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Facile synthesis and enhanced luminescence behavior of ZnO:Reduced graphene oxide(rGO) hybrid nanostructures
Journal of Luminescence 203 (2018) 1–6
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Facile synthesis and enhanced luminescence behavior of ZnO:Reduced graphene oxide(rGO) hybrid nanostructures
T
⁎
G. Jayalakshmia, , K. Saravanana, Jagnaseni Pradhana,b, P. Magudapathya, B.K. Panigrahib,c a
Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, Tamilnadu, India Homi Bhabha National Institute, Kalpakkam 603102, Tamilnadu, India c Electronics and Instrumentation Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnO:rGO hybrid nanostructures Photoluminescence Trap states XANES Charge transfer
The structural, electronic and luminescence behavior of ZnO nanorods and ZnO-reduced graphene oxide (ZnO:rGO) hybrid nanostructures have been studied using X-diffraction (XRD), Raman scattering, X-ray absorption near edge structure (XANES) spectroscopy and photoluminescence (PL) studies. XRD and Raman analyses reveal the formation of hybrid nanostructures composed of ZnO and rGO. The morphology and elemental composition of the grown nanostructures were investigated using FESEM and EDX measurements. The hybridization of rGO with ZnO provides an additional path way for the transfer of charge from the defect states of ZnO to rGO as evidenced from the increase in unoccupied density of states in the O-K edge XANES spectra and the enhanced UV, blue emissions as well as the suppressed visible emission in the PL spectra, respectively. Our systematic experimental investigations demonstrate the hybridization of rGO with ZnO is an effective approach to tailor the optical properties of ZnO and offers the prospective way for the fabrication of novel optoelectronic devices.
1. Introduction ZnO is a promising semiconducting material because of its fascinating properties such as direct wide band gap (3.37 eV), high exciton binding energy (60 meV) at room temperature, high surface area, high mechanical strength, thermal stabilities, and high oscillator strength of excitonic transitions find applications in various technological devices including optoelectronics devices, solar cells, sensors, spintronic devices, photocatalysis, UV photodetectors and ultraviolet lasers [1–7]. The one-dimensional nanostructures such as nanorods, nanowires, nanotubes, nanoflowers, nanobelts have been attracted due to their remarkable physical and chemical properties that can be used in various electronic and optoelectronic devices [8,9]. The luminescent behavior in ultraviolet (UV) region is of great interest due to its potential applications in optical and optoelectronic devices such as UV lasers, UV photodetectos and UV light emitting diodes etc. Several synthesis routes have been employed for the growth of ZnO nanostructures including sol–gel [10], hydrothermal [11], co-precipitation [12], physical vapour deposition [5,13] techniques etc. Among the synthesis routes, hydrothermal technique is simple, low cost technique, large scale production and the shape, dimensions of ZnO nanostructures can be controlled by the growth parameters. However, the dangling bonds on the surfaces,
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Corresponding author. E-mail address: [email protected] (G. Jayalakshmi).
https://doi.org/10.1016/j.jlumin.2018.06.023 Received 19 April 2018; Received in revised form 7 June 2018; Accepted 8 June 2018 0022-2313/ © 2018 Published by Elsevier B.V.
impurities and large number of intrinsic defects in ZnO, such as Zn interstitials (Zni), oxygen vacancies (Vo), oxygen interstitials (Oi), and oxygen antisite (OZn) are act as electron trapping centers, which potentially quenches the luminescence efficiency in the ultraviolet region [1,14]. In order to obtain the high efficient UV emission, it is essential to enhance the radiative recombination centers and to reduce the defects in the ZnO nanostructures. The hybrid nanostructures comprising of low-dimensional semiconducting nanostructures and 2D carbon based materials offer intriguing optoelectronic properties [15–18]. The hybridization of 2D carbon based materials with ZnO has been potentially investigated for the fabrication of supercapacitors, photocatalysis, photodetectors and solar cell applications [15,19–21]. Reduced graphene oxide (rGO), a 2D sp2-hybridized carbon atoms composed of oxygen containing functional groups such as epoxy, hydroxyl and carboxylic groups in the basal planes as well as edges of graphene facilitate the chemical bonding between rGO and ZnO due to the plasmonic effect of graphene, similar to that of the surface plasmons induced by the metal nanoparticles [22,23]. Recently, an enhanced photoluminescence in ZnO nanostructures assisted by surface plasmon resonance of reduced graphene oxide nanoflakes have been proven an effective means to improve the efficiency of optoelectronic devices such as light emitters [16,24–27].
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improve the crystallinity. The as-synthesized ZnO NRs powders were dispersed in ethanol with the concentration of 1 mg/ml and ultrasonicated for 30 min to produce uniform dispersion and subsequently drop-casted onto the clean Si substrate and then annealed at 500 °C in an air ambient.
Han et al. have reported the photoluminescence behavior of ZnO nanorods by capping with the reduced graphene oxide sheets and the observed enhanced UV emission is attributed to the interfacial charge transfer from ZnO nanorods to rGO [28]. Ding et al. have reported the photoluminescence behavior of ZnO nanorods coated with rGO and found that the intensity of UV emission is significantly increased [8]. However, none of the experimental reports dealing the alternation of local electronic structure of ZnO upon hybridization with the rGO sheets. Further, the correlation of local electronic structure of ZnO:rGO with its photoluminescence behavior has not been reported so far. In the present study, we have highlighted the facile synthesis of ZnO:rGO hybrid nanostructures and investigated its electronic structural and photoluminescence behavior. Finally, a model which demonstrates the synergistic effects between ZnO and rGO via the interfacial charge transfer from the defect states that lies below the conduction band of ZnO to rGO, which could effectively stimulate the coupling of optical transition between ZnO nanorods (NRs) and the sp2hybridized carbon in the sp3- matrix of graphene oxide nanoflakes is proposed.
2.3. Synthesis of reduced graphene oxide (rGO) Graphene oxide was prepared by using modified Hummer's method [27]. Briefly, graphite powder (2 g) and NaNO3 (2 g) was put into a solution of conc. H2SO4 (100 ml) in a beaker and stirred vigorously for 1 h with in an ice bath, whose temperature was maintained below 10 °C. 12 g KMnO4 was gradually added to the above reaction mixture and continued the stirring for 30 mins. The ice bath was then removed and the solution was kept in continuous stirring for 3 h at room temperature. The suspension was then diluted with 250 ml of distilled water followed by addition of 30 ml of H2O2 (30%) to reduce the residual permanganate and manganese dioxide to colourless soluble manganese sulphate. The GO deposit was then centrifuged at 4000 rpm for 10 mins to eliminate the trace amount of un-exfoliated graphite particles and repeatedly washed with deionized water until the pH = 7 was achieved. To obtain reduced graphene oxide (rGO), the exfoliated GO solution of 1 g in 1000 ml was mixed with 1 ml of hydrazine monohydrate under constant stirring and then the mixture was heated at 150 °C for 24 h. The as-obtained black precipitate was further centrifuged at 10,000 rpm for 30 mins and the final product was collected from the centrifugation and then washed with the distilled water and dried in a hot air oven at 90 °C for 4 h.
2. Experimental details 2.1. Growth Mechanism of ZnO NRs The growth of ZnO NRs structure by hydrothermal method consists of hydrolysis of zinc nitrate hexahydrate (Zn(NO3)2·6H2O) in water in the presence of hexamethylenetetramine (C6H12N4) in deionized water and heating the solution constantly at 90 °C for 12 h. The set of chemical reactions (Eqs. (1)–(8)) driven the growth of ZnO NRs architecture is described below: [26]
C6 H12 N4 + 6H2 O ↔ 6HCHO + 4NH3→
(1)
NH3 + H2 O ↔ NH3. H2 O→
(2)
NH3. H2 O ↔ NH4+ + OH−→
(3)
Zn (NO3)2 → Zn2 + + 2NO3−→
(4)
Zn2 + + 2OH− ↔ Zn (OH )2→
(5)
Zn (OH )2 +
↔ [Zn (OH )4
ZnO NRs powder synthesized by using hydrothermal method, described in Section 2.2 and GO powder prepared from graphite powder using modified Hummer's method, described in Section 2.3 were admixed in the ethanol solution with the concentration of 1 mg/ml and 0.5 mg/ml respectively, and the admixed solution was stirred for 1 h and then drop casted onto the Si substrate. Finally, it annealed at 500 °C for 1 h in air ambient to enhance the chemical bonding of few layer reduced graphene oxide sheets with ZnO NRs.
(6)
Zn (OH )2 ↔ ZnO + H2 O→ OH−
2.4. Synthesis of ZnO:rGO hybrid nanostructure on Si substrate
]2 − →
[Zn (OH )4]2 − ↔ ZnO + H2 O + OH−→
(7) (8)
2.5. Characterization
In the hydrothermal process, HMTA serves as a precursor for the growth of ZnO NRs and acts as a buffer layer for the release of OH− ions. It initially hydrolyzed gradually with water and form formaldehyde and ammonia (NH3 ). The NH3 reacts with the water, producing OH− ions and decomposition of zinc nitrate hexahydrate salts provides Zn2+ ions, which are required for the building blocks of ZnO NRs. The electrostatic interaction between the negatively charged, OH− anions and positively charged, Zn2+cations form an intermediate compound, [Zn (OH )4]2 −, which play an important role in the crystal growth of ZnO by dehydration process.
The structural studies of ZnO NRs and ZnO:rGO hybrid nanostructures have been performed by grazing incidence X-ray diffraction GIXRD measurements using a Inel Equinox 2000 × -ray diffractometer operating at 40 kV and a current of 30 mA with Cu Kα radiation of wavelength, 1.5406 Å. Raman analyses of the samples were carried out using WITech Alpha 300 RA spectrometer equipped with a 532 nm laser as an excitation source in the backscattering geometry. The morphology of ZnO and ZnO:rGO hybrid nanostructures were examined using field emission scanning electron microscope (FE-SEM; Carl Zeiss-Neon 40). The elemental composition of the samples was analyzed by energydispersive X-ray (EDX) spectroscopy using an EDX detector (INCA, Oxford). The chemical bonds and composition in the ZnO, ZnO:rGO nanostructures were investigated using a FTIR spectrometer (Bruker Tensor II) equipped with an attenuated total reflectance (ATR) diamond crystal in the range from 500 to 4000 cm−1 with a resolution of 4 cm−1. X-ray absorption near-edge structure (XANES) measurements have been performed using SXAS beam line at Indus-2, Raja Ramanna Center for Advanced Technology, Indore. The room temperature photoluminescence (PL) measurements were recorded using WITech Alpha 300 RA spectrometer with laser excitation of 355 nm and power of 1 mW in the backscattering geometry.
2.2. Synthesis of ZnO NRs on Si substrate ZnO NRs were synthesized by a simple hydrothermal method and then drop-casted onto the Si substrate. Before the deposition, the Si substrates of dimension of 1 cm2 was successively rinsed with ethanol, acetone and distilled water using ultrasonic bath and then dried. Briefly, 0.2 M equimolar aqueous solution of zinc acetate dihydrate and hexamethylenetetramine solutions were admixed, and stirred at room temperature, and then subjected to the hydrothermal reaction at 90 °C for 12 h. The final product was then collected by filtration, washed with deionized water for three times and then annealed at 450 °C for 4 h to 2
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Fig. 2. Raman spectra of ZnO and ZnO:rGO hybrid nanostructures. Fig. 1. XRD pattern of ZnO and ZnO:rGO hybrid nanostructures. The ZnO JCPDS Card No. 36-1451 is given for the comparison.
perpendicular to the sample surface [29]. The absence of E1(LO) phonon mode at 588 cm−1 is related to the oxygen deficiency, which indicates the grown ZnO and ZnO:rGO hybrid nanostructures have high optical quality [30]. The Raman band appeared at 1088 cm−1 and 1156 cm−1 are attributed to the A1(TO+LO) and 2A1(LO) modes respectively. The ZnO:rGO hybrid nanostructure exhibits prominent peaks that corresponds to the D band at ~ 1345 cm–1, G band at ~ 1577 cm–1, 2D band at 2684 cm–1, D+G band at 2928 cm–1, respectively [31]. The G band originates from the first-order scattering of doubly degenerate E2 g phonons in the Brillouine zone center, whereas D band is attributed to the structural imperfections created by the attachment of oxygenated groups on the basal planes [32]. The 2D and D +G bands are the second-order of D band and combination of D and G bands, respectively. The superposition of ZnO phonon modes with the rGO phonon modes confirms the formation of ZnO:rGO hybrid nanostructures. Fig. 2 The surface morphology and elemental composition of ZnO NRs, ZnO:rGO hybrid nanostructures were studied using FE-SEM and EDS analyses, respectively and are presented in Fig. 3. The top panel of Fig. 3 presents the surface morphology of the grown ZnO, ZnO:rGO hybrid nanostructures obtained using the solution phase deposition. FESEM micrograph shows the well flatted hexagonal faceted ZnO nanorods of different diameters, which are uniformly distributed, randomly oriented onto the Si substrate, and are indicated by the yellow arrow marks. The length of the nanorods is found to be ranging from 40 to 250 nm. It is a strange to measure the diameter of the rod as it is in the random orientation, because the ZnO NRs were drop casted and dried onto a Si substrate to prepare a thin film. The hybridization of rGO sheets with ZnO leads the aggregation of small clusters of ZnO NRs in conjugation with rGO sheets. The FE-SEM micrograph of ZnO:rGO hybrid nanostructures clearly reveals that the rGO sheets were predominantly consist of single, double and few-layer graphene sheets, with some of them being overlapped and are folded at the edges with numerous wrinkles on its surface. The chemical composition of ZnO NRs and ZnO:rGO hybrid nanostructures investigated using the EDX analyses is shown in bottom panel of Fig. 3. EDX spectrum clearly shows the presence of Zn and O elements in the ZnO NRs. The inset of Fig. 3 shows the composition profile of the elements present in the grown nanostructures. In addition to Zn and O elements, the presence
3. Results and discussion GIXRD pattern of ZnO NRs and ZnO: rGO hybrid nanostructures is shown in Fig. 1. The diffraction peaks appeared at 2θ values of 31.76°, 34.51°, 36.33°, 47.69°, 56.86°, 63.10°, 66.68°, 68.22°, and 69.32° corresponds to (100), (002), (101), (102), (110), (103), (200), (112), and (201) planes of hexagonal wurtzite structure of ZnO. The diffraction peaks were indexed according to the JCPDS No. 36–1451. No characteristic peaks of other impurities and secondary/intermediate phases were detected in the XRD pattern. The additional peak found at 2θ of 27.92° in ZnO:rGO hybrid nanostructures are accounted for the reflection from the (002) plane of the rGO. It is found that there is no considerable shift in the peak position of ZnO upon hybridization with rGO, which implies rGO is in better conjugation with the surface of the ZnO NRs, without destructing its crystalline structure. Further, the coupling of rGO with the ZnO NRs has prevented the restacking of rGO sheets in the ZnO:rGO nanostructure. From the XRD result, it is infer that ZnO:rGO hybrid nanostructures is composed of ZnO with a hexagonal wurtzite structure and rGO. Raman spectroscopy is widely used tool for the structural characterization of nanomaterials. The hexagonal wurtzite structure of ZnO has the space group of C64v with two formula units in the primitive cell with all atoms occupying the C3v sites. The zone-center optical phonons can be classified as the following irreducible representations:
Γ = A1 + 2B1 + E1 + 2E2
(9)
where, A1 and E1 modes are polar and split into the transverse optical (TO) and longitudinal optical (LO) phonons. Raman spectra of ZnO samples clearly show E2high and E2low and A1(LO) mode along with the overlapping of the Si substrates modes. The E2 mode consists of two modes: E2high is associated with the vibration of oxygen atoms and E2low is associated with the Zn sublattice [28]. The dominant E2low mode at 101 cm−1 and E2high mode at 443 cm−1 indicates high crystalline quality of the sample. Raman mode centered at 332 cm−1 is ascribed to the E2high − E2low mode merged with the Si substrate peak. The A1(LO) phonons appears only when the c-axis of wurtzite ZnO is parallel to the sample surface and E1(LO) phonons appears when the light is 3
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Fig. 3. FE-SEM micrographs of ZnO and ZnO:rGO hybrid nanostructures and its corresponding elemental composition recorded using EDX spectra is given below with the composition profile in the inset.
the O–H bending vibration, respectively. The band centered at ~ 1660 cm−1 corresponds to the C˭C skeletal vibration from the un-oxidized graphitic domains [35]. Due to the strong hybridization between the ZnO and rGO in the ZnO:rGO composite nanostructure, the stretching vibration of ZnO shifted from ~ 516 cm−1 to ~ 520 cm−1. Further, in the ZnO:rGO composites nanostructure, it is observed that the O–H stretching vibration at ~ 3345 cm−1 are almost disappeared due to the deoxygenation/reduction of GO and in the addition to the stretching vibrations of ZnO, peak centered at ~ 1660 cm−1 indicating the hybridization of rGO sheets with ZnO. Fig. 5 presents O K-edge XANES spectra of ZnO NRs, ZnO:rGO hybrid nanostructures recorded using the surface sensitive total electron yield (TEY) mode. The O K-edge XANES spectra probes the spectral features of O-2p density of states in the conduction band hybridized with different Zn orbitals. The observed spectral features in the energy region between 530 and 540 eV are labelled A and B are ascribed to the hybridization of O 2p with highly dispersive Zn 3d/4 s states, which form the bottom of the conduction band. The spectral features in the region between 540 and 548 eV, are labelled C and D, respectively are assigned to the O 2p states hybridized with Zn 4p states and features above 548 eV are ascribed to the hybridization of O 2p states with Zn higher orbitals, respectively [36,37]. The drastic increase in intensity of spectral pre-edges, A and B of ZnO:rGO hybrid nanostructures is ascribed to the Zn 3d/4s states hybridized with C 2p states results in increase of unoccupied density of states in the conduction band. In other words, the strong hybridization of s-p-d orbitals causes the alternation in the electronic structure of ZnO:rGO by introducing the more unoccupied density of states in the conduction band. The PL characteristics of ZnO NRs and ZnO:rGO hybrid
of carbon is clearly seen in the spectra, which confirms the formation of ZnO:rGO hybrid nanostructures. No other impurities were detected besides the elements present, which indicates the high purity of the grown nanostructures. Fig. 4 shows the ATR-FTIR spectra of ZnO and ZnO: rGO composites. The broad band centered at ~ 516, 677, 1375 and 1585 cm−1 are attributed to the stretching vibrations of ZnO [33]. The bands centered at ~ 1046, 1118, 1796 cm−1 and 3345 cm−1 are ascribed to the C–O, C–O–C, C˭O, C–OH stretching vibrations, respectively [34,35]. The band centered at 2850 cm−1, ~2922 cm−1 and ~1398 cm−1 corresponds to the asymmetric and symmetric C–H stretching vibrations, and
Fig. 4. ATR-FTIR spectra of ZnO, ZnO:rGO hybrid nanostructures. 4
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demonstrating interfacial charge transfer that takes place from the trapping states of ZnO to rGO. The enhanced UV and the suppressed visible emission in ZnO:rGO hybrid nanostructures is attributed to the Zn2+ ions on the surface of the nanorods bonded with the negatively charged rGO sheets through the electrostatic interactions leading to the local creation of Zn–O–C bonding. The interfacial charge transfer from ZnO to rGO takes place through the defect levels due to the favourable matching of energy levels. In the energy level diagram of ZnO, the conduction band and valence band of ZnO lies about 4.05 eV and 7.25 eV (vs. vacuum), respectively, defect states such as Zni and Vo++ lies 0.22 eV and 2 eV below the conduction band, and the work function of rGO is 4.42 eV [41–43]. The energy level of Zni in ZnO nanostructures is comparable with the Fermi energy of the rGO facilitate the electron transfer from the Zni trap states of ZnO to rGO, thus the synergetic effect from both the components enhances the effective radiative recombination of electrons results in improved UV, blue emissions. The quenching of visible emission in ZnO:rGO hybrid nanostructures refers to the charge transfer from the trap states of ZnO to rGO occurs at the interface. Until recently, overall, very few reports have dealing the PL behavior of ZnO, ZnO:rGO composites. Ding et al. have reported the enhanced UV emission centered at ~ 390 nm and the relatively suppressed visible emission at ~442 nm, 450–600 nm for the ZnO nanorods coated with GO and RGO sheets [8]. Kim et al. have reported PL spectra of RGO-incorporated ZnO films with and without MgO spacer layers between ZnO and RGO layers and found that PL intensity monotonically decreases as the distance between the ZnO and RGO layers increases [22]. Hwang et al. have reported the substantially enhanced PL emission from hybrid structures of graphene/ZnO films with a different number of layers on the ZnO films and found that PL intensity in the UV region is stronger for the single layer graphene than that of the bilayer graphene on the ZnO [44]. But in the present study, it is interesting that the ZnO:rGO hybrid nanostructure exhibits an enhanced UV, blue emission and completely suppressed visible emission due to the interfacial charge transfer from ZnO to rGO takes place through the defect level in ZnO.
Fig. 5. XANES spectra of ZnO (red circles) and ZnO:rGO (blue line) hybrid nanostructures.
Fig. 6. PL spectra of ZnO and ZnO:rGO hybrid nanostructures.
nanostructures excited using 355 nm laser line is shown in Fig. 6. PL spectra show three main features at about 386 nm, 432 nm and 620 nm respectively. The narrow UV emission band at ~386 nm is attributed to the recombination of a hole in the valence band and an electron in the conduction band (so called excitonic emission). The emission in the visible region is well-known for the intrinsic defects in ZnO, such as Zn interstitials (Zni), oxygen vacancies (Vo), oxygen interstitials (Oi), and oxygen antisite (OZn) respectively, which are trapping centers for the electron-hole recombination [38]. The emission peak centered at ~ 432 nm, corresponds to the electron transition from the defect state such as Zni to the top of the valence band [8]. The broad emission peak centered at ~ 620 nm corresponds to the recombination of a conduction band electron with a doubly ionized oxygen-vacancy defects (Vo++) located 2 eV below the conduction band edge. It is well known that these defect state and the surface defects in ZnO nanostructures acts as electrons trap centers, which affects the luminescence efficiency of ZnO nanostructures [8,39]. Recent experimental reports reveal that rGO sheets itself exhibits blue luminescence centered at ~ 425 nm due to the recombination of electron-hole pairs from the large number of localized sp2-carbon clusters embedded within the sp3 matrix [35,40]. The hybridization of rGO with ZnO nanostructures significantly modifies the emission characteristics of ZnO nanostructures. The enhanced UV, blue emissions and the suppressed defect level emission is observed in ZnO: rGO hybrid nanostructures in comparison with ZnO nanostructures. Fig. 7(a) displays the schematic diagram illustrating the recombination of electrons in the conduction band (CB) to the holes in the valence band (VB) of ZnO, the recombination of electrons in the defect state, Zni to the VB, the recombination of electrons from the CB to the defect state, Vo++, respectively. Fig. 7(b) shows the band diagram
4. Conclusions In summary, ZnO NRs and ZnO:rGO hybrid nanostructures synthesized via the hydrothermal route were drop casted onto the Si substrates. The X-ray diffraction pattern reveals the evolution of diffraction from the hexagonal wurtzite structure of ZnO and (002) plane of the rGO. The evolution of phonon modes of ZnO along with the D band, G band of rGO implies the formation of ZnO:rGO hybrid nanostructures. FE-SEM microscopy analysis reveals hexagonal faceted ZnO nanorods having good interfacial contact with the rGO sheets and the EDX spectra clearly reveals the elemental composition present in the nanostructures. The O-K edge XANES spectra reveals the strong hybridization between the Zn 3d/4 s states hybridized with C 2p states results in increase of unoccupied density of states in the conduction band. The enhanced UV, blue emission and suppressed visible emission is ascribed to the interfacial charge transfer from Zni trapping state of ZnO to rGO. Our experimental results demonstrate that the hybrid nanostructures such as semiconductor/reduced graphene oxide hybrids is a promising potential candidate in energy harvesting devices including solar cells, photodetectors, sensors and photocatalysis with the improved performance. Acknowledgments One of the authors, G J wishes to thank the Science and Engineering Research Board (SERB), Govt. of India, for financial support through Fast-Track Young Scientist Scheme (Grant No. YSS/2015/000240) and also extended thanks to Dr. T. Arun, Institute of Physics, Bhubaneswar for FE-SEM, EDX measurements. K. S would like to acknowledge Dr. 5
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Fig. 7. (a) The schematic diagram illustrating the recombination of electrons in the conduction band (CB) to the holes in the valence band (VB) of ZnO, the recombination of electrons in the defect state, Zni to the VB, the recombination of electrons from the CB to the defect state, Vo++, respectively. (b) shows the band diagram demonstrating interfacial charge transfer that takes place from the trapping states (Zni) of ZnO to rGO.
Mukul Gupta for the help in XANES measurements.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Facile synthesis and enhanced luminescence behavior of ZnO:Reduced graphene oxide(rGO) hybrid nanostructures
T
⁎
G. Jayalakshmia, , K. Saravanana, Jagnaseni Pradhana,b, P. Magudapathya, B.K. Panigrahib,c a
Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, Tamilnadu, India Homi Bhabha National Institute, Kalpakkam 603102, Tamilnadu, India c Electronics and Instrumentation Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnO:rGO hybrid nanostructures Photoluminescence Trap states XANES Charge transfer
The structural, electronic and luminescence behavior of ZnO nanorods and ZnO-reduced graphene oxide (ZnO:rGO) hybrid nanostructures have been studied using X-diffraction (XRD), Raman scattering, X-ray absorption near edge structure (XANES) spectroscopy and photoluminescence (PL) studies. XRD and Raman analyses reveal the formation of hybrid nanostructures composed of ZnO and rGO. The morphology and elemental composition of the grown nanostructures were investigated using FESEM and EDX measurements. The hybridization of rGO with ZnO provides an additional path way for the transfer of charge from the defect states of ZnO to rGO as evidenced from the increase in unoccupied density of states in the O-K edge XANES spectra and the enhanced UV, blue emissions as well as the suppressed visible emission in the PL spectra, respectively. Our systematic experimental investigations demonstrate the hybridization of rGO with ZnO is an effective approach to tailor the optical properties of ZnO and offers the prospective way for the fabrication of novel optoelectronic devices.
1. Introduction ZnO is a promising semiconducting material because of its fascinating properties such as direct wide band gap (3.37 eV), high exciton binding energy (60 meV) at room temperature, high surface area, high mechanical strength, thermal stabilities, and high oscillator strength of excitonic transitions find applications in various technological devices including optoelectronics devices, solar cells, sensors, spintronic devices, photocatalysis, UV photodetectors and ultraviolet lasers [1–7]. The one-dimensional nanostructures such as nanorods, nanowires, nanotubes, nanoflowers, nanobelts have been attracted due to their remarkable physical and chemical properties that can be used in various electronic and optoelectronic devices [8,9]. The luminescent behavior in ultraviolet (UV) region is of great interest due to its potential applications in optical and optoelectronic devices such as UV lasers, UV photodetectos and UV light emitting diodes etc. Several synthesis routes have been employed for the growth of ZnO nanostructures including sol–gel [10], hydrothermal [11], co-precipitation [12], physical vapour deposition [5,13] techniques etc. Among the synthesis routes, hydrothermal technique is simple, low cost technique, large scale production and the shape, dimensions of ZnO nanostructures can be controlled by the growth parameters. However, the dangling bonds on the surfaces,
⁎
Corresponding author. E-mail address: [email protected] (G. Jayalakshmi).
https://doi.org/10.1016/j.jlumin.2018.06.023 Received 19 April 2018; Received in revised form 7 June 2018; Accepted 8 June 2018 0022-2313/ © 2018 Published by Elsevier B.V.
impurities and large number of intrinsic defects in ZnO, such as Zn interstitials (Zni), oxygen vacancies (Vo), oxygen interstitials (Oi), and oxygen antisite (OZn) are act as electron trapping centers, which potentially quenches the luminescence efficiency in the ultraviolet region [1,14]. In order to obtain the high efficient UV emission, it is essential to enhance the radiative recombination centers and to reduce the defects in the ZnO nanostructures. The hybrid nanostructures comprising of low-dimensional semiconducting nanostructures and 2D carbon based materials offer intriguing optoelectronic properties [15–18]. The hybridization of 2D carbon based materials with ZnO has been potentially investigated for the fabrication of supercapacitors, photocatalysis, photodetectors and solar cell applications [15,19–21]. Reduced graphene oxide (rGO), a 2D sp2-hybridized carbon atoms composed of oxygen containing functional groups such as epoxy, hydroxyl and carboxylic groups in the basal planes as well as edges of graphene facilitate the chemical bonding between rGO and ZnO due to the plasmonic effect of graphene, similar to that of the surface plasmons induced by the metal nanoparticles [22,23]. Recently, an enhanced photoluminescence in ZnO nanostructures assisted by surface plasmon resonance of reduced graphene oxide nanoflakes have been proven an effective means to improve the efficiency of optoelectronic devices such as light emitters [16,24–27].
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improve the crystallinity. The as-synthesized ZnO NRs powders were dispersed in ethanol with the concentration of 1 mg/ml and ultrasonicated for 30 min to produce uniform dispersion and subsequently drop-casted onto the clean Si substrate and then annealed at 500 °C in an air ambient.
Han et al. have reported the photoluminescence behavior of ZnO nanorods by capping with the reduced graphene oxide sheets and the observed enhanced UV emission is attributed to the interfacial charge transfer from ZnO nanorods to rGO [28]. Ding et al. have reported the photoluminescence behavior of ZnO nanorods coated with rGO and found that the intensity of UV emission is significantly increased [8]. However, none of the experimental reports dealing the alternation of local electronic structure of ZnO upon hybridization with the rGO sheets. Further, the correlation of local electronic structure of ZnO:rGO with its photoluminescence behavior has not been reported so far. In the present study, we have highlighted the facile synthesis of ZnO:rGO hybrid nanostructures and investigated its electronic structural and photoluminescence behavior. Finally, a model which demonstrates the synergistic effects between ZnO and rGO via the interfacial charge transfer from the defect states that lies below the conduction band of ZnO to rGO, which could effectively stimulate the coupling of optical transition between ZnO nanorods (NRs) and the sp2hybridized carbon in the sp3- matrix of graphene oxide nanoflakes is proposed.
2.3. Synthesis of reduced graphene oxide (rGO) Graphene oxide was prepared by using modified Hummer's method [27]. Briefly, graphite powder (2 g) and NaNO3 (2 g) was put into a solution of conc. H2SO4 (100 ml) in a beaker and stirred vigorously for 1 h with in an ice bath, whose temperature was maintained below 10 °C. 12 g KMnO4 was gradually added to the above reaction mixture and continued the stirring for 30 mins. The ice bath was then removed and the solution was kept in continuous stirring for 3 h at room temperature. The suspension was then diluted with 250 ml of distilled water followed by addition of 30 ml of H2O2 (30%) to reduce the residual permanganate and manganese dioxide to colourless soluble manganese sulphate. The GO deposit was then centrifuged at 4000 rpm for 10 mins to eliminate the trace amount of un-exfoliated graphite particles and repeatedly washed with deionized water until the pH = 7 was achieved. To obtain reduced graphene oxide (rGO), the exfoliated GO solution of 1 g in 1000 ml was mixed with 1 ml of hydrazine monohydrate under constant stirring and then the mixture was heated at 150 °C for 24 h. The as-obtained black precipitate was further centrifuged at 10,000 rpm for 30 mins and the final product was collected from the centrifugation and then washed with the distilled water and dried in a hot air oven at 90 °C for 4 h.
2. Experimental details 2.1. Growth Mechanism of ZnO NRs The growth of ZnO NRs structure by hydrothermal method consists of hydrolysis of zinc nitrate hexahydrate (Zn(NO3)2·6H2O) in water in the presence of hexamethylenetetramine (C6H12N4) in deionized water and heating the solution constantly at 90 °C for 12 h. The set of chemical reactions (Eqs. (1)–(8)) driven the growth of ZnO NRs architecture is described below: [26]
C6 H12 N4 + 6H2 O ↔ 6HCHO + 4NH3→
(1)
NH3 + H2 O ↔ NH3. H2 O→
(2)
NH3. H2 O ↔ NH4+ + OH−→
(3)
Zn (NO3)2 → Zn2 + + 2NO3−→
(4)
Zn2 + + 2OH− ↔ Zn (OH )2→
(5)
Zn (OH )2 +
↔ [Zn (OH )4
ZnO NRs powder synthesized by using hydrothermal method, described in Section 2.2 and GO powder prepared from graphite powder using modified Hummer's method, described in Section 2.3 were admixed in the ethanol solution with the concentration of 1 mg/ml and 0.5 mg/ml respectively, and the admixed solution was stirred for 1 h and then drop casted onto the Si substrate. Finally, it annealed at 500 °C for 1 h in air ambient to enhance the chemical bonding of few layer reduced graphene oxide sheets with ZnO NRs.
(6)
Zn (OH )2 ↔ ZnO + H2 O→ OH−
2.4. Synthesis of ZnO:rGO hybrid nanostructure on Si substrate
]2 − →
[Zn (OH )4]2 − ↔ ZnO + H2 O + OH−→
(7) (8)
2.5. Characterization
In the hydrothermal process, HMTA serves as a precursor for the growth of ZnO NRs and acts as a buffer layer for the release of OH− ions. It initially hydrolyzed gradually with water and form formaldehyde and ammonia (NH3 ). The NH3 reacts with the water, producing OH− ions and decomposition of zinc nitrate hexahydrate salts provides Zn2+ ions, which are required for the building blocks of ZnO NRs. The electrostatic interaction between the negatively charged, OH− anions and positively charged, Zn2+cations form an intermediate compound, [Zn (OH )4]2 −, which play an important role in the crystal growth of ZnO by dehydration process.
The structural studies of ZnO NRs and ZnO:rGO hybrid nanostructures have been performed by grazing incidence X-ray diffraction GIXRD measurements using a Inel Equinox 2000 × -ray diffractometer operating at 40 kV and a current of 30 mA with Cu Kα radiation of wavelength, 1.5406 Å. Raman analyses of the samples were carried out using WITech Alpha 300 RA spectrometer equipped with a 532 nm laser as an excitation source in the backscattering geometry. The morphology of ZnO and ZnO:rGO hybrid nanostructures were examined using field emission scanning electron microscope (FE-SEM; Carl Zeiss-Neon 40). The elemental composition of the samples was analyzed by energydispersive X-ray (EDX) spectroscopy using an EDX detector (INCA, Oxford). The chemical bonds and composition in the ZnO, ZnO:rGO nanostructures were investigated using a FTIR spectrometer (Bruker Tensor II) equipped with an attenuated total reflectance (ATR) diamond crystal in the range from 500 to 4000 cm−1 with a resolution of 4 cm−1. X-ray absorption near-edge structure (XANES) measurements have been performed using SXAS beam line at Indus-2, Raja Ramanna Center for Advanced Technology, Indore. The room temperature photoluminescence (PL) measurements were recorded using WITech Alpha 300 RA spectrometer with laser excitation of 355 nm and power of 1 mW in the backscattering geometry.
2.2. Synthesis of ZnO NRs on Si substrate ZnO NRs were synthesized by a simple hydrothermal method and then drop-casted onto the Si substrate. Before the deposition, the Si substrates of dimension of 1 cm2 was successively rinsed with ethanol, acetone and distilled water using ultrasonic bath and then dried. Briefly, 0.2 M equimolar aqueous solution of zinc acetate dihydrate and hexamethylenetetramine solutions were admixed, and stirred at room temperature, and then subjected to the hydrothermal reaction at 90 °C for 12 h. The final product was then collected by filtration, washed with deionized water for three times and then annealed at 450 °C for 4 h to 2
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Fig. 2. Raman spectra of ZnO and ZnO:rGO hybrid nanostructures. Fig. 1. XRD pattern of ZnO and ZnO:rGO hybrid nanostructures. The ZnO JCPDS Card No. 36-1451 is given for the comparison.
perpendicular to the sample surface [29]. The absence of E1(LO) phonon mode at 588 cm−1 is related to the oxygen deficiency, which indicates the grown ZnO and ZnO:rGO hybrid nanostructures have high optical quality [30]. The Raman band appeared at 1088 cm−1 and 1156 cm−1 are attributed to the A1(TO+LO) and 2A1(LO) modes respectively. The ZnO:rGO hybrid nanostructure exhibits prominent peaks that corresponds to the D band at ~ 1345 cm–1, G band at ~ 1577 cm–1, 2D band at 2684 cm–1, D+G band at 2928 cm–1, respectively [31]. The G band originates from the first-order scattering of doubly degenerate E2 g phonons in the Brillouine zone center, whereas D band is attributed to the structural imperfections created by the attachment of oxygenated groups on the basal planes [32]. The 2D and D +G bands are the second-order of D band and combination of D and G bands, respectively. The superposition of ZnO phonon modes with the rGO phonon modes confirms the formation of ZnO:rGO hybrid nanostructures. Fig. 2 The surface morphology and elemental composition of ZnO NRs, ZnO:rGO hybrid nanostructures were studied using FE-SEM and EDS analyses, respectively and are presented in Fig. 3. The top panel of Fig. 3 presents the surface morphology of the grown ZnO, ZnO:rGO hybrid nanostructures obtained using the solution phase deposition. FESEM micrograph shows the well flatted hexagonal faceted ZnO nanorods of different diameters, which are uniformly distributed, randomly oriented onto the Si substrate, and are indicated by the yellow arrow marks. The length of the nanorods is found to be ranging from 40 to 250 nm. It is a strange to measure the diameter of the rod as it is in the random orientation, because the ZnO NRs were drop casted and dried onto a Si substrate to prepare a thin film. The hybridization of rGO sheets with ZnO leads the aggregation of small clusters of ZnO NRs in conjugation with rGO sheets. The FE-SEM micrograph of ZnO:rGO hybrid nanostructures clearly reveals that the rGO sheets were predominantly consist of single, double and few-layer graphene sheets, with some of them being overlapped and are folded at the edges with numerous wrinkles on its surface. The chemical composition of ZnO NRs and ZnO:rGO hybrid nanostructures investigated using the EDX analyses is shown in bottom panel of Fig. 3. EDX spectrum clearly shows the presence of Zn and O elements in the ZnO NRs. The inset of Fig. 3 shows the composition profile of the elements present in the grown nanostructures. In addition to Zn and O elements, the presence
3. Results and discussion GIXRD pattern of ZnO NRs and ZnO: rGO hybrid nanostructures is shown in Fig. 1. The diffraction peaks appeared at 2θ values of 31.76°, 34.51°, 36.33°, 47.69°, 56.86°, 63.10°, 66.68°, 68.22°, and 69.32° corresponds to (100), (002), (101), (102), (110), (103), (200), (112), and (201) planes of hexagonal wurtzite structure of ZnO. The diffraction peaks were indexed according to the JCPDS No. 36–1451. No characteristic peaks of other impurities and secondary/intermediate phases were detected in the XRD pattern. The additional peak found at 2θ of 27.92° in ZnO:rGO hybrid nanostructures are accounted for the reflection from the (002) plane of the rGO. It is found that there is no considerable shift in the peak position of ZnO upon hybridization with rGO, which implies rGO is in better conjugation with the surface of the ZnO NRs, without destructing its crystalline structure. Further, the coupling of rGO with the ZnO NRs has prevented the restacking of rGO sheets in the ZnO:rGO nanostructure. From the XRD result, it is infer that ZnO:rGO hybrid nanostructures is composed of ZnO with a hexagonal wurtzite structure and rGO. Raman spectroscopy is widely used tool for the structural characterization of nanomaterials. The hexagonal wurtzite structure of ZnO has the space group of C64v with two formula units in the primitive cell with all atoms occupying the C3v sites. The zone-center optical phonons can be classified as the following irreducible representations:
Γ = A1 + 2B1 + E1 + 2E2
(9)
where, A1 and E1 modes are polar and split into the transverse optical (TO) and longitudinal optical (LO) phonons. Raman spectra of ZnO samples clearly show E2high and E2low and A1(LO) mode along with the overlapping of the Si substrates modes. The E2 mode consists of two modes: E2high is associated with the vibration of oxygen atoms and E2low is associated with the Zn sublattice [28]. The dominant E2low mode at 101 cm−1 and E2high mode at 443 cm−1 indicates high crystalline quality of the sample. Raman mode centered at 332 cm−1 is ascribed to the E2high − E2low mode merged with the Si substrate peak. The A1(LO) phonons appears only when the c-axis of wurtzite ZnO is parallel to the sample surface and E1(LO) phonons appears when the light is 3
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Fig. 3. FE-SEM micrographs of ZnO and ZnO:rGO hybrid nanostructures and its corresponding elemental composition recorded using EDX spectra is given below with the composition profile in the inset.
the O–H bending vibration, respectively. The band centered at ~ 1660 cm−1 corresponds to the C˭C skeletal vibration from the un-oxidized graphitic domains [35]. Due to the strong hybridization between the ZnO and rGO in the ZnO:rGO composite nanostructure, the stretching vibration of ZnO shifted from ~ 516 cm−1 to ~ 520 cm−1. Further, in the ZnO:rGO composites nanostructure, it is observed that the O–H stretching vibration at ~ 3345 cm−1 are almost disappeared due to the deoxygenation/reduction of GO and in the addition to the stretching vibrations of ZnO, peak centered at ~ 1660 cm−1 indicating the hybridization of rGO sheets with ZnO. Fig. 5 presents O K-edge XANES spectra of ZnO NRs, ZnO:rGO hybrid nanostructures recorded using the surface sensitive total electron yield (TEY) mode. The O K-edge XANES spectra probes the spectral features of O-2p density of states in the conduction band hybridized with different Zn orbitals. The observed spectral features in the energy region between 530 and 540 eV are labelled A and B are ascribed to the hybridization of O 2p with highly dispersive Zn 3d/4 s states, which form the bottom of the conduction band. The spectral features in the region between 540 and 548 eV, are labelled C and D, respectively are assigned to the O 2p states hybridized with Zn 4p states and features above 548 eV are ascribed to the hybridization of O 2p states with Zn higher orbitals, respectively [36,37]. The drastic increase in intensity of spectral pre-edges, A and B of ZnO:rGO hybrid nanostructures is ascribed to the Zn 3d/4s states hybridized with C 2p states results in increase of unoccupied density of states in the conduction band. In other words, the strong hybridization of s-p-d orbitals causes the alternation in the electronic structure of ZnO:rGO by introducing the more unoccupied density of states in the conduction band. The PL characteristics of ZnO NRs and ZnO:rGO hybrid
of carbon is clearly seen in the spectra, which confirms the formation of ZnO:rGO hybrid nanostructures. No other impurities were detected besides the elements present, which indicates the high purity of the grown nanostructures. Fig. 4 shows the ATR-FTIR spectra of ZnO and ZnO: rGO composites. The broad band centered at ~ 516, 677, 1375 and 1585 cm−1 are attributed to the stretching vibrations of ZnO [33]. The bands centered at ~ 1046, 1118, 1796 cm−1 and 3345 cm−1 are ascribed to the C–O, C–O–C, C˭O, C–OH stretching vibrations, respectively [34,35]. The band centered at 2850 cm−1, ~2922 cm−1 and ~1398 cm−1 corresponds to the asymmetric and symmetric C–H stretching vibrations, and
Fig. 4. ATR-FTIR spectra of ZnO, ZnO:rGO hybrid nanostructures. 4
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demonstrating interfacial charge transfer that takes place from the trapping states of ZnO to rGO. The enhanced UV and the suppressed visible emission in ZnO:rGO hybrid nanostructures is attributed to the Zn2+ ions on the surface of the nanorods bonded with the negatively charged rGO sheets through the electrostatic interactions leading to the local creation of Zn–O–C bonding. The interfacial charge transfer from ZnO to rGO takes place through the defect levels due to the favourable matching of energy levels. In the energy level diagram of ZnO, the conduction band and valence band of ZnO lies about 4.05 eV and 7.25 eV (vs. vacuum), respectively, defect states such as Zni and Vo++ lies 0.22 eV and 2 eV below the conduction band, and the work function of rGO is 4.42 eV [41–43]. The energy level of Zni in ZnO nanostructures is comparable with the Fermi energy of the rGO facilitate the electron transfer from the Zni trap states of ZnO to rGO, thus the synergetic effect from both the components enhances the effective radiative recombination of electrons results in improved UV, blue emissions. The quenching of visible emission in ZnO:rGO hybrid nanostructures refers to the charge transfer from the trap states of ZnO to rGO occurs at the interface. Until recently, overall, very few reports have dealing the PL behavior of ZnO, ZnO:rGO composites. Ding et al. have reported the enhanced UV emission centered at ~ 390 nm and the relatively suppressed visible emission at ~442 nm, 450–600 nm for the ZnO nanorods coated with GO and RGO sheets [8]. Kim et al. have reported PL spectra of RGO-incorporated ZnO films with and without MgO spacer layers between ZnO and RGO layers and found that PL intensity monotonically decreases as the distance between the ZnO and RGO layers increases [22]. Hwang et al. have reported the substantially enhanced PL emission from hybrid structures of graphene/ZnO films with a different number of layers on the ZnO films and found that PL intensity in the UV region is stronger for the single layer graphene than that of the bilayer graphene on the ZnO [44]. But in the present study, it is interesting that the ZnO:rGO hybrid nanostructure exhibits an enhanced UV, blue emission and completely suppressed visible emission due to the interfacial charge transfer from ZnO to rGO takes place through the defect level in ZnO.
Fig. 5. XANES spectra of ZnO (red circles) and ZnO:rGO (blue line) hybrid nanostructures.
Fig. 6. PL spectra of ZnO and ZnO:rGO hybrid nanostructures.
nanostructures excited using 355 nm laser line is shown in Fig. 6. PL spectra show three main features at about 386 nm, 432 nm and 620 nm respectively. The narrow UV emission band at ~386 nm is attributed to the recombination of a hole in the valence band and an electron in the conduction band (so called excitonic emission). The emission in the visible region is well-known for the intrinsic defects in ZnO, such as Zn interstitials (Zni), oxygen vacancies (Vo), oxygen interstitials (Oi), and oxygen antisite (OZn) respectively, which are trapping centers for the electron-hole recombination [38]. The emission peak centered at ~ 432 nm, corresponds to the electron transition from the defect state such as Zni to the top of the valence band [8]. The broad emission peak centered at ~ 620 nm corresponds to the recombination of a conduction band electron with a doubly ionized oxygen-vacancy defects (Vo++) located 2 eV below the conduction band edge. It is well known that these defect state and the surface defects in ZnO nanostructures acts as electrons trap centers, which affects the luminescence efficiency of ZnO nanostructures [8,39]. Recent experimental reports reveal that rGO sheets itself exhibits blue luminescence centered at ~ 425 nm due to the recombination of electron-hole pairs from the large number of localized sp2-carbon clusters embedded within the sp3 matrix [35,40]. The hybridization of rGO with ZnO nanostructures significantly modifies the emission characteristics of ZnO nanostructures. The enhanced UV, blue emissions and the suppressed defect level emission is observed in ZnO: rGO hybrid nanostructures in comparison with ZnO nanostructures. Fig. 7(a) displays the schematic diagram illustrating the recombination of electrons in the conduction band (CB) to the holes in the valence band (VB) of ZnO, the recombination of electrons in the defect state, Zni to the VB, the recombination of electrons from the CB to the defect state, Vo++, respectively. Fig. 7(b) shows the band diagram
4. Conclusions In summary, ZnO NRs and ZnO:rGO hybrid nanostructures synthesized via the hydrothermal route were drop casted onto the Si substrates. The X-ray diffraction pattern reveals the evolution of diffraction from the hexagonal wurtzite structure of ZnO and (002) plane of the rGO. The evolution of phonon modes of ZnO along with the D band, G band of rGO implies the formation of ZnO:rGO hybrid nanostructures. FE-SEM microscopy analysis reveals hexagonal faceted ZnO nanorods having good interfacial contact with the rGO sheets and the EDX spectra clearly reveals the elemental composition present in the nanostructures. The O-K edge XANES spectra reveals the strong hybridization between the Zn 3d/4 s states hybridized with C 2p states results in increase of unoccupied density of states in the conduction band. The enhanced UV, blue emission and suppressed visible emission is ascribed to the interfacial charge transfer from Zni trapping state of ZnO to rGO. Our experimental results demonstrate that the hybrid nanostructures such as semiconductor/reduced graphene oxide hybrids is a promising potential candidate in energy harvesting devices including solar cells, photodetectors, sensors and photocatalysis with the improved performance. Acknowledgments One of the authors, G J wishes to thank the Science and Engineering Research Board (SERB), Govt. of India, for financial support through Fast-Track Young Scientist Scheme (Grant No. YSS/2015/000240) and also extended thanks to Dr. T. Arun, Institute of Physics, Bhubaneswar for FE-SEM, EDX measurements. K. S would like to acknowledge Dr. 5
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Fig. 7. (a) The schematic diagram illustrating the recombination of electrons in the conduction band (CB) to the holes in the valence band (VB) of ZnO, the recombination of electrons in the defect state, Zni to the VB, the recombination of electrons from the CB to the defect state, Vo++, respectively. (b) shows the band diagram demonstrating interfacial charge transfer that takes place from the trapping states (Zni) of ZnO to rGO.
Mukul Gupta for the help in XANES measurements.
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