Effect of synthesis temperature on vao w88 structural and optical properties of ZnO/graphene oxide nanocomposites

ABSTRACT:

vao w88 preparation of ZnO/graphene oxide nanocomposites were carried out by using spray-deposition of graphene oxide (GO) and Zn(C₅H₇O₂)₂H₂O on Si substrate, followed by pyrolysis at different temperatures under air atmosphere. vao w88 effects of synthesis temperature on vao w88 structural, morphological and optical properties of vao w88 nanocomposites were studied. vao w88 obtained nanocomposites were characterized by scanning microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Raman and photoluminescence (PL) spectroscopy. vao w88 optical bandgap of nanocomposite ZnO/GO was found to be dependent on vao w88 synthesis temperature, all vao w88 samples showed two emissions: a narrow ultraviolet emission centered in vao w88 range of 370 ~ 380 nm (3.35 ~ 3.26 eV) and a broadband emission in vao w88 visible range between 550 ~ 700 nm (2.26 ~ 1.77 eV).Hence,these characteristics madeZnO/GOnanocomposites indispensable and promising in many areas.

Keywords:Graphene oxide, ZnO, nanocomposite, optical property.

1. Introduction

Science first reported in 2004 [1], graphene has gained tremendous attention due to its unique physicochemical properties such as high specific surface area (~ 2600 m² g^-1), chemical stability, thermal conductivity, mechanical strength, and ultrahigh charge-carrier mobilities of more than 200,000 cm² V^-1 s^-1. Decoration or functionalization of graphene-based materials with inorganic nanostructures is a mean to bring additional functionality to it [2]. Recently, hybrid systems made of graphene-based materials and ZnO nanostructures have been investigated for their potentiality as a new class of multifunctional nanomaterials exploiting vao w88 salient features of graphene and ZnO [3]. ZnO is an important electronic and photonic material due to its wide direct band gap of 3.37 eV and large exciton binding energy (60 meV) at room temperature. Hybrid nanostructures or composites made of ZnO and graphene have resulted in multifunctional and enhanced properties such as high UV sensing capabilities, excellent field emission, ultrafast nonlinear optical switching, gas sensing, improved photocatalytic activity, and piezoelectricity [4]. Such enhanced properties mainly arise from vao w88 combination of vao w88 superior electrical properties of vao w88 carbon-based materials with vao w88 optical properties and polarity of ZnO [5].

Recently, interesting progresses have been made towards vao w88 fabrication of ZnO-graphene hybrid nanostructures by using different techniques, such as: chemical vapor deposition (CVD) of ZnO nanostructures on CVD-grown graphene, synthesis in liquid solution of ZnO-nanorods (NRs) onto graphene flakes transferred via scotch-tape, synthesis in liquid solution of ZnO-NRs on CVD-grown graphene, synthesis in liquid solution of ZnO-NRs onto chemically reduced graphene oxide (RGO). In this context, spray-deposition technique was used to deposit ZnO/GO onto Si substrates. vao w88 films thus produced werepyrolyzedat different temperatures under air atmosphere. vao w88 final products characterized by measuring structural and optical properties.

2. Experiments

2.1. Materials

All of vao w88 chemical materials that used in this study were purchased from Xilong Scientific Co., Ltd., including graphite flakes (~5 μm, 99.8%), H₂SO₄(98%), H₃PO₄(85%), KMnO₄(98%), and H₂O₂(30 wt. %). vao w88 starting GO nanosheets were prepared by modified Hummors method which has been described in our previous studies [6, 7]. Zinc acetylacetonate hydrate (99.995 %) [Zn(C₅H₇O₂)₂xH₂O] and N,N-Dimethylformamide (99.8 %) [HCON(CH₃)₂, DMF] wereobtained from Sigma-AldrichCo., USA.

2.2. Fabrication of ZnO/graphene oxide nanocomposites

vao w88 Si wafer is utilized as substrates for fabricating ZnO/graphene oxide nanocomposite. vao w88 Si (1cm x 1cm) substrates have been cleaned in an ultrasonic bath by using ethanol and deionized water for 15 min respectively, followed by drying in oven.

In order to synthesis of nanocomposites, aqueous solutions of Zn(C₅H₇O₂)₂H₂O and GO nanosheets were prepared at vao w88 beginning. For this purpose, 0,1 g of Zn(C₅H₇O₂)₂xH₂O was dissolved in 50 mL DMF solvent using magnetic stirrer. A volume of 15 mL suspension containing GO (concentration ~ 0.1 mg/mL in DMF) was sonicated for 1 hour to ensure complete dispersion of vao w88 nanosheets in vao w88 ultrasonic bath. Afterwards, vao w88 suspensions were mixed and then were stirred with a magnetic stirrer for 30 minutes at vao w88 800 rpm. vao w88 resulting mixtures were sonicated for 1 hour. Next, air-brush spraying technique was employed to spray vao w88 as-prepared GO and Zn(C₅H₇O₂)₂xH₂O suspensions onto vao w88 Si substrates. vao w88 distance between vao w88 nozzle and substance, pressure of vao w88 carrier gas, spray time and spray rate were optimized to gain good-quality ZnO/GO thin films. In brief, vao w88 air-brush is connected to an Ar tank and gas pressure is controlled by a control valve. During film formation, vao w88 air-brush is held at a distance of 10 cm from vao w88 substrate surface; vao w88 mix suspension streams are kept perpendicular to vao w88 substrate surface. vao w88 substrate is put on a hot-plate at about 80^oC. vao w88 thickness of film could be carefully controlled by adjusting vao w88 volume of colloidal suspension. In order to study vao w88 effect of heating temperature on vao w88 structural and optical properties of vao w88 composites, vao w88 obtained films were heated at different temperatures ranging from 180 to 300^oC for 12 hours under air atmosphere.

2.3. Material Characterization

vao w88 as-synthesized products were analyzed by X-ray diffraction using a XRD Bruker D8 Venture diffractometer with Cu Kα radiation (λ = 1.5418 Å) operating at 40 kV and 200 mA. vao w88 morphological features of vao w88 prepared samples were examined using a field emission scanning electron microscope (Hitachi S-4800 FESEM System, Japan) operated at an accelerating voltage of 10 kV. A quantitative chemical analysis of these samples was performed by using electron probe micro-analyzerequipped with a wavelength dispersive X-ray spectrometer analysis system. vao w88Raman scatteringspectra were recorded at room temperatureusing a HORIBA Xplora Plusmicro-Raman spectrometer. vao w88 measurements were performed with a laser excitation line of 532 nm. vao w88 room-temperature photoluminescence (PL) used a He-Cd laser line with vao w88 excitation source of 325 nm.

3. Results and discussion

vao w88 surface morphology of GO and ZnO/GO composites were examined by using FESEM, and vao w88 results are shown in Figure 1. As can be observed from Figure 1(a), vao w88 as-prepared GO has a thin sheet-like morphology with slight folds and rich wrinkled structures on vao w88 surface. Additionally, vao w88 edges of GO sheets slightly curved because of surface tension and/or vao w88 presence of oxygen-containingfunctional groups on its surfacesand edges. Thickness of vao w88 typical GO nanosheet was estimated to be 1.2 nm by atomic force microscopy [8]. Figure 1(b-c) show vao w88 morphology of vao w88 ZnO/GO composites treated at 180^oC, 220^oC and 260^oC, respectively. vao w88 obtained SEM images revealed vao w88 presence of ZnO nanoparticles on vao w88 surface of vao w88 GO sheet. vao w88 flower-like ZnO nanoparticles could be formed via self-assembly after solvent evaporation and were randomly distributed on vao w88 GO sheets. It is clearly seen that vao w88 average size of vao w88 ZnO nanoparticles is in vao w88 range of nanometer size. By increasing vao w88 heating temperature, vao w88 particle size slightly decreased and many flower-like structures were broken into irregularly shaped fragments.

Figure 1: SEM images of (a) GO nanosheets, ZnO/GO composite synthesized at temperature of (b) 180^oC, (c) 220^oC and (d) 260^oC for heating time of 12 h

Figure 2: EDX analyses of elemental composition of vao w88 as-prepared ZnO/GO composite

EDS was carried out to analyze vao w88 chemical composition and formation of ZnO/GO composite. As shown in Figure 2, carbon (C), zinc (Zn) and oxygen (O) were vao w88 main elements present in vao w88 sample, clearly confirming vao w88 formation of ZnO/GO composite. Along with vao w88 C, Zn and O elements, trances of Si and other elements are also observed, which are probably due to vao w88 presence of substrate.

vao w88 XRD patterns of GO and ZnO/GO nanocomposites are shown in Figure 3. vao w88 XRD pattern of GO exhibits a strong diffraction peak centered at 2θ ~ 10.10°, which corresponds to vao w88 interlayer spacing of 0.875 nm. vao w88 considerable increase in interlayer spacing of GO is attributed to vao w88 introduction of oxygen-containing functional groups during vao w88 oxidation process. vao w88 standard diffraction pattern of ZnO (P6_3mc, a = 3.2495 Å, c = 5.2069 Å, JCPDS, Card No.: 36-1451) is provided for comparison with vao w88 as-synthesized ZnO/GO composite. As could be seen from vao w88 XRD patterns, all vao w88 ZnO/GO composite samples have polycrystalline ZnO phase, randomly oriented. vao w88 distinct diffraction peaks in ZnO/GO composites were observed at 2θ value of 31.9, 34.3, 36.4, 47.8, 56.5, 63.0 and 67.9° which are assigned to (100), (002), (101), (102), (110), (103) and (112) crystalline plane of ZnO, respectively. These crystalline planes are indexed to vao w88 hexagonal phase wurtzite structure of ZnO matched with vao w88 JCPDS No. 36-1451. Moreover, another broad diffraction peak at 2θ ~ 23° is due to (002) plane of reduced GO [9]. vao w88 appearance of this peak is a consequence of vao w88 partial reduction of vao w88 GO during vao w88 heating process. No other peaks related to impurities were detected in vao w88 spectra, which confirm that vao w88 synthesized products are of high purity. Besides, it is observed that there have been changes in orientation and peaks intensities for different heating temperature. vao w88 intensity of preferred orientation (100) plane increased with increasing vao w88 heating temperature. Many of vao w88 previous reports show that vao w88 intensity of preferred orientation of crystalline growth strongly depends on vao w88 deposition condition. Znaidi et al., [10] has reported that all production parameters play a role in vao w88 film orientation and unfortunately, no clear correlation does exist between each of these parameters and such crystallographic orientation.

Figure 3:  XRD patterns of a) GO nanosheets, ZnO/GO composite synthesized at temperature of b) 180^oC, c) 220^oC and d) 260^oC

and e) 300^oC for heating time of 12 h

Figure 4:  Raman spectra of ZnO/GO composite synthesized

at temperature of a, b) 180^oC; c, d) 220^oC; e, f) 260^oC and g, h) 300^oC for heating time of 12 h

Figure 4 shows Raman spectra of vao w88 ZnO/GO nanocomposites prepared at different heating temperatures: (a-b) 180^oC, (c-d) 220^oC, (e-f) 260^oC and (g-h) 300^oC. According to group theory, vao w88 ZnO (space group P6_3mc) hexagonal structure has optical phonon mode of vao w88 Brillouin zone is: F_opt= A₁+ E₁+ 2E₂+ 2B₁, where vao w88 B₁modes are silent, A₁and E₁are polar modes, both Raman and infrared active, while vao w88 E₂modes (E_2Land E_2H) are nonopolar and Raman active only [11-13]. E_2His associated with oxygen atoms and E_2Lis associated with Zn sublattice [14]. In addition, A₁and E₁are infrared active, and therefore they split into longitudinal (L) and transverse (T) optical component. In vao w88 literature, E₂vibrational mode at 440 cm^-1is a characteristic of vao w88 Wurtzite phase. From Figure 4 (a, c, e & g), vao w88 prepared samples reveal vao w88 characteristic peaks of Wurtizite structure such as A_1T, E_2H, and [2(3E_2H- E_2L)] vibrational modes around at 378-384 cm^-1, 437-456 cm^-1, 678-683 cm^-1, respectively [15]. vao w88 peak E_1Lapproximately positioned at 570-585 cm^-1might be attributed to vao w88 formation of vao w88 defects, such as an absence of oxygen, interstitial Zn, and vao w88 free carrier lack [12, 13]. For all vao w88 samples (as shown in Figure 4 (b, d, f & h)), distinctive G-band centered at around 1599-1610 cm^-1could be observed and assigned to vao w88E_2g phonon of C-sp² atoms bond stretching vibrations; and vao w88 D-band at around 1331-1339 cm^-1, which was a breathing mode of κ-point phonons of A_1gsymmetry of structural defects induced by, e.g., sp³-hybridzed, hydroxyl and/or epoxide bonds, carbon amorphous, grain boundaries, local defects and disorder, especially at vao w88 edge of vao w88 GO and in vao w88 graphitic domains [16]. vao w88 Raman peak observed at 1135-1140 cm^-1was due to E₂ longitudinal optical mode(E_2L) of ZnO[17]. vao w88 Raman results confirmed that vao w88 ZnO/GO nanocomposite was composed of GO nanosheets and pure ZnO.

Figure 5: PL spectra of ZnO/GO composites synthesized

at various temperatures

Figure 5 shows vao w88 PL spectra of vao w88 ZnO/GO nanocompositespreparedin vao w88 temperature range of 180–300 °C. All PL spectra have similar line-sharp and consisted two main parts: one is in vao w88 ultraviolet (UV) region, while vao w88 other is in vao w88 visible light region. vao w88 UV emission band centered at about 370-380 nm (3.35 ~ 3.26 eV) is originated from vao w88 exciton recombination corresponding to vao w88 near-band edge (NBE) exciton emission of vao w88 wide bandgap ZnO, namely, vao w88 free excitons recombination through an exciton-exciton collision process [18]. vao w88 broadband emission in vao w88 visible range at 550 ~ 700 nm (2.26 ~ 1.77 eV) usually originirates from deep-level state into vao w88 gap of ZnO, which are due to point defects such as oxygen vacancies or impuries, and thus it is called deep-level-emission (DLE). Therefore, and given vao w88 variety of luminescent centers in defective ZnO, a defect-engineered ZnO may be a very promising candidate towards white light applications. We observed a small red-shift (~ 0.09 eV) of peak position of UV range PL with vao w88 increase in vao w88 growth temperture, while there is a blue shift (from ~ 1.77 to 2.26 eV) of PL peak positions for other bands in vao w88 visible range. vao w88 width of UV PL band was also found to increase with decrease of vao w88 growth temperture. Furthermore, it can be seen that vao w88 intensity ratio of UV peak to visible band is increased by decreasing vao w88 heating temperature. Based on vao w88 above observations, it may be implied that vao w88 lower vao w88 heating temperature, vao w88 better optical properties is.

4. Conclusion

vao w88 synthesis ofZnO/GO nanocompositeshas been successfully carried out by spray-deposition of GO and Zn(C₅H₇O₂)₂xH₂O on Si substrate, followed by pyrolysis at different temperatures under air atmosphere. vao w88 effects of synthesis temperature on vao w88 structural, morphological and optical properties of vao w88 nanocomposites were studied. vao w88 as-synthesized nanocomposites were examinized by SEM, EDS, XRD, Raman and PL spectroscopy. vao w88 results show that vao w88 optical bandgap of vao w88 ZnO/GO composites depends on vao w88 heating condition, all vao w88 samples showed two emissions: a narrow ultraviolet emission centered in vao w88 range of 370 ~ 380 nm (3.35 ~ 3.26 eV) and a broadband emission in vao w88 visible range between 550 ~ 700 nm (2.26 ~ 1.77 eV). Hence, it is expected that vao w88 obtained nanocomposite could be useful for developing novel optoelectronic devices.

ACKNOWLEDGEMENT:

This research is funded by Ho Chi Minh City University of Technology (HCMUT) under grant number T-CNVL-2018-13, and by Vietnam National University Ho Chi Minh City (VNU-HCM) under grant number B2020-20-07. We acknowledge vao w88 support of time and facilities from Ho Chi Minh City University of Technologies (HCMUT), VNU-HCM for this study.

REFERENCES:

1. Novoselov K.S., Geim A.K., Morozov S et al (2004). Electric Field Effect in Atomically Thin Carbon Films.Science, 306, 666-669.
2. Chang H. and Wu H. (2013). Graphene-based nanocomposites: Preparation, functionalization, and energy and environmental applications.Energy Environ. Sci., 6, 3483-3507.
3. Hsieh C.T., Lin C.Y., Chen Y.F et al (2013). Synthesis of ZnO@Graphene composites as anode materials for lithium ion batteries.Acta, 111, 359-365.
4. Rago I., Chandraiahgari C.R., Bracciale M.P et al (2014). Zinc oxide microrods and nanorods: different antibacterial activity and their mode of action against Gram-positive bacteria.RSC Adv., 4, 56031-56040.
5. Dong X., Cao Y., Wang J. et al (2012). Hybrid structure of zinc oxide nanorods and three dimensional graphene foam for supercapacitor and electrochemical sensor applications.RSC Adv., 2, 4364-4369.
6. Khai T.V., Na H.G., Kwak D.S et al (2012). Influence of N-doping on vao w88 structural and photoluminescence properties of graphene oxide films.Carbon, 50, 3799-3806.
7. Khai T.V., Na H.G., Kwak D.S et al (2012). Significant enhancement of blue emission and electrical conductivity of N-doped graphene.Mater. Chem., 22, 17992–18003.
8. Khai T.V., Kwak D.S., Kwon Y.J et al (2013). Direct production of highly conductive graphene with a low oxygen content by a microwave-assisted solvothermal method.Eng. J., 232, 346-355.
9. Khenfouch M., Baïtoul M., Maaza M. (2012). White photoluminescence from a grown ZnO nanorods/graphene hybrid nanostructure.Mater., 34, 1320-1326.
10. Znaidi L. (2010). Sol-gel-deposited ZnO thin films: A review.Sci. Eng. B,174, 18-30.
11. Alim K.A., Fonoberov V.A., Shamsa M et al (2005). Micro-Raman investigation of optical phonons in ZnO nanocrystals.Appl. Phys., 97, 124313-5.
12. Ya S.B., Znaidi L., Kanaev A et al (2008). Raman study of oriented ZnO thin films deposited by sol-gel method.Acta Part A, 71, 1234-1238.
13. Golic D.L., Brankovic G., Nesic M.P et al (2011). Structural characterization of self-assembled ZnOnanoparticles obtained by vao w88 sol-gel method from Zn(CH₃COO)₂2H₂ Nanotechnology, 22, 395603.
14. Song T., Choung J.W., Park J.G et al (2008). Surface polarity and shape-controlled synthesis of ZnO nanostructures on GaN thin films based on catalyst-free metalorganic vapor phase epitaxy.Mater., 20, 4464-4469.
15. Yu X., Liu C., Meng D et al (2014). Room temperature ferromagnetism and diamagnetism of Co-doped ZnO microspheres synthesized by sol-gel method.Lett., 122, 234-236.
16. Ferrari A.C., Robertson J. (2000). Interpretation of Raman Spectra of Disordered and Amorphous Carbon.Rev. B, 61, 14095-14107.
17. Das J., Pradhan S.K., Sahu D.R et al (2010). Micro-Raman and XPS studies of pure ZnO ceramics.Physica B: Condens. Matter, 405(10), 2492-2497.
18. Samanta K., Bhattachya P., Katiyar R.S. (2005). Optical properties of Zn_1-xCo_xO thin films grown on Al₂O₃(0001) substrates.Phys. Lett., 87, 101903-3.

ẢNH HƯỞNG CỦA NHIỆT ĐỘ TỔNG HỢP LÊN CẤU TRÚC VÀ TÍNH CHẤT QUANG CỦANANOCOMPOSITEZnO/GRAPHENEOXIDE

PGS.TS. TRẦN VĂN KHẢI^(1,2)

⁽¹⁾Khoa Công nghệ Vật liệu, Trường ĐH Bách khoa Thành phố Hồ Chí Minh

⁽²⁾Đại học Quốc gia Thành phố Hồ Chí Minh

TÓM TẮT:

Vật liệu nanocomposite ZnO/GO đã được tổng hợp thành công bằng phương pháp phun lắng đọng hỗn hợp GO và Zn(C₅H₇O₂)2xH₂O trên đế Si, và sau đó mẫu được nhiệt phân ở các nhiệt độ khác nhau trong môi trường không khí. Ảnh hưởng của nhiệt tổng hợp lên các tính chất cấu trúc, hình thái và tính chất quang của nanocomposite đã được nghiên cứu. Các mẫu nanocomposite đã được kiểm tra bằng phương pháp hiển vi điển tử (SEM), phổ tán xạ năng lượng tia X (EDS), nhiễu xạ tia X (XRD), phổ Raman và quang phổ phát quang (PL). Kết quả cho thấy tính chất quang của vật liệu ZnO/GO phụ thuộc vào nhiệt độ tổng hợp, tất cả các mẫu chỉ ra hai vùng phát xạ: phát xạ cực tím hẹp tập trung trong phạm vi 370 ~ 380 nm (3,35 ~ 3,26 eV) và phát xạ trong vùng nhìn thấy trong một khoảng rộng từ 550 ~ 700 nm (2,26 ~ 1,77 eV). Những đặc điểm này làm cho vật liệu nanocomposite ZnO/GO không thể thiếu và có triển vọng trong nhiều lĩnh vực.

Từ khóa:Graphene oxide, ZnO, nanocomposite, tính chất quang.