ABSTRACT:                                                                 

The 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. The effects of synthesis temperature on the structural, morphological and optical properties of the nanocomposites were studied. The obtained nanocomposites were characterized by scanning microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Raman and photoluminescence (PL) spectroscopy. The optical bandgap of nanocomposite ZnO/GO was found to be dependent on the synthesis temperature, all the samples showed two emissions: a narrow ultraviolet emission centered in the range of 370 ~ 380 nm (3.35 ~ 3.26 eV) and a broadband emission in the visible range between 550 ~ 700 nm (2.26 ~ 1.77 eV). Hence, these characteristics made ZnO/GO nanocomposites 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 the 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 the combination of the superior electrical properties of the carbon-based materials with the optical properties and polarity of ZnO [5].

Recently, interesting progresses have been made towards the 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. The films thus produced were pyrolyzed at different temperatures under air atmosphere. The final products characterized by measuring structural and optical properties.

2. Experiments

2.1. Materials

All of the 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. %). The 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] were obtained from Sigma-Aldrich Co., USA.

2.2. Fabrication of ZnO/graphene oxide nanocomposites

The Si wafer is utilized as substrates for fabricating ZnO/graphene oxide nanocomposite. The 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 the 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 the nanosheets in the ultrasonic bath. Afterwards, the suspensions were mixed and then were stirred with a magnetic stirrer for 30 minutes at the 800 rpm. The resulting mixtures were sonicated for 1 hour. Next, air-brush spraying technique was employed to spray the as-prepared GO and Zn(C5H7O2)2xH2O suspensions onto the Si substrates. The distance between the nozzle and substance, pressure of the carrier gas, spray time and spray rate were optimized to gain good-quality ZnO/GO thin films. In brief, the air-brush is connected to an Ar tank and gas pressure is controlled by a control valve. During film formation, the air-brush is held at a distance of 10 cm from the substrate surface; the mix suspension streams are kept perpendicular to the substrate surface. The substrate is put on a hot-plate at about 80oC. The thickness of film could be carefully controlled by adjusting the volume of colloidal suspension. In order to study the effect of heating temperature on the structural and optical properties of the composites, the obtained films were heated at different temperatures ranging from 180 to 300 oC for 12 hours under air atmosphere.

2.3. Material Characterization

The 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. The morphological features of the 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-analyzer equipped with a wavelength dispersive X-ray spectrometer analysis system. The Raman scattering spectra were recorded at room temperature using a HORIBA Xplora Plus micro-Raman spectrometer. The measurements were performed with a laser excitation line of 532 nm. The room-temperature photoluminescence (PL) used a He-Cd laser line with the excitation source of 325 nm.

3. Results and discussion

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

sem_images_of_go_nanosheets_zno-go_composite_synthesized_at_temperature

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

edx_analyses_of_elemental_composition_of_the_as-prepared_zno-go_composite

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

The XRD patterns of GO and ZnO/GO nanocomposites are shown in Figure 3. The XRD pattern of GO exhibits a strong diffraction peak centered at 2θ ~ 10.10°, which corresponds to the interlayer spacing of 0.875 nm. The considerable increase in interlayer spacing of GO is attributed to the introduction of oxygen-containing functional groups during the oxidation process. The standard diffraction pattern of ZnO (P63mc, a = 3.2495 Å, c = 5.2069 Å, JCPDS, Card No.: 36-1451) is provided for comparison with the as-synthesized ZnO/GO composite. As could be seen from the XRD patterns, all the ZnO/GO composite samples have polycrystalline ZnO phase, randomly oriented. The 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 the hexagonal phase wurtzite structure of ZnO matched with the JCPDS No. 36-1451. Moreover, another broad diffraction peak at 2θ ~ 23° is due to (002) plane of reduced GO [9]. The appearance of this peak is a consequence of the partial reduction of the GO during the heating process. No other peaks related to impurities were detected in the spectra, which confirm that the synthesized products are of high purity. Besides, it is observed that there have been changes in orientation and peaks intensities for different heating temperature. The intensity of preferred orientation (100) plane increased with increasing the heating temperature. Many of the previous reports show that the intensity of preferred orientation of crystalline growth strongly depends on the deposition condition. Znaidi et al., [10] has reported that all production parameters play a role in the 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

xrd_patterns_of_go_nanosheets

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

raman_spectra_of_zno-go_composite_synthesized_1

raman_spectra_of_zno-go_composite_synthesized_2

Figure 4 shows Raman spectra of the 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, the ZnO (space group P63mc) hexagonal structure has optical phonon mode of the Brillouin zone is: Fopt = A1 + E1 + 2E2 + 2B1, where the B1 modes are silent, A1 and E1 are polar modes, both Raman and infrared active, while the E2 modes (E2L and E2H) are nonopolar and Raman active only [11-13]. E2H is associated with oxygen atoms and E2L is associated with Zn sublattice [14]. In addition, A1 and E1 are infrared active, and therefore they split into longitudinal (L) and transverse (T) optical component. In the literature, E2 vibrational mode at 440 cm-1 is a characteristic of the Wurtzite phase. From Figure 4 (a, c, e & g), the prepared samples reveal the characteristic peaks of Wurtizite structure such as A1T, E2H, and [2(3E2H - E2L)] vibrational modes around at 378-384 cm-1, 437-456 cm-1, 678-683 cm-1, respectively [15]. The peak E1L approximately positioned at 570-585 cm-1 might be attributed to the formation of the defects, such as an absence of oxygen, interstitial Zn, and the free carrier lack [12, 13]. For all the samples (as shown in Figure 4 (b, d, f & h)), distinctive G-band centered at around 1599-1610 cm-1 could be observed and assigned to the E2g phonon of C-sp2 atoms bond stretching vibrations; and the D-band at around 1331-1339 cm-1, which was a breathing mode of κ-point phonons of A1g symmetry of structural defects induced by, e.g., sp3-hybridzed, hydroxyl and/or epoxide bonds, carbon amorphous, grain boundaries, local defects and disorder, especially at the edge of the GO and in the graphitic domains [16]. The Raman peak observed at 1135-1140 cm-1 was due to E2 longitudinal optical mode (E2L) of ZnO [17]. The Raman results confirmed that the ZnO/GO nanocomposite was composed of GO nanosheets and pure ZnO.  

Figure 5: PL spectra of ZnO/GO composites synthesized

at various temperatures

pl_spectra_of_zno_go_composites_synthesized_at_various_temperatures

Figure 5 shows the PL spectra of the ZnO/GO nanocomposites prepared in the temperature range of 180–300 °C. All PL spectra have similar line-sharp and consisted two main parts: one is in the ultraviolet (UV) region, while the other is in the visible light region. The UV emission band centered at about 370-380 nm (3.35 ~ 3.26 eV) is originated from the exciton recombination corresponding to the near-band edge (NBE) exciton emission of the wide bandgap ZnO, namely, the free excitons recombination through an exciton-exciton collision process [18]. The broadband emission in the visible range at 550 ~ 700 nm (2.26 ~ 1.77 eV) usually originirates from deep-level state into the 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 the 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 the increase in the growth temperture, while there is a blue shift (from ~ 1.77 to 2.26 eV) of PL peak positions for other bands in the visible range. The width of UV PL band was also found to increase with decrease of the growth temperture. Furthermore, it can be seen that the intensity ratio of UV peak to visible band is increased by decreasing the heating temperature. Based on the above observations, it may be implied that the lower the heating temperature, the better optical properties is.

4. Conclusion

The synthesis of ZnO/GO nanocomposites has 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. The effects of synthesis temperature on the structural, morphological and optical properties of the nanocomposites were studied. The as-synthesized nanocomposites were examinized by SEM, EDS, XRD, Raman and PL spectroscopy. The results show that the optical bandgap of the ZnO/GO composites depends on the heating condition, all the samples showed two emissions: a narrow ultraviolet emission centered in the range of 370 ~ 380 nm (3.35 ~ 3.26 eV) and a broadband emission in the visible range between 550 ~ 700 nm (2.26 ~ 1.77 eV). Hence, it is expected that the 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 the support of time and facilities from Ho Chi Minh City University of Technologies (HCMUT), VNU-HCM for this study.

 

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ẢNH HƯỞNG CỦA NHIỆT ĐỘ TỔNG HỢP LÊN CẤU TRÚC VÀ TÍNH CHẤT QUANG CỦA NANOCOMPOSITE ZnO/GRAPHENE OXIDE

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.