison in the UV–visible light absorption spectroscopy results of the bare TiO2 and TiO2/CdSe heterostructured nanowires. The TiO2/CdSe nanowires show an apparent increase in the light absorbance of the visible light ranging from ∼400 to ∼700 nm. This much enhanced visiblerange light absorption in the TiO2/CdSe nanowires is attributed to the excitation of electrons in the CdSe. Since the size distribution of the CdSe nanocrystals attached to the surface of

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the TiO2 nanowires was ∼5–10 nm, their energy band gap ranges from 2.48 to 1.74 eV according to the so-called, quantum confinement effect,35–38 by which electrons and holes are threedimensionally confined in a nanocrystal quantum dot and its energy band gap increases with decrease in the size. Therefore, the corresponding light absorption wavelength ranges from 500 to 714 nm, and almost full visible-range light can be absorbed by the CdSe nanocrystals. The exited electrons in the nanocrystals can be effectively injected into the TiO2 due to the strong interfacial bonding between them as shown in Figure 5a. The bare TiO2 and the TiO2/CdSe nanowires show lightabsorption peaks at 335 and 353 nm, which correspond to energy band gaps of 3.70 and 3.52 eV, respectively. Both of the nanostructures show quantum confinement effect, since the pure rutile possesses an energy band gap of 3.2 eV. The slight red shift in the UV–visible light absorption of the TiO2/CdSe nanowires can come from the leakage of the electron wave function of TiO2 into CdSe.39,40 The TiO2/CdSe nanowires show a second light absorption peak around 662 nm, which corresponds to an energy band gap of 1.86 eV and the average particle size of 8.2 nm of the CdSe nanocrystals. Photocatalytic efficiency of the TiO2/CdSe nanowires was evaluated using the degradation of a methylene blue (MB) solution, and it was compared with that of the bare TiO2. The MB solution containing the nanostructures was irradiated by ultraviolet (UV) light for photocatalytic reactions. The decrease in the intensity of visible light absorbance peaks at 664 nm, corresponding to the decomposition of the MB, was monitored according to the UV-light exposure time period. Figure 7a shows the normalized variation in the absorbance of the MB solutions containing the bare TiO2 and TiO2/CdSe nanowires. The TiO2/ CdSe nanowires almost completely decomposed the MB solution within 80 min, while the bare TiO2 nanowires decomposed it within 110 min. It is certain that the visible-light absorption could contribute to the photodecomposition reaction in the heterostructured TiO2/CdSe nanowires. The possible mechanism for the enhanced photocatalytic activity even under visible-range light irradiation was considered using the energy band diagram for the heterostructured TiO2/ CdSe nanowires as also indicated in Figure 7b. The diagram explains how the loaded CdSe nanocrystals can act as a catalyst for both the reductive and oxidative reactions. The essential point is that the energy barrier height at the TiO2/CdSe interface is changeable by the light illumination. When the light is illuminated, photoexcited electrons in the conduction band of CdSe mostly enter the Fermi level of TiO2, and they act as catalyst for a reductive reaction. Absorption of a unit of the visible-range light, associated with the formation of a conduction band free electron and a valence band hole, occurs in the CdSe nanocrystals during the visible-light irradiation, and the migration of the photogenerated electron to the surface and the transfer to the TiO2 nanowire occurs subsequently. There is avery low concentration of holes with which to recombine, and therefore the electron has high opportunity to participate in the reduction reaction to form oxygen radicals, which are very strong oxidants and can decompose organic substances effectively. Also, the photogenerated holes in the CdSe nanocrystals theoretically migrate to the surface and participate in the oxidation reaction to form hydroxyl radicals, which are more effective than oxygen radicals to decompose organic substances. The TiO2 nanowire itself can show photodecomposition activity only under UVlight irradiation due to the high energy band gap. The holes created by UV light irradiation of the TiO2 can transfer to the

Enhanced Photochemical Response of TiO2/CdSe Nanowires

CdSe, and the opportunities for electron–hole recombination can be substantially reduced. Conclusions In this study, straight and high-density single-crystalline TiO2 nanowires were successfully grown on titanium substrates at a remarkably lowered temperature over a reduced time period. In addition to the TiCl4 gas, the Ti vapor source from the titanium substrates can accelerate the TiO2 nanowire growth. More possibly, the rutile layer formed by the oxidation of the titanium substrates could induce epitaxy for the nucleation of rutile nanowires. The lowered activation energy for nucleation by epitaxy can significantly enhance the nucleation and growth kinetics of the TiO2 nanowires. Heterostructured TiO2/CdSe nanostructures were achieved through overcoating the surface of the single-crystalline TiO2 nanowires with CdSe nanocrystalcontaining solution and subsequent annealing at 600 °C. The TiO2/CdSe nanowires show the absorption of UV–visible light ranging from 353 to 614 nm. The enhanced visible light absorption in the TiO2/CdSe is attributed to the charge separation between the CdSe and TiO2. Due to visible-light absorbing capability, the TiO2/CdSe nanowires show enhanced photocatalytic efficiency compared with the bare TiO2 nanowires. This TiO2/CdSe heterostructure also can be a strong candidate for dye-sensitized solar cells due to the high quantum yield from the charge separation under visible-light irradiation. Acknowledgment. This work was supported by the Korea Research Foundation (KRF) Grant by the Korean Government (MOEHRD; KRF-2006-311-D00568). Also, this work was supported through the Post-BK 21 program by the Korean Government (MOEHRD).

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