ARTICLE IN PRESS Journal of Luminescence 129 (2009) 434–438
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Optical characterization of ZnO nanoparticles capped with various surfactants M.L. Singla , Muhamed Shafeeq M, Manish Kumar Central Scientific Instruments Organization (CSIO), (Council of Scientific and Industrial Research, New Delhi), Materials Research and Bio-Nanotechnology Division, Sector-30/C, Chandigarh-160 030, India
a r t i c l e in f o
a b s t r a c t
Article history: Received 9 July 2008 Received in revised form 6 November 2008 Accepted 11 November 2008 Available online 16 December 2008
The presence of surfactants (Hexamine, tetraethylammonium bromide (TEAB), cetyltrimethylammonium bromide (CTAB), tetraoctylammonium bromide (TOAB) and PVP) on the surface of zinc oxide (ZnO) nanoparticles resulted variation in their optical properties. The optical properties of each surfactant-capped zinc oxide nanoparticles were investigated using UV–visible absorption and fluorescence techniques. The particle size of these nanoparticles were calculated from their absorption edge, and found to be in the quantum confinement range. The absorption spectra and fluorescent emission spectra showed a significant blue shift compared to that of the bulk zinc oxide. Large reduction in the intensity of visible emission of zinc oxide/surfactant was observed and these emissions were vanished more quickly, with the decrease in excitation energy, for the smaller nanoparticles. Out of the four surfactants (other than PVP), CTAB-capped zinc oxide has smallest particle size of 2.4 nm, as calculated from the absorption spectrum. Thus the presence of surfactant on the surface of zinc oxide plays a significant role in reducing defect emissions. Furthermore, ZnO/PVP nanoparticles showed no separate UV emission peak; however, the excitonic UV emission and the visible emission at 420 nm overlap to form a single broad band around 420 nm. & 2008 Elsevier B.V. All rights reserved.
Keywords: ZnO nanoparticles Surfactant Fluorescence Absorption edge Quantum-size effect
1. Introduction Quantum-sized nanodots represent a class of zero-dimensional nanostructures, which show interesting properties that differ from those of bulk crystals [1]. These nanomaterials have potential applications as important components and interconnects in nanodevices [2]. A burst of research activities have been emerging in the synthesis and characterization of semiconducting nanostructures including ZnO, ZnS, PbS, CdS, CdSe and TiO2 [3–6] because of their path-breaking applications in the fields of electronics [7], optoelectronics [8–11], chemical and bio-sensors [12–15], photonic catalysis [16], etc. In this regard, zinc oxide nanocrystals and their derivatives have been extensively studied by several researchers. Zinc Oxide (ZnO) is one of the most promising material for short-wavelength light emitting devices and for a wide range of technological applications due to its wide band gap energy of 3.37 eV [17], high exciton binding energy (60 meV) [18] and optical transparency [19]. It is a direct band gap n-type semiconductor with crystal structure similar to that of GaN. ZnO
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nanoparticles are one of the few oxides that show quantum confinement effect in an experimentally accessible range [20]. The tunability of the properties of ZnO with the particle size may find applications in formulating new composite materials with optimized properties for various applications. The large excitonic binding energy of ZnO leads to extreme stability of excitons at room temperature [21] and enables devices to function at low threshold voltages. The optical properties of ZnO are more interesting since confinement of charge carriers in the restricted volume of the small particles can lead to effects such as enlargement of band gap [22]. In the quantum-size region, the absorption of UV or visible light depends on the size of the nanoparticles [20]. The luminescent spectra of ZnO contain typically a UV band-edge emission and one or more broad emission peaks in the visible region. The visible emission is mainly attributed to the surface defects of the crystal, resulting in large variations in the emission peaks depending on the fabrication conditions. A lot of work has been devoted in decreasing the defect emission by various techniques. Luminescence emission of ZnO particles depends on the method of fabrication [18,23], annealing conditions in various environments [24,25], presence of dopants [26] and on PVP coating [19,27,28]. However, to the best of our knowledge, there have been no studies on the optical behavior of zinc oxide nanoparticles capped
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with surfactant molecules like hexamine, tetraethylammonium bromide (TEAB), cetyltrimethylammonium bromide (CTAB) and tetraoctylammonium bromide (TOAB). In this study, we investigate the influence of these surfactants on ZnO nanoparticles using UV–visible absorption and fluorescence emission techniques. For this study, nanoparticles have been prepared by the solvothermal method in isopropanol medium in the presence of different surfactants. The average particle size of each sample has been calculated from absorption spectra using the absorption edge v/s diameter calibration curve established by Meulenkamp [20].
2. Experimental details 2.1. Chemicals used The following chemicals were purchased from commercial suppliers and used as received: zinc acetate dihydrate (Zn(ac)2 2H2O) (Loba Chemie, M.W. ¼ 219.5), sodium hydroxide (S.D. Fine-chem Ltd., MW ¼ 40.0), PVP (Loba Chemie, MW ¼ 40,000), hexamine (Loba Chemie, MW ¼ 140.19), TEAB (Loba Chemie, MW ¼ 210.17), CTAB (Loba Chemie, MW ¼ 364.46), TOAB (Spactrochem, MW ¼ 546.81) and isopropanol (Spectrochem, MW ¼ 60.10). 2.2. Synthesis of ZnO nanoparticles ZnO nanoparticles were synthesized using adapted literature procedure, proposed by Bahnemann et al. [29] and have also been adopted by various other groups [30–32]. However, the volume of the solvent has been reduced in our synthesis procedure. In a typical synthesis, 1 mM (0.0505 g) Zn(ac)2 2H2O was dissolved completely in 20 ml isopropanol by vigorous stirring at about 50 1C. The solution was then diluted to a total volume of 230 ml with isopropanol followed by chilling to 0 1C in an ice-bath. 20 mM (0.032 g) NaOH solution (40 ml) in isopropanol (NaOH was dissolved in isopropanol by vigorous stirring at 60 1C) was added slowly drop-wise using a dropping funnel, to the above-mentioned cooled solution under stirring. The reaction was maintained for 15 min and finally the reaction vessel was immersed in a pre-heated water bath at 50 1C for 2 h. A clear solution was
formed, which was found to be stable, at ambient conditions, even after 1 month. The average size of the ZnO nanoparticles prepared by this method is below 10 nm [29–32]. 2.3. Synthesis of surfactant-coated ZnO nanoparticles In 40 ml isopropanol, 1 mM Zn(ac)2 2H2O was dissolved and diluted to 500 ml followed by chilling to 0 1C, as described above. The solution was then divided to five aliquot volumes, each of which was taken in separate flask numbered as 2 to 6. Specific surfactant was added into the above pre-marked solution under vigorous stirring (Zn2+/surfactant, molar ratio 5:3). 15 ml of NaOH solution in isopropanol was added drop-wise to each of the mixture within 5 min. Each flask was then immersed in the water bath at 50 1C for 2 h. The final solutions were kept below 4 1C overnight to attain stabilization. 2.4. Analytical measurements Optical absorption spectra were taken on a Perkin Elmer, Lambda-35 UV/vis spectrometer. For fluorescence studies, the emission spectra were recorded on a Varian Fluorescence Spectrophotometer (Carrier Eclipse). The average particle size of each sample was determined from UV/vis absorption spectra using absorption edge v/s diameter calibration curve established by Meulenkamp [20].
3. Results and discussions Absorption and fluorescence spectroscopy are powerful nondestructive techniques to explore the optical properties of semiconducting nanoparticles. The optical properties of pure and capped ZnO nanoparticles have been analyzed with hexamine, TEAB, CTAB or TOAB as capping agent. The absorption spectra of ZnO/isopropanol solution in the UV and visible range are presented in Fig. 1. A blank solution of isopropanol and/or respective surfactant is taken as reference. All absorption curves exhibit an intensive absorption in the range 200–370 nm, with the absorption edge in between 300 and 370 nm, owing to the relatively large exciton binding energy. It can be seen that there is
absorbance (a.u)
1 ZnO (no surfactant) 2 ZnO+Hexamine 3 ZnO +CTAB 4 ZnO +TOAB 5 ZnO +TEAB 6 ZnO +PVP
3 4
5
435
2
1
6
210 220 230 240 250 260 270 280 290 300 310 320 330 340 350 360 370 380 390 400 wavelength (nm) Fig. 1. UV–visible absorption spectra of pure and surfactant-capped ZnO nanoparticles.
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where A, B and C are constants whose values are A ¼ 3.301, B ¼ 294.0 and C ¼ 1.09 for ZnO nanoparticles [20]. Using the above equation, the calculated particle sizes are given in Table 1. These values reveal that the size of nanoparticles reduced considerably when prepared in the presence of surfactants. Fig. 2 contains the photo-induced fluorescence spectra of each sample at 2571 1C. Curves 1 to 5 exhibit a weak emission in the UV range from 350 to 372 nm (the enlarged portion of Fig. 2) and broad emission in the visible range, when excited by a light of 325 nm wavelength. The UV emission around 361 nm for these five samples can be attributed to the near band-edge emission (NBE), coming from the radiative recombination of electrons in the conduction band and holes in the valance band [38]. The intensity of this emission reduced when the ZnO nanoparticles are capped with surfactants (Fig. 2; curves 2 to 5), which may be due to the decrease in their particle size. The band-edge optical properties of pure and surfactant-capped ZnO nanoparticles are summarized in the Table 1. The Stokes shift measured in the Table 1 shows that CTAB-capped ZnO nanoparticles has lowest Stokes shift of 0.58 eV. The particle size is also smallest in this case. These values indicate that CTAB can act as an excellent capping agent during the formation of ZnO nanoparticles. The broad emission in the visible range for non-capped ZnO, centered at 542 nm (Fig. 2; curve 1), can be ascribed to the singly ionized oxygen vacancy. This green emission arises when a photogenerated hole (O–) trapped at a deep level above the valence band recombines with an electron trapped at a shallow level below the conduction band [27]. However, the relative
a significant blue shift in the excitonic absorption, for the nanoparticles capped with surfactant compared to that of the bulk ZnO. The excitonic absorption for bulk ZnO is at 373 nm as reported earlier [17,33]. The blue shift in the excitation absorption clearly indicates the quantum confinement property of nanoparticles. In the quantum confinement range, the band gap of the particles increases resulting in the shift of absorption edge to lower wavelength, as the particle size decreases. A weak absorption band has also been observed above 370 nm, for samples 2, 3, 4 and 5 in Fig. 1, which may be due to the presence of ligands [27] or from other functional groups. All the five ZnO samples (curves 1 to 5; Fig. 1) exhibit excellent UV absorption capacity and high transparency in the visible range. However, the relative absorption for the ZnO capped with PVP (curve 6; Fig. 1) is much lower, indicating the morphology change of the nanoparticles, as described later in this section. Since the reported particle size of ZnO, following our synthesis route [29–32], is below 10 nm (quantum confinement range), in this study, it was directly measured from the absorption spectrum of each sample. Various methods have been adopted to find the particle diameter from such measurements [29,34–37]. A convenient and practical method, described by Meulenkamp [20], has been applied to calculate the particle size. It involves equating diameter, D (A˚), with the wavelength at which the absorption is 50% of that at the excitonic peak, denoted as l1/2 (nm). The formula can be written as 1240=l1=2 ¼ A þ B=D2 C=D
Table 1 Measured diameters and the band-edge optical properties of pure and surfactant-capped ZnO nanoparticles. Sample
Absorption edge, l (nm)
l1/2 (nm)
Diameter (nm)
Emission peak (nm)
Stokes shift (eV)
ZnO (no surfactant) ZnO+Hexamine ZnO+CTAB ZnO+TOAB ZnO+TEAB
335 307 309 305 304
355 336 323 335 334
4.2 2.9 2.4 2.9 2.8
361 361 361 361 361
0.27 0.61 0.58 0.63 0.64
A
Intensity (a.u.)
6 1
1 ZnO (no surfacent) 2
2 ZnO+Hexamine 3 ZnO+CTAB
Intensity (a.u.)
3 5 350
360 370 Wavelength (nm)
4 ZnO+TOAB
4
1
5 ZnO+TEAB 380
6 ZnO+PVP
3 4 5
2 6 A
400
500 Wavelength (nm)
600
Fig. 2. Fluorescence properties of pure and capped ZnO nanoparticles at an excitation wavelength of 325 nm. (Inset contains the enlarged portion of the marked area).
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ZnO
Polar head Non-polar tail
Isopropanol medium Fig. 3. Scheme of capping of surfactants (CTAB, TEAB and TOAB) on the surface of ZnO nanoparticles.
Intensity (a.u.)
1 ZnO (no surfactant) 2 ZnO +Hex/TOAB/TEAB 3 ZnO +CTAB
2 3
400
450
500 550 Wavelength (nm)
600
Fig. 4. Emission spectra of different samples for an excitation wavelength of 350 nm.
Excitation at
5
Intensity (a.u.)
intensity of the emission peaks in the visible region decreased considerably when different surfactants were used (Fig. 2; curve 2 to 5). This reveals that the surfactant helps in reducing the trap emissions of ZnO nanoparticles. Here, we can see that the surfactant on the surface of ZnO plays two roles in order to reduce the visible emission: firstly, it helps to form defect-free nanocrystals during nucleation and secondly, it attaches to the surface of nanoparticles so as to keep the particle size minimum. The capping of surfactants on ZnO nanoparticles is shown in Fig. 3. The chain length of non-polar moiety in the CTAB surfactant is sufficient to form proper capping, resulting in the formation of smaller sized nanoparticles. In case of TEAB surfactant, polar head group is stronger than that of the non-polar hydrocarbon moiety. Therefore it may lead to instability at the particle surface, resulting in the formation of slightly large-sized nanoparticles. TOAB contains four long hydrocarbon chains of equal length, which may hinder the interaction with other TOAB molecules due to steric effect. This results in the increase of core size, hence the particle size may be still large. When the particle size increases, the intensity of the visible emission also increases due to the fast hole trap process [27]. It is evident from Fig. 2 that the visible emission peak is blue shifted in accordance with the particle size, even though the shift in the band-edge emission is not appreciable. This result confirms the previous report that the energetic positions of the maxima of both emission bands depend on the size of the ZnO nanoparticles [31]. The fluorescent spectra have been further used to investigate the depth of visible emission at different excitation energies. The emission spectra for all the five samples except PVP-capped nanoparticles are shown in Fig. 4, for an excitation wavelength of 350 nm (3.55 eV). It can be seen that the sample capped with CTAB (curve 3) shows lowest visible emission among all the ZnO samples as CTAB-capped ZnO nanoparticles have lowest particle size also (Table 1). On increasing the excitation wavelength to 360 nm, the ZnO sample capped with TEAB and CTAB showed lowest visible emission. Emission spectra have been recorded by increasing the excitation wavelength up to 380 nm and then the visible emission of other samples containing TOAB and hexamine also showed minimum intensity. The visible emission for the non-
437
3
2
1 300 nm 2 320 nm 3 330 nm 4 350 nm 5 pure PVP (excitation at 350 nm)
4 1
350
400
450 500 Wavelength (nm)
550
Fig. 5. Emission spectra of PVP-capped ZnO nanoparticles at different excitation wavelengths.
capped ZnO sample was present even at the excitation wavelength of 380 nm. The disappearances of visible emission for various samples were in the order (ZnO capped with) CTAB, TEAB, TOAB and hexamine. It can be seen from Table 1 that the increase in particle size also follows the same order. Since the visible emission is due to the presence of trap levels arising from surface defects, indicating that the large-sized nanoparticles have various trap levels. Some of the trap levels are present in the nanoparticles, which can be excited even at low energies. When the particle size decreases, most of the trap levels exist in almost the same energy level and this range also decreases as the size decreases. The fluorescent emission spectrum (Fig. 2; curve 6) and the absorption spectrum (Fig. 1; curve 6) for the ZnO/PVP sample exhibit different behaviors in comparison with other surfactantcapped nanoparticles (Fig. 1 and 2; curves 1 to 5). This anomalous behavior of PVP-capped ZnO is in agreement with an earlier report by Guo et al. [30] where Zn2+ and PVP (MW ¼ 10,000) were taken in the molar ratio 5:8. The similarity in result with that of Guo et al. inspite of different molar concentration (5:3) may be due to the high polymer chain length of PVP (MW ¼ 40,000) used. Guo et al. [30] reported that the presence of PVP on the surface of ZnO favors the formation of nanorods. We analyzed the emission spectra of ZnO/PVP at different excitation wavelengths, which are given in Fig. 5. Curves 1 to 3 (Fig. 5) show two broad emission
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bands in the blue–green region (visible): one centered at 420 nm and the other at around 495 nm but with no emission peak in the UV region. The latter is due to trap emission as in the case of other ZnO samples. For an excitation with 300 nm (curve 1), both peaks have nearly equal intensities. It is interesting to see that when the excitation energy decreases, the first peak becomes more prominent and the second one disappears, showing the property of energy-selective luminescence. From Fig. 2, it is evident that the UV emission is still present in the nanoparticles. At 420 nm, the UV emission and the visible emission overlap to form a single band. The fluorescence spectrum of pure PVP solution in isopropanol (Fig. 5; curve 5) showed a single broad emission at 410 nm and hence the visible emission at 420 nm for the PVPcapped ZnO can be attributed to the presence of PVP.
4. Conclusions In summary, we have prepared ZnO nanoparticles capped with various surfactants, and their optical properties were investigated using absorption and fluorescence techniques. Quantum-size effects are evident from the optical absorption spectra, which showed blue shifts in the excitonic feature as compared to the bulk ZnO. The particle size calculations from the absorption edge showed that CTAB-capped ZnO nanoparticle has smallest particle size and lowest Stokes shift. This reveals the capacity of CTAB to act as an excellent capping agent for ZnO nanoparticles. The fluorescence emission spectra show blue shift both in the UV and in the visible regions, indicating the dependence of both emissions on the size of the nanoparticles. The PVP-capped ZnO nanoparticle gives two visible emission peaks centered at 420 and 495 nm. The UV emission peak is not visible as the presence of a strong band at 420 nm. This arises due to the presence of PVP in the sample. The intensities of the two visible emissions (excitonic and trap emissions) of the ZnO/PVP depend on the excitation energies, which may find applications in the energy-selective luminescent devices.
Acknowledgements The author Muhd. Shafeeq M. would like to thank The coordinator, Nanotechnology program, AMU-Aligarh and The Director, CSIO for allowing to do the project work at CSIO, Chandigarh.
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