Facile Continuous Flow Injection Process for High Quality Long ZnO

Jan 12, 2012 - Rajesh Kumar , Ahmad Umar , Girish Kumar , Hari Singh Nalwa , Anil Kumar , M. S. Akhtar. Journal of Materials Science 2017 52 (9), 4743...
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Facile Continuous Flow Injection Process for High Quality Long ZnO Nanowire Arrays Synthesis Liang-Yih Chen* and Yu-Tung Yin Department of Chemical Engineering, National Taiwan University of Science and Technology, 43, Section 4, Keelung Road, Taipei 106, Taiwan S Supporting Information *

ABSTRACT: This paper reports a facile continuous flow injection (CFI) process to synthesize high-quality long zinc oxide nanowire arrays (ZnO-NAs) using a hydrothermal method. In previous related studies, the photoluminescence (PL) spectra of ZnO-NAs synthesized using a batch process exhibit highly visible emission caused by defect structures. In contrast to the batch process, zinc oxide nanowire arrays grown using the CFI process can reduce the visible emission in PL spectra effectively, demonstrating the qualities of zinc oxide nanowire arrays as superior to those grown using a batch process. To understand the difference between batch processes and the CFI process, inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to analyze the zinc precursor concentration regarding growth duration. According to the concentration variation of both processes, we could determine that zinc precursor concentration is maintained at a constant level to promote ZnO-NAs growth continuously in the CFI process. Long ZnO nanowire arrays can be obtained easily using the CFI process. The results demonstrate that the CFI process is a facile process for growing high-quality long ZnO nanowire arrays.

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solution.12 Recently, Gao et al. also proposed a similar concept to moderately decrease the degree of supersaturation.6c To prevent the fusion of ZnO nanowires at the roots and to obtain sufficiently long ZnO nanowire arrays, a multiple stage and a self-assembled monolayer (SAM) coating were used to protect the wires that were grown in the previous stage from widening and fusion in the next growth stage.13 On the basis of previous studies, a batch process was used to grow a one-dimensional ZnO nanostructure. A multiple refreshing process was necessary to increase the length of the ZnO nanowire arrays because the zinc salt concentration was consumed to reduce the growth rate with time. However, the reaction conditions change as a function of time because the composition evolves continuously during crystal growth. In contrast to the batch process, a continuous flow injection (CFI) process offers the advantage of providing reaction conditions that are time-invariant. Ito et al. used the liquid film method to deposit ZnO and CdS films by flowing the heated solution over a substrate that was placed inside a low-profile reactor.14 Baxter el al. reported the use of a continuous flow microreactor to grow ZnO nanowire arrays.15 However, the composition changes along the length of the reactor. In this study, we propose a CFI process to grow ZnO nanowire arrays with a high crystal quality because of the supply of a stable zinc

inc oxide (ZnO) is a promising functional material because of its catalytic, electrical, optoelectronic, and piezoelectric properties.1 In particular, the one-dimensional ZnO nanostructure has attracted notable attention because of its applications to nanodevices, such as light-emitting diodes,2 field-effect transitions,3 ultraviolet lasers,4 chemical sensors,5 and solar cells.6 However, the development of a simple and economical method of synthesizing ZnO nanomaterials is vital for industrial mass production. Various synthesis methods have recently been developed for one-dimensional ZnO nanomaterial growth, including physical vapor deposition,7 laser ablation,8 and the solution method.9 In 2001, Vayssieres et al. proposed an inexpensive fabrication for highly oriented ZnO nanorod arrays directly on various types of substrates.9c An aqueous solution that contained zinc salt and organic amine, such as hexamethylenetetramine (HMTA, (CH2)6N4), was used. Due to precursor consumption, multiple refreshing processes were necessary to increase the length of the ZnO nanowire arrays. Subsequently, several researchers proposed a number of strategies to obtain a long and high aspect ratio ZnO nanowire array for its applications. Law et al. improved the aspect ratio of ZnO nanowire arrays to above 125 by using polyethylenimine (PEI) to hinder the lateral growth of the nanowires in the solution.10 Tak and Yong replaced HMTA with ammonium hydroxide (NH4OH) to control the zinc−amine complex relation rate to reduce the homogeneous nucleation in the bulk solution.11 Hwang et al. obtained long ZnO nanowire arrays by using a preheating process, which reduced the precursor concentration to prevent the high supersaturation in the growth © 2012 American Chemical Society

Received: October 3, 2011 Revised: December 27, 2011 Published: January 12, 2012 1055

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Figure 1. Schematic figure of the continuous flow injection apparatus. Reservoir A and reservoir B are the aqueous solutions of zinc acetate dihydrate and hexamethylenetetramine (HMTA), respectively.

precursor concentration during the growth of ZnO nanowire arrays. The CFI process is illustrated schematically in Figure 1. Parts a and b of Figure 2 display the SEM images of the ZnO nanowires that were grown on ITO glass substrates by using

Figure 2. Cross-sectional SEM images of ZnO nanowire arrays grown by (a) the batch process and (b) the continuous flow injection process. The zinc precursor concentration is 0.025 M, and the growth time is 24 h.

the batch process and the CFI process, respectively. A high density of ZnO nanowire arrays grew vertically on the ITO substrates. The diameters of the nanowires ranged from 100 to 150 nm for both processes. The ZnO nanowire arrays that were grown by the CFI process were 15 μm in length, which was longer than those grown by the batch process (approximately 10 μm). Cross-sectional TEM was conducted to confirm the growth direction of the ZnO nanowire arrays by the CFI process, as illustrated in Figure 3a. The corresponding SAED pattern and high-resolution transmission electron microscopy (HRTEM) are illustrated in Figure 3b and c. The c-axis growth in the CFI process was confirmed by the SAED pattern. The result was consistent with the lattice fringes of the HRTEM with an interplanar spacing of approximately 0.52 nm. As for ZnO nanowire arrays grown by the batch process, the morphology is similar, as shown in the Supporting Information (Figure S1). Figure 4a illustrates the room-temperature microphotoluminescence (μ-PL) spectra of the as-grown ZnO nanowire arrays that were synthesized by the batch process and the CFI process.

Figure 3. (a) Cross-sectional TEM image, (b) corresponding selected area diffraction pattern, and (c) high-resolution TEM image of ZnO nanowire arrays grown by the continuous flow injection process.

The PL spectra of the as-grown samples are illustrated by the UV peak at 378 nm and an orange-red emission centered at 620 nm. The UV peak is generally attributed to radiative excitonic recombinations,16 and the orange-red emission is attributed to radiative transitions involving the defect-related energy levels that are located in the band gap. From previous studies, the orange-red emission is assumed to involve interstitial oxygen ions (Oi), which are found in ZnO that is grown electrochemically and hydrothermally.17 For the batch process, the as-grown ZnO nanowire arrays exhibited a strong and broad orange-red emission and the PL intensity ratio of the UV emission to the visible emission was low, which indicated the presence of high density point defects in the ZnO nanowire arrays. In contrast to the batch process, the ZnO nanowire arrays that were prepared 1056

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Figure 4. Corresponding (a) micro-PL and (b) micro-Raman spectra of ZnO nanowire arrays grown by the batch and continuous flow injection processes. (c) A1(LO)/E1(LO) peak of micro-Raman spectroscopy enlarged from part b. [(1): batch process; (2) continuous flow injection process].

PL spectra were shown in Figure S2(a)−(d) of the Supporting Information, and the average diameters and lengths of ZnO nanowire arrays were also listed in Table 1. From Figure S2(d),

by the CFI process displayed visible emission; however, the PL intensity ratio of the UV emission to the visible emission was higher than that of the sample prepared by the batch process. The intensity ratio of the UV−visible emissions demonstrated the optical qualities of the ZnO nanowire arrays. The higher intensity ratio of the UV emission to the visible emission indicates the higher optical quality of the ZnO nanowire arrays. In addition, a fundamental understanding of the thermal and electrical properties requires precise knowledge of the vibrational modes of the crystal, which are related to the mechanical properties. The vibrational properties of the ZnO nanowire arrays that were prepared by the two methods may be investigated by using μ-Raman spectroscopy, as illustrated in Figure 4b. From theoretical prediction, the ZnO has 4 atoms per unit cell, which results in 12 photon branches (9 optical and 4 symmetry. Eight sets are 3 acoustic) due to its space group, C6v found near the center of the Brillouin zone, as follows: E2(L), E2(H), 2A1, 2E1, and 2B1. Both E2 sets, one A1 set and one E1 set, are Raman active, and the B1 set is silent. With the E2(H) peak normalized, the integrated intensity of the A1(LO)/ E1(LO) mode of the ZnO nanowire arrays that were prepared by the batch process was higher than that prepared by the continuous flow injection process, as illustrated in Figure 4c. This peak was attributed to a defect-induced band, which was reported to be markedly dependent on the oxygen stoichiometry.18 To further identify the quality of ZnO nanowire arrays grown via the CFI process, three different zinc precursor concentrations (5 mM, 10 mM, 25 mM) were used to grow ZnO nanowire arrays and these samples were analyzed by μ-PL analysis. The cross-sectional SEM images and

Table 1. Average Diameter and Length of ZnO Nanowire Arrays Grown by the CFI Process with Three Different Zinc Precursor Concentrations conc

5 mM

10 mM

25 mM

diameter (nm) length (μm)

120 20

200 16

230 15

the intensity of the visible band emission of as-grown ZnO nanowire arrays increases with decreasing zinc precursor concentration due to the small diameter (high surface area). However, the intensity of the visible band emission is still lower than that of the UV band emission. The result demonstrated that the crystal quality of ZnO nanowire arrays grown by using the CFI process is good enough. To understand the difference between the continuous flow injection process and the batch process, we studied the changes in the molar concentration of the zinc precursor in the reactor for both processes by using an inductively coupled plasma atomic emission spectroscope (ICP-AES), as illustrated in Figure 4. In this study, the growth time of the batch and CFI processes was controlled at 24 h and the concentration of the zinc precursor and HMTA was 0.025 M. From ICP-AES analysis, as illustrated in Figure4, the concentration of the zinc precursor in the batch process reduced rapidly to a lower value. However, the concentration of the zinc precursor may be 1057

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photogenerated electrons repeatedly interact with a distribution of traps as they undertake a random walk through the film. One promising solution is to increase the electron diffusion length in the photoanode by replacing the nanoparticle film with an array of oriented single crystalline nanowires. Although the nanowire photoanode has good transport properties, the dye adsorption amount is low compared to that of a nanoparticles photoanode with the same thickness.10 To obtain a high-performance nanowire photoanode, a large surface area for dye adsorption is necessary. Yang et al. proposed a sufficient density of long, ZnO nanowires as photoanode by using polyethyleneimine (PEI), a cationic polyelectrolyte to hinder only the lateral growth of ZnO nanowires in the solution.10 Gao et al. coated the surface of nanowires with a self-assembled monolayer (SAM) formed from the precursor octadecyltrichlorosilane (OTS) and used the multilayer assembly method to grow long ZnO nanowire arrays (∼40 μm).13 The long ZnO nanowire arrays were used to fabricate DSSCs with power conversion efficiencies up to 7%. From previous studies, we can find long ZnO nanowire arrays, as the photoanode can provide high dye adsorption amount to obtain high conversion efficiency for DSSCs. However, the long ZnO nanowire arrays are always formed by using a refreshing process in batch processes because of the rapid consumption of zinc precursors. For the CFI process, the length and the diameter of ZnO nanowire arrays could be controlled easily using the injection concentration and growth time. In this work, 55 μm ZnO nanowire arrays could be grown without requiring a refreshing process for long enough growth time, as shown in Figure S3. In conclusion, we proposed a facile growth method to grow ZnO nanowire arrays on seeded substrates. The concentration of the zinc precursor in this method was easily controlled by the injection concentration and volumetric flow rate. The qualities of the ZnO nanowire arrays were efficiently controlled due to the stable maintenance of the precursor concentration in the reactor. The length of the ZnO nanowire arrays was easily increased by continually injecting the precursor.

maintained at a stable value to supply the zinc species to continually grow ZnO nanowire arrays on the substrate. The batch and CPI processes can be described by the following model:

S=

1 {DS0 − [DS0 − (D + μ)m]e−(D +μ)t } D+μ

(1)

where S is the zinc precursor concentration in the reactor (mol/L), S0 is the input concentration of the zinc precursor (mol/L), m is initial zinc concentration, μ is the specific growth rate of ZnO (1/h), and D is the ratio of volumetric flow rate (F) to reactor volume (V). (The detailed derivation is in the Supporting Information). Equation 1 was used to effectively fit the ICP-AES analysis (as represented by the solid line) and demonstrated that the zinc precursor maintained a stable level to supply the growth of ZnO nanowire arrays. For the batch process, the assumption of F = 0 was used to fit the ICP-AES analysis (as represented by the short-dash line). From the μ-PL and μ-Raman analysis, we found that the optical and structural qualities of the ZnO nanowire arrays that were grown by the CFI process were superior to those that were grown by the batch process. The high oxygen-related defects, such as oxygen interstitials, were observed in the samples that were prepared by the batch process. This was due to the rapid consumption of the precursor in the batch reactor with growth time, as illustrated in Figure 5. The zinc species

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ASSOCIATED CONTENT

S Supporting Information *

Experimental and characterization details, models for the batch and continuous flow injection processes, and TEM and SEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 5. Time variation of zinc concentration profiles during the growth period in the batch and CFI processes. The scatters (●, ○) were zinc precursor concentrations in the batch and CFI processes measured by ICP-AES. The lines (---, ―) were the changes of zinc concentrations in the batch and CFI processes by using modeling.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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rapidly reduced to a low concentration and caused the deficiency of the zinc precursors during the growth of the ZnO nanowire arrays. In contrast to the case of the batch process, the zinc precursors were maintained at a stable concentration level to continuously supply the growth of ZnO nanowire arrays in the CFI process. The as-grown ZnO nanowire arrays maintained excellent optical and structural qualities without any thermal anneal treatment due to the stable precursor supply. Currently, dye-sensitized solar cells (DSSCs) are among the most promising devices for low-cost solar electricity energy conversion. The nature of electron transport in nanoparticle networks proceeds by a trap-limiting diffusion process, in which

ACKNOWLEDGMENTS We are grateful for the financial support provided by the National Science Council under Contract No. NSC 99-2221-E011-123.

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