Unraveling Oxygen Transfer at the Graphene OxideâZnO Nanorod

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Unraveling Oxygen Transfer at the Graphene Oxide–ZnO Nanorod Interface Dae-Hwang Yoo, Tran Viet Cuong, Seung Hwan Lee, Wan Sik Hwang, Won Jong Yoo, Chang-Hee Hong, and Sung Hong Hahn J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5039046 • Publication Date (Web): 08 Jul 2014 Downloaded from http://pubs.acs.org on July 10, 2014

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The Journal of Physical Chemistry

Unraveling Oxygen Transfer at the Graphene Oxide–ZnO Nanorod Interface Dae-Hwang Yoo†a, Tran Viet Cuong‡a, Seunghwan Lee§, Wan Sik Hwang∥, Won Jong Yoo§, Chang-Hee Hong‡*, and Sung Hong Hahn†* †

Department of Physics and Energy Harvest-Storage Research Center, University of Ulsan, Ulsan 680-749, Republic of Korea ‡

Semiconductor Physics Research Center, School of Semiconductor and Chemical Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea

§

Department of Nano Science and Technology, SKKU Advanced Institute of Nano

Technology (SAINT), Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do 440-746 Korea ∥

Department of Materials Engineering, Korea Aerospace University, Seoul 412-791, Republic of Korea

Abstract

ZnO nanorods (NRs) were grown on graphene oxide (GO) with hydrothermal method and their structural, optical, and electrical properties were compared with those of ZnO NRs grown on quartz without GO. The enhancement of the crystallinity and the reduction of the oxygen vacancies of ZnO NRs on GO (ZnO/GO) were observed. TEM/EDS observation demonstrated the increase of oxygen concentration in ZnO NR by growing on GO due to the diffusion from the GO during solution synthesis process. From the conductivity measurement of a ZnO NR, however, the conductivity of ZnO NR on GO was turn out to be much lower than that of ZnO NR on quartz. This confirms that diffused oxygen from GO fill out the

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oxygen vacancies inside ZnO NRs and that it leads to a decrease of the conductivity which is caused by the reduction of the charge-carrier concentration in ZnO NRs. These findings offer insightful information to apply high performance ZnO/GO hybrid devices via process optimization.

Keywords: ZnO nanorod; graphene oxide; hydrothermal method, oxygen transfer

* Corresponding author. Tel: +82 52 259 2330; Fax: +82 52 259 1693 E-mail address: [email protected] (C. H. Hong), [email protected] (S. H. Hahn) a

Dae-Hwang Yoo and Tran Viet Cuong contributed equally to this work


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Introduction Graphene has attracted tremendous interest in the field of materials science due to its extraordinary physical and chemical properties.1,2 It is regarded as an important building block for the synthesis of various functional composite materials. Among graphene-based semiconductor composites, hybrids of one-dimensional (1D) ZnO nanorods/nanowires with two-dimensional (2D) graphene layers have emerged as a new architectural platform. In this prototype, 1D ZnO works as efficient channels for carrier transport and electrical pumping for radiative recombination while 2D graphene acts as a transparent conducting electrode. Therefore, this combination would allow for the unique properties of the heterostructures to be exploited in versatile device applications including sensors,3 piezoelectric power sources,4 UV-photodetectors,5 and solar cells.6 Over the past several years, many studies have been performed using chemical vapor deposition (CVD) and solution techniques to synthesize 1D ZnO on graphene7 and graphene oxide (GO),8 respectively. Compared to graphene, an exfoliated single layer of GO sheets can be well dispersed in different aqueous solutions. Thus, GO thin films prepared from solution processes are more compatible with solutionbased synthesis of ZnO/GO hybrid nanostructures at low temperatures and low costs. This is in contrast to high temperature (typically from 450 to 900oC)-assisted CVD growth of ZnO nanostructures on graphene, which not only prevents the use of important substrates such as glass and plastics but also damages the interface between ZnO and graphene. It should be noted that earlier attempts often focused on making devices for special applications and unfortunately, there are no detailed studies aimed at assessing the effects of GO on the optical and electrical properties of 1D ZnO. In this study, 1D ZnO nanorods were grown on quartz and GO using a solution method and their optical and electrical properties were examined. Diffused oxygen from GO is believed to fill out the oxygen vacancy defects in ZnO. To the



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best of our knowledge, this study is the first attempt to investigate oxygen diffusion during a solution synthesis process. Consequently, ZnO grown on GO showed significantly improved crystalline quality, but resulted in stumpy n-type semiconducting behavior. These findings offer insightful information for achieving high performance ZnO/GO hybrid devices via process optimization.

Experimental Methods

GO was synthesized by the modified Hummer method using natural flake graphite.9 To grow ZnO NRs on GO, 4 ml of a GO solution was sprayed onto a 2x2 cm2 quartz plate. To prepare the ZnO seed layer, 0.2 ml of an ethanolic solution of 5 mM zinc acetate dehydrate was spin coated on the GO film and heated to 350 °C in an Ar gas atmosphere for 60 min. The growth of the ZnO NRs was carried out by placing the substrate in a 100 ml aqueous solution of 16 mM zinc nitrate hexahydrate and 25 mM methanamine followed by heating the solution to 90 ºC in an oven for 2 hrs. The substrates covered with ZnO NRs were rinsed with water and then dried in a nitrogen stream. The structural and optical properties of the ZnO NRs grown on GO (ZnO/GO) were compared to those of ZnO NRs grown on quartz (ZnO/Q) by X-ray diffraction (XRD, Philips PW3710), optical absorption, and photoluminescence (PL) measurements. A transmission electron microscopy (TEM, JEOL JEM-2100F) with an energy dispersive spectrometer (EDS) was used to examine the morphologies and the composition ratio of the elements in the ZnO NRs. In order to fabricate the field effect transistors (FETs), ultrasonic agitation of the ZnO NRs grown on GO and quartz in isopropyl alcohol (IPA) was conducted. To allow drying, they were deposited at 100 °C on heavily doped n-type Si wafers with a thermally grown 300 nm thick oxide layer (Si/SiO2 wafer), which was pretreated with a piranha solution. The source and drain contacts were then


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defined by electron beam lithography (EBL) using Ti/Au (5/100 nm) contacts. The electrical characteristics were measured using a Keithley 4200 semiconductor characterization system.

Results and Discussion Figure 1(a) shows the Raman spectra of ZnO/GO. Peaks at 330 and 438 cm−1 were observed, which correspond to A1 symmetry in the TO mode and to the E2 high frequency phonon mode of ZnO, respectively. The Raman shifts observed at 1,340 cm−1 and 1,584 cm−1 correspond to the D and G bands of GO, respectively. The D band is due to the existence of the functional groups such as hydroxyl and carbonyl groups on the graphite structure. The G band indicates the presence of sp2 bonds in the structure. Figure 1(b) shows the XRD patterns of ZnO/Q and ZnO/GO. The patterns demonstrate the well constructed structure of the ZnO NRs. Diffraction peaks for GO were not observed, which may be related to the low amount and low diffraction intensity of GO. The peaks for ZnO correspond to a wurtzite structure and show that the ZnO NRs grew along the c-axis with high orientation. This is due to the easy formation of the (002) texture followed by the lowest surface energy density of the (002) orientation.10 The intensity of the (002) peak of ZnO grown on GO, however, was higher than that of ZnO grown on quartz. This means that GO helps the ZnO NRs to grow with high orientation and crystallinity. The Scherrer formula, D = 0.9λ/(βcosθ), was used to calculate the size of the crystal, where λ is the X-ray wavelength and β and θ are FWHM and diffraction angle, respectively. By growing ZnO on GO, there was a slight decrease of the crystal size from 55.97 to 53.75 nm. Figure 2(a) shows the absorption spectra of ZnO/Q and ZnO/GO. Compared to the absorption spectrum of ZnO/Q, which showed a gradual decrease to the band edge, the absorption spectrum of ZnO/GO showed a more rapid decrease to the band edge. Absorption peaks were



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observed for ZnO/Q and ZnO/GO at 368 nm and 353 nm, respectively, demonstrating that the absorption peak shifted to the shorter wavelength region for ZnO/GO, as shown at the inset in Fig. 2(a). It appears that the rapid decrease and blue shift stem from the difference of oxygen vacancy defects. That is, the presence of many oxygen defects in ZnO/Q results in the shift of the band edge to the longer wavelength region. Figure 2(b) shows the PL spectra of the samples. A sharp peak at 382 nm and a broad peak at about 580 nm were observed, which correspond to the band gap and oxygen vacancies in the ZnO NRs, respectively. The higher intensity of the UV peak of ZnO/GO shows that ZnO NRs were grown with a high crystallinity on GO compared to on quartz, whereas the broad peak in the visible region was decreased for ZnO/GO. This may be caused by a reduction in the recombination of excited electrons and holes which occurs in the heterojunction system of ZnO and GO. Because the conduction band is higher than that of GO, some electrons excited by photons have a tendency to transfer to the conduction band of GO.11 This results in a reduction of the recombination between electrons and holes, which leads to a decrease of the broad peak intensity. Another possible primary factor related to the decrease of the peak intensity is the reduction of oxygen vacancy defects in ZnO NRs. The oxygen concentration in ZnO NRs is expected to be influenced by GO, which may be evidence of a reduction of defects. As a result, the oxygen concentration was evaluated using TEM and EDS. Figure 3 shows TEM images of the ZnO NR samples. The insets show HRTEM images and the selected area electron diffraction (SAED) pattern of the ZnO NRs. The HTREM image in Fig. 3(a) shows that ZnO NRs grew with a z-axis direction with a lattice size of 0.27 nm. The SAED pattern demonstrates the crystallization of the ZnO NRs. The HRTEM image in Fig. 3(b) shows the same growth direction of ZnO/GO with ZnO/Q. However, its SAED pattern shows a higher crystallinity of the ZnO NRs. Figure 3(c) represents the EDS analysis at some positions of the ZnO NRs of ZnO/Q. The atomic concentration of oxygen at all positions was


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about 50%. However, the EDS analysis of the ZnO NRs of ZnO/GO in Fig. 3(d) shows that the atomic concentration of oxygen is about 60% at all positions, which demonstrates that the oxygen concentration increased by about 20% compared to the ZnO NRs in ZnO/Q. This can be attributed to the diffusion of oxygen from GO into ZnO.12 The diffused oxygen is believed to fill out the oxygen vacancy defects in the ZnO NRs, resulting in significant crystalline quality improvements. A simple mechanism for oxygen transfer from GO to ZnO NRs is represented in Fig. 4. During the growth of ZnO NRs by a hydrothermal method, diffused oxygen from GO is believed to fill out the oxygen vacancy defects in ZnO. This is due to the functional groups in GO which contain an abundance of oxygen. At higher temperatures, oxygen leaves these functional groups and is transferred to ZnO during the growth where it contributes to the increase of the oxygen concentration in ZnO, as shown at EDS analysis results in Figs. 3(c) and (d). We further studied the conductivities of ZnO/Q and ZnO/GO as a comparative parameter to clarify the effect of diffused oxygen. Optical microscopy images of the ZnO/Q and ZnO/GObased FET devices are shown in Figs. 5(a) and (b), respectively. The conductivity was obtained from the relation σ = LIds/Vdsπr2, where L is the channel length of the nanorod FET and r is radius of a nanorod (r = W/2). Based on this equation, the conductivities of the ZnO/Q and ZnO/GO FETs were plotted as a function of the gate voltage, as shown in Figs. 5(c) and (d), respectively. Unfortunately, poor ZnO NRs transistor characteristics were observed. There are several possible origins for this result such as defects at the interface between ZnO and SiO2 as well as a large contact resistance or surface-mediated effects including chemisorption and carrier scattering or trap processes by surface states. Both graphs show typical p-type nanorod transistor behavior that may result from the annealing effect.13,14 A significant finding in our study is that the conductivity of ZnO/Q was measured to be 16.7 (Ωm)-1, which is thirteen times higher than that of ZnO/GO at Vgs = 0 V (1.27



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(Ωm)-1, Vds = 5 V). At a negative drain voltage of -5 V, a similar phenomenon was also observed. The conductivity of ZnO/GO was much lower than that of ZnO/Q. This confirms that diffused oxygen from GO must fill out the oxygen vacancies inside ZnO nanorods. Therefore, it leads to a decrease of the conductivity, which can be attributed to the reduction of the charge-carrier concentration in the ZnO NRs.

Conclusions

In conclusion, 1D ZnO NRs were grown on quartz and GO using a solution method and their structural, optical, and electrical properties were examined. The crystallinity of ZnO was enhanced and the oxygen vacancy defects decreased. These results are caused by the diffusion of oxygen from GO during the growth of ZnO NRs in the solution synthesis process. ZnO grown on GO showed a reduction in electrical conductivity. A decrease of oxygen vacancies by the supplement of oxygen leads to a reduction of the charge-carrier concentration in the ZnO NRs on GO. These results provide insightful information to achieve high performance ZnO/GO hybrid devices via process optimization.

Acknowledgement This work was supported by the Priority Research Centers Program (2009-0093818) and by the Human Resource Training Project for Regional Innovation (2012H1B8A2026179) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education.

References


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Figure Captions Fig. 1. (a) Raman shift of ZnO/GO and (b) XRD patterns of ZnO/Q and ZnO/GO. Fig. 2. (a) Absorbance and (b) PL spectra of ZnO/Q and ZnO/GO. Fig. 3. (a) and (b) TEM image of ZnO/Q and ZnO/GO. (c) and (d) EDS analyses for atomic concentration of the elements in ZnO NR. Fig. 4. Schematic diagram to explain oxygen transfer from GO to ZnO NRs. Fig. 5. (a) (b) OM images for device and (c) (d) I-V characteristics of ZnO/Q and ZnO/GO.



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(a) Fig. 1


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(b) Fig. 1



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(a) Fig. 2


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(b) Fig. 2



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Fig. 3


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Fig. 4



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Fig. 5


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Unraveling Oxygen Transfer at the Graphene OxideâZnO Nanorod

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