Controllable and Rapid Synthesis of Long ZnO Nanowire Arrays for

Jun 27, 2014 - Chengxian College, Southeast University, 6 Dongda Rd Pukou, Nanjing 210088, China. ABSTRACT: The hydrothermal method is widely used ...
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Controllable and Rapid Synthesis of Long ZnO Nanowire Arrays for Dye-Sensitized Solar Cells Liqing Liu,† Kunquan Hong,*,† Xing Ge,† Dongmei Liu,‡ and Mingxiang Xu*,† †

Department of Physics and Key Laboratory of MEMS of the Ministry of Education, Southeast University, Si Pai Lou 2, Nanjing 210096, China ‡ Chengxian College, Southeast University, 6 Dongda Rd Pukou, Nanjing 210088, China

ABSTRACT: The hydrothermal method is widely used to synthesize ZnO nanowires for electrical and optical devices. However, the rapid synthesis of long vertically aligned ZnO nanowire arrays on a transparent conductive oxide substrate is still a challenge and also time-consuming. In this paper, we report a controllable and rapid growth of long ZnO nanowire arrays by a microwave heating method with fresh precursor solution continuously injected into the reactor. This method can avoid the growth stoppage and keep the concentration of the reactants in dynamic equilibrium during the whole reaction. It is found that the length of the nanowires increases linearly with growth time, and the growth rate is as high as 58−78 nm/min, producing ZnO nanowires with a length over 10 μm after growing for 2−3 h. When these nanowire arrays were used as the photoanodes of dyesensitized solar cells (DSSC), the power conversion efficiency of these ZnO nanowire-based DSSCs increases with the length of the nanowires, which is mainly attributed to the enlarged internal surface area and therefore dye-loading amount enhancement in the longer ZnO nanowires. This controllable and rapid method is useful for synthesizing ZnO or other ultralong 1D nanostructure for nanodevices.

1. INTRODUCTION ZnO nanowire arrays have attracted tremendous attention in various applications due to their unique optical and electrical properties, such as ultraviolet (UV) nanowire lasers,1 UV photodetectors,2 light-emitting diodes,3,4 solar cells,5−7 fieldeffect transistors,8 biosensors,9 and so forth. To realize these applications, the synthesis of ZnO nanowire arrays with desired morphology and properties is highly expected. So a great number of methods have been developed for synthesizing ZnO nanowire arrays,10,11 including molecular beam epitaxy (MBE),12 chemical vapor deposition (CVD),13,14 pulsed laser deposition (PLD),15 hydrothermal,16 and thermal evaporation17,18 methods, among others. Among these methods, the hydrothermal method draws a particular interest for the advantages of low experimental temperature, high efficiency, easy adoption, and low cost,19−22 with a potential for large scale. So ZnO nanowires were frequently synthesized by the hydrothermal method and widely used in electrical and optical devices. For example, Kao et al. obtained ZnO nanowires with different lengths from 1 to 3 μm, reporting that the short circuit current density (Jsc), open circuit voltage (Voc), and energy © XXXX American Chemical Society

conversion efficiency (η) of DSSC increase with the length of ZnO nanowires.23 This result was explained by the larger internal surface area in the longer nanowires. The enlarged surface area increases the dye loading to enhance the absorption of light.23 However, fabrication of long ZnO nanowire arrays (such as with a length exceeding 10 μm) by a hydrothermal method is still a challenge and also timeconsuming. The length growth rate of the nanowires was retarded because the growth solution becomes inactive for long-time growth. The reported reaction time for long ZnO nanowire synthesis is commonly spanning from several hours to days. For example, Kao et al. prepared ZnO nanowires up to 11.4 μm, and the solution was maintained at 90 °C for 12 h.24 Qiu et al. obtained ZnO nanowires with length of 10 μm by sealing and heating the reactor to 95 °C for 10 h.25 Law et al. synthesized ZnO nanowire arrays with a length of 17 μm by repeatedly introducing the substrate to a fresh solution bath Received: December 7, 2013 Revised: June 23, 2014

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every 2.5 h and total reaction time is up to 50 h.5 Therefore, it is very imperative to develop a controllable and rapid wet chemical method to synthesize long ZnO nanowire arrays for nanodevices. In this paper, a controllable microwave-assisted hydrothermal method was developed to synthesize ZnO nanowire arrays rapidly with the length exceeding 15 μm. This method has advantages of avoiding the growth stoppage and keeping the reactant concentration in dynamic equilibrium during the process of reaction through continuously injecting fresh precursor solutions into the reactor. Vertically aligned ZnO nanowire arrays with various lengths (3.4−17.2 μm) have been obtained. SEM images show the length of the nanowires increases almost linearly with growth time, and the growth rate is as high as 58−78 nm/min. When these nanowire arrays were used as the photoanodes of DSSCs, the photocurrent density− voltage (J−V) curves show that the power conversion efficiency improves with the length of the ZnO nanowires because of the larger internal surface area for more N719 dye absorption.

DSSCs was characterized under AM 1.5G (100 mW/cm2) simulated sunlight.

3. CHARACTERIZATION The morphologies of the as-grown samples were characterized by field emission scanning electron microscopy (FESEM, FEI Inspect F50). X-ray diffraction (XRD, Rigaku Smartlab) with Cu Kα radiation (λ = 0.15406 nm) was employed to measure the crystal structure of ZnO nanowire arrays. The optical properties of the samples were studied by photoluminescence (PL, HJY-FL3-211-TCSPC) spectra with a 450 W Xe lamb as the excitation source and excitation wavelength of 340 nm. The amount of N719 adsorption was determined by detaching the N719 from the surface of ZnO nanowire arrays in 0.01 M NaOH solution, and then the absorptions were measured by UV−vis spectrophotometer (U3900). 4. RESULTS AND DISCUSSION Figure 1 displays the XRD patterns of the samples with different growth times, in which the curves correspond to the

2. EXPERIMENTAL SECTION 2.1. Synthesis of ZnO Nanowire Arrays. The ZnO nanowire arrays were synthesized by a ZnO nanoseed-mediated microwave heating growth method. At first, a thin layer of ZnO was deposited on a fluorine-doped tin oxide (FTO) glass substrate by pulsed laser deposition (PLD) technique. The pulse energy was 300 mJ, and the total pulse number was set at 2000 to spurt the ZnO target to obtain a ∼100 nm ZnO thin film on FTO. Second, this substrate with a layer of ZnO was dipped into a beaker with 150 mL of 12.5 mM zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and 25 mM hexamethylenetetramine (HMTA, C6H12N4) mixture solution in deionized (DI) water and heated with a commercially available microwave oven (Galanz, 2.45 GHz). A hole was drilled into the top of this microwave oven, through which three Teflon tubes connected with tube pumps were inserted into the microwave oven. The precursor solutions were continuously injected into the reactor at a flow rate of 7 mL/min through two Teflon tubes. The third Teflon tube was fixed upon the surface of the solution to pump out the extra solution to maintain the volume of solution at 150 mL during the whole experimental process. This arrangement can avoid the stoppage of growth and keeping dynamic equilibrium of reactant concentration in the process of reaction. During the growth, a turntable inside the microwave oven revolved at 5 rpm to heat the solution homogeneously. This heating process was performed for 0.5 to 5.0 h with a power setting of 640 W. Then the substrate was taken out from the growth solution, rinsed with DI water softly several times, and dried in air flow. 2.2. Fabrication of DSSCs. The area of the ZnO nanowire arrays was limited to 0.5 cm2. The ZnO nanowire arrays were first calcined at 450 °C for 1 h to eliminate any residual organics and moisture absorbed from ambient air. The ZnO nanowire arrays were sensitized in a solution of 0.5 mM (Bu4N)2[Ru(4,4′-(COOH)-2,2,-bipyridine)2 (NCS)2] (N719 dye) in ethanol for 1 h, and then the sandwiched structure was assembled with the FTO glass coated with platinum film as the counter electrode.5 The distance between the two electrodes is 50 μm. The electrolytic solution consisting of 0.1 M LiI, 0.1 M I2, 0.5 M tert-butylpyridine, and 0.6 M tetrabutylammonium iodide in acetonitrile. It was introduced into the gap formed by the two electrodes by capillary force. The performance of the

Figure 1. XRD patterns of the as-grown ZnO nanowire arrays grown for different times.

Figure 2. Cross-sectional SEM images of ZnO nanowire arrays with different lengths. Wire length (a) 3.4 μm, (b) 7.3 μm, (c) 10.8 μm, (d) 14.7 μm, (e) 17.2 μm. Scale bar, 5 μm.

samples growing for 1, 2, 3, 4, and 5 h, respectively. All the diffraction peaks of the samples can be indexed to the wurtzite B

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Figure 6. Room-temperature PL spectra of the ZnO nanowires grown for different times.

Figure 3. Average length of the nanowires as the function of the growth time.

Figure 7. Current density−voltage characteristics of DSSCs fabricated by ZnO nanowire with various lengths. Figure 4. Cross-sectional SEM images of ZnO nanowire arrays with different solution injection rate. Solution injection rates of (a) 4.5 mL/ min, (b) 6.0 mL/min, (c) 7.5 mL/min, (d) 9.0 mL/min, (e) 10.5 mL/ min, and (f) 12 mL/min. Scale bar, 3 μm.

(100), (103), and (004) planes were located at 31.8, 62.8 and 72.6°, respectively. No impurity peak is found in the XRD patterns, in which it can be seen that ZnO nanowires synthesized by our method are pure phase. Figure 2a−e show the typical cross-sectional FESEM images of the vertically aligned ZnO nanowire arrays with a length of 3.4, 7.3, 10.8, 14.7, and 17.2 μm after growing for 1, 2, 3, 4, and 5 h, respectively. Note that the samples consist of high-density nanowires which are vertically aligned to the substrate. The length of the ZnO nanowire arrays increases with the growth time. Figure 3 shows the length of nanowires as the function of the growth time, which fits well with a linear function with a slope of 3.5 μm/h, indicating that the growth rate is relatively constant in length. This suggests that the length of the nanowires increases by extending the growth time. Therefore, ultralong ZnO nanowire arrays can be obtained. The aspect ratio of length to diameter of the ZnO nanowire arrays increases with the growth time leading to the enlargement of the internal surface area. So this method is useful for synthesizing ultralong nanowire arrays with larger internal surface area for nanodevices. Insight into the reactions involved during the formation of ZnO is helpful to understand the mechanism of the rapid growth of these nanowires. In the growth process, ammonia (NH3) is produced as a result of the decomposition of HMTA heated in the solution. It is concluded in the literature that

Figure 5. Length of the nanowires as a function of flow rate.

phase (JCPDS card no. 36-1451) of ZnO with the lattice constants of a = 3.250 Å and c = 5.207 Å. These patterns reveal that the ZnO nanowires grow preferentially along the [002] direction. The strong diffraction density of the (002) peaks shows these ZnO nanowires are well-crystallized and highly oriented. The other peaks of the ZnO nanowires growing along C

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Table 1. Photovoltaic Parameters of the ZnO Nanowire-Based DSSC growth time (h)

wire length (μm)

Jsc (mA/cm2)

Voc (V)

FF

η (%)

1 2 3 4 5

3.4 7.3 10.8 14.7 17.2

1.9 3.2 4.2 5.0 6.0

0.537 0.589 0.614 0.547 0.583

0.34 0.41 0.32 0.42 0.36

0.35 0.76 0.83 1.16 1.25

solution is 94, 93, and 91 °C when the injection rate is 4.5, 7.5, and 9 mL/min, respectively. Meanwhile, pump out speed is also increased with the injection rate. Thus, the injected fresh solution does not react fully at lower solution temperature and higher pumping out speed, which leads a decrease in length growth rate when the injection rate exceeds 10.5 mL/min. Figure 6 shows the PL spectra of as-grown ZnO nanowires with different length at room temperature. For these samples, the PL spectra peaks possess weak near-band-edge (NBE) emission and the bright green emission with peak positions at about 380 and 550 nm, respectively. The NBE emission is the intrinsic characteristic of the near-band-edge transition of ZnO that is attributed to the free exciton transitions. The strong green emission in the visible range is attributed to the defects in the nanowires.8 With the increase of the growth times, the green emission peaks intensity increases. The defect-related emission intensity increases with the surface area of the ZnO nanowires.24 Figure 7 presents the photocurrent−voltage (J−V) curves of DSSCs fabricated by ZnO nanowire arrays with various lengths (3.4−17.2 μm). From these results, short circuit current density Jsc gradually increases from 1.9 to 6.0 mA/cm2 when the length of the nanowires increases from 3.4 to 17.2 μm. The open circuit voltage Voc is in the range of 0.54 to 0.62 V. The lengths of ZnO nanowire arrays, short circuit current density, open circuit voltage, fill factor (FF), and power conversion efficiency η are summarized in Table 1. The J−V curves show that the η gradually increases with the lengths (3.4−17.2 μm) of the ZnO nanowires. It can be explained that the enlarged internal surface area in the lengthened nanowire arrays results in the increase of N719 dye-loading amount absorbed on the surface of ZnO nanowires. In this work, UV−vis spectrophotometer was employed to measure the absorption of N719 dye on these nanowires. Figure 8 shows the UV−vis absorption spectra of the N719 dye detached from ZnO nanowire arrays with solvent of 0.01 M NaOH aqueous solution. It is clearly seen the absorption of N719 dye increases with the length of the ZnO nanowire arrays, indicating high dye loading in the lengthened nanowire. In this way, more photons can be absorbed by the N719 dye molecule on the surface of the nanowires to excite more electrons. Then these electrons inject from the N719 dye molecules to the conduction band of the ZnO nanowires. These excited electrons diffuse through the nanowires to FTO and then to the external circuit. As a result, the Jsc and power conversion efficiency are improved.

Figure 8. UV−vis absorption spectra of the N719 dye detached from ZnO nanowire arrays with different growth times of 1−5 h corresponding to various lengths of 3.4−17.2 μm, respectively.

HMTA plays the role to buffer the pH value of the solution by slowly releasing OH− ions.26−28 The OH− ion reacts with Zn2+ to produce the insoluble zinc hydroxide (Zn(OH)2), which then decomposes to ZnO acting as the growth unit of ZnO nanostructures.29 So it is reasonable that the releasing rate of OH− ions decreases with time because more and more HMTA is decomposed. At the same time, the Zn2+ ions are also depleted slowly. This phenomenon retards the growth of ZnO nanowire arrays. As a result, the growth rate in length is decreased with growth time. Because fresh precursor solutions are more active to increase the growth rate, replacement of the solution is frequently adopted to grow long nanowires. In our work, along with the continuous injection of fresh solution into the reactor, part of the heated reactant is pumped out of the reactor, and the concentrations of the species in the solution gradually become stabilization and finally in equilibrium. This process ensures that the length of nanowires increases linearly with growth time. Then a constant growth rate is reached, suggesting that the length of the nanowires can be extended monotonously by increasing the growth time. To prove the role of the injection rate of the fresh precursor, experiments were conducted with the same conditions as the sample shown in Figure 2, except for adjusting the rate of raw solution to 4−12 mL/min. The growth times were maintained for 2 h. Figure 4 shows the SEM images of the samples. It is found that samples with similar morphologies are obtained, except for the length of the nanowires. In the curve of length as a function of the injection rate (Figure 5), the length increases at first and then decreases with an increasing injection rate. A maximum length growth rate is reached at an injection rate of 10.5 mL/min. Because fresh precursor solutions are more active to increase the growth rate, it is reasonable that the length growth rate increases with the injection rate at first. However, the temperature of the solution is also gradually reduced when the injection rate increases. For example, the temperature of the

5. CONCLUSION In summary, we have developed a controllable and rapid growth method for synthesizing ZnO nanowire arrays. This method can avoid growth stoppage of nanowires and keep the concentration of the reactants in dynamic equilibrium through continuously injecting fresh precursor solutions into the reactor in the process of reaction. Various lengths of ZnO nanowire D

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arrays have been obtained. When these nanowires arrays were used as the photoanodes of DSSCs, the performance of the DSSCs improves with the increase of length of the ZnO nanowire arrays (shown in J−V curves). The Jsc and η reach 6.0 mA/cm2 and 1.25% when the length of the nanowires is up to 17.2 μm. This owes to the long nanowires possessing larger internal surface area, resulting in an increase of N719 dye loading amount. It is known that increasing the length of nanowires is an effective means to improve the surface area and then increase the power conversion efficiency of DSSCs. Therefore, our method is an effective process for synthesizing long ZnO nanowires or other nanomaterials in a short time for the nanodevice.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (973 Program) (grant no. 2014CB932103), Foundation for Climax Talents Plan in Six-Big Fields of Jiangsu Province of China (grant no. 1107020070) and Large Equipment Grants of Southeast University, China.

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