TiOx nanoscale heterojunctions

TiOx nanoscale heterojunctions

ARTICLE IN PRESS Journal of Crystal Growth 307 (2007) 348–352 www.elsevier.com/locate/jcrysgro Fabrication and characterization of ZnO/TiOx nanoscal...

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ARTICLE IN PRESS

Journal of Crystal Growth 307 (2007) 348–352 www.elsevier.com/locate/jcrysgro

Fabrication and characterization of ZnO/TiOx nanoscale heterojunctions Y.F. Hsua, A.B. Djurisˇ ic´a,, K.H. Tama, K.Y. Cheunga, W.K. Chanb a

Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong

b

Received 3 February 2007; received in revised form 12 June 2007; accepted 6 July 2007 Communicated by J.M. Redwing Available online 12 July 2007

Abstract ZnO nanorods have been grown by a hydrothermal method on titania/titanate nanowires. The resulting branched structures were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and photoluminescence (PL) measurements. The influence of seeding on the nanorod growth was investigated, and the large-scale synthesis of branched ZnO/TiOx heterostructures has been demonstrated. The ZnO nanorods grow along [0 0 0 1] direction and emit UV and visible (yellow–green) luminescence, as expected for samples grown by a hydrothermal method. r 2007 Elsevier B.V. All rights reserved. PACS: 81.07.Vb Keywords: B1. Nanomaterials; B1. Oxides

1. Introduction One-dimensional nanomaterials have been a subject of intense study in recent years due to their great potential for a variety of practical applications, such as electronic and optoelectronic nanodevices. Fabrication and characterization of different nanoscale heterojunctions is of particular interest for device applications. Various types of such heterojunctions have been reported in recent years, such as carbon nanotubeZnO heterojunctions [1,2], GaP/GaAsP [3], ZnS/SiO2 [4], InAs/GaAs [5], and ZnO/GaN, ZnO/GaP, ZnO/SiC [6]. Among various material combinations, ZnO–TiO2 as a junction of two wide band gap materials used in photovoltaic [7] and photocatalysis [8] applications is of particular interest. For such applications, branched structures are expected to result in increased efficiency due to considerable increase in the surface area [9]. Although ZnO-based solar cells typically have worse performance than TiO2-based solar cells, there is Corresponding author. Tel.: +852 2859 7946; fax: +852 2559 9152.

E-mail address: [email protected] (A.B. Djurisˇ ic´). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.07.010

significant interest in the use of ZnO due to its higher electron mobility compared to TiO2 [10]. Some promising results have been obtained for ZnO nanowires coated with TiO2 [7] and TiO2/ZnO films [11]. It has also been shown that ZnO enhanced photocatalytic activity of titanate [12]. Therefore, there is considerable interest in growth and characterization of ZnO/TiOx nanoscale heterojunctions with different morphologies. While some nanoscale heterojunctions can be grown in a single growth step with a carefully optimized starting materials and growth conditions [4], more common method for growing branched or dendritic nanoscale heterojunctions is a multistage growth, where catalyst clusters deposited on 1D nanostructures result in the growth of branched nanostructures by vapor–liquid–solid (VLS) mechanism [3,5,6]. While successful growth of ZnO nanorods on various 1D nanostructrures has been reported using chemical vapor deposition (CVD) [6], this process requires a growth temperature of 500 1C and the presence of metal catalysts. Unlike CVD growth, hydrothermal growth of ZnO nanostructures [13–17] occurs at

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significantly lower temperatures (o100 1C) and it does not require metallic catalysts. In this paper, hydrothermal method is used to grow zinc oxide rods on titania/titanate nanowires. The influence of the seeding process and growth conditions on the morphology of the obtained branched heterostructures has been investigated, and the growth mechanisms have been discussed. Photoluminescence (PL) spectra of the fabricated branched structures exhibit UV and visible emission, in good agreement with previous reports on hydrothermally grown ZnO nanorods. 2. Experimental details Titania nanowires were fabricated on Ti foils (Aldrich, 99.6% purity, 0.127 mm) or Ti films on quartz substrates by KF and moisture-assisted oxidation process, as reported previously [18]. A drop of 1 wt% potassium fluoride (Aldrich, 499%) aqueous solution was placed at the center of the substrate. The substrate and a boat filled with deionized water were placed into a quartz tube, which was evacuated to 7.5 Torr. Then, 0.1 Lpm argon gas was flushed into the tube for 5 min and the furnace was heated to 150 1C with a rate of 5 1C/min. This temperature was maintained for 2 h, and then the temperature was raised to 650 1C at a rate of 30 1C/min. After 2 h at 650 1C, the furnace was cooled down at a rate of 1 1C/min. The fabricated nanowires were then cleaned by stirring in the deionized water for 24 h in order to remove any excess potassium fluoride. Zinc oxide rods were fabricated on the cleaned samples by hydrothermal deposition. Zinc acetate/ethanol solution (5 mM) was first dropped on the substrates and rinsed in ethanol for 10 s, followed by argon gas drying. This process was repeated six times. Then the samples were heated at 200 1C for 20 min. Followed by the heat treatment, the procedures described above were repeated. Zero times, two times and four times of acetate solution pre-treatments were tried in this work in order to compare the effect of zinc acetate seeds on ZnO nanorod growth. Finally, 25 mM solution containing polyethylenimine (Aldrich, 50 wt% solution in water), zinc nitrate hydrate (Aldrich, 99.999%) and hexamethylenetetramine (Aldrich, 499% ACS reagent) was used to grow zinc oxide rods. The temperature of the reaction was kept at 90 1C for 2.5 h at atmospheric pressure. The samples were then placed in ultrasonic bath for 10 s in order to remove residues and dried in an oven. The morphology of the fabricated samples was examined by scanning electron microscopy (SEM) (Leo 1530 field emission SEM) and transmission electron microscopy (TEM) (Philips Tecnai-20 TEM and JEOL 2010F TEM). The crystal structure of the samples was studied using X-ray diffraction (XRD) (Bruker AXS SMART CCD diffractometer). For PL measurement a He–Cd laser (325 nm) was used as the excitation source, while the PL spectra were collected using a fiberoptic spectrometer PDA-512-USB (Control Development Inc.).

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3. Results and discussion Fig. 1 shows the representative SEM images of TiOx nanowires which served as the substrate for ZnO nanorods, and ZnO/TiOx nanostructures for different ZnO seeding procedures. Without zinc acetate solution for the formation of ZnO nanoseeds, no ZnO nanorods are grown on TiOx nanowires. The nanorod coverage is directly proportional to the amount of ZnO nanoseeds, which can be controlled by repeating the zinc acetate solution application procedure. On the other hand, no significant dependence of ZnO nanorod coverage on the growth solution concentration was observed. Fig. 2 shows the representative SEM images for ZnO nanorods grown from solutions with 100 mM concentration. It can be observed that the ZnO/TiOx morphology is similar to that obtained from 25 mM solutions for the same seeding procedure. The ZnO nanorod length can be increased by placing the substrates into a fresh solution and repeating the growth procedure, as shown in Fig. 3. However, while such process can give very long ZnO nanowires on flat, planar substrates [16], the nanorod length in this case is limited by the available space between TiOx nanowires. To investigate the growth mechanism in more detail, the fabricated heterojunctions were characterized by TEM. Fig. 4 shows the representative SEM and TEM images of a TiOx nanowire with ZnO nanorods. It can be observed that nanorods can nucleate on all facets of the nanowire, indicating that nanowires were coated with a number of seeds derived from zinc acetate solutions. HRTEM images of acetate seed and ZnO nanorods on TiOx nanowires are shown in Fig. 5. It can be observed that there is a clear interface between the ZnO seed and TiOx nanowire, and consequently there is a clear interface between ZnO nanorods and TiOx nanowire. For ZnO nanorods grown on TiOx nanowires, clear lattice fringes can be observed. The nanowires grown by KF and waterassisted oxidation of Ti typically consist of a mixture of rutile titania and monoclinic potassium titanate nanowires (with potassium content in the range 5–9%), with very few nanowires having anatase crystal structure [18]. From HRTEM images, we can identify lattice spacings of 7.4 A˚ corresponding to [1 1 0] growth direction for titanate nanowire and 2.6 A˚ corresponding to (0 0 0 2) planar spacing for ZnO nanorods. Thus, ZnO nanorods grow along [0 0 0 1] direction, in agreement with previous studies of hydrothermally grown ZnO nanorods [14,15]. XRD peaks of TiOx nanowire, as shown in Fig. 6, correspond to a mixture of rutile titania and potassium titanate nanowires, as discussed in detail in our previous work [18]. The titanate peaks are in good agreement with previous studies on potassium titanate crystal structure [19–21]. After ZnO nanorods growth, clear peaks corresponding to hexagonal ZnO can also be observed in addition to those corresponding to TiOx nanowires. Optical properties of ZnO nanostructures fabricated by different methods have been extensively studied in the

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Fig. 1. (a) Representative SEM image of TiOx nanowires. (b), (c) and (d) Representative SEM images of zinc oxide rods growing on nanowires without acetate seeds, two layers of acetate seeds and four layers of acetate seeds, respectively. Solution concentration for ZnO growth is 25 mM.

Fig. 2. SEM images of ZnO rods growing on nanowires with (a) two layers of acetate seeds and (b) four layers of acetate seeds. Solution concentration for ZnO growth is 100 mM.

Fig. 3. SEM images of ZnO rods with solution growth repeated (a) two times and (b) three times. Solution concentration for ZnO growth is 25 mM.

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Fig. 4. Representative SEM image of ZnO/TiOx nanostructures is shown in (a), while representative TEM images are shown in (b) and (c).

Fig. 5. Representative HRTEM image of (a) ZnO/TiOx nanostructures and (b) acetate seeds on TiOx nanowires.

literature [15,17,22–25] (for a recent review, see Ref. [25]). The optical properties of the ZnO/TiOx samples in this work were studied by PL spectroscopy, and the obtained spectra are shown in Fig. 7. No emission is observed from bare TiOx nanowires, while after ZnO nanorods growth weak UV emission and broad yellow–green emission can be observed, in agreement with previously reported PL spectra of hydrothermally grown ZnO [15,17]. The defect emission has been previously assigned to interstitial oxygen

[15,17], but recent results indicate that the presence of OH groups on ZnO surface can play a significant role in the yellow defect emission from the hydrothermally grown samples [22]. Therefore, we annealed ZnO/TiOx heterostructures in Ar gas flow at 200 and 600 1C, respectively. The obtained spectra are also shown in Fig. 7. It can be observed that the UV-to-visible emission ratio is increased after annealing at 200 1C, but the defect emission still dominates. On the other hand, significant increase in the

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the two materials. PL spectra exhibited emission peaks typical for hydrothermally grown ZnO nanorods, while no emission from TiOx nanowires was observed. Acknowledgments This work was supported by the Research Grants Council of Hong Kong (Project nos. HKU 7008/04P, 7019/04P, 7010/05P). Financial support from the Strategic Research Theme, University Development Fund, Science Faculty Development Fund (University of Hong Kong) and Seed Funding Grant (administrated by The University of Hong Kong) and Outstanding Young Researcher Award are also acknowledged. Fig. 6. XRD spectrum of ZnO/TiOx nanostructures and TiOx nanowires.

Fig. 7. PL spectra of TiOx nanowires and ZnO/TiOx nanostructures with and without annealing (200 and 600 1C, Ar gas flow, 30 min).

UV emission and small red shift in the defect emission are observed after annealing at 600 1C. This behavior is different from the ZnO nanorods grown on Si substrate [22], indicating that the substrate may affect the defect types and concentrations in ZnO nanorods. The seeding technique can affect the UV to visible emission ratio, and ZnO nanorods grown with ZnO nanoparticles seeds [15,17] typically have lower UV-to-visible emission ratio compared to ZnO nanorods grown with zinc acetate-derived seeds [22]. However, the influence of substrate and seeding method on the optical properties of ZnO nanorods requires further study. 4. Conclusions ZnO nanorods have been grown on TiOx nanowires by a hydrothermal method. The rod density could be controlled by zinc acetate seed density, while the growth solution concentration did not significantly affect the ZnO nanorod density. Both TiOx nanowires and ZnO nanorods were found to be single crystalline, with clear interface between

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