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Cite this: Phys. Chem. Chem. Phys., 2015, 17, 31822
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Enhanced field-emission of silver nanoparticle– graphene oxide decorated ZnO nanowire arrays Guojing Wang,a Zhengcao Li,*ab Mingyang Li,ac Jiecui Liao,a Chienhua Chen,a Shasha Lva and Chuanqing Shia This work presents a new method to improve the field emission (FE) properties of semiconductors decorated with low-cost graphene oxide (GO) nanosheets and trace amounts of noble metal. The Ag/GO/ZnO composite emitter exhibited efficient FE properties with a low turn-on field of 1.4 V mm1 and a high field enhancement factor of 7018. The excellent FE properties of the Ag/GO/ZnO composite can be attributed to the tunneling effect of electrons through the heterojunction. The FE properties of the Ag/GO/ZnO composite are slightly better than those of the Ag/ZnO composite which forms an
Received 24th August 2015, Accepted 30th October 2015
energy well that collects electrons on interfaces when an electric field is applied. This behavior is
DOI: 10.1039/c5cp05036g
arrays (NWAs) and Ag/GO, which leads to easier electron transfer. High-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) were employed to characterise the
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connection and evolution of the ZnO NWAs and Ag/GO composites.
associated with heterostructures that offer more contact points and protrusions between ZnO nanowire
1 Introduction Due to the wide employment of high resolution electron microscopes, the fabrication of flat panel displays, and the process of discharging spacecraft from induced charges,1,2 field electron emission, also called field emission (FE), have attracted considerable attention in recent years. One-dimensional (1D) semiconductor nanostructures, with interesting photonic and electronic properties, and used as important emitter tips, are one focus in FE research. There are numerous studies on nanotips and nanowires (NWs) of Si, III–V systems and oxide systems,3–5 including SnO2,6 GeO2,7 ZnO/ZnWO4,8 and ZnO NWs.9,10 Among these semiconductors, ZnO11,12 is a wide band gap semiconductor that is nontoxic, inexpensive and has excellent physical, chemical and biocompatibility properties. According to previous theoretical and experimental research, vertically aligned 1D ZnO nanostructure arrays show better FE performances than disordered nanostructures.13 The performances of 1D ZnO nanostructure arrays are also affected by many other factors, including the uniformity, density, and taper morphology of the emitters, which are closely related to the local emitting intensity and the interaction among emitters.13
a
State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China. E-mail: [email protected] b Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China c Department of Engineering Physics, Tsinghua University, Beijing 100084, China
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However, so far, the ability to control 1D ZnO nanostructure arrays and the further improvement of their FE performances are still problems that urgently need to be solved. Recently, Zhai et al. developed an effective route (with the help of a self-assembled template) towards the full control of ZnO nanowire arrays (NWAs) with the turn-on field (Eto) first decreasing sharply from 12 V mm1 to 2.4 V mm1.14,15 The nature of FE is that electrons near the Fermi level are emitted into vacuum by quantum mechanical tunneling from a conducting/semiconducting emitter with the application of a very high external electric field (B105 V cm1), and the FE properties are closely linked with the work function or the negative electron affinity (NEA). Thus, many researchers turned their attention to improving the energy band structure by doping or decorating. The turn-on fields are decreased to about 1.9 and 2.6 V mm1 for ZnO nanorods decorated with Pt and Ag particles respectively.16 More recently, Hsueh et al. reported that ZnO NWAs modified with Au nanoparticles (NPs) have a smaller Eto (around 1 V mm1).17 However, these works normally use noble metal which would increase costs. Graphene, a 2D material, has attracted tremendous attention due to its extraordinary electrical, thermal, optical and mechanical properties.1,18–21 Recently, graphene was frequently reported to enhance FE properties.22–25 In addition, low-cost graphene oxide (GO) nanosheets decorated with metal nanoparticles or metal oxides have also attracted a great deal of attention due to the potential applications in many technological fields.18,22,26–32 In this work, the investigations on the FE properties of ZnO NWAs grown on Si substrates, ZnO NWAs decorated with Ag
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(Ag/ZnO), ZnO NWAs decorated with GO (GO/ZnO) and ZnO NWAs decorated with silver NPs and GO (Ag/GO/ZnO) have been carried out. ZnO NWAs show a high turn-on voltage (410 V mm1). GO decoration reduces the turn-on voltage to 2.3 V mm1. Ag NPs and GO or Ag decoration reduces the turn-on voltage significantly (1.4 V mm1).
2 Experimental The fabrication process for the ZnO NWAs, Ag/GO, GO/ZnO, Ag/GO/ZnO and Ag/ZnO nanocomposites is illustrated in Fig. 1. Before all procedures were preformed, the ZnO NWAs and Ag/GO nanocomposites were prepared. First, the ZnO film was deposited on single crystal p-Si(100) substrates (thickness: 0.5 mm) at room temperature by radio frequency (RF) magnetron sputtering technology for 10 min. The deposition parameters were base pressure 5 105 Pa, RF power 80 W, Ar flow rate 30 sccm, O2 flow rate 10 sccm, and working pressure 0.6 Pa. The film used was commercially available ZnO (ZhongNuo Advanced Material (Beijing) Technology Co., Ltd; 99.99% purity). The thickness of the film was B10 nm measured by an ellipsometer. Next, the samples were put into a 150 ml aqueous solution of zinc nitrate [Zn(NO3)2 6H2O] (0.025 M) and hexamethylenetetramine (C6H12N4), and a hydrothermal process was conducted at 95 1C for 5 h. After this reaction, the samples (ZnO NWAs) were removed from the solution, by rinsing with distilled water and drying.
Fig. 1
The GO used for the composite fabrication was obtained by the ultra-sonication of graphite oxide.33 GO dispersion was prepared by ultrasonically dispersing 0.02 g GO flakes in 50 ml distilled water for 30 min. After that, 10 ml of the aqueous solution of AgNO3 (20 mM) was added into the aqueous solution of GO, followed by magnetic stirring for 1 h. Next, the mixed solution was centrifuged with a speed of 10 000 rpm for 30 min followed by cleaning with ethanol. Finally, the precipitate was dispersed into 50 ml distilled water, transferred into a Teflon-lined stainless steel autoclave and then heated in an electric oven at 120 1C for 10 h. Then, we obtained Ag/GO nanocomposites. A Ag/GO nanocomposite dispersion of 0.5 ml was dropped onto a ZnO NWAs/Si substrate, and dried at 100 1C for 5 min. The dropping/drying process was repeated for 10 cycles to obtain the Ag/GO/ZnO composites. GO/ZnO nanocomposites were synthesised to act as a control. The GO dispersion prepared in a previous step was dropped onto a ZnO NWAs/Si substrate, and dried at 100 1C for 5 min. The dropping/drying process was repeated for 10 cycles to obtain the GO/ZnO composites. Ag/ZnO nanocomposites were also synthesised to act as a control. A ZnO NWAs/Si substrate was immersed in a 3 ml aqueous solution of 20 mM AgNO3 and illuminated with a 100 W Xe light for 10 s. The morphology and structure of the samples were characterised by field emission scanning electron microscopy (FE-SEM,
Schematic depiction of the fabrication process.
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JEOL-JSM 7001F) and high-resolution transmission electron microscopy (HRTEM, JEOL-JSM 2011). Energy-dispersive X-ray spectroscopy (EDS) was used to confirm the elements in the samples. The elemental and chemical states of the samples were evaluated by X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi, Thermofisher Scientific). Raman scattering experiments were performed using a HORIBA Jobin Yvon HR800 Spectrometer with a spectral resolution of r2 cm1 in the wave number range from 200 to 2000 cm1. Field emission properties of the samples were measured in a vacuum chamber with base pressures below 2 105 Pa. Phosphor was deposited on a transparent conductive material (indium tin-oxide), to serve as an anode electrode in the vacuum system. The distance between the sample and the anode electrode was 300 mm.
3 Results and discussion 3.1
Morphological observations
Fig. 2a and b show the typical top and cross section SEM images of ZnO NWAs. The length and the diameter of these ZnO NWAs are around 1.5 mm and 80 nm, respectively. Fig. 2c shows a typical top section SEM image of the Ag/GO nanocomposites. The white dots dispersed in the GO flakes are the Ag NPs. This has been confirmed by energy-dispersive X-ray spectroscopy (EDS) (Fig. 2d). It is clearly seen that all the peaks on the EDS curve of the composites correspond to Ag, O, Si (resulting from the substrates) and C. Fig. 3 displays the SEM and EDS mapping images of the Ag/ZnO (a–c), GO/ZnO (d–f) and Ag/GO/ZnO (g–i) composites (a, d and g: top images; b, e and h: mapping images of the top images; c, f and i: cross section images). The Ag NPs randomly filled the spaces between the NWs. This can be confirmed by the mapping image of Ag/ZnO. Due to immersion in aqueous AgNO3 solution, Ag NPs agglomerated into clusters (as shown in Fig. 3c), leading to FE properties (as discussed in section 3.3) that are better than those previously reported.16 The surface of
Fig. 2 SEM images of the ZnO NWAs for (a) the top section, and (b) the cross section. (c) A top section SEM image of the Ag/GO nanocomposites. (d) Energy-dispersive X-ray spectroscopy of the Ag/GO nanocomposites.
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the NWs led to the formation of similarly shaped and highdensity protrusions in the GO. The elements of Zn, O and C distributed uniformly. The Ag/GO nanocomposites uniformly attached to the tops of the ZnO NWAs similarly to GO. The Ag NPs partly clustered as shown by the white dots in Fig. 3g. This can be confirmed by the mapping image of Ag/GO/ZnO as circled by the red line. TEM images of the Ag/GO/ZnO composites, as shown in Fig. 4a, further confirm the results obtained from SEM observation. As seen in the bright-field image of Fig. 4a, GO flakes were connected to ZnO NWs. Notably, it can be seen that dark Ag NPs of B200 nm diameter connected to the GO flakes. Fig. 4b shows a HRTEM image of the connection (marked in Fig. 4a) of the GO composites and ZnO NWs. The surface of the ZnO NWs connected to GO flakes has a rougher morphology. The plane fringe with a crystalline plane spacing of 0.263 nm is assigned to the (0002) planes of ZnO with a hexagonal wurtzite structure. Fig. 4c and d are the selected area electron diffraction (SAED) patterns of the ZnO NWs and GO flakes marked in Fig. 4a. This result also demonstrates that the ZnO NWs covered conformally by GO flakes are single crystals with wurtzite structures with a preferential [0002] growth direction. The SAED pattern of the GO flakes shows multi-ring characteristics, indicating polycrystalline structures. 3.2
Raman and XPS analysis
Generally, Raman scattering is considered to be the most powerful nondestructive technique to study the crystalline quality, structural disorder, and defects in materials.34,35 ZnO crystallises in a wurtzite structure with C6v4 (P63mc) space group symmetry, having six Raman active optical phonon modes at the center of the first Brillouin zone.36 The frequencies of the fundamental optical modes in ZnO are E2(low) = 101 cm1, E2(high) = 437 cm1, A1(TO) = 380 cm1, A1(LO) = 574 cm1, E1(TO) = 407 cm1, and E1(LO) = 583 cm1. Fig. 5 shows the Raman spectra of the ZnO NWs, Ag/GO, Ag/ZnO, GO/ZnO and Ag/GO/ZnO composites. The peaks located at 300 cm1 and 522 cm1 correspond to Si modes. The peak observed around 950 cm1 derives from SiO2.37 The Raman spectra of ZnO NWs have been widely studied.36,38,39 On the whole, our results are comparable with the results of these works. Due to the effect of Si, some of the ZnO scattering peaks have been incorporated into the Si mode. The only observed peaks located around 435.7, 616.8, and 1044.9 cm1 are attributed to the E2 (high), A1(TA + TO) and A1(TO + LO) modes of ZnO. It is well known that GO exhibits two main characteristic peaks as a D band located at 1335 cm1 and a G band located at 1582 cm1. The G band is attributed to the doubly degenerate in-plane optical vibration (E2g mode) of the sp2 bonded carbon atoms while the D band suggests a disorder-induced mode (including sp3 defects).40 It was observed that the Ag/GO composite showed a relatively high intensity of the D to G band, whereas the intensity ratio of D/G (ID/IG) of the GO/ZnO and Ag/GO/ZnO composites decreased (the short blue line in Fig. 5 is the result magnified two times), which confirmed
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Fig. 3 SEM and EDS mapping images of the Ag/ZnO (a–c), the GO/ZnO (d–f) and the Ag/GO/ZnO (g–i) composites (a, d and g: top images; b, e and h: mapping images of the top images; c, f and i: cross section images).
Fig. 5 Raman spectra of ZnO, Ag/GO, Ag/ZnO, GO/ZnO and Ag/GO/ZnO composites.
Fig. 4 TEM characterisation of the Ag/GO/ZnO composites. (a) Bright-field image; (b) high-resolution image from area b in (a) for the surface of ZnO NWs; (c and d) selected area electron diffraction patterns from area c and d in (a) for the ZnO NWs and GO flakes, respectively.
the reduction of the GO sheets during the dropping/drying processes.41 Notably, the sample which contained Ag showed remarkably intense Raman scattering. To clarify the elemental and chemical states of the composites, XPS measurements were performed on the Ag/ZnO and the
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Ag/GO/ZnO composites (as shown in Fig. 6). All the peaks on XPS curve for the composites are ascribed to Ag, Zn, O and C (shown in Fig. 6a). It is notable that the observed Ag peak for the Ag/GO/ZnO composites was weak, indicating that their Ag content is much less than Ag/ZnO. High-resolution spectra of Zn, Ag, and C species obtained from the Ag/ZnO and the Ag/GO/ZnO composites are shown in Fig. 6b, c, and d, respectively. From Fig. 6b, it can be seen that the Zn 2p3/2 peaks have values of about 1021.5 and 1022.7 eV for Ag/ZnO and Ag/GO/ZnO respectively. The peak value for Ag/ZnO (1021.15 eV) is similar to that of pure ZnO nanorods; this finding confirms that Zn mainly exists in a Zn2+ chemical state on the sample surface.42 The Zn 2p3/2 peaks of Ag/GO/ZnO are
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Fig. 6 (a) Complete XPS spectra of Ag/ZnO and Ag/GO/ZnO composites. High-resolution spectra of samples for the elements of (b) Zn, (c) Ag, and (d) C. The inset of (c) is the high-resolution spectra of Ag/GO/ZnO for the elements Ag. The inset of (d) is the high-resolution spectra and the fitting results of Ag/GO/ZnO for the element C.
shifted to higher binding energies. This may be because the ZnO NWs reacted with the carboxyl groups (–COOH) on the surface of GO as:41 ZnO + 2R–COOH - (R–COO)2Zn2+ + H2O
(1)
Fig. 6c illustrates the XPS spectra of Ag 3d regions of the Ag/ZnO and Ag/GO/ZnO composites. The Ag 3d peaks for bulk Ag locate at 368.2 and 374.2 eV for Ag 3d5/2 and Ag 3d3/2, respectively.42 For Ag/ZnO, the Ag 3d peaks shift remarkably to lower binding energies (Ag 3d5/2, 367.6 eV; Ag 3d3/2, 373.5 eV); this phenomenon is similar to results obtained from dendritelike ZnO@Ag heterostructures,43 Ag–ZnO heterostructure nanofibers,44 and worm-like Ag/ZnO core–shell heterostructures.42 Attributed to the electron transfer from metallic Ag to ZnO crystals, the binding energy for Ag shifts to low energy. The Fermi levels of the two components equilibrate when the metal particles come into contact with a semiconductor, such as Au/ZnO or Ag/ZnO heterostructures. Accordingly, when Ag (work function = 4.26 eV) is attached to ZnO (work function = 5.3 eV), some of the electrons transfer from Ag to ZnO at the interfaces of the Ag/ZnO composites, resulting in a higher
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valence of Ag. The binding energy of univalent Ag is lower than that of zero-valent Ag; therefore, the shift to low binding energies of the Ag 3d5/2 and Ag 3d3/2 peaks further verifies the formation of Ag/ZnO composites. However, for Ag/GO/ZnO the Ag 3d peak positions are similar to bulk Ag (Ag 3d5/2, 368.2 eV; Ag 3d3/2, 374.2 eV) as shown in the inset of Fig. 6c. This result implies that the Ag dispersed in GO is not in direct contact with ZnO and that Ag exists as nanoparticles. Moreover, the content of Ag in Ag/GO/ZnO (0.58 at%) is much lower than in the Ag/ZnO composites (7.91 at%). Fig. 6d displays the XPS spectra of C 1s regions of the Ag/ZnO and Ag/GO/ZnO composites. It is clearly seen that the C 1s peak has a value of about 284.7 eV for Ag/ZnO. Meanwhile, there is an asymmetric peak for Ag/GO/ZnO. Moreover, careful examination reveals that two sets of humps appear at the high binding energy wing of the peak. To extract their peak positions more accurately, three Gaussian peaks were used to fit the experimental data of C 1s for Ag/GO/ZnO. The resultant Gaussian peaks and the comparison between the fitting result and the experimental data are shown in the inset of Fig. 6d. It can be seen that the fitting result is reasonable. The peaks have values of about 284.5, 285.8 and 287.9 eV corresponding to C–C, C–O and CQO chemical bonds.26
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Table 1 Comparative table of the field emission properties of modified ZnO NWs
Material (NWAs)
Turn-on field (V mm1)
Current density (mA cm2)
ZnO14,15 Orderly ZnO14,15 Pt/ZnO16 Ag/ZnO16 Au/ZnO17 Graphene/ZnO22 Ag/ZnO in present work Ag/GO/ZnO in present work
12 2.4 1.9 2.6 2 5.4 1.4 1.4
0.1 0.1 10 10 B1 10 1 1
b 5750 29 450 1100 7414 7018
Fig. 7 The J–E behavior of the ZnO, Ag/GO, Ag/ZnO, GO/ZnO and Ag/GO/ZnO composites. The inset is the F–N plots of the Ag/ZnO and Ag/GO/ZnO composites. The solid lines are the fitting results.
3.3
Field emission analysis
Fig. 7 displays the curves of FE current density versus applied electric field of ZnO NWs, Ag/ZnO, Ag/GO/ZnO, Ag/GO and GO/ZnO nanocomposites. The Ag/ZnO and Ag/GO/ZnO NWs have lower turn-on fields (around 1.4 V mm1), followed by GO/ZnO (around 2.3 V mm1). The higher turn-on field of the ZnO NWs (410 V mm1) was beyond the measurement range of our equipment.17 Additionally, FE capacity was not observed in Ag/GO nanocomposites. Generally, the FE characteristics of the materials were analyzed using the Fowler–Nordheim (FN) equation: 2 2 b E BF3=2 J¼A exp (2) F bE where J is the current density, E is the applied electric field, b is the effective field enhancement factor, A = 1.56 106 (A eV V2), B = 6.83 103 (V mm1 eV2/3), and F is the work function of the ZnO NWs (eV), whose value is around 5.37 eV.17 The F–N equation can be used to describe the linear relationship between ln( J/E2) and 1/E. 2 BF3=2 Ab 2 ln J=E ¼ ln (3) bE F ln( J/E2) was plotted as a function of 1/E, as shown in the inset of Fig. 7. The two straight lines representing the two gradients have slopes that are equal to BF3/2/b. The calculated b values are B7414 and B7018 for ZnO NWs with Ag NPs and ZnO NWs with Ag/GO nanocomposites, respectively. A comparison of the FE properties of our results with the other published work on modified 1D ZnO nanostructure arrays is provided in Table 1. We can see that the turn-on field and b obtained in our study is better than most of the values. Furthermore, the FE performances of decorated ZnO NWAs are much better than those of pure ZnO NWAs. This is a result of oxygen molecules (O2) absorbed on the surface of the ZnO NWAs capturing free electrons (e) from the ZnO NWAs and becoming (O2), i.e., O2 + e - O2.17 Even in a high vacuum (105) chamber, this reaction can happen and result in a
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Fig. 8 The proposed energy band structure and FE mechanism.
depletion region near the surface of the ZnO NWAs. Thus, a potential barrier is built and heightens the turn-on field. The structure could change the energy band structure.5,24,45 With Ag NP modification, some of the electrons transfer from Ag to ZnO at the interfaces and replenish the loss of electrons on the surface. Moreover, due to the attachment of Ag (work function = 4.26 eV) to ZnO (work function = 5.3 eV), the Fermi levels of the ZnO NWs are lifted and the two components equilibrate when the metal particles come into contact with the semiconductor. Meanwhile, an energy well forms on the surfaces of Ag/ZnO which collects electrons when an electric field is applied. Hence, Ag NPs modification can improve the FE properties. Except for the improvement of the energy structure, the improvement of the FE properties with GO or Ag/GO decoration can also be attributed to the tunneling effect of electrons through the heterojunction.46 Besides, the GO or Ag/GO sheets on ZnO NWs have lower turn-on fields, because such structures offer more contact points and protrusions between the ZnO NWs and GO or Ag/GO, which leads to easier electron transfer. Fig. 8 shows the proposed mechanisms.
4 Conclusions This work shows a new method to perfect the FE properties of semiconductors modified by low-cost GO nanosheets and trace amounts of noble metal. The morphology, elemental and chemical states of ZnO NWAs, Ag/GO, Ag/ZnO, GO/ZnO and Ag/GO/ZnO composites were confirmed by SEM, TEM, XPS and Raman tests. XPS results have shown that the ZnO NWAs reacted with carboxyl groups (–COOH) on the surface of GO and Ag nanoparticles dispersed on GO are not in direct contact with ZnO. The XPS results also indicated that electrons transfer from metallic Ag to ZnO in the Ag/ZnO composites. The Ag/ZnO
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and the Ag/GO/ZnO composite emitters exhibited efficient FE with low turn-on fields of 1.4 and 1.4 V mm1 and high field enhancement factors of 7414 and 7018, respectively. This is because an energy well forms on the surface of Ag/ZnO, which collects electrons when an electric field is applied. For the Ag/GO/ZnO composites, the excellent FE properties can be attributed to the tunneling effect of electrons through the Ag/GO/ZnO heterojunction. Besides, the GO or Ag/GO sheets on ZnO NWs have lower turn-on fields, because such structures offer more contact points and protrusions between the ZnO NWs and GO or Ag/GO, which leads to easier electron transfer. It is also notable that the Ag/GO/ZnO composites have shown slightly better FE properties compared with the Ag/ZnO composites.
Acknowledgements The authors are grateful for the financial support of the National Science and Technology Major Project, and the Natural Science Foundation of China (under Grant No. 61176003).
References 1 J. Wu, X. P. Shen, L. Jiang, K. Wang and K. M. Chen, Appl. Surf. Sci., 2010, 256, 2826–2830. 2 K. K. Naik, R. Khare, D. Chakravarty, M. A. More, R. Thapa, D. J. Late and C. S. Rout, Appl. Phys. Lett., 2014, 105, 233101. 3 S. S. Lv, Z. C. Li, S. M. Su, L. H. Lin, Z. J. Zhang and W. Miao, RSC Adv., 2014, 4, 31729–31734. 4 S. S. Lv, Z. C. Li, J. C. Liao, Z. J. Zhang and W. Miao, J. Vac. Sci. Technol., B: Nanotechnol. Microelectron.: Mater., Process., Meas., Phenom., 2015, 33, 02B101. 5 S. S. Lv, Z. C. Li, C. H. Chen, J. C. Liao, G. J. Wang, M. Y. Li and W. Miao, ACS Appl. Mater. Interfaces, 2015, 7, 13564–13568. 6 Z. R. Dai, Z. W. Pan and Z. L. Wang, Adv. Funct. Mater., 2003, 13, 9–24. 7 Z. G. Bai, D. P. Yu, H. Z. Zhang, Y. Ding, Y. P. Wang, X. Z. Gai, Q. L. Hang, G. C. Xiong and S. Q. Feng, Chem. Phys. Lett., 1999, 303, 311–314. 8 B. R. Huang, T. C. Lin, K. T. Chu, Y. K. Yang and J. C. Lin, Surf. Coat. Technol., 2013, 231, 289–292. 9 R. R. Devarapalli, D. R. Shinde, B.-B. Fatiha, S. G. Yenchalwar, R. Boukherroub, M. A. Moreb and M. V. Shelke, J. Mater. Chem., 2012, 22, 22922–22928. 10 C. X. Zhao, K. Huang, S. Z. Deng, N. S. Xu and J. Chen, Appl. Surf. Sci., 2013, 270, 82–89. 11 W. B. Zou, J. W. Zhu, Y. X. Sun and X. Wang, Mater. Chem. Phys., 2011, 125, 617–620. 12 P. Zhan, W. P. Wang, Z. Xie, Z. C. Li, Z. J. Zhang, P. Zhang, B. Y. Wang and X. Z. Cao, Appl. Phys. Lett., 2012, 101, 031913. 13 X. D. Wang, J. Zhou, C. S. Lao, J. H. Song, N. S. Xu and Z. L. Wang, Adv. Mater., 2007, 19, 1627–1631.
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14 H. Zeng, X. J. Xu, Y. Bando, U. K. Gautam, T. Y. Zhai, X. S. Fang, B. D. Liu and D. Golberg, Adv. Funct. Mater., 2009, 19, 3165–3172. 15 T. Y. Zhai, L. Li, Y. Ma, M. Y. Liao, X. Wang, X. S. Fang, J. N. Yao, Y. Bandoa and D. Golberga, Chem. Soc. Rev., 2011, 40, 2986–3004. 16 C. H. Ye, Y. Bando, X. S. Fang, G. Z. Shen and D. Golberg, J. Phys. Chem. C, 2007, 111, 12673–12676. 17 S. Y. Lin, S. J. Chang and T. J. Hsueh, ECS J. Solid State Sci. Technol., 2013, 2, N149–N151. 18 J. Ding, X. Yan and Q. Xue, Mater. Chem. Phys., 2012, 133, 405–409. 19 C. X. Wu, F. S. Li, Y. A. Zhang and T. L. Guo, Carbon, 2012, 50, 3622–3626. 20 K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666–669. 21 Z. C. Yang, Q. Zhao, Y. X. Ou, W. Wang, H. Li and D. P. Yu, Appl. Phys. Lett., 2012, 101, 173107. 22 J. M. Lee, Y. B. Pyun, J. Yi, J. W. Choung and W. I. Park, J. Phys. Chem. C, 2009, 113, 19134–19138. 23 W. T. Zheng, Y. M. Ho, H. W. Tian, M. Wen, J. L. Qi and Y. A. Li, J. Phys. Chem. C, 2009, 113, 9164–9168. 24 S. S. Lv, Z. C. Li, J. C. Liao, G. J. Wang, M. Y. Li and W. Miao, Sci. Rep., 2015, 5, 15035. 25 C. X. Wu, F. S. Li, Y. A. i. Zhang and T. L. Guo, Carbon, 2012, 50, 3622–3626. 26 G. Williams, B. Seger and P. V. Kamat, ACS Nano, 2008, 2, 1487–1491. 27 R. Muszynski, B. Seger and P. V. Kamat, J. Phys. Chem. C, 2008, 112, 5263–5266. 28 J. Yao, X. P. Shen, B. Wang, H. K. Liu and G. X. Wang, Electrochem. Commun., 2009, 11, 1849–1852. 29 W. T. Zheng, Y. M. Ho, H. W. Tian, M. Wen, J. L. Qi and Y. A. Li, J. Phys. Chem. C, 2009, 113, 9164–9168. 30 C. Xu, X. Wang and J. W. Zhu, J. Phys. Chem. C, 2008, 112, 19841–19845. 31 Y. P. Zhang, H. B. Li, L. K. Pan, T. Lu and Z. Sun, J. Electroanal. Chem., 2009, 634, 68–71. 32 C. Xu, X. Wang, L. C. Yang and Y. P. Wu, J. Solid State Chem., 2009, 182, 2486–2490. 33 Z. Y. Guo, M. X. Wang, Z.-H. Huang and F. Y. Kang, Carbon, 2015, 87, 282–291. 34 X. Z. Sun, R. Tao, L. H. Lin, Z. C. Li, Z. J. Zhang and J. Y. Feng, Appl. Surf. Sci., 2011, 257, 3861–3866. 35 X. Z. Sun, L. H. Lin, Z. C. Li, Z. J. Zhang and J. Y. Feng, Appl. Surf. Sci., 2009, 256, 916–920. 36 Q. Zhou, X. Zhang, Y. Huang, Z. C. Li, Y. P. Zhao and Z. J. Zhang, Appl. Phys. Lett., 2012, 100, 113101. 37 M. Z. M. Yusoff, A. Mahyuddin, Z. Hassan, H. A. Hassan, M. J. Abdullah, M. Rusop, S. M. Mohammad and N. M. Ahmed, Mater. Sci. Semicond. Process., 2015, 29, 231–237. 38 S. Khosravi-Gandomania, R. Yousefi, F. Jamali-Sheinic and N. M. Huang, Ceram. Int., 2014, 40, 7957–7963. 39 H. Q. Bian, S. Y. Ma, Z. M. Zhang, J. M. Gao and H. B. Zhu, J. Cryst. Growth, 2014, 394, 132–136.
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40 H. J. Jeong, H. Y. Kim, H. D. Jeong, S. Y. Jeong, J. T. Han and G.-W. Lee, J. Mater. Chem., 2012, 22, 11277–11283. 41 R. C. Wang, Y. C. Chen, S. J. Chen and Y. M. Chang, Carbon, 2014, 70, 215–223. 42 H. R. Liu, G. X. Shao, J. F. Zhao, Z. X. Zhang, Y. Zhang, J. Liang, X. G. Liu, H. S. Jia and B. S. Xu, J. Phys. Chem. C, 2012, 116, 16182–16190.
This journal is © the Owner Societies 2015
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43 C. D. Gu, C. Cheng, H. Y. Huang, T. L. Wong, N. Wang and T. Y. Zhang, Cryst. Growth Des., 2009, 9, 3278–3285. 44 D. D. Lin, H. Wu, R. Zhang and W. Pan, Chem. Mater., 2009, 21, 3479–3484. 45 L. H. Lin, Z. C. Li, J. Y. Feng and Z. J. Zhang, Phys. Chem. Chem. Phys., 2013, 15, 6063–6067. 46 L. Sun, X. T. Zhou, Y. A. Zhang and T. L. Guo, J. Alloys Compd., 2015, 632, 604–608.
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Cite this: Phys. Chem. Chem. Phys., 2015, 17, 31822
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Enhanced field-emission of silver nanoparticle– graphene oxide decorated ZnO nanowire arrays Guojing Wang,a Zhengcao Li,*ab Mingyang Li,ac Jiecui Liao,a Chienhua Chen,a Shasha Lva and Chuanqing Shia This work presents a new method to improve the field emission (FE) properties of semiconductors decorated with low-cost graphene oxide (GO) nanosheets and trace amounts of noble metal. The Ag/GO/ZnO composite emitter exhibited efficient FE properties with a low turn-on field of 1.4 V mm1 and a high field enhancement factor of 7018. The excellent FE properties of the Ag/GO/ZnO composite can be attributed to the tunneling effect of electrons through the heterojunction. The FE properties of the Ag/GO/ZnO composite are slightly better than those of the Ag/ZnO composite which forms an
Received 24th August 2015, Accepted 30th October 2015
energy well that collects electrons on interfaces when an electric field is applied. This behavior is
DOI: 10.1039/c5cp05036g
arrays (NWAs) and Ag/GO, which leads to easier electron transfer. High-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) were employed to characterise the
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connection and evolution of the ZnO NWAs and Ag/GO composites.
associated with heterostructures that offer more contact points and protrusions between ZnO nanowire
1 Introduction Due to the wide employment of high resolution electron microscopes, the fabrication of flat panel displays, and the process of discharging spacecraft from induced charges,1,2 field electron emission, also called field emission (FE), have attracted considerable attention in recent years. One-dimensional (1D) semiconductor nanostructures, with interesting photonic and electronic properties, and used as important emitter tips, are one focus in FE research. There are numerous studies on nanotips and nanowires (NWs) of Si, III–V systems and oxide systems,3–5 including SnO2,6 GeO2,7 ZnO/ZnWO4,8 and ZnO NWs.9,10 Among these semiconductors, ZnO11,12 is a wide band gap semiconductor that is nontoxic, inexpensive and has excellent physical, chemical and biocompatibility properties. According to previous theoretical and experimental research, vertically aligned 1D ZnO nanostructure arrays show better FE performances than disordered nanostructures.13 The performances of 1D ZnO nanostructure arrays are also affected by many other factors, including the uniformity, density, and taper morphology of the emitters, which are closely related to the local emitting intensity and the interaction among emitters.13
a
State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China. E-mail: [email protected] b Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China c Department of Engineering Physics, Tsinghua University, Beijing 100084, China
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However, so far, the ability to control 1D ZnO nanostructure arrays and the further improvement of their FE performances are still problems that urgently need to be solved. Recently, Zhai et al. developed an effective route (with the help of a self-assembled template) towards the full control of ZnO nanowire arrays (NWAs) with the turn-on field (Eto) first decreasing sharply from 12 V mm1 to 2.4 V mm1.14,15 The nature of FE is that electrons near the Fermi level are emitted into vacuum by quantum mechanical tunneling from a conducting/semiconducting emitter with the application of a very high external electric field (B105 V cm1), and the FE properties are closely linked with the work function or the negative electron affinity (NEA). Thus, many researchers turned their attention to improving the energy band structure by doping or decorating. The turn-on fields are decreased to about 1.9 and 2.6 V mm1 for ZnO nanorods decorated with Pt and Ag particles respectively.16 More recently, Hsueh et al. reported that ZnO NWAs modified with Au nanoparticles (NPs) have a smaller Eto (around 1 V mm1).17 However, these works normally use noble metal which would increase costs. Graphene, a 2D material, has attracted tremendous attention due to its extraordinary electrical, thermal, optical and mechanical properties.1,18–21 Recently, graphene was frequently reported to enhance FE properties.22–25 In addition, low-cost graphene oxide (GO) nanosheets decorated with metal nanoparticles or metal oxides have also attracted a great deal of attention due to the potential applications in many technological fields.18,22,26–32 In this work, the investigations on the FE properties of ZnO NWAs grown on Si substrates, ZnO NWAs decorated with Ag
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(Ag/ZnO), ZnO NWAs decorated with GO (GO/ZnO) and ZnO NWAs decorated with silver NPs and GO (Ag/GO/ZnO) have been carried out. ZnO NWAs show a high turn-on voltage (410 V mm1). GO decoration reduces the turn-on voltage to 2.3 V mm1. Ag NPs and GO or Ag decoration reduces the turn-on voltage significantly (1.4 V mm1).
2 Experimental The fabrication process for the ZnO NWAs, Ag/GO, GO/ZnO, Ag/GO/ZnO and Ag/ZnO nanocomposites is illustrated in Fig. 1. Before all procedures were preformed, the ZnO NWAs and Ag/GO nanocomposites were prepared. First, the ZnO film was deposited on single crystal p-Si(100) substrates (thickness: 0.5 mm) at room temperature by radio frequency (RF) magnetron sputtering technology for 10 min. The deposition parameters were base pressure 5 105 Pa, RF power 80 W, Ar flow rate 30 sccm, O2 flow rate 10 sccm, and working pressure 0.6 Pa. The film used was commercially available ZnO (ZhongNuo Advanced Material (Beijing) Technology Co., Ltd; 99.99% purity). The thickness of the film was B10 nm measured by an ellipsometer. Next, the samples were put into a 150 ml aqueous solution of zinc nitrate [Zn(NO3)2 6H2O] (0.025 M) and hexamethylenetetramine (C6H12N4), and a hydrothermal process was conducted at 95 1C for 5 h. After this reaction, the samples (ZnO NWAs) were removed from the solution, by rinsing with distilled water and drying.
Fig. 1
The GO used for the composite fabrication was obtained by the ultra-sonication of graphite oxide.33 GO dispersion was prepared by ultrasonically dispersing 0.02 g GO flakes in 50 ml distilled water for 30 min. After that, 10 ml of the aqueous solution of AgNO3 (20 mM) was added into the aqueous solution of GO, followed by magnetic stirring for 1 h. Next, the mixed solution was centrifuged with a speed of 10 000 rpm for 30 min followed by cleaning with ethanol. Finally, the precipitate was dispersed into 50 ml distilled water, transferred into a Teflon-lined stainless steel autoclave and then heated in an electric oven at 120 1C for 10 h. Then, we obtained Ag/GO nanocomposites. A Ag/GO nanocomposite dispersion of 0.5 ml was dropped onto a ZnO NWAs/Si substrate, and dried at 100 1C for 5 min. The dropping/drying process was repeated for 10 cycles to obtain the Ag/GO/ZnO composites. GO/ZnO nanocomposites were synthesised to act as a control. The GO dispersion prepared in a previous step was dropped onto a ZnO NWAs/Si substrate, and dried at 100 1C for 5 min. The dropping/drying process was repeated for 10 cycles to obtain the GO/ZnO composites. Ag/ZnO nanocomposites were also synthesised to act as a control. A ZnO NWAs/Si substrate was immersed in a 3 ml aqueous solution of 20 mM AgNO3 and illuminated with a 100 W Xe light for 10 s. The morphology and structure of the samples were characterised by field emission scanning electron microscopy (FE-SEM,
Schematic depiction of the fabrication process.
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JEOL-JSM 7001F) and high-resolution transmission electron microscopy (HRTEM, JEOL-JSM 2011). Energy-dispersive X-ray spectroscopy (EDS) was used to confirm the elements in the samples. The elemental and chemical states of the samples were evaluated by X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi, Thermofisher Scientific). Raman scattering experiments were performed using a HORIBA Jobin Yvon HR800 Spectrometer with a spectral resolution of r2 cm1 in the wave number range from 200 to 2000 cm1. Field emission properties of the samples were measured in a vacuum chamber with base pressures below 2 105 Pa. Phosphor was deposited on a transparent conductive material (indium tin-oxide), to serve as an anode electrode in the vacuum system. The distance between the sample and the anode electrode was 300 mm.
3 Results and discussion 3.1
Morphological observations
Fig. 2a and b show the typical top and cross section SEM images of ZnO NWAs. The length and the diameter of these ZnO NWAs are around 1.5 mm and 80 nm, respectively. Fig. 2c shows a typical top section SEM image of the Ag/GO nanocomposites. The white dots dispersed in the GO flakes are the Ag NPs. This has been confirmed by energy-dispersive X-ray spectroscopy (EDS) (Fig. 2d). It is clearly seen that all the peaks on the EDS curve of the composites correspond to Ag, O, Si (resulting from the substrates) and C. Fig. 3 displays the SEM and EDS mapping images of the Ag/ZnO (a–c), GO/ZnO (d–f) and Ag/GO/ZnO (g–i) composites (a, d and g: top images; b, e and h: mapping images of the top images; c, f and i: cross section images). The Ag NPs randomly filled the spaces between the NWs. This can be confirmed by the mapping image of Ag/ZnO. Due to immersion in aqueous AgNO3 solution, Ag NPs agglomerated into clusters (as shown in Fig. 3c), leading to FE properties (as discussed in section 3.3) that are better than those previously reported.16 The surface of
Fig. 2 SEM images of the ZnO NWAs for (a) the top section, and (b) the cross section. (c) A top section SEM image of the Ag/GO nanocomposites. (d) Energy-dispersive X-ray spectroscopy of the Ag/GO nanocomposites.
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the NWs led to the formation of similarly shaped and highdensity protrusions in the GO. The elements of Zn, O and C distributed uniformly. The Ag/GO nanocomposites uniformly attached to the tops of the ZnO NWAs similarly to GO. The Ag NPs partly clustered as shown by the white dots in Fig. 3g. This can be confirmed by the mapping image of Ag/GO/ZnO as circled by the red line. TEM images of the Ag/GO/ZnO composites, as shown in Fig. 4a, further confirm the results obtained from SEM observation. As seen in the bright-field image of Fig. 4a, GO flakes were connected to ZnO NWs. Notably, it can be seen that dark Ag NPs of B200 nm diameter connected to the GO flakes. Fig. 4b shows a HRTEM image of the connection (marked in Fig. 4a) of the GO composites and ZnO NWs. The surface of the ZnO NWs connected to GO flakes has a rougher morphology. The plane fringe with a crystalline plane spacing of 0.263 nm is assigned to the (0002) planes of ZnO with a hexagonal wurtzite structure. Fig. 4c and d are the selected area electron diffraction (SAED) patterns of the ZnO NWs and GO flakes marked in Fig. 4a. This result also demonstrates that the ZnO NWs covered conformally by GO flakes are single crystals with wurtzite structures with a preferential [0002] growth direction. The SAED pattern of the GO flakes shows multi-ring characteristics, indicating polycrystalline structures. 3.2
Raman and XPS analysis
Generally, Raman scattering is considered to be the most powerful nondestructive technique to study the crystalline quality, structural disorder, and defects in materials.34,35 ZnO crystallises in a wurtzite structure with C6v4 (P63mc) space group symmetry, having six Raman active optical phonon modes at the center of the first Brillouin zone.36 The frequencies of the fundamental optical modes in ZnO are E2(low) = 101 cm1, E2(high) = 437 cm1, A1(TO) = 380 cm1, A1(LO) = 574 cm1, E1(TO) = 407 cm1, and E1(LO) = 583 cm1. Fig. 5 shows the Raman spectra of the ZnO NWs, Ag/GO, Ag/ZnO, GO/ZnO and Ag/GO/ZnO composites. The peaks located at 300 cm1 and 522 cm1 correspond to Si modes. The peak observed around 950 cm1 derives from SiO2.37 The Raman spectra of ZnO NWs have been widely studied.36,38,39 On the whole, our results are comparable with the results of these works. Due to the effect of Si, some of the ZnO scattering peaks have been incorporated into the Si mode. The only observed peaks located around 435.7, 616.8, and 1044.9 cm1 are attributed to the E2 (high), A1(TA + TO) and A1(TO + LO) modes of ZnO. It is well known that GO exhibits two main characteristic peaks as a D band located at 1335 cm1 and a G band located at 1582 cm1. The G band is attributed to the doubly degenerate in-plane optical vibration (E2g mode) of the sp2 bonded carbon atoms while the D band suggests a disorder-induced mode (including sp3 defects).40 It was observed that the Ag/GO composite showed a relatively high intensity of the D to G band, whereas the intensity ratio of D/G (ID/IG) of the GO/ZnO and Ag/GO/ZnO composites decreased (the short blue line in Fig. 5 is the result magnified two times), which confirmed
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Fig. 3 SEM and EDS mapping images of the Ag/ZnO (a–c), the GO/ZnO (d–f) and the Ag/GO/ZnO (g–i) composites (a, d and g: top images; b, e and h: mapping images of the top images; c, f and i: cross section images).
Fig. 5 Raman spectra of ZnO, Ag/GO, Ag/ZnO, GO/ZnO and Ag/GO/ZnO composites.
Fig. 4 TEM characterisation of the Ag/GO/ZnO composites. (a) Bright-field image; (b) high-resolution image from area b in (a) for the surface of ZnO NWs; (c and d) selected area electron diffraction patterns from area c and d in (a) for the ZnO NWs and GO flakes, respectively.
the reduction of the GO sheets during the dropping/drying processes.41 Notably, the sample which contained Ag showed remarkably intense Raman scattering. To clarify the elemental and chemical states of the composites, XPS measurements were performed on the Ag/ZnO and the
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Ag/GO/ZnO composites (as shown in Fig. 6). All the peaks on XPS curve for the composites are ascribed to Ag, Zn, O and C (shown in Fig. 6a). It is notable that the observed Ag peak for the Ag/GO/ZnO composites was weak, indicating that their Ag content is much less than Ag/ZnO. High-resolution spectra of Zn, Ag, and C species obtained from the Ag/ZnO and the Ag/GO/ZnO composites are shown in Fig. 6b, c, and d, respectively. From Fig. 6b, it can be seen that the Zn 2p3/2 peaks have values of about 1021.5 and 1022.7 eV for Ag/ZnO and Ag/GO/ZnO respectively. The peak value for Ag/ZnO (1021.15 eV) is similar to that of pure ZnO nanorods; this finding confirms that Zn mainly exists in a Zn2+ chemical state on the sample surface.42 The Zn 2p3/2 peaks of Ag/GO/ZnO are
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Fig. 6 (a) Complete XPS spectra of Ag/ZnO and Ag/GO/ZnO composites. High-resolution spectra of samples for the elements of (b) Zn, (c) Ag, and (d) C. The inset of (c) is the high-resolution spectra of Ag/GO/ZnO for the elements Ag. The inset of (d) is the high-resolution spectra and the fitting results of Ag/GO/ZnO for the element C.
shifted to higher binding energies. This may be because the ZnO NWs reacted with the carboxyl groups (–COOH) on the surface of GO as:41 ZnO + 2R–COOH - (R–COO)2Zn2+ + H2O
(1)
Fig. 6c illustrates the XPS spectra of Ag 3d regions of the Ag/ZnO and Ag/GO/ZnO composites. The Ag 3d peaks for bulk Ag locate at 368.2 and 374.2 eV for Ag 3d5/2 and Ag 3d3/2, respectively.42 For Ag/ZnO, the Ag 3d peaks shift remarkably to lower binding energies (Ag 3d5/2, 367.6 eV; Ag 3d3/2, 373.5 eV); this phenomenon is similar to results obtained from dendritelike ZnO@Ag heterostructures,43 Ag–ZnO heterostructure nanofibers,44 and worm-like Ag/ZnO core–shell heterostructures.42 Attributed to the electron transfer from metallic Ag to ZnO crystals, the binding energy for Ag shifts to low energy. The Fermi levels of the two components equilibrate when the metal particles come into contact with a semiconductor, such as Au/ZnO or Ag/ZnO heterostructures. Accordingly, when Ag (work function = 4.26 eV) is attached to ZnO (work function = 5.3 eV), some of the electrons transfer from Ag to ZnO at the interfaces of the Ag/ZnO composites, resulting in a higher
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valence of Ag. The binding energy of univalent Ag is lower than that of zero-valent Ag; therefore, the shift to low binding energies of the Ag 3d5/2 and Ag 3d3/2 peaks further verifies the formation of Ag/ZnO composites. However, for Ag/GO/ZnO the Ag 3d peak positions are similar to bulk Ag (Ag 3d5/2, 368.2 eV; Ag 3d3/2, 374.2 eV) as shown in the inset of Fig. 6c. This result implies that the Ag dispersed in GO is not in direct contact with ZnO and that Ag exists as nanoparticles. Moreover, the content of Ag in Ag/GO/ZnO (0.58 at%) is much lower than in the Ag/ZnO composites (7.91 at%). Fig. 6d displays the XPS spectra of C 1s regions of the Ag/ZnO and Ag/GO/ZnO composites. It is clearly seen that the C 1s peak has a value of about 284.7 eV for Ag/ZnO. Meanwhile, there is an asymmetric peak for Ag/GO/ZnO. Moreover, careful examination reveals that two sets of humps appear at the high binding energy wing of the peak. To extract their peak positions more accurately, three Gaussian peaks were used to fit the experimental data of C 1s for Ag/GO/ZnO. The resultant Gaussian peaks and the comparison between the fitting result and the experimental data are shown in the inset of Fig. 6d. It can be seen that the fitting result is reasonable. The peaks have values of about 284.5, 285.8 and 287.9 eV corresponding to C–C, C–O and CQO chemical bonds.26
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Table 1 Comparative table of the field emission properties of modified ZnO NWs
Material (NWAs)
Turn-on field (V mm1)
Current density (mA cm2)
ZnO14,15 Orderly ZnO14,15 Pt/ZnO16 Ag/ZnO16 Au/ZnO17 Graphene/ZnO22 Ag/ZnO in present work Ag/GO/ZnO in present work
12 2.4 1.9 2.6 2 5.4 1.4 1.4
0.1 0.1 10 10 B1 10 1 1
b 5750 29 450 1100 7414 7018
Fig. 7 The J–E behavior of the ZnO, Ag/GO, Ag/ZnO, GO/ZnO and Ag/GO/ZnO composites. The inset is the F–N plots of the Ag/ZnO and Ag/GO/ZnO composites. The solid lines are the fitting results.
3.3
Field emission analysis
Fig. 7 displays the curves of FE current density versus applied electric field of ZnO NWs, Ag/ZnO, Ag/GO/ZnO, Ag/GO and GO/ZnO nanocomposites. The Ag/ZnO and Ag/GO/ZnO NWs have lower turn-on fields (around 1.4 V mm1), followed by GO/ZnO (around 2.3 V mm1). The higher turn-on field of the ZnO NWs (410 V mm1) was beyond the measurement range of our equipment.17 Additionally, FE capacity was not observed in Ag/GO nanocomposites. Generally, the FE characteristics of the materials were analyzed using the Fowler–Nordheim (FN) equation: 2 2 b E BF3=2 J¼A exp (2) F bE where J is the current density, E is the applied electric field, b is the effective field enhancement factor, A = 1.56 106 (A eV V2), B = 6.83 103 (V mm1 eV2/3), and F is the work function of the ZnO NWs (eV), whose value is around 5.37 eV.17 The F–N equation can be used to describe the linear relationship between ln( J/E2) and 1/E. 2 BF3=2 Ab 2 ln J=E ¼ ln (3) bE F ln( J/E2) was plotted as a function of 1/E, as shown in the inset of Fig. 7. The two straight lines representing the two gradients have slopes that are equal to BF3/2/b. The calculated b values are B7414 and B7018 for ZnO NWs with Ag NPs and ZnO NWs with Ag/GO nanocomposites, respectively. A comparison of the FE properties of our results with the other published work on modified 1D ZnO nanostructure arrays is provided in Table 1. We can see that the turn-on field and b obtained in our study is better than most of the values. Furthermore, the FE performances of decorated ZnO NWAs are much better than those of pure ZnO NWAs. This is a result of oxygen molecules (O2) absorbed on the surface of the ZnO NWAs capturing free electrons (e) from the ZnO NWAs and becoming (O2), i.e., O2 + e - O2.17 Even in a high vacuum (105) chamber, this reaction can happen and result in a
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Fig. 8 The proposed energy band structure and FE mechanism.
depletion region near the surface of the ZnO NWAs. Thus, a potential barrier is built and heightens the turn-on field. The structure could change the energy band structure.5,24,45 With Ag NP modification, some of the electrons transfer from Ag to ZnO at the interfaces and replenish the loss of electrons on the surface. Moreover, due to the attachment of Ag (work function = 4.26 eV) to ZnO (work function = 5.3 eV), the Fermi levels of the ZnO NWs are lifted and the two components equilibrate when the metal particles come into contact with the semiconductor. Meanwhile, an energy well forms on the surfaces of Ag/ZnO which collects electrons when an electric field is applied. Hence, Ag NPs modification can improve the FE properties. Except for the improvement of the energy structure, the improvement of the FE properties with GO or Ag/GO decoration can also be attributed to the tunneling effect of electrons through the heterojunction.46 Besides, the GO or Ag/GO sheets on ZnO NWs have lower turn-on fields, because such structures offer more contact points and protrusions between the ZnO NWs and GO or Ag/GO, which leads to easier electron transfer. Fig. 8 shows the proposed mechanisms.
4 Conclusions This work shows a new method to perfect the FE properties of semiconductors modified by low-cost GO nanosheets and trace amounts of noble metal. The morphology, elemental and chemical states of ZnO NWAs, Ag/GO, Ag/ZnO, GO/ZnO and Ag/GO/ZnO composites were confirmed by SEM, TEM, XPS and Raman tests. XPS results have shown that the ZnO NWAs reacted with carboxyl groups (–COOH) on the surface of GO and Ag nanoparticles dispersed on GO are not in direct contact with ZnO. The XPS results also indicated that electrons transfer from metallic Ag to ZnO in the Ag/ZnO composites. The Ag/ZnO
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and the Ag/GO/ZnO composite emitters exhibited efficient FE with low turn-on fields of 1.4 and 1.4 V mm1 and high field enhancement factors of 7414 and 7018, respectively. This is because an energy well forms on the surface of Ag/ZnO, which collects electrons when an electric field is applied. For the Ag/GO/ZnO composites, the excellent FE properties can be attributed to the tunneling effect of electrons through the Ag/GO/ZnO heterojunction. Besides, the GO or Ag/GO sheets on ZnO NWs have lower turn-on fields, because such structures offer more contact points and protrusions between the ZnO NWs and GO or Ag/GO, which leads to easier electron transfer. It is also notable that the Ag/GO/ZnO composites have shown slightly better FE properties compared with the Ag/ZnO composites.
Acknowledgements The authors are grateful for the financial support of the National Science and Technology Major Project, and the Natural Science Foundation of China (under Grant No. 61176003).
References 1 J. Wu, X. P. Shen, L. Jiang, K. Wang and K. M. Chen, Appl. Surf. Sci., 2010, 256, 2826–2830. 2 K. K. Naik, R. Khare, D. Chakravarty, M. A. More, R. Thapa, D. J. Late and C. S. Rout, Appl. Phys. Lett., 2014, 105, 233101. 3 S. S. Lv, Z. C. Li, S. M. Su, L. H. Lin, Z. J. Zhang and W. Miao, RSC Adv., 2014, 4, 31729–31734. 4 S. S. Lv, Z. C. Li, J. C. Liao, Z. J. Zhang and W. Miao, J. Vac. Sci. Technol., B: Nanotechnol. Microelectron.: Mater., Process., Meas., Phenom., 2015, 33, 02B101. 5 S. S. Lv, Z. C. Li, C. H. Chen, J. C. Liao, G. J. Wang, M. Y. Li and W. Miao, ACS Appl. Mater. Interfaces, 2015, 7, 13564–13568. 6 Z. R. Dai, Z. W. Pan and Z. L. Wang, Adv. Funct. Mater., 2003, 13, 9–24. 7 Z. G. Bai, D. P. Yu, H. Z. Zhang, Y. Ding, Y. P. Wang, X. Z. Gai, Q. L. Hang, G. C. Xiong and S. Q. Feng, Chem. Phys. Lett., 1999, 303, 311–314. 8 B. R. Huang, T. C. Lin, K. T. Chu, Y. K. Yang and J. C. Lin, Surf. Coat. Technol., 2013, 231, 289–292. 9 R. R. Devarapalli, D. R. Shinde, B.-B. Fatiha, S. G. Yenchalwar, R. Boukherroub, M. A. Moreb and M. V. Shelke, J. Mater. Chem., 2012, 22, 22922–22928. 10 C. X. Zhao, K. Huang, S. Z. Deng, N. S. Xu and J. Chen, Appl. Surf. Sci., 2013, 270, 82–89. 11 W. B. Zou, J. W. Zhu, Y. X. Sun and X. Wang, Mater. Chem. Phys., 2011, 125, 617–620. 12 P. Zhan, W. P. Wang, Z. Xie, Z. C. Li, Z. J. Zhang, P. Zhang, B. Y. Wang and X. Z. Cao, Appl. Phys. Lett., 2012, 101, 031913. 13 X. D. Wang, J. Zhou, C. S. Lao, J. H. Song, N. S. Xu and Z. L. Wang, Adv. Mater., 2007, 19, 1627–1631.
31828 | Phys. Chem. Chem. Phys., 2015, 17, 31822--31829
Paper
14 H. Zeng, X. J. Xu, Y. Bando, U. K. Gautam, T. Y. Zhai, X. S. Fang, B. D. Liu and D. Golberg, Adv. Funct. Mater., 2009, 19, 3165–3172. 15 T. Y. Zhai, L. Li, Y. Ma, M. Y. Liao, X. Wang, X. S. Fang, J. N. Yao, Y. Bandoa and D. Golberga, Chem. Soc. Rev., 2011, 40, 2986–3004. 16 C. H. Ye, Y. Bando, X. S. Fang, G. Z. Shen and D. Golberg, J. Phys. Chem. C, 2007, 111, 12673–12676. 17 S. Y. Lin, S. J. Chang and T. J. Hsueh, ECS J. Solid State Sci. Technol., 2013, 2, N149–N151. 18 J. Ding, X. Yan and Q. Xue, Mater. Chem. Phys., 2012, 133, 405–409. 19 C. X. Wu, F. S. Li, Y. A. Zhang and T. L. Guo, Carbon, 2012, 50, 3622–3626. 20 K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666–669. 21 Z. C. Yang, Q. Zhao, Y. X. Ou, W. Wang, H. Li and D. P. Yu, Appl. Phys. Lett., 2012, 101, 173107. 22 J. M. Lee, Y. B. Pyun, J. Yi, J. W. Choung and W. I. Park, J. Phys. Chem. C, 2009, 113, 19134–19138. 23 W. T. Zheng, Y. M. Ho, H. W. Tian, M. Wen, J. L. Qi and Y. A. Li, J. Phys. Chem. C, 2009, 113, 9164–9168. 24 S. S. Lv, Z. C. Li, J. C. Liao, G. J. Wang, M. Y. Li and W. Miao, Sci. Rep., 2015, 5, 15035. 25 C. X. Wu, F. S. Li, Y. A. i. Zhang and T. L. Guo, Carbon, 2012, 50, 3622–3626. 26 G. Williams, B. Seger and P. V. Kamat, ACS Nano, 2008, 2, 1487–1491. 27 R. Muszynski, B. Seger and P. V. Kamat, J. Phys. Chem. C, 2008, 112, 5263–5266. 28 J. Yao, X. P. Shen, B. Wang, H. K. Liu and G. X. Wang, Electrochem. Commun., 2009, 11, 1849–1852. 29 W. T. Zheng, Y. M. Ho, H. W. Tian, M. Wen, J. L. Qi and Y. A. Li, J. Phys. Chem. C, 2009, 113, 9164–9168. 30 C. Xu, X. Wang and J. W. Zhu, J. Phys. Chem. C, 2008, 112, 19841–19845. 31 Y. P. Zhang, H. B. Li, L. K. Pan, T. Lu and Z. Sun, J. Electroanal. Chem., 2009, 634, 68–71. 32 C. Xu, X. Wang, L. C. Yang and Y. P. Wu, J. Solid State Chem., 2009, 182, 2486–2490. 33 Z. Y. Guo, M. X. Wang, Z.-H. Huang and F. Y. Kang, Carbon, 2015, 87, 282–291. 34 X. Z. Sun, R. Tao, L. H. Lin, Z. C. Li, Z. J. Zhang and J. Y. Feng, Appl. Surf. Sci., 2011, 257, 3861–3866. 35 X. Z. Sun, L. H. Lin, Z. C. Li, Z. J. Zhang and J. Y. Feng, Appl. Surf. Sci., 2009, 256, 916–920. 36 Q. Zhou, X. Zhang, Y. Huang, Z. C. Li, Y. P. Zhao and Z. J. Zhang, Appl. Phys. Lett., 2012, 100, 113101. 37 M. Z. M. Yusoff, A. Mahyuddin, Z. Hassan, H. A. Hassan, M. J. Abdullah, M. Rusop, S. M. Mohammad and N. M. Ahmed, Mater. Sci. Semicond. Process., 2015, 29, 231–237. 38 S. Khosravi-Gandomania, R. Yousefi, F. Jamali-Sheinic and N. M. Huang, Ceram. Int., 2014, 40, 7957–7963. 39 H. Q. Bian, S. Y. Ma, Z. M. Zhang, J. M. Gao and H. B. Zhu, J. Cryst. Growth, 2014, 394, 132–136.
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Paper
40 H. J. Jeong, H. Y. Kim, H. D. Jeong, S. Y. Jeong, J. T. Han and G.-W. Lee, J. Mater. Chem., 2012, 22, 11277–11283. 41 R. C. Wang, Y. C. Chen, S. J. Chen and Y. M. Chang, Carbon, 2014, 70, 215–223. 42 H. R. Liu, G. X. Shao, J. F. Zhao, Z. X. Zhang, Y. Zhang, J. Liang, X. G. Liu, H. S. Jia and B. S. Xu, J. Phys. Chem. C, 2012, 116, 16182–16190.
This journal is © the Owner Societies 2015
PCCP
43 C. D. Gu, C. Cheng, H. Y. Huang, T. L. Wong, N. Wang and T. Y. Zhang, Cryst. Growth Des., 2009, 9, 3278–3285. 44 D. D. Lin, H. Wu, R. Zhang and W. Pan, Chem. Mater., 2009, 21, 3479–3484. 45 L. H. Lin, Z. C. Li, J. Y. Feng and Z. J. Zhang, Phys. Chem. Chem. Phys., 2013, 15, 6063–6067. 46 L. Sun, X. T. Zhou, Y. A. Zhang and T. L. Guo, J. Alloys Compd., 2015, 632, 604–608.
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