- Email: [email protected]
ZnO heterostructure: surface functional groups induced electrical properties
Electrochimica Acta 196 (2016) 558–564
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
In-situ fabrication of reduced graphene oxide (rGO)/ZnO heterostructure: surface functional groups induced electrical properties Xinyi Chen* , Hongchen Guo, Tun Wang, Miao Lu* , Taihong Wang Pen-Tung Sah Institute of Micro-Nano Science & Technology, Xiamen University, 361005, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 December 2015 Received in revised form 7 February 2016 Accepted 27 February 2016 Available online 2 March 2016
Reduced graphene oxide (rGO)/ZnO heterostructure was prepared via electrochemical deposition directly on Hummers method derived rGO membranes and the corresponding diodes had been fabricated. Rectifying I–V curve was obtained by modifying the functional groups on the surface of rGO. Further investigation for GO based transistors showed that the conductivity of rGO could vary from n-type to p-type under different annealing conditions. Based on Lerf-Klinowski model and X-Ray photoelectron spectroscopy, it was found that the C-sp2, hydroxyl and epoxy in rGO would be responsible for the change of electrical properties. It was also concluded that reasonable p-type conductivity of rGO for obtaining rectifying rGO/ZnO heterostructure occurred while the percentage of C-sp2 content was about 54% with the C-sp2/(OH + C-O-C) ratio around 1.6. ã 2016 Elsevier Ltd. All rights reserved.
Keywords: reduced graphene oxide ZnO hydroxyl epoxy heterostructure
1. Introduction Wide bandgap oxide such as ZnO has declaimed to be very promising in optoelectronic applications such as solar cells and light emitting diodes, due to its low cost, varieties of synthesis methods and nanostructured morphologies, as well as the tunable optical and electrical properties. However, the difficulty in p-type doping of ZnO due to the rich defect chemistry, has hindered the further development of ZnO-based optoelectronic devices. Accordingly, another candidate to develop ZnO based optoelectronic devices is to use heterojunction device structure [1,2]. The most commonly used p-type materials for ZnO based heterojunction is Mg:GaN because of the same hexagonal structure and the reasonably low lattice mismatch (1.5%) [1,3]. However, the carrier concentration and mobility of Mg:GaN are significantly lower than that of ZnO, in which case the recombination of carriers would occur on GaN side of the heterojunction, and the device performance are thus dominated by the material properties of GaN. On the other hand, graphene, a one-atom-thick material with closely packed sp2 bonded carbon honeycomb structure, has exhibited excellent and tunable conductivity. Graphene has a rich defect chemistry inducing vacancy, impurity, surface absorbate,
* Corresponding authors. E-mail addresses: [email protected] (X. Chen), [email protected] (M. Lu). http://dx.doi.org/10.1016/j.electacta.2016.02.201 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.
grain boundary and topological defects [4–20], which results in varieties of the electrical and mechanical properties of graphene. For example, boron and phosphine can be used as the p-type and n-type dopant, respectively [5–7]. It has also been reported that p-type conductivity of graphene could be introduced in two ways: broadened band gap and confinement of electron transport due to quasi-one-dimensional size effect [4,14], or the adsorption of ambient gases [20]. Accordingly, the adjustable and controllable conductivity of graphene might provide an alternatively new approach to p-type materials for ZnO heterojunctions. Among various kinds of graphene synthesis, Hummers method has provided a low cost, fast and productive approach for graphene via oxidizing graphite to dispersed GO sheet, which can be further reduced to reduced GO (rGO) via annealing or chemical reduction [17]. Different from graphene obtained by vapor deposition and exfoliation, the Hummers derived rGO has abundant hydration related defects uniquely. Recently, Lerf-Klinowski model has been commonly used to describe the structure of GO [18], which proposed that there are three hydration related functional groups forming chemical bonds with C atom: hydroxyl, epoxy and carboxyl. It has also been reported that annealing can promote the generation or extinction of these hydration related functional groups via different temperature, pressure and gas ambience [19–21]. Meanwhile, those kind of oxygen related functional groups such as hydroxyl widely exist as surface defects on the polar facets {0001} of ZnO, which are the dominant facets for growth. Oxygen
X. Chen et al. / Electrochimica Acta 196 (2016) 558–564
vacancy and interstitial are also very common in metal oxides including ZnO [1]. The similarity in defect chemistry of graphene and metal oxides opens up the prospect of developing graphene/ ZnO (or other metal oxides) heterostructures [1,22]. Graphene was usually mixed with metal oxides in powder or stamped to other as-prepared oxides to obtain hybrid materials [23,24], in which cases the surface defects could not promote the interfacial growth, and the interface between graphene and ZnO would not be influenced by only surface defects of graphene and ZnO, but also the gels used for stamping. Moreover, direct growth of ZnO nanostructure on graphene was rarely reported, which usually required a pre-fabricated seed layer [25]. In this work, rGO/ ZnO heterostructure with diode architecture is developed by an in-situ deposition of ZnO nanorods directly on Hummers method derived rGO membranes. Electrochemistry, which could be applied in biological and environmental analysis [26–28], is employed to
559
synthesize ZnO nanorods on rGO. The interface between graphene and ZnO is modified to achieve rectifying I–V properties. It has been found that the post-growth annealing can change the chemical components of absorbed functional groups on the surface of graphene and then affect the electrical property in rGO/ZnO interface. 2. Experimental Graphene was obtained by reduction of Hummers method derived graphene oxide as follows: 0.5 g expanded graphite, 3 g KMnO4 and 0.5 g NaNO3 (Xilong Chemical Co., Ltd., China) were mixed in 60 ml concentrated sulfur acid and the solution was stirred for at least 12 h until the color changed to dark green, then the solution was diluted by deionized (DI) water to 200 ml. After that 12 ml H2O2 was introduced. Then the solution was centrifuged
Fig. 1. (a) Schematic of the assembly of rGO substrate and the electrodepositon; SEM images of rGO/ZnO heterostructure i.e. ZnO nanorods on (b) as-prepared; and (c) 200 C for 30 min.; (d) 200 C for 2 h; (e) 400 C for 30 min. annealed rGO substrates.
560
X. Chen et al. / Electrochimica Acta 196 (2016) 558–564
at 10000 rpm for 5 times to remove the impurities. Once graphene oxide dispersive solution (0.25 mg/ml) was prepared, N2H4 (35%, 5 ml for each 10 ml solution, Xilong Chemical Co., Ltd., China) and NH3 (25%, 35 ml for each 10 ml solution, Xilong Chemical Co., Ltd., China) were added. The reduction was performed at 95 C for 1 h thus rGO could be obtained [29]. The rGO solution was consequently filtrated under pumping and rGO film formed on the filtration membrane, which could be mechanically lifted off from filtration membrane. The previous research on ZnO based heterojunction showed that starting material for growing ZnO, such as substrate, catalyst and seed layer, had obvious influence on the properties of ZnO including morphology, emission, turn-on voltage and external quantum effect [30,31]. In that case, post-
growth annealing in hydrogen/nitrogen atmosphere was applied to the as-prepared rGO membranes, which were then fixed on glass substrates using polydimethylsiloxane (PDMS, Xilong Chemical Co., Ltd., China) as described in Fig. 1(a). Those fixed rGO on glass would be used for the growth of ZnO nanorods. ZnO nanorods were grown on rGO via electrochemical deposition: the solution for electrochemical deposition containing 8 mM Zn(NO3)2xH2O (Xilong Chemical Co., Ltd., China) and hexamethylenetetramine (C6H12N4, HMT, Xilong Chemical Co., Ltd., China) was heated up to 80 C, and rGO membrane on glass was the working electrode, while Pt foil was used as the counter electrode [3]. A bias at 2 V was applied to the system for 30 min. and ZnO nanorods were grown on rGO membranes. Post-growth
Fig. 2. (a) TEM image of ZnO nanorods, the inset show the SAED pattern and HRTEM image, which indicates a typical growth of ZnO along [0001]; (b) XRD patterns of all the samples; (c) detailed XRD pattern in the range from 15 to 45 of GO and rGO.
X. Chen et al. / Electrochimica Acta 196 (2016) 558–564
annealing in 5% H2/95% N2 forming gas with different parameters was performed to rGO films before ZnO growth to explore the influence of surface defect and impurities. Scanning electron microscopy (SEM, Carl Zeiss, Germany), transmission electron microscopy (TEM, JEOL 2100, Japan), X-Ray diffraction (XRD, Rigaku, Japan) and X-Ray photoelectron spectroscopy (XPS, PHI QUANTUM 2000, USA) were employed to characterize the morphology, components and structure of the as-grown samples. Device for rGO/ZnO heterostucture was prepared as described: a 100 nm Au layer was used as the contact electrode for rGO side, and indium tin oxide (ITO, 100 nm) was used as the contact electrode for ZnO side. An insulating layer of 240 nm made up of poly-methyl methacrylate (PMMA 600 K, Allresit) was spin-coated on rGO/ZnO at 2000 rpm and then annealed at 200 C for 60 s, to form an insulating layer to avoid leakage current between rGO and ITO, after which oxygen plasma was applied for 10 s to etch the top surface of PMMA to expose ZnO tips. An Agilent 4156C analyzer equipped with four probes was employed to measure the electrical properties. GO based transistors were fabricated on boron doped silicon <100> with an oxide layer around 100 nm, while Si was used as the gate electrode directly. Lithography and sputtering was employed to prepare the two Au/Cr (200 nm/20 nm) electrodes on SiOx side as source and drain, respectively. Then 10 mL GO solution (20 mg/ mL) was dropped on the SiOx side of the device, while a square wave AC signal (200 kHz, 8 Vpp) was applied between source and drain electrodes for 4 minutes. Then the device was rinsed by DI water and dried by nitrogen flow. 3. Results and Discussion Fig. 1 shows the SEM images of ZnO nanorods electrodeposited on rGO substrates. It is shown that the density of ZnO nanorods increases with the annealing temperature and time for rGO substrates, which indicates an increased conductivity of rGO substrates via annealing (SEM images of bare rGO and electrochemical deposited ZnO nanorods on other conductive substrates are shown in Fig. S1 in the supporting information). Fig. 2(a) shows the morphology and structure of electrodeposited ZnO nanorods in TEM, which indicated that ZnO nanorods exhibited a normal hexagonal lattice structure with the grown direction along [0001] that had also been proved by the XRD pattern shown in Fig. 2(b). Detailed XRD pattern in the range from 15 to 45 of GO and rGO is shown in Fig. 2(c). It can be seen that the diffraction peak at around 24.8 degree, which revealed a wider layer distance than that of
561
graphite along [0001] direction, was corresponding to the distance between rGO layers and became narrower and sharper with higher annealing temperature and longer annealing time, indicating the uniformization of rGO layers during thermal treatment in hydrogen/nitrogen environment. The electrical properties of rGO/ZnO hybrid structure were characterized through diode devices and the related I–V curves are shown in Fig. 3. It can be observed that the I–V curves of 200 C annealed rGO based rGO/ZnO were not rectifying regardless of the annealing time, so was the unannealed rGO. On the other hand, 400 C annealed rGO based rGO/ZnO hybrid structure exhibited rectifying conductivity. Diode equation was used to study the electrical property of 400 C annealed rGO/ZnO in the range of (0 2 V) as follows: I ¼ Is ½expðqV=nkTÞ 1 As a result, a huge ideality factor 59.9 could be achieved, which was applied to a nonideal diode model that had been reported for ZnO and carbon based heterojunctions, indicating a strong involvement of interfacial defects and contact resistance [3,32,33]. Meanwhile, for higher bias (2 5 V), the electrical property could be described as I Vb where b = 2.08 was obtained that was close to the common b = 2 relationship for wide band gap materials based diodes, which was consistent with the nature of ZnO [3]. In addition, the rectification ratio of 400 C annealed rGO/ZnO is 2.7 at 4 V, higher than that of reported rGO/ZnO device [34]. It also exhibited higher current under positive bias than reported multijunction rGO/ZnO device [34] and rGO/ZnO based field effect transistor [35]. Although the rectifying property of our 400 C annealed rGO/ZnO device was not that perfect as the latest reported Al-rGO/TiO2 Schottky diode, in which rGO did not act as p-type materials but was just mixed with TiO2 to be the semiconductor part of the Schottky device, the current at +1 V for these two kinds of devices was similar (3 mA) [36]. The increased conductivity and rectifying property of rGO/ZnO via annealing indicates that change of surface defects of rGO influences the electrical property of rGO/ZnO heterostructure, while ZnO nanorods are synthesized under the same condition for all the devices. To further investigate the electrical properties of GO and rGO, the Hummers method derived GO samples were integrated to transistor via electrophoresis as the above mentioned, shown in
Fig. 3. I–V curves of rGO/ZnO heterostructure. The inset shows the device diagram.
562
X. Chen et al. / Electrochimica Acta 196 (2016) 558–564
Fig. 4. (a) Device diagram of GO based transistor; Ids-Vds under different Vgs of (b) as-prepared; (c) 200 C for 30 min.; (d) 200 C for 2 h; (e) 400 C for 30 min. annealed GO transistors, the inset shows the detailed I–V in a smaller range.
Fig. 4(a). Fig. 4 shows the Ids-Vds under different gate voltages for as-prepared and annealed GO transistors. It can be seen that the unannealed GO exhibited n-type conductivity i.e. the resistance decreased with the increase of gate voltage. The devices exhibited lower resistance with longer annealing time or higher annealing temperature. Particularly, it should be noticed from Fig. 4 that both 200 C, 30 min. and 400 C, 30 min. annealed devices exhibited p-type conductivity, but only 400 C annealed rGO/ZnO heterostructure could exhibit rectifying I–V properties. The resistance of 200 C, 30 min. annealed sample was still too large (about 30 times to that of 400 C annealed sample) and limit the recombination of carriers in the interface of the heterostructure [31]. On the other hand, the p-type conductivity had been eliminated for the 200 C, 2 h annealed device. It revealed that a long time annealing would probably damage the chemical components that could contribute positively to the p-type conductivity of rGO. According to Lerf-Klinowski model, oxygen exists in rGO via C-O sp3 as hydroxyl, epoxy and carboxyl, which can be evaluated by C/O ratio [18,37,38]. The recent research showed that hydroxyl could exist stably and widely in GO, epoxy had poor thermal stability so that it would decompose after annealing. In addition, the amount of carboxyl is usually small and it can hardly exist in the bulk [18,39]. To further investigate the chemical components of C-O sp3 based functional groups as well as their influence on the electrical properties of rGO/ZnO heterostructure, XPS was applied to GO, asprepared rGO and annealed rGO samples, as shown in Fig. 5. There were four peaks could be identified for all the GO and rGO samples:
284.7 eV, 285.1–285.7 eV, 286.4–287.1 eV and 288.9–289.5 eV, which were considered to be corresponding to C-sp2 bonding, hydroxyl (-OH), epoxy (C-O-C) and carboxyl (-COOH) functional groups, respectively [40–45]. It can be observed from the inset of Fig. 5(a) that epoxy existed abundantly in GO, which was obviously reduced after N2H4 reduction and hydrogen annealing. It was also obvious that the amount of different C-O sp3 groups varied with the annealing conditions. The details are shown in Table 1, which indicates that the percentage of carboxyl did not change much for all the samples. In that case, C-sp2/(OH + C-O-C) ratio was used instead of the normal C/O ratio to focus the change of hydroxyl and epoxy during annealing. The 200 C, 30 min. and 400 C, 30 min. annealed samples had similar C-sp2 percentage and C-sp2/(OH + CO-C) ratio, while both of these exhibited p-type conductivity in the transistors according to Fig. 4. In 200 C, 2 h annealed sample, the percentage of both hydroxyl and epoxy increased and that of C-sp2 decreased, which might hold responsibility for the extinction of the p-type conductivity of this sample. On the other hand, although the C-sp2 percentage and C-sp2/(OH + C-O-C) ratio were close in 200 C, 30 min. and 400 C, 30 min. annealed samples, hydroxyl was dominant in the 400 C annealed sample. Referring to the SEM images, the density of ZnO nanorods on 400 C annealed rGO was a bit higher than that on 200 C annealed rGO, which might promote the combination of carriers on the rGO/ZnO interface. Correspondingly, the 400 C annealed rGO had higher conductivity and exhibited rectifying I–V property in rGO/ZnO heterostructure. Table I revealed that epoxy could be generated on the surface of
X. Chen et al. / Electrochimica Acta 196 (2016) 558–564
563
Fig. 5. XPS of (a) as-prepared; (b) 200 C for 30 min.; (c) 200 C for 2 h; (d) 400 C for 30 min. annealed rGO.
rGO membrane during 200 C annealing, which might due to the diffusion of oxygen from the inner layers of rGO membrane. It is also clear that epoxy would decompose to hydroxyl at higher temperature (400 C in this work) [19]. On the other hand, it should be noticed that the C-sp2 percentage dropped apparently in 200 C, 2 h annealed rGO, indicating that annealing overtime could damage the C-sp2 bonding. Conclusively, C-sp2 percentage at about 54% and C-sp2/(OH + C-O-C) ratio at about 1.6 could result in the p-type conductivity of rGO. It is well known that an ideal graphene with very few oxygen related defects should exhibit metallic conductivity, while C-sp2 is the absolutely dominant bond and electron is the absolutely dominant carrier. So it might be inferred that the p-type conductivity of rGO would be eliminated when C-Sp2 percentage is too high. On the other hand, although the unannealed rGO had comparable C-sp2 percentage and C-sp2/ (OH + C-O-C) ratio, the resistance of unannealed rGO was too high to form effective band alignment with ZnO to promote the recombination of carriers [31]. Thus unannealed rGO based rGO/ ZnO could not exhibit rectifying I–V property. According to the Table 1 Chemical components of GO and rGO. C-sp2 GO rGO (as-prepared) rGO (200 C, 30 min.) rGO (200 C, 2 h.) rGO (400 C, 30 min.)
Hydroxyl Epoxy Carboxyl C-sp2 to (OH + COC)
47.2% 15.7% 60.7% 20.3%
26.6% 6.7%
10.5% 12.3%
1.1 2.2
18.3%
14.4%
13.4%
1.6
40.3% 28.5%
16.5%
14.7%
0.9
54.7% 26.7%
7.5%
11.1%
1.6
53.9%
comparison of SEM images and electrical characterization, higher conductivity of rGO and higher density of ZnO nanorods were also proved to be helpful for the presence of rectifying I–V property of rGO/ZnO heterostructure. 4. Conclusion In summary, rGO/ZnO heterostructure was in-situ fabricated on rGO substrates electrochemically and rectifying I–V curves was obtained by modifying the functional groups on rGO surface and thus the interface between rGO and ZnO via annealing. It was found that a proper C-sp2 percentage (54%) and C-sp2/(OH + C-OC) ratio (1.6) could result in p-type conductivity of rGO, which make rGO a promising p-type candidate for ZnO heterojunctions. Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (No. 20720150084 and No. 20720140512), the NSFC under project (No. 61071010 and No. 61306087), the Natural Science Foundation of Fujian Province of China (No. 2015J05130), and Young Teachers’ Education and Research Project of Fujian Province (No. JA14007). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.02.201.
564
X. Chen et al. / Electrochimica Acta 196 (2016) 558–564
References [1] A.B. Djuriši c, A.M.C. Ng, X.Y. Chen, ZnO nanostructures for optoelectronics: material properties and device applications, Prog. Quantum Electron. 34 (2010) 191–259. [2] X.Y. Chen, F. Fang, A.M.C. Ng, A.B. Djuriši c, K.W. Cheah, C.C. Ling, W.K. Chan, P. W.K. Fong, H.F. Lui, C. Surya, Nitrogen doped-ZnO/n-GaN heterojunctions, J. Appl. Phys. 109 (2011) 084330. [3] X.Y. Chen, A.M.C. Ng, F. Fang, Y.H. Ng, A.B. Djuriši c, H.L. Tam, K.W. Cheah, S. Gwo, W.K. Chan, P.W.K. Fong, H.F. Lui, C. Surya, ZnO nanorod/GaN lightemitting diodes: the origin of yellow and violet emission bands under reverse and forward bias, J. Appl. Phys. 110 (2011) 094513. [4] X.L. Li, X.R. Wang, L. Zhang, S. Lee, H.J. Dai, Chemically, derived ultrasmooth graphene nanoribbon semiconductors, Science 319 (2008) 1229–1232. [5] X.R. Wang, X.L. Li, L. Zhang, Y. Yoon, P.K. Weber, H.L. Wang, J. Guo, H.J. Dai, Ndoping of graphene through electrothermal reactions with ammonia, Science 324 (2009) 768–771. [6] H. Wang, Y. Zhou, D. Wu, L. Liao, S.L. Zhao, H.L. Peng, Z.F. Liu, Synthesis of boron-doped graphene monolayers using the sole solid feedstock by chemical vapor deposition, Small 9 (2013) 1316–1320. [7] M.Y. Xing, W.Z. Fang, X.L. Yang, B.Z. Tian, J.L. Zhang, Highly-dispersed borondoped graphene nanoribboons with enhanced conductibility and photocatalysis, Chem. Comm. 50 (2014) 6637–6640. [8] B.T. McGrail, B.J. Rodier, E. Pentzer, Rapid functionalization of graphene oxide in water, Chem. Mater. 26 (2014) 5806–5811. [9] W. Gao, L.B. Alemany, L.J. Ci, P.M. Ajayan, New insights into the structure and reduction of graphite oxide, Nature Chem. 1 (2009) 403–408. [10] C. Gómez-Navarro, J.C. Meyer, R.S. Sundaram, A. Chuvilin, S. Kurasch, M. Burghard, K. Kern, U. Kaiser, Atomic structure of reduced graphene oxide, Nano Lett. 10 (2010) 1144–1148. [11] W.H. Nam, B.B. Kim, S.G. Seo, Y.S. Lim, J.Y. Kim, W.S. Seo, W.K. Choi, H.H. Park, J. Y. Lee, Structurally nanocrystalline-electrically single crystalline ZnO reduced graphene oxide composites, Nano Lett. 14 (2014) 5104–5109. [12] J.L. Garcia-Pomar, A.Yu. Nikitin, L. Martin-Moreno, Scattering of graphene plasmons by defects in the graphene sheet, ACS Nano 7 (2013) 4988–4994. [13] O.C. Compton, B. Jain, D.A. Dikin, A. Abouimrane, K. Amine, S.T. Nguyen, Chemically active reduced graphene oxide with tunable C/O ratios, ACS Nano 5 (2011) 4380–4391. [14] X.R. Wang, Y.J. Ouyang, X.L. Li, H.L. Wang, J. Guo, H.J. Dai, Room-temperature all-semiconducting sub-10-nm graphene nanoribbon field-effect transistors, Phys. Rev. Lett. 100 (2008) 206803. [15] H.J. Zhang, G. Lee, C. Gong, L. Colombo, K. Cho, Grain boundary effect on electrical transport properties of graphene, J. Phys. Chem. C 118 (2014) 2338– 2343. [16] L. Ciri c, A. Sienkiewicz, R. Gaál, J. Ja cimovi c, C. Vâju, A. Magrez, L. Forró, Defects and localization in chemically-derived graphene, Phys. Rev. B 86 (2012) 195139. [17] W.S. Hummers, R.E. Offeman, Preparation of Graphitic Oxide, J. Am. Chem. Soc. 80 (1958) 1339. [18] A. Lerf, H.Y. He, M. Forster, J. Klinowski, Structure of graphite oxide revisited, J. Phys. Chem. B 102 (1998) 4477–4482. [19] Z. Xu, Y. Bando, L. Liu, W.L. Wang, X.D. Bai, D. Golberg, Electrical conductivity, chemistry, and bonding alternations under graphene oxide to graphene transition as revealed by in situ TEM, ACS Nano 5 (2011) 4401–4406. [20] H.E. Romero, N. Shen, P. Joshi, H.R. Gutierrez, S.A. Tadigadapa, J.O. Sofo, P.C. Eklund, n-Type behavior of graphene supported on Si/SiO2 substrates, ACS Nano 2 (2008) 2037–2044. [21] W.J. Xie, L.T. Weng, K.M. Ng, C.K. Chan, C.M. Chan, Clean graphene surface through high temperature annealing, Carbon 94 (2015) 740–748. [22] W.Q. Fan, X.Q. Yu, H.C. Lu, H.Y. Bai, C. Zhang, W.D. Shi, Fabrication of TiO2/RGO/ Cu2O heterostructure for photoelectrochemical hydrogen production, Appl. Catal. B: Environ. 181 (2016) 7–15. [23] K. Anand, O. Singh, M.P. Singh, J. Kaur, R.C. Singh, Hydrogen sensor based on graphene/ZnO nanocomposite, Sens. Actu. B: Chem. 195 (2014) 409–415.
[24] L.B. Gao, G.X. Ni, Y.P. Liu, B. Liu, A.H. Castro Neto, K.P. Loh, Face-to-face transfer of wafer-scale graphene films, Nature 505 (2014) 190–194. [25] M.Y. Lu, Y.M. Ruan, C.Y. Chiu, Y.P. Hsieh, M.P. Lu, Direct growth of ZnO nanowire arrays on UV-irradiated graphene, CrystEngComm 17 (2015) 9097–9101. [26] H. Beitollahi, I. Sheikhshoaie, Selective voltammetric determination of norepinephrine in the presence of acetaminophen and folic acid at a modified carbon nanotube paste electrode, J. Electroanal. Chem. 661 (2011) 336–342. [27] H. Beitollahi, A. Mohadesi, S. Mohammadi, A. Akbari, Electrochemical behavior of a carbon paste electrode modified with 5-amino-30 ,40 -dimethyl-biphenyl-2ol/carbon nanotube and its application for simultaneous determination of isoproterenol, acetaminophen and N-acetylcysteine, Electrochim Acta 68 (2012) 220–226. [28] H. Beitollahi, I. Sheikhshoaie, Electrocatalytic and simultaneous determination of isoproterenol, uric acid and folic acid at molybdenum (VI) complex-carbon nanotube paste electrode, Electrochim. Acta 56 (2011) 10259–10263. [29] D. Li, M.B. Müller, S. Gilje, R.B. Kaner, G.G. Wallace, Processable aqueous dispersions of graphene nanosheets, Nature Nanotech. 3 (2008) 101–105. [30] X.Y. Chen, A.M.C. Ng, F. Fang, A.B. Djuriši c, W.K. Chan, H.L. Tam, K.W. Cheah, P. W.K. Fong, H.F. Lui, C. Surya, The influence of the ZnO seed layer on the ZnO Nanorod/GaN LEDs, J. Electrochem. Soc. 157 (2010) H308–H311. [31] X.Y. Chen, A.M.C. Ng, F. Fang, Y.H. Ng, A.B. Djuriši c, H.L. Tam, K.W. Cheah, S. Gwo, W.K. Chan, P.W.K. Fong, H.F. Lui, C. Surya, ZnO nanorod/GaN lightemitting diodes: The origin of yellow and violet emission bands under reverse and forward bias, J. Appl. Phys. 110 (2011) 094513. [32] M. Brötzmann, U. Vetter, H. Hofsäss, BN/ZnO heterojunction diodes with apparently giant ideality factors, J. Appl. Phys. 106 (2009) 063704. [33] M. Brötzmann, U. Vetter, H. Hofsäss, ta-C/Si heterojunction diodes with apparently giant ideality factors, Phys. Status Solidi C 7 (2010) 256–259. [34] B.D. Boruah, D.B. Ferry, A. Mukherjee, A. Misra, Few-layer graphene/ZnO nanowires based high performance UV photodetector, Nanotechnology 26 (2015) 235703. [35] Z. Sun, D. Huang, Z. Yang, X.L. Li, N.T. Hu, C. Yang, H. Wei, G.L. Yin, D.N. He, Y.F. Zhang, IEEE Electron Dev. Lett. 36 (2015) 1376–1379. [36] M. Das, J. Datta, A. Dey, R. Jana, A. Layek, S. Middya, P.P. Ray, One step hydrothermal synthesis of a rGO–TiO2 nanocomposite and its application on a Schottky diode: improvement in device performance and transport properties, RSC Adv. 5 (2015) 101582–101592. [37] D.R. Dreyer, S.J. Park, C.W. Bielawski, R.S. Ruoff, The chemistry of graphene oxide, Chem. Soc. Rev. 39 (2010) 228–240. [38] H. Bai, C. Li, G.Q. Shi, Functional composite materials based on chemically converted graphene, Adv. Mater. 23 (2011) 1089–1115. [39] M. Mermoux, Y. Chabre, A. Rousseau, FTIR and 13C NMR study of graphite oxide, Carbon 29 (1991) 469–474. [40] A. Siokou, F. Ravani, S. Karakalos, O. Frank, M. Kalbac, C. Galiotis, Surface refinement and electronic properties of graphene layers grown on copper substrate: An XPS, UPS and EELS study, Appl. Surf. Sci. 257 (2011) 9785–9790. [41] L. Stobinski, B. Lesiak, A. Malolepszy, M. Mazurkiewicz, B. Mierzwa, J. Zemek, P. Jiricek, I. Bieloshapk, Graphene oxide and reduced graphene oxide studied by the XRD, TEM and electron spectroscopy methods, J. Electron Spect. Rel. Phenom. 195 (2014) 145–154. [42] W. Ai, Z.M. Luo, J. Jiang, J.H. Zhu, Z.Z. Du, Z.X. Fan, L.H. Xie, H. Zhang, W. Huang, T. Yu, Nitrogen and sulfur codoped graphene: multifunctional electrode materials for high-performance Li-ion batteries and oxygen reduction reaction, Adv. Mater 26 (2014) 6186–6192. [43] A. Ganguly, S. Sharma, P. Papakonstantinou, J. Hamilton, Probing the thermal deoxygenation of graphene oxide using high-resolution in situ X-ray-based spectroscopies, J. Phys. Chem. C 115 (2011) 17009–17019. [44] T.V. Khai, H.G. Na, D.S. Kwak, Y.J. Kwon, H. Ham, K.B. Shim, H.W. Kim, Significant enhancement of blue emission and electrical conductivity of Ndoped graphene, J. Mater. Chem. 22 (2012) 17992–18003. [45] D. Chen, H.B. Feng, J.H. Li, Graphene oxide: preparation, functionalization, and electrochemical applications, Chem. Rev. 112 (2012) 6027–6053.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
In-situ fabrication of reduced graphene oxide (rGO)/ZnO heterostructure: surface functional groups induced electrical properties Xinyi Chen* , Hongchen Guo, Tun Wang, Miao Lu* , Taihong Wang Pen-Tung Sah Institute of Micro-Nano Science & Technology, Xiamen University, 361005, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 December 2015 Received in revised form 7 February 2016 Accepted 27 February 2016 Available online 2 March 2016
Reduced graphene oxide (rGO)/ZnO heterostructure was prepared via electrochemical deposition directly on Hummers method derived rGO membranes and the corresponding diodes had been fabricated. Rectifying I–V curve was obtained by modifying the functional groups on the surface of rGO. Further investigation for GO based transistors showed that the conductivity of rGO could vary from n-type to p-type under different annealing conditions. Based on Lerf-Klinowski model and X-Ray photoelectron spectroscopy, it was found that the C-sp2, hydroxyl and epoxy in rGO would be responsible for the change of electrical properties. It was also concluded that reasonable p-type conductivity of rGO for obtaining rectifying rGO/ZnO heterostructure occurred while the percentage of C-sp2 content was about 54% with the C-sp2/(OH + C-O-C) ratio around 1.6. ã 2016 Elsevier Ltd. All rights reserved.
Keywords: reduced graphene oxide ZnO hydroxyl epoxy heterostructure
1. Introduction Wide bandgap oxide such as ZnO has declaimed to be very promising in optoelectronic applications such as solar cells and light emitting diodes, due to its low cost, varieties of synthesis methods and nanostructured morphologies, as well as the tunable optical and electrical properties. However, the difficulty in p-type doping of ZnO due to the rich defect chemistry, has hindered the further development of ZnO-based optoelectronic devices. Accordingly, another candidate to develop ZnO based optoelectronic devices is to use heterojunction device structure [1,2]. The most commonly used p-type materials for ZnO based heterojunction is Mg:GaN because of the same hexagonal structure and the reasonably low lattice mismatch (1.5%) [1,3]. However, the carrier concentration and mobility of Mg:GaN are significantly lower than that of ZnO, in which case the recombination of carriers would occur on GaN side of the heterojunction, and the device performance are thus dominated by the material properties of GaN. On the other hand, graphene, a one-atom-thick material with closely packed sp2 bonded carbon honeycomb structure, has exhibited excellent and tunable conductivity. Graphene has a rich defect chemistry inducing vacancy, impurity, surface absorbate,
* Corresponding authors. E-mail addresses: [email protected] (X. Chen), [email protected] (M. Lu). http://dx.doi.org/10.1016/j.electacta.2016.02.201 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.
grain boundary and topological defects [4–20], which results in varieties of the electrical and mechanical properties of graphene. For example, boron and phosphine can be used as the p-type and n-type dopant, respectively [5–7]. It has also been reported that p-type conductivity of graphene could be introduced in two ways: broadened band gap and confinement of electron transport due to quasi-one-dimensional size effect [4,14], or the adsorption of ambient gases [20]. Accordingly, the adjustable and controllable conductivity of graphene might provide an alternatively new approach to p-type materials for ZnO heterojunctions. Among various kinds of graphene synthesis, Hummers method has provided a low cost, fast and productive approach for graphene via oxidizing graphite to dispersed GO sheet, which can be further reduced to reduced GO (rGO) via annealing or chemical reduction [17]. Different from graphene obtained by vapor deposition and exfoliation, the Hummers derived rGO has abundant hydration related defects uniquely. Recently, Lerf-Klinowski model has been commonly used to describe the structure of GO [18], which proposed that there are three hydration related functional groups forming chemical bonds with C atom: hydroxyl, epoxy and carboxyl. It has also been reported that annealing can promote the generation or extinction of these hydration related functional groups via different temperature, pressure and gas ambience [19–21]. Meanwhile, those kind of oxygen related functional groups such as hydroxyl widely exist as surface defects on the polar facets {0001} of ZnO, which are the dominant facets for growth. Oxygen
X. Chen et al. / Electrochimica Acta 196 (2016) 558–564
vacancy and interstitial are also very common in metal oxides including ZnO [1]. The similarity in defect chemistry of graphene and metal oxides opens up the prospect of developing graphene/ ZnO (or other metal oxides) heterostructures [1,22]. Graphene was usually mixed with metal oxides in powder or stamped to other as-prepared oxides to obtain hybrid materials [23,24], in which cases the surface defects could not promote the interfacial growth, and the interface between graphene and ZnO would not be influenced by only surface defects of graphene and ZnO, but also the gels used for stamping. Moreover, direct growth of ZnO nanostructure on graphene was rarely reported, which usually required a pre-fabricated seed layer [25]. In this work, rGO/ ZnO heterostructure with diode architecture is developed by an in-situ deposition of ZnO nanorods directly on Hummers method derived rGO membranes. Electrochemistry, which could be applied in biological and environmental analysis [26–28], is employed to
559
synthesize ZnO nanorods on rGO. The interface between graphene and ZnO is modified to achieve rectifying I–V properties. It has been found that the post-growth annealing can change the chemical components of absorbed functional groups on the surface of graphene and then affect the electrical property in rGO/ZnO interface. 2. Experimental Graphene was obtained by reduction of Hummers method derived graphene oxide as follows: 0.5 g expanded graphite, 3 g KMnO4 and 0.5 g NaNO3 (Xilong Chemical Co., Ltd., China) were mixed in 60 ml concentrated sulfur acid and the solution was stirred for at least 12 h until the color changed to dark green, then the solution was diluted by deionized (DI) water to 200 ml. After that 12 ml H2O2 was introduced. Then the solution was centrifuged
Fig. 1. (a) Schematic of the assembly of rGO substrate and the electrodepositon; SEM images of rGO/ZnO heterostructure i.e. ZnO nanorods on (b) as-prepared; and (c) 200 C for 30 min.; (d) 200 C for 2 h; (e) 400 C for 30 min. annealed rGO substrates.
560
X. Chen et al. / Electrochimica Acta 196 (2016) 558–564
at 10000 rpm for 5 times to remove the impurities. Once graphene oxide dispersive solution (0.25 mg/ml) was prepared, N2H4 (35%, 5 ml for each 10 ml solution, Xilong Chemical Co., Ltd., China) and NH3 (25%, 35 ml for each 10 ml solution, Xilong Chemical Co., Ltd., China) were added. The reduction was performed at 95 C for 1 h thus rGO could be obtained [29]. The rGO solution was consequently filtrated under pumping and rGO film formed on the filtration membrane, which could be mechanically lifted off from filtration membrane. The previous research on ZnO based heterojunction showed that starting material for growing ZnO, such as substrate, catalyst and seed layer, had obvious influence on the properties of ZnO including morphology, emission, turn-on voltage and external quantum effect [30,31]. In that case, post-
growth annealing in hydrogen/nitrogen atmosphere was applied to the as-prepared rGO membranes, which were then fixed on glass substrates using polydimethylsiloxane (PDMS, Xilong Chemical Co., Ltd., China) as described in Fig. 1(a). Those fixed rGO on glass would be used for the growth of ZnO nanorods. ZnO nanorods were grown on rGO via electrochemical deposition: the solution for electrochemical deposition containing 8 mM Zn(NO3)2xH2O (Xilong Chemical Co., Ltd., China) and hexamethylenetetramine (C6H12N4, HMT, Xilong Chemical Co., Ltd., China) was heated up to 80 C, and rGO membrane on glass was the working electrode, while Pt foil was used as the counter electrode [3]. A bias at 2 V was applied to the system for 30 min. and ZnO nanorods were grown on rGO membranes. Post-growth
Fig. 2. (a) TEM image of ZnO nanorods, the inset show the SAED pattern and HRTEM image, which indicates a typical growth of ZnO along [0001]; (b) XRD patterns of all the samples; (c) detailed XRD pattern in the range from 15 to 45 of GO and rGO.
X. Chen et al. / Electrochimica Acta 196 (2016) 558–564
annealing in 5% H2/95% N2 forming gas with different parameters was performed to rGO films before ZnO growth to explore the influence of surface defect and impurities. Scanning electron microscopy (SEM, Carl Zeiss, Germany), transmission electron microscopy (TEM, JEOL 2100, Japan), X-Ray diffraction (XRD, Rigaku, Japan) and X-Ray photoelectron spectroscopy (XPS, PHI QUANTUM 2000, USA) were employed to characterize the morphology, components and structure of the as-grown samples. Device for rGO/ZnO heterostucture was prepared as described: a 100 nm Au layer was used as the contact electrode for rGO side, and indium tin oxide (ITO, 100 nm) was used as the contact electrode for ZnO side. An insulating layer of 240 nm made up of poly-methyl methacrylate (PMMA 600 K, Allresit) was spin-coated on rGO/ZnO at 2000 rpm and then annealed at 200 C for 60 s, to form an insulating layer to avoid leakage current between rGO and ITO, after which oxygen plasma was applied for 10 s to etch the top surface of PMMA to expose ZnO tips. An Agilent 4156C analyzer equipped with four probes was employed to measure the electrical properties. GO based transistors were fabricated on boron doped silicon <100> with an oxide layer around 100 nm, while Si was used as the gate electrode directly. Lithography and sputtering was employed to prepare the two Au/Cr (200 nm/20 nm) electrodes on SiOx side as source and drain, respectively. Then 10 mL GO solution (20 mg/ mL) was dropped on the SiOx side of the device, while a square wave AC signal (200 kHz, 8 Vpp) was applied between source and drain electrodes for 4 minutes. Then the device was rinsed by DI water and dried by nitrogen flow. 3. Results and Discussion Fig. 1 shows the SEM images of ZnO nanorods electrodeposited on rGO substrates. It is shown that the density of ZnO nanorods increases with the annealing temperature and time for rGO substrates, which indicates an increased conductivity of rGO substrates via annealing (SEM images of bare rGO and electrochemical deposited ZnO nanorods on other conductive substrates are shown in Fig. S1 in the supporting information). Fig. 2(a) shows the morphology and structure of electrodeposited ZnO nanorods in TEM, which indicated that ZnO nanorods exhibited a normal hexagonal lattice structure with the grown direction along [0001] that had also been proved by the XRD pattern shown in Fig. 2(b). Detailed XRD pattern in the range from 15 to 45 of GO and rGO is shown in Fig. 2(c). It can be seen that the diffraction peak at around 24.8 degree, which revealed a wider layer distance than that of
561
graphite along [0001] direction, was corresponding to the distance between rGO layers and became narrower and sharper with higher annealing temperature and longer annealing time, indicating the uniformization of rGO layers during thermal treatment in hydrogen/nitrogen environment. The electrical properties of rGO/ZnO hybrid structure were characterized through diode devices and the related I–V curves are shown in Fig. 3. It can be observed that the I–V curves of 200 C annealed rGO based rGO/ZnO were not rectifying regardless of the annealing time, so was the unannealed rGO. On the other hand, 400 C annealed rGO based rGO/ZnO hybrid structure exhibited rectifying conductivity. Diode equation was used to study the electrical property of 400 C annealed rGO/ZnO in the range of (0 2 V) as follows: I ¼ Is ½expðqV=nkTÞ 1 As a result, a huge ideality factor 59.9 could be achieved, which was applied to a nonideal diode model that had been reported for ZnO and carbon based heterojunctions, indicating a strong involvement of interfacial defects and contact resistance [3,32,33]. Meanwhile, for higher bias (2 5 V), the electrical property could be described as I Vb where b = 2.08 was obtained that was close to the common b = 2 relationship for wide band gap materials based diodes, which was consistent with the nature of ZnO [3]. In addition, the rectification ratio of 400 C annealed rGO/ZnO is 2.7 at 4 V, higher than that of reported rGO/ZnO device [34]. It also exhibited higher current under positive bias than reported multijunction rGO/ZnO device [34] and rGO/ZnO based field effect transistor [35]. Although the rectifying property of our 400 C annealed rGO/ZnO device was not that perfect as the latest reported Al-rGO/TiO2 Schottky diode, in which rGO did not act as p-type materials but was just mixed with TiO2 to be the semiconductor part of the Schottky device, the current at +1 V for these two kinds of devices was similar (3 mA) [36]. The increased conductivity and rectifying property of rGO/ZnO via annealing indicates that change of surface defects of rGO influences the electrical property of rGO/ZnO heterostructure, while ZnO nanorods are synthesized under the same condition for all the devices. To further investigate the electrical properties of GO and rGO, the Hummers method derived GO samples were integrated to transistor via electrophoresis as the above mentioned, shown in
Fig. 3. I–V curves of rGO/ZnO heterostructure. The inset shows the device diagram.
562
X. Chen et al. / Electrochimica Acta 196 (2016) 558–564
Fig. 4. (a) Device diagram of GO based transistor; Ids-Vds under different Vgs of (b) as-prepared; (c) 200 C for 30 min.; (d) 200 C for 2 h; (e) 400 C for 30 min. annealed GO transistors, the inset shows the detailed I–V in a smaller range.
Fig. 4(a). Fig. 4 shows the Ids-Vds under different gate voltages for as-prepared and annealed GO transistors. It can be seen that the unannealed GO exhibited n-type conductivity i.e. the resistance decreased with the increase of gate voltage. The devices exhibited lower resistance with longer annealing time or higher annealing temperature. Particularly, it should be noticed from Fig. 4 that both 200 C, 30 min. and 400 C, 30 min. annealed devices exhibited p-type conductivity, but only 400 C annealed rGO/ZnO heterostructure could exhibit rectifying I–V properties. The resistance of 200 C, 30 min. annealed sample was still too large (about 30 times to that of 400 C annealed sample) and limit the recombination of carriers in the interface of the heterostructure [31]. On the other hand, the p-type conductivity had been eliminated for the 200 C, 2 h annealed device. It revealed that a long time annealing would probably damage the chemical components that could contribute positively to the p-type conductivity of rGO. According to Lerf-Klinowski model, oxygen exists in rGO via C-O sp3 as hydroxyl, epoxy and carboxyl, which can be evaluated by C/O ratio [18,37,38]. The recent research showed that hydroxyl could exist stably and widely in GO, epoxy had poor thermal stability so that it would decompose after annealing. In addition, the amount of carboxyl is usually small and it can hardly exist in the bulk [18,39]. To further investigate the chemical components of C-O sp3 based functional groups as well as their influence on the electrical properties of rGO/ZnO heterostructure, XPS was applied to GO, asprepared rGO and annealed rGO samples, as shown in Fig. 5. There were four peaks could be identified for all the GO and rGO samples:
284.7 eV, 285.1–285.7 eV, 286.4–287.1 eV and 288.9–289.5 eV, which were considered to be corresponding to C-sp2 bonding, hydroxyl (-OH), epoxy (C-O-C) and carboxyl (-COOH) functional groups, respectively [40–45]. It can be observed from the inset of Fig. 5(a) that epoxy existed abundantly in GO, which was obviously reduced after N2H4 reduction and hydrogen annealing. It was also obvious that the amount of different C-O sp3 groups varied with the annealing conditions. The details are shown in Table 1, which indicates that the percentage of carboxyl did not change much for all the samples. In that case, C-sp2/(OH + C-O-C) ratio was used instead of the normal C/O ratio to focus the change of hydroxyl and epoxy during annealing. The 200 C, 30 min. and 400 C, 30 min. annealed samples had similar C-sp2 percentage and C-sp2/(OH + CO-C) ratio, while both of these exhibited p-type conductivity in the transistors according to Fig. 4. In 200 C, 2 h annealed sample, the percentage of both hydroxyl and epoxy increased and that of C-sp2 decreased, which might hold responsibility for the extinction of the p-type conductivity of this sample. On the other hand, although the C-sp2 percentage and C-sp2/(OH + C-O-C) ratio were close in 200 C, 30 min. and 400 C, 30 min. annealed samples, hydroxyl was dominant in the 400 C annealed sample. Referring to the SEM images, the density of ZnO nanorods on 400 C annealed rGO was a bit higher than that on 200 C annealed rGO, which might promote the combination of carriers on the rGO/ZnO interface. Correspondingly, the 400 C annealed rGO had higher conductivity and exhibited rectifying I–V property in rGO/ZnO heterostructure. Table I revealed that epoxy could be generated on the surface of
X. Chen et al. / Electrochimica Acta 196 (2016) 558–564
563
Fig. 5. XPS of (a) as-prepared; (b) 200 C for 30 min.; (c) 200 C for 2 h; (d) 400 C for 30 min. annealed rGO.
rGO membrane during 200 C annealing, which might due to the diffusion of oxygen from the inner layers of rGO membrane. It is also clear that epoxy would decompose to hydroxyl at higher temperature (400 C in this work) [19]. On the other hand, it should be noticed that the C-sp2 percentage dropped apparently in 200 C, 2 h annealed rGO, indicating that annealing overtime could damage the C-sp2 bonding. Conclusively, C-sp2 percentage at about 54% and C-sp2/(OH + C-O-C) ratio at about 1.6 could result in the p-type conductivity of rGO. It is well known that an ideal graphene with very few oxygen related defects should exhibit metallic conductivity, while C-sp2 is the absolutely dominant bond and electron is the absolutely dominant carrier. So it might be inferred that the p-type conductivity of rGO would be eliminated when C-Sp2 percentage is too high. On the other hand, although the unannealed rGO had comparable C-sp2 percentage and C-sp2/ (OH + C-O-C) ratio, the resistance of unannealed rGO was too high to form effective band alignment with ZnO to promote the recombination of carriers [31]. Thus unannealed rGO based rGO/ ZnO could not exhibit rectifying I–V property. According to the Table 1 Chemical components of GO and rGO. C-sp2 GO rGO (as-prepared) rGO (200 C, 30 min.) rGO (200 C, 2 h.) rGO (400 C, 30 min.)
Hydroxyl Epoxy Carboxyl C-sp2 to (OH + COC)
47.2% 15.7% 60.7% 20.3%
26.6% 6.7%
10.5% 12.3%
1.1 2.2
18.3%
14.4%
13.4%
1.6
40.3% 28.5%
16.5%
14.7%
0.9
54.7% 26.7%
7.5%
11.1%
1.6
53.9%
comparison of SEM images and electrical characterization, higher conductivity of rGO and higher density of ZnO nanorods were also proved to be helpful for the presence of rectifying I–V property of rGO/ZnO heterostructure. 4. Conclusion In summary, rGO/ZnO heterostructure was in-situ fabricated on rGO substrates electrochemically and rectifying I–V curves was obtained by modifying the functional groups on rGO surface and thus the interface between rGO and ZnO via annealing. It was found that a proper C-sp2 percentage (54%) and C-sp2/(OH + C-OC) ratio (1.6) could result in p-type conductivity of rGO, which make rGO a promising p-type candidate for ZnO heterojunctions. Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (No. 20720150084 and No. 20720140512), the NSFC under project (No. 61071010 and No. 61306087), the Natural Science Foundation of Fujian Province of China (No. 2015J05130), and Young Teachers’ Education and Research Project of Fujian Province (No. JA14007). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.02.201.
564
X. Chen et al. / Electrochimica Acta 196 (2016) 558–564
References [1] A.B. Djuriši c, A.M.C. Ng, X.Y. Chen, ZnO nanostructures for optoelectronics: material properties and device applications, Prog. Quantum Electron. 34 (2010) 191–259. [2] X.Y. Chen, F. Fang, A.M.C. Ng, A.B. Djuriši c, K.W. Cheah, C.C. Ling, W.K. Chan, P. W.K. Fong, H.F. Lui, C. Surya, Nitrogen doped-ZnO/n-GaN heterojunctions, J. Appl. Phys. 109 (2011) 084330. [3] X.Y. Chen, A.M.C. Ng, F. Fang, Y.H. Ng, A.B. Djuriši c, H.L. Tam, K.W. Cheah, S. Gwo, W.K. Chan, P.W.K. Fong, H.F. Lui, C. Surya, ZnO nanorod/GaN lightemitting diodes: the origin of yellow and violet emission bands under reverse and forward bias, J. Appl. Phys. 110 (2011) 094513. [4] X.L. Li, X.R. Wang, L. Zhang, S. Lee, H.J. Dai, Chemically, derived ultrasmooth graphene nanoribbon semiconductors, Science 319 (2008) 1229–1232. [5] X.R. Wang, X.L. Li, L. Zhang, Y. Yoon, P.K. Weber, H.L. Wang, J. Guo, H.J. Dai, Ndoping of graphene through electrothermal reactions with ammonia, Science 324 (2009) 768–771. [6] H. Wang, Y. Zhou, D. Wu, L. Liao, S.L. Zhao, H.L. Peng, Z.F. Liu, Synthesis of boron-doped graphene monolayers using the sole solid feedstock by chemical vapor deposition, Small 9 (2013) 1316–1320. [7] M.Y. Xing, W.Z. Fang, X.L. Yang, B.Z. Tian, J.L. Zhang, Highly-dispersed borondoped graphene nanoribboons with enhanced conductibility and photocatalysis, Chem. Comm. 50 (2014) 6637–6640. [8] B.T. McGrail, B.J. Rodier, E. Pentzer, Rapid functionalization of graphene oxide in water, Chem. Mater. 26 (2014) 5806–5811. [9] W. Gao, L.B. Alemany, L.J. Ci, P.M. Ajayan, New insights into the structure and reduction of graphite oxide, Nature Chem. 1 (2009) 403–408. [10] C. Gómez-Navarro, J.C. Meyer, R.S. Sundaram, A. Chuvilin, S. Kurasch, M. Burghard, K. Kern, U. Kaiser, Atomic structure of reduced graphene oxide, Nano Lett. 10 (2010) 1144–1148. [11] W.H. Nam, B.B. Kim, S.G. Seo, Y.S. Lim, J.Y. Kim, W.S. Seo, W.K. Choi, H.H. Park, J. Y. Lee, Structurally nanocrystalline-electrically single crystalline ZnO reduced graphene oxide composites, Nano Lett. 14 (2014) 5104–5109. [12] J.L. Garcia-Pomar, A.Yu. Nikitin, L. Martin-Moreno, Scattering of graphene plasmons by defects in the graphene sheet, ACS Nano 7 (2013) 4988–4994. [13] O.C. Compton, B. Jain, D.A. Dikin, A. Abouimrane, K. Amine, S.T. Nguyen, Chemically active reduced graphene oxide with tunable C/O ratios, ACS Nano 5 (2011) 4380–4391. [14] X.R. Wang, Y.J. Ouyang, X.L. Li, H.L. Wang, J. Guo, H.J. Dai, Room-temperature all-semiconducting sub-10-nm graphene nanoribbon field-effect transistors, Phys. Rev. Lett. 100 (2008) 206803. [15] H.J. Zhang, G. Lee, C. Gong, L. Colombo, K. Cho, Grain boundary effect on electrical transport properties of graphene, J. Phys. Chem. C 118 (2014) 2338– 2343. [16] L. Ciri c, A. Sienkiewicz, R. Gaál, J. Ja cimovi c, C. Vâju, A. Magrez, L. Forró, Defects and localization in chemically-derived graphene, Phys. Rev. B 86 (2012) 195139. [17] W.S. Hummers, R.E. Offeman, Preparation of Graphitic Oxide, J. Am. Chem. Soc. 80 (1958) 1339. [18] A. Lerf, H.Y. He, M. Forster, J. Klinowski, Structure of graphite oxide revisited, J. Phys. Chem. B 102 (1998) 4477–4482. [19] Z. Xu, Y. Bando, L. Liu, W.L. Wang, X.D. Bai, D. Golberg, Electrical conductivity, chemistry, and bonding alternations under graphene oxide to graphene transition as revealed by in situ TEM, ACS Nano 5 (2011) 4401–4406. [20] H.E. Romero, N. Shen, P. Joshi, H.R. Gutierrez, S.A. Tadigadapa, J.O. Sofo, P.C. Eklund, n-Type behavior of graphene supported on Si/SiO2 substrates, ACS Nano 2 (2008) 2037–2044. [21] W.J. Xie, L.T. Weng, K.M. Ng, C.K. Chan, C.M. Chan, Clean graphene surface through high temperature annealing, Carbon 94 (2015) 740–748. [22] W.Q. Fan, X.Q. Yu, H.C. Lu, H.Y. Bai, C. Zhang, W.D. Shi, Fabrication of TiO2/RGO/ Cu2O heterostructure for photoelectrochemical hydrogen production, Appl. Catal. B: Environ. 181 (2016) 7–15. [23] K. Anand, O. Singh, M.P. Singh, J. Kaur, R.C. Singh, Hydrogen sensor based on graphene/ZnO nanocomposite, Sens. Actu. B: Chem. 195 (2014) 409–415.
[24] L.B. Gao, G.X. Ni, Y.P. Liu, B. Liu, A.H. Castro Neto, K.P. Loh, Face-to-face transfer of wafer-scale graphene films, Nature 505 (2014) 190–194. [25] M.Y. Lu, Y.M. Ruan, C.Y. Chiu, Y.P. Hsieh, M.P. Lu, Direct growth of ZnO nanowire arrays on UV-irradiated graphene, CrystEngComm 17 (2015) 9097–9101. [26] H. Beitollahi, I. Sheikhshoaie, Selective voltammetric determination of norepinephrine in the presence of acetaminophen and folic acid at a modified carbon nanotube paste electrode, J. Electroanal. Chem. 661 (2011) 336–342. [27] H. Beitollahi, A. Mohadesi, S. Mohammadi, A. Akbari, Electrochemical behavior of a carbon paste electrode modified with 5-amino-30 ,40 -dimethyl-biphenyl-2ol/carbon nanotube and its application for simultaneous determination of isoproterenol, acetaminophen and N-acetylcysteine, Electrochim Acta 68 (2012) 220–226. [28] H. Beitollahi, I. Sheikhshoaie, Electrocatalytic and simultaneous determination of isoproterenol, uric acid and folic acid at molybdenum (VI) complex-carbon nanotube paste electrode, Electrochim. Acta 56 (2011) 10259–10263. [29] D. Li, M.B. Müller, S. Gilje, R.B. Kaner, G.G. Wallace, Processable aqueous dispersions of graphene nanosheets, Nature Nanotech. 3 (2008) 101–105. [30] X.Y. Chen, A.M.C. Ng, F. Fang, A.B. Djuriši c, W.K. Chan, H.L. Tam, K.W. Cheah, P. W.K. Fong, H.F. Lui, C. Surya, The influence of the ZnO seed layer on the ZnO Nanorod/GaN LEDs, J. Electrochem. Soc. 157 (2010) H308–H311. [31] X.Y. Chen, A.M.C. Ng, F. Fang, Y.H. Ng, A.B. Djuriši c, H.L. Tam, K.W. Cheah, S. Gwo, W.K. Chan, P.W.K. Fong, H.F. Lui, C. Surya, ZnO nanorod/GaN lightemitting diodes: The origin of yellow and violet emission bands under reverse and forward bias, J. Appl. Phys. 110 (2011) 094513. [32] M. Brötzmann, U. Vetter, H. Hofsäss, BN/ZnO heterojunction diodes with apparently giant ideality factors, J. Appl. Phys. 106 (2009) 063704. [33] M. Brötzmann, U. Vetter, H. Hofsäss, ta-C/Si heterojunction diodes with apparently giant ideality factors, Phys. Status Solidi C 7 (2010) 256–259. [34] B.D. Boruah, D.B. Ferry, A. Mukherjee, A. Misra, Few-layer graphene/ZnO nanowires based high performance UV photodetector, Nanotechnology 26 (2015) 235703. [35] Z. Sun, D. Huang, Z. Yang, X.L. Li, N.T. Hu, C. Yang, H. Wei, G.L. Yin, D.N. He, Y.F. Zhang, IEEE Electron Dev. Lett. 36 (2015) 1376–1379. [36] M. Das, J. Datta, A. Dey, R. Jana, A. Layek, S. Middya, P.P. Ray, One step hydrothermal synthesis of a rGO–TiO2 nanocomposite and its application on a Schottky diode: improvement in device performance and transport properties, RSC Adv. 5 (2015) 101582–101592. [37] D.R. Dreyer, S.J. Park, C.W. Bielawski, R.S. Ruoff, The chemistry of graphene oxide, Chem. Soc. Rev. 39 (2010) 228–240. [38] H. Bai, C. Li, G.Q. Shi, Functional composite materials based on chemically converted graphene, Adv. Mater. 23 (2011) 1089–1115. [39] M. Mermoux, Y. Chabre, A. Rousseau, FTIR and 13C NMR study of graphite oxide, Carbon 29 (1991) 469–474. [40] A. Siokou, F. Ravani, S. Karakalos, O. Frank, M. Kalbac, C. Galiotis, Surface refinement and electronic properties of graphene layers grown on copper substrate: An XPS, UPS and EELS study, Appl. Surf. Sci. 257 (2011) 9785–9790. [41] L. Stobinski, B. Lesiak, A. Malolepszy, M. Mazurkiewicz, B. Mierzwa, J. Zemek, P. Jiricek, I. Bieloshapk, Graphene oxide and reduced graphene oxide studied by the XRD, TEM and electron spectroscopy methods, J. Electron Spect. Rel. Phenom. 195 (2014) 145–154. [42] W. Ai, Z.M. Luo, J. Jiang, J.H. Zhu, Z.Z. Du, Z.X. Fan, L.H. Xie, H. Zhang, W. Huang, T. Yu, Nitrogen and sulfur codoped graphene: multifunctional electrode materials for high-performance Li-ion batteries and oxygen reduction reaction, Adv. Mater 26 (2014) 6186–6192. [43] A. Ganguly, S. Sharma, P. Papakonstantinou, J. Hamilton, Probing the thermal deoxygenation of graphene oxide using high-resolution in situ X-ray-based spectroscopies, J. Phys. Chem. C 115 (2011) 17009–17019. [44] T.V. Khai, H.G. Na, D.S. Kwak, Y.J. Kwon, H. Ham, K.B. Shim, H.W. Kim, Significant enhancement of blue emission and electrical conductivity of Ndoped graphene, J. Mater. Chem. 22 (2012) 17992–18003. [45] D. Chen, H.B. Feng, J.H. Li, Graphene oxide: preparation, functionalization, and electrochemical applications, Chem. Rev. 112 (2012) 6027–6053.