Controllable Growth of ZnO Nanorod Arrays on NiO Nanowires and

Article pubs.acs.org/crystal

Controllable Growth of ZnO Nanorod Arrays on NiO Nanowires and Their High UV Photoresponse Current Ting Guo, Yidong Luo, Yujun Zhang, Yuan-Hua Lin,* and Ce-Wen Nan School of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China ABSTRACT: Three-dimensional architectures composed of one-dimensional nanoscale building blocks are well-known to be able to integrate properties of materials with different dimensions and to derive more advanced performance. In this experiment, ZnO nanorod arrays are controlled to grow on NiO nanowires (NWs) to construct typical comblike nanocomposites by means of ZnO seeding on the surface of NiO NWs and subsequent seeds growth in hydrothermal conditions. The growth mechanism of this complex structure is proposed. Our results indicate that the photoresponse current of UV photosensors of these nanocomposites can be tuned by UV irradiance intensity. The sensor possesses improved UV photoresponse current (about 3.5 mA) under UV illumination (365 nm) of 400 mW cm−2, due to the advantages of composite structure and fine morphology, which reveals that this nanocomposite can be a promising candidate for UV photosensors.

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INTRODUCTION As a wide-band-gap (3.37 eV) multifunctional semiconductor with a large exciton binding energy (60 meV), zinc oxide has triggered great interest for a diverse range of practical applications due to its unique properties in light-emitting diodes,1,2 solar cells,3,4 gas sensors,5 resistive random access memory (RRAM),6 and so on.7−9 Stimulated by the discovery of room-temperature ultraviolet lasing from ZnO NWs,10 substantial effort has been dedicated to the fabrication of vertically grown ZnO nanorod or NW arrays. These arrays are the promising candidates for applications in nanostructured solar cells, field emission displays, biochemosensors, and other areas. Previous reports11−19 have presented a variety of techniques to fabricate vertically aligned ZnO nanorod arrays. Ng et al.20 grew vertically well-aligned ZnO nanorods making use of the vapor−liquid−solid (VLS) growth mechanism. In the process, gold nanoparticles served as the catalyst for the growth of nanorods. The relatively high temperatures (850− 1000 °C) of the growth were the least of perfection. Wu et al.17 demonstrated a metal organic chemical vapor deposition (MOCVD) method to obtain the well-aligned ZnO nanorod arrays at a low temperature (around 500 °C). Using zinc acetylacetonate hydrate as the zinc source in the system, the ZnO nanorod arrays were grown on a silicon substrate in a low pressure. Law et al.21 dip-coated the F-doped SnO2 conductive glass (FTO) substrates in a concentrated ethanol solution to form a thick film of ZnO nanocrystal seeds on the substrates. Then ZnO nanorod arrays could be obtained by immersing the substrates in ZnO growth solutions at 92 °C for 2.5 h, in which crytal seeds coated on substrates successfully grew to nanorods. This method was favored for its low cost and the ease of scale-up. Recently, much research works have focused on threedimensional (3D) superstructures composed of one-dimensional (1D) nanoscale building blocks.4,22−24 For instance, Wang et al.25 introduced a new type of stretchable strain sensor with ultrahigh © 2014 American Chemical Society

tolerable strain based on a ZnO NWs/polystyrene nanofiber (PSNF) hybrid structure on a polydimethylsiloxane (PDMS) film. The device can withstand strain up to 50%, with high durability, fast response, and high gauge factors. Unalan et al.26 controlled the grown of ZnO NWs on carbon fibers making use of the vapor transport and condensation approach and used it as an anode material of flexible dye-sensitized solar cells (DSSCs). In contrast to nanostructures with an ordinary geometrical morphology, these structures possess a larger aspect ratio and offer more unique characteristics combining both 1D and collective physical properties. In addition, owing to their inherent tunable spatial distribution and anisotropic nature, these superstructures will create unique physical properties. In this paper, we control epitaxial crystal growth of ZnO nanorod arrays on NiO NWs to construct a typical ZnO and NiO nanocomposites (labeled as ZnO@NiO NCs) by the hydrothermal method and make an analysis of the growth mechanism. Subsequently, the UV photosensors based on the ZnO@NiO NCs are demonstrated to possess enhanced photoresponse current at the milliampere (mA) level. The results indicate ZnO@NiO NCs to be a promising candidate for UV photosensors with excellent performance.

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EXPERIMENTAL SECTION

Method for Electrospinning NiO NWs. Under stirring conditions, 0.45 g of Ni(Ac)2·4H2O was put slowly into 10 mL of alcohol. Then 0.8 g of polyvinylpyrrolidone (PVP) powder was added slowly into the former solution to form the precursor sol solution. Subsequently, the precursor sol solution was loaded into a 10 mL plastic syringe for electrospinning. During the electrospinning process, the distance between the needle tip and the collector was held in 12 cm Received: January 8, 2014 Revised: February 19, 2014 Published: February 25, 2014 2329

dx.doi.org/10.1021/cg500031t | Cryst. Growth Des. 2014, 14, 2329−2334

Crystal Growth & Design

Article

Figure 1. Fabrication and measurement processes of ZnO@NiO NCs photosensor. and a direct current voltage of 12 kV was applied to the syringe needle and collector with a feed rate of 0.8 mL h−1. This process results in a small piece of PVP/Ni(Ac)2 composite film collected on the tin foil. Finally, pure NiO NWs were obtained by calcination at 550 °C for 3 h in the air to completely get rid of PVP. The sample was labeled as NiO@Si. Preparation of ZnO Seed Solution. First, 1.10 g of zinc acetate dihydrate was dissolved in 50.0 mL of isopropyl alcohol to form 100 mM ZnO seed solution, and then the solution was stirred vigorously for 10 min. After that, 700 μL of triethylamine was dropwise added to the solution. Under the room temperature, the final solution was stirred for another 2 h, of which the pH was 6.3 (pH meter) in the end. Preparation of ZnO Growth Solution. Equimolar aqueous solutions of zinc nitrate hexahydrate and hexamethylenetetramine were used to grow ZnO nanorods on NiO NWs. First, 7.71 g of hexamethylenetetramine was dissolved in 550 mL of deionized (DI) water to form a 100 mM solution. After dissolving, under the room temperature, 16.4 g of zinc nitrate hexahydrate was added to the solution, and the final solution was stirred for another 2 h. Growth of ZnO Nanorods. First, the NiO@Si swatch was dipcoated with the ZnO seed solution for 8 min (seeding proccess). Then the swatch was annealed at 350 °C for 2 h to promote the seed crystallinity. After that, the swatch was immersed in the growth solution and incubated at 95 °C for 8 h in an oven (seeds growth process). Finally, the swatch was removed from the growth solution, rinsed with DI water thoroughly, and annealed at 350 °C for 2 h again. Characterization. The crystal structure of as-prepared nanostructures was checked by X-ray diffraction (D/MAX-2500 V) with Cu Kα radiation. The XRD patterns were recorded in the 2θ range of 20−80° with a scanning step of 8° min−1. The morphologies and chemical composition of the ZnO@NiO NCs were examined by field emission scanning electron microscopy (SEM) with an energy-dispersive X-ray spectrometer (EDX). The detailed microscopic structure of ZnO nanorods was characterized using transmission electron microscopy (TEM), highresolution TEM, and selected area electron diffraction (SAED). Fabrication and Measurement of UV Photosensors. The fabrication and measurement processes of ZnO@NiO NCs photosensors are illustrated in Figure 1. As shown, first, with interdigitated comblike Pt electrodes on the front side of the Si substrate, NiO fiber

mats were deposited on the substrate by electrospinning. Whereafter, the hot-pressing (100 °C) step was applied to acquire fine adhesion to the substrates. After that, the swatch was calcined for the purpose of decomposing the organic components and crystallizing NiO. The inset shows the SEM image of the as-prepared NiO after calcination. Finally, the UV photosensors were obtained by the following steps: ZnO seeding process, ZnO seeds growth process, and calcination. For measurement of electrical and photoelectric properties of the photosensor, a Keithley 2410 sourcemeter and UV LED (365 nm) with a maximum power density of 400 mW cm−2 were used.

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RESULTS AND DISCUSSION

Figure 2, pattern a, demonstrates the structural characteristics of the as-synthesized NiO after calcination at 550 °C for 3 h investigated by XRD. The diffraction peaks typical to NiO are clearly observed, which agree with those of standard NiO of cubic structure (JCPDS Card No. 47-1049). Figure 2, pattern b, shows the structural characteristics of ZnO@NiO NCs. The

Figure 2. XRD patterns of the as-prepared NiO and ZnO@NiO NCs after calcination. 2330



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Figure 3. SEM images of the ZnO@NiO NCs.

Figure 4. EDX elemental mapping patterns of Zn, Ni, and O of ZnO@NiO NCs.

Besides the morphology of the ZnO@NiO NCs, a corresponding EDX elemental mapping of Zn, Ni, and O is also characterized (Figure 4). Although postsintering treatment makes Ni element diffuse into the lattice of ZnO nanorods, Ni element is inclined to distribute along the nanowire as before. Figure 5b shows the high-resolution TEM image of the selected area in Figure 5a. The results show that the spacing (around 0.52 nm) between adjacent lattice planes is consistent with the d value of the (0001) facets of hexagonal wurtzite ZnO, demonstrating that the ZnO nanorods grow along the

result exhibits that only peaks corresponding to the ZnO wurtzite structure could be observed, and no impurity phase peaks appear in the sample. NiO peaks are not observed because the thickness of the ZnO is too large for X-ray. The typical SEM images of the ZnO@NiO NCs are indicated in Figure 3. As shown, every NiO nanowire is wrapped by closely packed nanometer-sized ZnO nanorods, with a hexagonal flat top, to form typical comblike morphologies. These nanorods attach vertically to the sidewall of NWs, with a length of 1−1.2 μm and a diameter of about 100 nm. 2331



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The reactions among the seed solution are categorized as nonaqueous solution reactions.27,28 The dissolution of zinc acetate dihydrate accompanies solvation of zinc and acetate ions by isopropanol (IsOH). Hydrated water molecules form hydrogen bonding with IsOH. The zinc ion has a coordination number of six; this feature leads it to form an octahedral inner coordination sphere, [Zn(IsOH)6]2+. The autoprotolysis constant of isopropanol is 10−20.8, so its pH is equal to 10.4 at a neutral condition. The pH of the as-synthesized solution was measured to be 6.3 at room temperature, so the present solution is relatively acidic. Therefore, the reaction 1 can be involved: [Zn(IsOH)6 ]2 + + Ac− ↔ [Zn(IsOH)5 − m (IsO)m Ac]1 − m + (IsOH 2)+

(1)

The formation of the alkoxy group gives the credit to the intensive acidity of the solvating IsOH that leads to the deprotonation by free IsOH. In addition, the reaction between the alkoxyacetate complexes and water should be taken into account. With lower concentration, water acts as a nucleophilic reactant. The nucleophilic substitution leads to hydrolysis of the complexes: [Zn(IsOH)5 − m (IsO)m Ac]1 − m + (m + 1)H 2O ↔ [Zn(IsOH)5 − m (OH)m + 1]1 − m + mIsOH + AcH (2)

When triethylamine is added, of which the alkalinity is slightly stronger than Ac− in the solution, the H+ of acetic acid will be captured by triethylamine, which will urge the reaction 2 toward the right. In general, a polymerization reaction will occur between complexes that have hydroxyl groups bonded to the metal ions. During the polymerization, an oxolation reaction constructs a bridge between the two metal ions. Labeling the zinc hydroxide complex simply as Zn−OH, the oxolation reaction could be expressed:

Figure 5. TEM photographs of ZnO nanorods growing on NiO NWs.

Zn−OH + HO−Zn ↔ Zn−(OH)−Zn + OH

[0001] direction. Meanwhile, the associated selected area electron diffraction (SAED) pattern (inset of Figure 5b) implies that these ZnO nanorods belong to single crystals.

(3)

In the present case, polymerization among the complexes, to which more hydroxyl groups are bonded, will generate layered

Figure 6. Illustration of the forming process of LBZA. 2332



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basic zinc acetate (LBZA), Zn5(OH)8Ac2·2H2O, which has been presented and demonstrated by research before.29 The whole reaction processes discussed above are illustrated in Figure 6. Afterward, LBZA will undergo both hydrolysis and deprotonation processes and transform into solid ZnO in the end. The hydrolysis process results in the removal of the intercalated Ac− ions. Zn5(OH)8 layers in the LBZA structure will transform into zinc hydroxyl complexes, namely, [Zn(OH)2−n n ]x . In the absence of OH−, the solution contains Ac− ions, whose amount is double that of zinc. Because of the smaller dissociation constant of acetic acid, the Ac− ions can be considered as a strong base. In the solution, Ac− ions bring a higher chemical potential of basic species. Therefore, the following equilibrium is conceivable: OH− + Ac− ↔ O2 − + AcH

The deprotonation reaction of described as

(4)

[Zn(OH)n2−n]x

can be

[Zn(OH)n2 − n ]x + x Ac− ↔ [ZnO(OH)1n−−n1]x + x AcH (5)

The complexes finally transformed into solid ZnO, which is slightly soluble in IsOH. The solution becomes turbid after a period of time, and the seed solution tends to form. The whole growth processes of ZnO@NiO NCs are illustrated in Figure 7. At first, ZnO seeds disperse randomly

Figure 7. The whole growth processes of ZnO@NiO NCs.

on the surface of NiO NWs. Subsequently, in growth solution, the seeds develop to typical hexagon platforms during the early stage. Finally, the platforms grow along the [0001] direction of hexagonal wurtzite to form nanorods. The photoelectric property of ZnO@NiO NCs UV photosensors are shown in Figure 8. The I−V curves of the device under dark and UV illumination conditions (UV irradiance of 400 mW cm−2) are shown in Figure 8a. The curves present a slightly nonlinear characteristic, which demonstrate a small Schottky barrier existing between NiO@ZnO NCs and the Pt interdigital electrode. Figure 8b exhibits UV photoresponse currents of the device under a series of illumination intensities at a fixed bias of 1 V. Under certain UV illumination, the current of the device initially rises sharply and gradually arrives to saturation. When the illumination is shut off, the current reduces sharply and gradually arrives to saturation. The results indicate that peak photoresponse current will increase with the increasing of UV intensity. In general, UV photosensors based on ZnO NW have a very low photoresponse current causing high-precision measurement systems to be indispensable to detect this weak signal and making the cost go up significantly. The time-resolved photoresponse current curve of the device in

Figure 8. (a) I−V curves of ZnO@NiO NCs UV photosensors with and without UV illumination. (b) UV photoresponse current of the device under a series of illumination intensities at a fixed bias of 1 V. (c) The time-resolved photoresponse current curve of the device in “on” and “off” states (5 V bias and UV irradiance of 400 mW cm−2).

“on” and “off” states (5 V bias and UV irradiance of 400 mW cm−2) is shown in Figure 8c. A maximum photoresponse current was located around 3.5 mA, which was large enough to be detected by a passive mechanical ammeter. The large photoresponse current should be attributed to two factors. First, it is well-known that ZnO NWs have exhibited excellent performance for UV photosensors,30−33 which could be explained by the hole-trapping mechanism.34 When ZnO NWs are placed under air condition, they will adsorb oxygen molecules on the surface, capturing the free electrons from 2333



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(15) Ma, T.; Guo, M.; Zhang, M.; Zhang, Y.; Wang, X. Nanotechnology 2007, 18, 035605. (16) Kim, K. S.; Jeong, H.; Jeong, M. S.; Jung, G. Y. Adv. Funct. Mater. 2010, 20, 3055−3063. (17) Wu, J.-J.; Liu, S.-C. Adv. Mater. 2002, 14, 215. (18) Zheng, Z.; Lim, Z. S.; Peng, Y.; You, L.; Chen, L.; Wang, J. Sci. Rep. 2013, 3, 2434. (19) Gu, Z.; Paranthaman, M. P.; Xu, J.; Pan, Z. W. ACS Nano 2009, 3, 273−278. (20) Ng, H. T.; Li, J.; Smith, M. K.; Nguyen, P.; Cassell, A.; Han, J.; Meyyappan, M. Science 2003, 300, 1249−1249. (21) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455−459. (22) Wang, X.; Hu, P.; Fangli, Y.; Yu, L. J. Phys. Chem. C 2007, 111, 6706−6712. (23) Athauda, T. J.; Ozer, R. R. Cryst. Growth Des. 2013, 13, 2680− 2686. (24) Elias, J.; Levy-Clement, C.; Bechelany, M.; Michler, J.; Wang, G. Y.; Wang, Z.; Philippe, L. Adv. Mater. 2010, 22, 1607−1612. (25) Xiao, X.; Yuan, L.; Zhong, J.; Ding, T.; Liu, Y.; Cai, Z.; Rong, Y.; Han, H.; Zhou, J.; Wang, Z. L. Adv. Mater. 2011, 23, 5440−5444. (26) Unalan, H. E.; Wei, D.; Suzuki, K.; Dalal, S.; Hiralal, P.; Matsumoto, H.; Imaizumi, S.; Minagawa, M.; Tanioka, A.; Flewitt, A. J.; Milne, W. I.; Amaratunga, G. A. J. Appl. Phys. Lett. 2008, 93, 133116. (27) Hosono, E.; Fujihara, S.; Kimura, T.; Imai, H. J. Colloid Interface Sci. 2004, 272, 391−398. (28) Hosono, E.; Fujihara, S.; Kimura, T.; Imai, H. J. Sol-Gel Sci. Technol. 2004, 29, 71−79. (29) Poul, L.; Jouini, N.; Fiévet, F. Chem. Mater. 2000, 12, 3123− 3132. (30) Menzel, A.; Subannajui, K.; Güder, F.; Moser, D.; Paul, O.; Zacharias, M. Adv. Funct. Mater. 2011, 21, 4342−4348. (31) Peng, L.; Zhai, J.; Wang, D.; Zhang, Y.; Wang, P.; Zhao, Q.; Xie, T. Sens. Actuators, B 2010, 148, 66−73. (32) Liu, Z.; Wang, Y.; Wang, B.; Li, Y.; Liu, Z.; Han, J.; Guo, K.; Li, Y.; Cui, T.; Han, L.; Liu, C.; Li, G. Int. J. Hydrogen Energy 2013, 38, 10226−10234. (33) Kou, H.; Jia, L.; Wang, C. Carbon 2012, 50, 3522−3529. (34) Soci, C.; Zhang, A.; Xiang, B.; Dayeh, S. A.; Aplin, D.; Park, J.; Bao, X.; Lo, Y.-H.; Wang, D. Nano Lett. 2007, 7, 1003−1009.

the conduction band (CB) (reaction 6), which brings about the formation of a low-conductivity depletion layer near the surface. Once UV illumination is on, electrons of the valence band (VB) will be stimulated to the CB, generating electron− hole pairs. Soon afterward, holes will be trapped at the surface and discharge the adsorbed oxygen ions (reaction 7), making oxygen photodesorbed from the surface and leaving behind unpaired electrons, which will participate in the charge transport of ZnO and increase the conductivity. O2 + e− → O2 −

(6)

h+ + O2 − → O2

(7)

Second, the morphology of 3D architectures plays an essential role in maximizing the surface area for photon absorption. This action could also enhance the photoresponse current.

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CONCLUSIONS In summary, ZnO@NiO NCs are successfully obtained by a two-step route, which contains a ZnO seeding step and a subsequent seeds growth process. The results of the photoelectric property of UV photosensors based on ZnO@NiO NCs indicate that the sensor possesses improved UV photoresponse current (around 3.5 mA) under UV illumination of 400 mW cm−2, and the peak photoresponse current can be tuned by UV irradiance intensity.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSF of China (51272121, 51221291, 51328203, and 51025205). REFERENCES

(1) Jeong, M. C.; Oh, B. Y.; Ham, M. H.; Lee, S. W.; Myoung, J. M. Small 2007, 3, 568−572. (2) Park, W. I.; Yi, G.-C. Adv. Mater. 2004, 16, 87. (3) Ko, S. H.; Lee, D.; Kang, H. W.; Nam, K. H.; Yeo, J. Y.; Hong, S. J.; Grigoropoulos, C. P.; Sung, H. J. Nano Lett. 2011, 11, 666−671. (4) McCune, M.; Zhang, W.; Deng, Y. Nano Lett. 2012, 12, 3656− 3662. (5) Spencer, M. J. S.; Yarovsky, I. J. Phys. Chem. C 2010, 114, 10881. (6) Chen, G.; Song, C.; Chen, C.; Gao, S.; Zeng, F.; Pan, F. Adv. Mater. 2012, 24, 3515−3520. (7) Park, W. I.; Kim, J. S.; Yi, G.-C.; Lee, H.-J. Adv. Mater. 2005, 17, 1393−1397. (8) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 1289−1292. (9) Das, S. N.; Kar, J. P.; Choi, J.-H.; Lee, T. I.; Moon, K.-J.; Myoung, J.-M. J. Phys. Chem. C 2010, 114, 1689−1693. (10) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897−1899. (11) Chen, L.-Y.; Yin, Y.-T. Cryst. Growth Des. 2012, 12, 1055−1059. (12) Zeng, H.; Xu, X.; Bando, Y.; Gautam, U. K.; Zhai, T.; Fang, X.; Liu, B.; Golberg, D. Adv. Funct. Mater. 2009, 19, 3165−3172. (13) Jean, J.; Chang, S.; Brown, P. R.; Cheng, J. J.; Rekemeyer, P. H.; Bawendi, M. G.; Gradecak, S.; Bulovic, V. Adv. Mater. 2013, 25, 2790− 2796. (14) Zhang, X.-M.; Lu, M.-Y.; Zhang, Y.; Chen, L.-J.; Wang, Z. L. Adv. Mater. 2009, 21, 2767−2770. 2334


Controllable Growth of ZnO Nanorod Arrays on NiO Nanowires and

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