CuO nanostructures on cellulose paper and their p–n junction properties

Materials Letters 116 (2014) 64–67

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Facile synthesis of ZnO/CuO nanostructures on cellulose paper and their p–n junction properties Goli Nagaraju, Yeong Hwan Ko, Jae Su Yu n Department of Electronics and Radio Engineering, Institute for Laser Engineering Kyung Hee University, 1 Seocheon-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Republic of Korea

art ic l e i nf o

a b s t r a c t

Article history: Received 24 September 2013 Accepted 24 October 2013 Available online 31 October 2013

A cellulose paper based p–n junction device was fabricated by synthesizing the ZnO/CuO nanostructures via a hydrothermal process. The CuO nanospindles (NSs) were decorated on the surface of cellulose paper substrate by a simple soaking process. Subsequently, ZnO nanorod arrays were grown on the surface of the CuO NSs decorated cellulose paper (i.e., CuO NSs/paper) by the hydrothermal process. The synthesized ZnO/CuO nanostructures on cellulose paper exhibited good crystal and optical properties and favorably formed the p–n heterojunction with a rectifying behavior. & 2013 Elsevier B.V. All rights reserved.

Keywords: Cellulose paper ZnO nanorod arrays CuO nanospindle Hydrothermal synthesis p–n heterojunction

1. Introduction Recently, cellulose papers have been attracted considerable attention in electronic devices including paper batteries, nanogenerators, sensors, field-effect transistors, and micro fluidic devices [1–5]. Also, various cellulose papers are being developed day-today for global economic requirements because of its large-scale production, naturally heightened material, low cost, flexible, and environmentally friendliness [2,6]. Furthermore, the cellulose paper based devices provide the reasonable electrical performances similar to the devices based on other conventional substrates. On the other hand, the investigations of nanostructured semiconductor materials have been extensively studied in many application fields due to their chemical and optoelectrical properties compared to the bulk materials [7]. Among them, lowdimensional nanostructured materials are potentially convenient for diverse nanodevices owing to their unique physical properties with high surface area [8]. Particularly, ZnO and CuO nanostructures have been commonly used for multiple applications in various devices of solar cells, photodetectors, and light-emitting diodes [9,10]. Naturally, ZnO and CuO are n-type and p-type semiconductor materials with different crystal structures, respectively. By using chemical synthesis techniques, various morphologies of nanostructures such as nanorods, nanotubes, nanopetals, and nanoleafs could be formed [11,12]. The CuO/ZnO hierarchical nanostructures have been synthesized for enhancing the efficiency

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of devices in several fields of applications. In order to fabricate these hierarchical nanostructures, however, it needed somewhat expensive, long-time, and complex processes by the use of deposition techniques, and thermal decomposition methods [13–15]. In this work, we synthesized the ZnO nanorod arrays (NRAs) on the surface of CuO nanospindles (NSs) decorated cellulose paper (i.e. CuO NSs/paper) by a simple and lowtemperature hydrothermal method. 2. Experimental procedure Fig. 1 shows the schematic and photographic images for the hydrothermally synthesized ZnO NRAs on the CuO NSs/paper. For the synthesis of CuO NSs, 15 mM of copper nitrate trihydrate and hexamethylenetetramine (HMTA) were dissolved in 900 ml of deionized (DI) water at 75 1C and it was continuously stirred at 75 1C for 6 h. Then, the precipitate solution (i.e., CuO solution) was used for coating the cellulose paper. As shown in Fig. 1(i), the paper substrate (Whatman qualitative-1) was soaked in CuO solution for
10–15 min at room temperature, and then it was thermally dried in oven at 75 1C for 2 h to enhance the adhesion between the CuO NSs and the paper substrate. By rinsing with DI water, the unadhered CuO NSs were removed and the decorated CuO NSs could remain firmly attached to the surface of cellulose paper fibers (Fig. 1(ii)). To synthesize the ZnO NRAs on the CuO NSs/ paper, the ZnO seed layer was coated on the surface of the CuO NSs/paper using the ZnO seed solution which was simply prepared by dissolving of 30 mM of zinc acetate dehydrate in 30 ml of ethanol at 40 1C. Then, the seed solution was dropped on the CuO

G. Nagaraju et al. / Materials Letters 116 (2014) 64–67

NSs/paper using a pipette at 110 1C on hotplate. For the conversion of zinc acetate dehydrates to ZnO seed particles, the sample was placed in the oven at 150 1C for 5 h as shown in Fig. 1(iii). To grow ZnO NRAs on the CuO NSs/paper, the seed coated CuO NSs/paper was immersed into the growth solution by dissolving 50 mM of zinc nitrate hexahydrate and HMTA in 900 ml of DI water at 85–90 1C for 3 h. After that, the immersed sample was carefully pulled out, rinsed with DI water and air dried. The morphology and crystallinity were characterized by the field-emission scanning electron microscope (FE-SEM: LEO SUPRA 55, Carl Zeiss) image and X-ray diffraction (XRD: Mac Science, M18XHF-SRA), respectively. To evaluate the optical and current–voltage characteristics, the photoluminescence (PL: RPM2000, Accent Optics) and semiconductor characterization (Keithley 4200-SCS, USA) systems, respectively, were employed.

(i)

Soaked paper in CuO solution

Drying at 75ºC for 2 h

(ii)

Cellulose fiber

CuO NSs decorated paper substrate (i.e., CuO NSs/paper)

CuO NSs

3. Results and discussion Fig. 2 shows the FE-SEM images of the (a) CuO NSs/paper, (b) CuO NSs on the interstitial regions of cellulose fibers (marked by red circle), and (c) decorated CuO NSs on cellulose fibers. In Fig. 2(a), the paper surface consisted of cellulose fibers was decorated with CuO NSs. As shown in Fig. 2(b) and (c), it is clearly observed that the CuO NSs were well decorated on the surface of cellulose fibers as well as their interstitial regions. The sizes of CuO NSs with
80–150 nm (width and length) were uniformly covered over the whole surface of cellulose fibers. In Fig. 2(b), it is found that each CuO NS was assembled by the combination of many tiny particles. In the hydrothermal synthesis of CuO NSs, indeed, the HMTA in the growth solution acts as a building block for the self-assembling of nanoparticles. The reaction mechanism in the formation of CuO NSs shape can be understood by the simple chemical reaction process. Initially, in the growth solution, the OH  anions and Cu2 þ cations were released, which leads to the immediate precipitation of Cu(OH)2 units. Subsequently, the

ZnO seed coating and Hydrothermal synthesis

(iii) ZnO seed coated CuO NSs/paper

ZnO NRAs

ZnO NRAs on CuO NSs/paper

Fig. 1. Schematic and photographic images of the hydrothermally synthesized ZnO NRAs on the CuO NSs/paper.

20 µm

65

30 nm

200 nm 30.0k (111) (111)

JCPDS CuO #05-0661 Cu2O #65-3288

Intensity (a. u.)

25.0k 20.0k 15.0k

Cu2O (110)

(202) (113) (113) (202) (222)

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(112)

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CuO (110)

2 µm

100 nm

(311) (311)

0.0

20

30

40

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2 (degree) Fig. 2. FE-SEM images of the (a) CuO NSs/paper, (b) CuO NSs on the interstitial regions of cellulose fibers (marked by red circle), (c) decorated CuO NSs on cellulose fibers, and (d) 2θ scan XRD pattern of the corresponding CuO NSs. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)

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CuO NSs were formed by the thermal decomposition of Cu(OH)2 units at prolonged reaction temperature and time by the selfassembling of large number of tiny particles. Fig. 2(d) shows the 2θ scan XRD pattern of the synthesized CuO NSs powder. All the diffraction peaks were assigned to the monoclinic structure of CuO NSs, unless the starting peak of (110) Cu2O. This secondary XRD peak of (110) was previously observed [16] and it could be influenced by the synthesis condition during growth. The sharp peaks at 2θ values around 35.5 and 38.91 correspond to the diffractions of (111) and (111) crystal planes of monoclinic CuO structure, respectively.

Fig. 3 shows the perspective/magnified-view FE-SEM images of (a) and (b) the ZnO seed coated CuO NSs/paper and (c) and (d) the ZnO NRAs grown on the CuO NSs/paper. In Fig. 3(a), the seed layer was coated on the overall surface of the CuO NSs/paper. The seed particles were compactly coated on the CuO NSs/paper fibers with their average diameters of 30–70 nm (Fig. 3(b)). In Fig. 3(c), the perspective view of FE-SEM image shows the integrated ZnO NRAs on the seeded CuO NSs/paper substrate. The ZnO NRAs were grown densely with a vertically aligned structure as shown in Fig. 3(d). The sizes and diameters of the grown ZnO nanorods were in the range of approximately 25–100 nm. From the 2θ scan XRD

1 µm

20 µm

200 nm

1 µm

200 nm

20 µm

40k

ZnO NRAs on CuO NSs/paper ZnO : JCPDS card (89-1397)

ZnO (002)

20k

PL Intensity (a. u.)

Intensity (a. u.)

30k

0.08 ZnO NRAs on CuO NSs/paper

0.06 0.04 0.02 0.00

Cellulose

(101) (100)

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400

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Cu2O (110) CuO (111) 10

20

30

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(202) 50

(130) (112)

(202) 60

(113) 70

(311) 80

90

2 (degree) Fig. 3. Perspective/magnified-view FE-SEM images of (a) and (b) ZnO seed coated CuO NSs/paper, (c) and (d) ZnO NRAs on the CuO NSs/paper, and (e) 2θ scan XRD pattern of ZnO NRAs on the CuO NSs/paper. The inset of (e) shows the PL spectrum of the same sample.

G. Nagaraju et al. / Materials Letters 116 (2014) 64–67

temperature. As shown in Fig. 4(b), when the voltage was swept from 15 to 15 V, the output current clearly exhibited the p–n junction characteristics with rectifying behavior. Under the forward bias, the turn-on voltage was estimated to be about 0.93 V and the reverse leakage current was
10  7 A at the bias of  15 V. This fabrication method can be expected to facilely produce the functional cellulose papers with p–n junction characteristics.

PET

ITO ZnO NRAs CuO NSs Paper

Ag paste

4. Conclusion

1.2x10-6

6.0x10-7 4.0x10-7

Current (A)

Current (A)

1.0x10-6 8.0x10-7

67

10

-6

10

-7

The cellulous paper based p–n junction device by the growth of ZnO NRAs on the CuO NSs/paper was successfully fabricated by a simple hydrothermal method. The soaking and thermal treatment with CuO solution ensured the well-decorated CuO NSs on cellulose fibers. By using the hydrothermal synthesis with ZnO seed coated CuO NSs/paper, the ZnO NRAs were also well integrated. Additionally, the fabricated device showed the I–V characteristics with rectifying behavior at a turn-on voltage of 0.93 V. This facile process for the ZnO NRAs grown on CuO NSs/papers could be a useful technique for nanostructures and papers based on electronic device applications.

10-8 10-9 -15 -10 -5

2.0x10-7

0

5

10 15

Voltage (V)

0.0 -15

-10

-5

0

5

10

15

Voltage (V) Fig. 4. (a) Schematic of the cellulous paper based p  n junction device with ZnO NRAs grown on the CuO NSs/paper and (b) I  V characteristics of the corresponding sample.

pattern of the ZnO NRAs grown on the CuO NSs/paper of Fig. 3(e), the XRD peaks of ZnO, CuO and cellulose were observed. At 2θ value of 34.61, the dominant XRD peak of ZnO (002) confirmed that the ZnO NRAs were grown along the c-axis of wurtzite structure, exhibiting a good crystallinity. As shown in the inset of Fig. 3(e), the strong and sharp PL emission peak in the ultraviolet (UV) region was observed at 383.7 nm and the visible PL emission with a weak and broad peak was obtained at 570 nm. These UV and visible PL emissions are attributed to the radial recombination of free excitons through the near band-edge emission and deep defect level emission of vacancy in the ionized oxygen of ZnO crystal lattice. This high ratio of UV and visible PL emissions indicated that the grown ZnO NRAs have a good optical property. Fig. 4(a) shows the schematic of the cellulous paper based p–n junction device with ZnO NRAs grown on the CuO NSs/paper for the electrical characterization. For p–n junction device, the silver (Ag) and indium tin oxide (ITO) coated polyethylene terephthalate (PET) were used for each electrode. The Ag paste was sufficiently coated on the back side of substrate and the ITO coated PET was used as the top electrode. The current–voltage (I–V) curve of the fabricated p–n junction device was characterized at room

Acknowledgments This research was supported by the Basic Science Research Program through the NRF of Korea funded by the Ministry of Science, ICT and Future Planning (No. 2013-010037). References [1] Nishide H, Oyaizu K. Science 2008;319:737–8. [2] Manekkathodi A, Lu M, Wang C, Chen L. Adv Funct Mater 2010;22:4059–63. [3] Gimenez AJ, Yañez-Limón JM, Seminario JM. J Phys Chem C 2011;115: 18829–34. [4] Fortunato E, Correia N, Barquinha P, Pereira L, Gonçalves G, Martins R. IEEE Electron Device Lett 2008;29:988–90. [5] Martinez AW, Philips ST, Whitesides GM. Anal Chem 2010;82:3–10. [6] Wegner TH, Jones PE. Cellulose 2006;13:115–8. [7] Willander M, ul Hasan K, Nur O, Zainelabdin A, Zaman S, Amin G. J Mater Chem 2012;22:2337–50. [8] Athauda TJ, Ozer RR. Cryst Growth Des 2013;13:2680–6. [9] Valls IG, Cantu ML. Energy Environ Sci 2009;2:19–34. [10] Wang SB, Hsiao CH, Chang SJ, Lam KT, Wen KH, Hung SC, et al. Sens Actuat A 2011;171:207–11. [11] Soomro MY, Hussain I, Bano N, Jun Lu, Hultman L, Nur O, et al., J Nanotechnol. 2012:2012:1-6. [12] Lbupoto ZH, Khun K, Lu J, Willander M. Appl Phys Lett 2013;102:103701–5. [13] Saji KJ, Populoh S, Tiwari AN, Romanyuk YE. Phys Stat Solidi A 2013;210:1386–91. [14] Xu CH, Lui HF, Surya C. Mater Lett 2011;65:27–30. [15] Wang P, Zhao X, Li B. Opt Expr 2011;19:11271–9. [16] Chen JT, Zhang F, Wang J, Zhang GA, Miao BB, Fan XY, et al. J Alloys Compd 2008;454:268–73.