PAM nanowire arrays prepared by seed-layer-assisted electrochemical deposition

Thin Solid Films 520 (2011) 1532–1540

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Ordered ZnO/AZO/PAM nanowire arrays prepared by seed-layer-assisted electrochemical deposition Yu-Min Shen a, Chih-Huang Pan a, Sheng-Chang Wang b,⁎, Jow-Lay Huang a, c, d,⁎⁎ a

Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Department of Mechanical Engineering, Southern Taiwan University, Tainan 710, Taiwan Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan d Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 701, Taiwan b c

a r t i c l e

i n f o

Available online 6 October 2011 Keywords: Porous alumina membranes Al-doped ZnO thin films ZnO nanowire arrays Seed-layer-assisted growth

a b s t r a c t An Al-doped ZnO (AZO) seed layer is prepared on the back side of a porous alumina membrane (PAM) substrate by spin coating followed by annealing in a vacuum at 400 °C. Zinc oxide in ordered arrays mediated by a high aspect ratio and an ordered pore array of AZO/PAM is synthesized. The ZnO nanowire array is prepared via a 3-electrode electrochemical deposition process using ZnSO4 and H2O2 solutions at a potential of − 1 V (versus saturated calomel electrode) and temperatures of 65 and 80 °C. The microstructure and chemical composition of the AZO seed layer and ZnO/AZO/PAM nanowire arrays are characterized by field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), and energy-dispersive X-ray spectroscopy (EDS). Results indicate that the ZnO/AZO/PAM nanowire arrays were assembled in the nanochannel of the porous alumina template with diameters of 110–140 nm. The crystallinity of the ZnO nanowires depends on the AZO seed layer during the annealing process. The nucleation and growth process of ZnO/AZO/PAM nanowires are interpreted by the seed-layer-assisted growth mechanism. © 2011 Elsevier B.V. All rights reserved.

1. Introduction In recent years, one-dimensional nanomaterials have attracted a lot of attention due to their unique opto-electrical, mechanical, and piezoelectrical properties [1,2]. With the development of optical and electric devices, controllable nanowire arrays are becoming increasingly important. Porous alumina membranes (PAMs) have attracted considerable attention due to their ability to control nanowire shape, size, and uniformity [3,4]. PAMs are mainly composed of a honeycomb array of channels, with a pore in the middle of each cell. The interpore distance and pore size are in the range of ~30 nm to several hundred nanometers depending on the electrolyte and voltage [5–7]. Since the development of the two-step anodization process by Masuda et al. [8], which produces highly ordered pore arrays and high-aspect-ratio PAMs, a lot of work has been devoted to synthesizing metal [9,10] and metal oxide nanowire arrays via PAM templates [11–13]. Zinc oxide, which crystallizes into a wurzite hexagonal structure, exhibits a direct band gap energy of 3.2 eV and a high excitation energy of ~60 meV. ZnO is thus widely applied in transparent conductive films [14], short-wave-emission materials [15], ultraviolet (UV)-lasers ⁎ Corresponding author. ⁎⁎ Correspondence to: J-L Huang, Coatings and Ceramics Lab. Department of Materials Science and Engineering, National Cheng Kung University Tel./fax: + 886 6 2754410. E-mail addresses: [email protected] (S.-C. Wang), [email protected] (J.-L. Huang). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.09.066

[16], and sensors [17]. Ordered ZnO nanowire arrays embedded into PAMs by tuning the electrochemical deposition parameters, such as annealing time [18], deposition potential and temperature [19], electrolyte solution [20], and additives [21,22], have already reported. In addition to growth of ZnO nanowire arrays by electrochemical deposition, Ku [23] and Kuo [24] have reported ZnO nanowire array growth via a seed-layer-assisted method. However, the nanowire's size, shape, and uniformity are difficult to control by seed layers assisted method. The present study thus combines template with the seed-layer-assisted to synthesize ZnO nanowire arrays. The microstucture of ZnO nanowire arrays embedded in AZO/PAMs is investigated for various synthesis parameters. 2. Experimental details 2.1. Preparation of porous alumina membranes (PAMs) For the synthesis of porous alumina membranes, high-purity Al foils (99.9995%) were first degreased in ethanol. Before the anodic process, the Al foils were annealed in an argon atmosphere of 10− 2 Torr at 500 °C for 1 h and etched by HF–HNO3–HCl–H2O (1:10:20:69 vol.%) solution. The Al foils were then degreased in acetone in an ultrasonic bath for 10 min. The samples were electro-polished in a mixture of HClO4– C2H5OH (1:4 vol.%) at 10 °C, with a current density of 100 mA/cm 2 for 1 min. The anodization was carried out in 0.3 M oxalic acid at a constant voltage of 80 V using Pt foil as a counter electrode. The electrolyte was

Y.-M. Shen et al. / Thin Solid Films 520 (2011) 1532–1540

rigorously stirred, and its temperature was kept at 3 °C during anodization. After 5 h of anodization, the alumina film was selectively etched away in a mixture of H3PO4–CrO3–H2O (2 g–3.5 mL–100 mL) at 70 °C for 40 min. After further anodization under the same conditions for 18 h, the remaining Al was dissolved by a saturated HgCl2 solution. The released membranes were etched in 5 vol.% H3PO4 solution at 60 °C for 30 min to dissolve the barrier layer on the bottom side of the PAM and at room temperature for 20 min to widen the pores (as shown in Fig. 4(a)).

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a

2.2. Preparation of Al-doped zinc oxide (AZO) seed layers 500 nm

For synthesis of AZO seed layers, 0.3 M ZnO solution was prepared by mixing 9.877 g of zinc acetate, 150 mL of ethanol, and 2.7 mL of ethanolamine. The mixing solution was heated to 60 °C and stirred for 30 min followed by stirring under room temperature for 24 h. The AZO solution was obtained by mixing ZnO solution and 0.1688 g of aluminum nitrate and then heating the resulting solution to 60 °C for 30 min. A glass substrate was cleaned in H3PO4–H2SO4 (3:1 vol.%) solution at 60 °C for 20 min. It was then rinsed in DI water in an ultrasonic bath for 1 min, and finally dried in N2 gas. Initially, the AZO seed layer was coated onto the glass substrate by spin coating and annealing in a vacuum at various temperatures (400, 600 °C) for various durations (0, 2 h). Finally, the PAMs were degreased in 0.05 M H2SO4 solution in an acid bath for 1 min and the AZO seed layers were coated by spin coating.

b

200 nm

2.3. Preparation of ZnO/AZO/PAMs nanowire arrays

c

Before the deposition of nanowire arrays, layers of Ti (10 nm) and Ag (100 nm) were coated onto one side of the membrane by the electron-beam vaporization method and the PAMs/AZO/Ti/Ag structure was sticked onto a Cu substrate to serve as the working electrode (cathode). Pt foil (anode) and a saturated calomel electrode (SCE, 0.241 V) served as the counter and reference electrodes, respectively, in a three-electrode electrochemical deposition system. The PAMs/ AZO/Ti/Ag/Cu structure was degassed using a mixture of 0.15 M ZnSO4 and 1.5 M H2O2 solution deposited under − 1 V at 65 and 80 °C for 3 h, respectively. After electrodeposition, the composites were heated in air at 400 °C for various durations (4 and 8 h) to obtain ZnO/AZO/PAMs nanowire arrays. 2.4. Characterization of ZnO/AZO/PAMs nanowire arrays

500 nm

Fig. 1. Plane-view SEM images of AZO seed layers at annealing temperatures and durations of (a) 400 °C, 0 h (b) 600 °C, 0 h, and (c) 400 °C, 2 h in a vacuum.

3. Results and discussion

temperature promotes grain growth. The loose structure at 600 °C restrained the conductivity of electrons, which caused an increase in the average sheet resistance. In addition, when the annealing time was increased to 2 h, the average sheet resistance increased to 1023 Ω/cm 2. As shown in Fig. 1 (c), the porosity of AZO films was increased. This indicates that resistance increases with increasing in annealing temperature and duration. Fig. 2 shows the average sheet resistance for various numbers of layers of the AZO seed. The resistance decreases with increasing number of layers. The relationship between the AZO seed layer and resistance can be expressed as:

3.1. Characterization of AZO seed layers

Rs ¼ 1=σt

Before the synthesis of ZnO/AZO/PAMs nanowire arrays, the AZO films and PAMs were characterized. Fig. 1 shows SEM images of AZO films coated on the glass substrate with annealing at 400 and 600 °C for 0 and 2 h in a vacuum, respectively. The average sheet resistances of AZO films annealed at 400 °C and 600 °C were found to be 129.37 and 326.34 Ω/cm 2, respectively. From Fig. 1 (a) and (b), the average grain sizes were calculated to be 21.8 and 44.2 nm, respectively, which indicates that an increase in the annealing

where, Rs is the average sheet resistance, σ is the conductance of the seed layer films, and t is the grain size. The conductivity of a thin film can be expressed as:

The surface and cross-sectional morphology were observed using field-emission scanning electron microscopy (FE-SEM). For surface morphology, the samples were prepared by being immersed in 3 M NaOH solution for 15 min to dissolve the PAM template. For crosssectional morphology, the samples were cut using a focused ion beam (FIB). The crystal structure of a single nanowire and ZnO/AZO/ PAMs composites was investigated using high-resolution transmission electron microscopy (HR-TEM), for which the samples were prepared by being dispersed in ethanol and cut by FIB, respectively.

σ ¼ nqμ

ð1Þ

ð2Þ

where n is the thickness of the thin film, q is the electric charge, and μ is the mobility of an electron. These equations suggest that the conductivity

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Average sheet resistance (Ω/cm2)

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3500 3000

diffraction pattern implies that point A has a ZnO structure, in which the planes are (100), (101), (102), and (110).

2500

3.2. Synthesis of nanowire arrays

2000

Fig. 5 shows SEM observations of the (a) PAMs and (b, c) crosssectional images, in which nanowire arrays were deposited in the PAMs under −1 V/SCE at 65 and 80 °C. In Fig. 5 (a), the channel diameter of the two-step anodized PAMs is about 110–140 nm. For the deposition temperature of 65 °C (Fig. 5(b)), there are many particles in the PAMs channel; obvious nanowires formed at 80 °C (Fig. 5(c)). Thus, the size and shape is controlled by the PAMs channel. The crystallinity of nanowires was investigated from TEM observation, as shown in Fig. 6 (a) (b) for deposition temperatures of 65 and 80 °C, respectively. Individual nanowires formed from particles that aggregated in the PAMs channel. The high resolution image results indicate the amorphous structure of the nanowire. In the other ways, compared the TEM results between 65 and 80 °C, a modicum fibrillar structure was observed in 80 °C expect for 65 °C. Comparing the contrast of TEM and high-resolution images between the nanowire prepared at 65 °C and 80 °C, it is observed that the contrast color nanowire prepared at 65 °C was shallower than that prepared at 80 °C. Some parts of the nanowire prepared at 80 °C was began to crystallize observed from high resolution images. This indicates that the crystallinity of the nanowire prepared at 80 °C is greater than that of the nanowire prepared at 65 °C. The formation of the nanowire can be described as:

1500 1000 500 0 0

1

2

3

4

5

Number of AZO seed layer Fig. 2. Plot of average sheet resistance versus the number of AZO seed films at annealed at 400 °C for 0 h in a vacuum.

of thin films can be improved by increasing the thickness of the thin film. However, the AZO films undergo heat treatment after each spin coating process, which may destroy. A 5-layer AZO seed was chosen to coat the back side of PAMs for the electrochemical deposition electrode. Fig. 3 shows TEM images of (a, b) AZO/PAMs composite, (c) a high-resolution image, and (d) the electron diffraction pattern. The AZO/PAMs TEM sample was prepared by focused ion beams (FIB). In Fig. 3(a), point 1 indicates the carbon films, which protected the sample during the cutting process, point 2 indicates the AZO films, and point 3 indicates the PAMs. An interface (shown as B layer) formed between points A and C in Fig. 3(b). The high-resolution image indicates that point A was polycrystalline, and points B and C were amorphous structures. The TEM results also show that the AZO films were well-adhered onto the PAMs. The EDX results (shown in Fig. 4) indicate that the Zn composition decreased and the Al composition increased from A to C. In addition, the electron

a

b

Zn

2þ

−

þ 2e →Zn −

−

2þ

ð3Þ

0

ð4Þ

E ¼ 0:694 VðvsSCEÞ

H2 O2 þ 2e →2OH Zn

0

E ¼ −1:01 VðvsSCEÞ

−

þ 2OH →ZnðOHÞ2

ð5Þ

c

d

Fig. 3. TEM images of AZO/PAMs composite. (a) Low magnification, (b) high-magnification, (c) high-resolution image, and (d) electron diffraction pattern. Regions 1, 2, and 3 in (a) are carbon films, AZO films, and PAMs, respectively.

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a

Zn

channel, the whiskery structure was observed at 4 h and the nanowire was filled entirely at 8 h. This whiskery structure was formed due to under dehydration course, i.e., there was insufficient time for the diffusion of H2O from the PAMs channel to the surface. In contrast, a dense nanowire was obtained after 8 h of heat treatment. To examine the surface morphology of the nanowire arrays, 3 M NaOH was used to remove the template. Fig. 7(c) shows the top-view image of a specimen heat treated for 8 h, which indicates that the size and distribution of the nanowires are well incorporated onto the PAMs. Fig. 8 shows TEM and electron diffraction results of individual nanowires from samples heat treated for 4 and 8 h. As shown in the TEM images, the nanocrystallinity decreased gradually to form the fully crystalline with increasing in the annealing time. The ZnO crystalline ring was found in the electron diffraction patterns of samples heat treated for 4 and 8 h. The ring pattern is more obvious for the 8-h sample. A spot pattern was also observed (as shown in Fig. 8 (c) and (d)). This result shows that during the dehydration process, ZnO was formed and the crystallinity improved. Fig. 9 shows a TEM image of ZnO/AZO/PAMs nanowire arrays with heat treatment at 400 °C for 8 h. Points A, B, and C correspond to ZnO/ PAMs, the interface of ZnO/AZO, and AZO seed layers, respectively. As shown in the electron diffraction pattern, a zone pattern with a hexagonal structure was found in the AZO seed layers (point C), which was formed by submicron grains. The electron diffraction patterns (point A and B) indicate that the crystallinity of the ZnO nanowire improved when the nanowire was close to the AZO seed layers. The d-spacing of the ZnO nanowire was 2.5 Å, and the growth direction was [001], which is close to that of the AZO seed layers (d = 2.7 Å), as shown in the high-resolution images. This result implies that during the electrochemical deposition and annealing processes, ZnO aggregated at the seed layers. Thus, the heterogeneous nuclei of the AZO seed layers improved the growth of ZnO nanowire. The ZnO nanowire approached to weak crystallinity, when the nanowire was farther seed layers. Therefore, the results indicate that the formation of ZnO nanowires is preferred in the AZO seed layers.

EDX-A Zn Zn

O Cu

Zn

Al

Energy

b

KeV

Ga Ga

EDX-B

O

Al C

Zn

Ga

Cu P

Energy

c O

KeV

Ga Ga

EDX-C

Ga

Al

C

Cu

4. Conclusion Zn

Zn

P

Energy

KeV

Fig. 4. EDX spectra of AZO/PAMs composite. (a) Point A, (b) point B, and (c) point C (see Fig. 3).

ZnðOHÞ2 →ZnO þ H2 O

1535

ð6Þ

Eq. (3) and Eq. (4) show the reduction potentials of Zn and OH−, respectively. In this work, the deposition potential was −1 V, so Zn2+ was not reduced to Zn metal instead of Zn2+ assembling on the surface of the working electrode. OH− formed on the electrode and reacted with Zn2+ to form the Zn(OH)2 hydrate (as shown in Eq.(5)). The equations show that the ZnO should be passed though dehydration process. The fibrillar structure is a transitional product during crystallization. This result implies that the fibrillar structure formed at 80 °C was obtained due to the partial dehydration of Zn(OH)2 to ZnO. 3.3. Effect of annealing time The above results indicate that polycrystalline and highly concentrated nanowire arrays were synthesized at 80 °C. This temperature was thus selected for subsequent electrochemical deposition. Before the annealing process, the PAMs were not removed. Fig. 7 (a) and (b) shows cross sectional SEM images of nanowire arrays after 400° C heat treatment for 4 and 8 h in air, respectively. It was found that in PAMs

Uniform and ordered polycrystalline ZnO nanowire arrays were fabricated using AZO seed layers in PAMs. The average sheet resistance of a 5-layer AZO seed was 129.37 Ω/cm2 under 400 °C annealing in a vacuum. The AZO seed layers were polycrystalline, and well-adhered to PAMs. The Zn(OH)2 hydrate was obtained during electrochemical deposition at −1 V and 65 and 80 °C. A partial ZnO compound formed at 80 °C during the dehydration process, which improved crystallinity. The shape and size of the ZnO nanowire arrays were controlled by the PAMs, which had a diameter of 110–140 nm. The crystallinlty of ZnO depended on the AZO seed layers, which grew along the PAMs channel. The heterogeneous nuclei of AZO seed layers improved the growth of ZnO nanowires. Acknowledgments This work was supported by the National Science Council of Taiwan under grant NSC 98-2221-E-006-008. References [1] [2] [3] [4] [5] [6] [7]

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a

b

PAMs

65 o C

c

80 o C

Fig. 5. Plane-view SEM image of (a) PAMs and cross sectional images of nanowire arrays deposited in the PAMs at − 1V/SCE and (b) 65 and (c) 80 °C.

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a

c

65 o C

b

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80 o C

d

Fig. 6. TEM and high-resolution TEM images of a single ZnO nanowire deposited at temperatures of (a, c) 65 °C and (b, d) 80 °C.

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a

4 hr

c

b

8 hr

8 hr

Fig. 7. Cross-sectional and SEM images of ZnO nanowire after 400 °C heat treatment in air for (a) 4, (b) 8 h, and (c) 8 h (top-view).

Y.-M. Shen et al. / Thin Solid Films 520 (2011) 1532–1540

a

c

4 hr

b

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8 hr

d

Fig. 8. TEM images and selected area electron diffraction (SAED) patterns of a single ZnO nanowire with 400 °C heat treatment for (a, c) 4 h and (b, d) 8 h.

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Point A -HR

2.5 Å [001]

Point C -HR

2.7 Å [001]

Point A -SAED

Point B -SAED

Point C -SAED

Fig. 9. TEM image of ZnO/AZO/PAMs nanowire arrays with heat treatment at 400 °C for 8 h. The sample was prepared by focused ion beam (FIB). Points A, B and C correspond to ZnO/PAMs, interface between ZnO and AZO, and AZO, respectively.