Synthesis, characterization, and luminescent properties of ZnO – SiO2 : PbS O. M. Ntwaeaborwa,a兲 H. C. Swart, R. E. Kroon, and J. J. Terblans Department of Physics, University of the Free State, Bloemfontein, ZA9300, South Africa
P. H. Holloway Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611-64
共Received 28 October 2008; accepted 20 January 2009; published 29 June 2009兲 PbS and ZnO nanoparticles prepared separately by a sol-gel process were incorporated in SiO2 by an ex situ method, resulting in a green emitting ZnO – SiO2 : PbS powder phosphor. Particle morphology, structure, and chemical composition of the ZnO nanoparticles, PbS nanoparticles, and ZnO – SiO2 : PbS composite were analyzed with transmission electron microscopy, x-ray diffraction, and energy dispersive spectroscopy 共EDS兲, respectively. With or without ZnO nanoparticles, green photoluminescence with a peak at 540 nm was observed from SiO2 : PbS when excited with a 325 nm He–Cd laser in air at room temperature. This peak was different from defect-related emission from ZnO nanoparticles at 580 nm and red-orange emission from SiO2 : Pb2+ with a broad peak at 600– 750 nm. Again, the 540 nm peak was different from the band edge emission from pure PbS nanoparticles at 1200 nm. Note that this emission 共540 nm兲 was enhanced considerably when ZnO nanoparticles were incorporated. Photoluminescence properties of ZnO – SiO2 : PbS is discussed. © 2009 American Vacuum Society. 关DOI: 10.1116/1.3086645兴
INTRODUCTION There has been growing research interest in the study of the synthesis and characterization of lead sulfide 共PbS兲 nanoparticles incorporated in transparent and amorphous matrices such as silica 共SiO2兲 and zirconia 共ZrO2兲 using different synthetic methods. For example, luminescent properties of PbS nanoparticles in a sol-gel ZrO2 film were reported by Hayakawa et al.1 Optical properties of PbS nanoparticles in SiO2 prepared by either sol-gel or reverse micelle processes were investigated by Nogami et al.2 and Bang et al.3,4 In some of these studies, PbS nanoparticles were incorporated in situ in the SiO2 or ZrO2 host. That is, the nanoparticles were formed at the same time as the host matrix. The disadvantage of the in situ incorporation is that it is difficult to prove whether or not PbS nanoparticles were formed in glassy SiO2 / ZrO2. It is also difficult to prove whether or not luminescence comes from the host matrix 共SiO2 / ZrO2兲 or PbS or Pb2+ ions. In this study, PbS nanoparticles and ZnO nanoparticles prepared separately by a sol-gel process were incorporated in SiO2 sol by an ex situ method. Photoluminescence data of PbS nanoparticles, SiO2 : PbS, ZnO nanoparticles, SiO2 : Pb2+, and ZnO – SiO2 : PbS were compared. The objective of this study was to investigate the origin of green emission from SiO2 : PbS and to evaluate energy transfer from ZnO nanoparticles to SiO2 : PbS. EXPERIMENT ZnO nanoparticles were prepared by dissolving zinc acetate in boiling ethanol with vigorous stirring at ⬃80– 100 ° C. The solution was then cooled in ice water and sodium hydroxide dissolved in ethanol was added dropwise. a兲
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The mixture was stirred vigorously in ice water for an additional 30 min. The final solution was stored at room temperature overnight followed by repeated washing in a mixture of heptane and ethanol with a volume ratio of 2:1. The precipitated ZnO nanoparticles were either dispersed in ethanol for ex situ embedding in SiO2 or dried at 90 ° C for characterization. A similar approach was used to prepare PbS nanoparticles using lead acetate and sodium sulfide as precursors. A SiO2 sol was prepared by mixing 10 ml of tetraethylorthosilicate 共TEOS兲 with 8 ml of ethanol, 20 ml of water, and 5 ml of nitric acid at room temperature and stirred for 1 h. The PbS nanoparticles dispersed in ethanol were then added dropwise to the SiO2 sol with vigorous stirring. After 1 h, the ZnO nanoparticles dispersed in ethanol were added and the mixture was stirred vigorously 共for ⬃10 h兲 until a gel formed. The molar ratios of ZnO : SiO2 : PbS, SiO2 : PbS, ZnO – SiO2, and SiO2 : Pb2+ were 1:19:6, 19:6, 1:19, and 19:6, respectively. A SiO2 : Pb2+ gel was also prepared by mixing SiO2 sol with lead acetate dissolved in ethanol. The gels were dried at room temperature for 3 – 5 days, were calcined in air at 600 ° C for 2 h, and ground to get powders using a pestle and mortar. Structure and chemical composition of the powders were examined by x-ray diffraction 共XRD兲 and x-ray energy dispersive spectroscopy 共EDS兲, respectively. Particle morphology of PbS and ZnO nanoparticles were analyzed by transmission electron microscopy 共TÉM兲. Photoluminescence data were collected by using a 325 nm He–Cd laser and a 325 nm monochromatized xenon lamp as excitation sources. RESULTS AND DISCUSSIONS Figure 1 shows the XRD spectra of 共1兲 ZnO nanoparticles, 共2兲 PbS nanoparticles, 共3兲 SiO2, and 共4兲 ZnO – SiO2 : PbS. Spectra 共1兲 and 共2兲 are consistent with the hexagonal and cubic structures of ZnO and PbS, respectively.
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FIG. 3. TEM images of 共a兲 ZnO and 共b兲 PbS nanoparticles. Scale = 0.05 m 共50 nm兲.
FIG. 1. XRD patterns of 共1兲 ZnO, 共2兲 PbS, 共3兲 SiO2, and 共4兲 ZnO – SiO2 : PbS.
The average particle sizes calculated from the broadened x-ray diffraction peaks using the Scherer equation were 5 and 6 nm in diameters for ZnO and PbS nanoparticles, respectively. A familiar broadband of SiO2 is at 2 = 25° in spectrum 共3兲 and it is clear that SiO2 remained amorphous even after calcining at 600 ° C for 2 h. Spectrum 共4兲 is consistent with the XRD pattern of amorphous SiO2, except for the low intensity crystalline peaks detected at 2 = 30, 43, 50, and 52 and are consistent with peaks from the PbS spectrum 关i.e., spectrum 共2兲兴. The EDS spectrum of ZnO – SiO2 – PbS powder is shown in Fig. 2. All the elements 共Zn, Si, Pb, O, and S兲 were detected consistent with successful incorporation of ZnO and PbS nanoparticles in the SiO2 host. The TEM images of 共a兲 ZnO and 共b兲 PbS nanoparticles in Fig. 3 show an agglomeration of spheroidal particles with average diameters of 5 and 6 nm, respectively, consistent with the XRD data. Figure 4 shows the defect related green emission from
FIG. 2. The EDS spectrum of ZnO – SiO2 : PbS. J. Vac. Sci. Technol. A, Vol. 27, No. 4, Jul/Aug 2009
ZnO nanoparticles with a maximum at 580 nm. As widely reported in the literature, this emission is associated with anionic oxygen vacancies 共VO兲 in ZnO. Figure 5 shows direct bandgap emission 共exe = 325 nm, He–Cd laser兲 associated with excitonic recombination in PbS at 1240 nm 共0.91 eV兲. Probably due to quantum confinement effects, this emission is blueshifted from that of bulk PbS at 3000 nm 共0.41 eV兲 reported in Ref. 5. Figure 6 shows the PL emission spectra of 共1兲 SiO2 共2兲 SiO2 : PbS, 共3兲 ZnO – SiO2 : PbS, 共4兲 ZnO – SiO2, and 共5兲 SiO2 : Pb2+. A broad emission peak; with a maximum at 490 nm and a shoulder at 417 nm were observed from pure SiO2 in spectrum 共1兲. In both spectra 共1兲 and 共2兲, the broad emission peak maximizes at 545 nm with a shoulder at 446 nm. In spectrum 共3兲, the peak at 545 nm is more intense than that at 446 nm. The 446 and 545 nm peaks were stable in spectra 共2兲–共4兲. Red-orange photoluminescence from SiO2 : Pb2+ 关spectrum 共5兲兴 was observed at 600– 750 nm range with a maximum at 685 nm, and was attributed to either 3 P0,1 → 3 P1 transition of Pb2+ or charge transfer.6,7 Note that the green-yellow emission at 580 nm from ZnO nanoparticles and the red-orange emission from Pb2+ were not detected from either ZnO – SiO2 : PbS 关spectrum 共3兲兴 or SiO2 : PbS 关spectrum 共5兲兴 powders. Also, no peak related to PbS nanoparticles was detected at 1240 nm from either SiO2 : PbS 关spectrum 共2兲兴 or ZnO : SiO2 : PbS powders. These data suggest that the stable green photoluminescence at
FIG. 4. PL emission spectrum of dried ZnO nanoparticles.
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emission from sol-gel derived SiO2 with major emission peaks located at 420– 660 nm has been reported.8–10 These emission peaks are associated with structural defects in the SiO2 network or charge transfer between O and Si atoms.9,11,12 The shifting of the main emission peak from 490 nm in pure SiO2 关spectrum 共1兲兴 to 545 nm in SiO2 : PbS 关spectrum 共2兲兴 and ZnO – SiO2 : PbS 关spectrum共3兲兴 was possibly caused by the presence of PbS and ZnO nanoparticles in the SiO2 network. The PL intensity at 545 nm from SiO2 : PbS and ZnO – SiO2 : PbS was compared for the PL data collected under the same conditions and it was noticed that ZnO nanoparticles act to enhance the intensity possibly by harvesting excitation energy and transferring it to SiO2 by phonon mediated processes or by stabilizing structural defects, in the SiO2 network, responsible for this emission. The actual mechanism by which this enhancement occurs as well as the role of PbS nanoparticles is yet to be determined. FIG. 5. PL emission spectrum of PbS nanoparticles.
545 nm in spectra 共2兲 and 共3兲 does not come from ZnO, Pb2+, or PbS. Notice that, except for the shifting of major emission peaks, spectrum 共1兲 共from pure SiO2兲 is similar to spectra 共2兲–共4兲. Since ZnO, Pb2+, and PbS are probably not responsible for this emission, it is therefore réasonable to attribute the emission to structural defects in SiO2. This conclusion is consistent with tunable photoluminescence observed from sol-gel derived silica xerogels by Lin and Baener.8 They observed the shifting of the main emission peak between 437 and 503 nm from pure SiO2 samples calcined at different temperatures and excited by different wavelengths. Other than different calcining temperatures and excitation wavelengths they attributed tunability of photoluminescence to the type of solvent, concentration of precursors, catalyst, pH, and the ratio of water to TEOS. Visible
CONCLUSION PbS and ZnO nanoparticles were successfully incorporated in a SiO2 sol by an ex situ process. Stable green photoluminescence from SiO2 : PbS and ZnO – SiO2 : PbS powders with a maximum at 545 nm was detected. The data suggest that this emission did not come from ZnO, PbS, or Pb2+. The emission was attributed to structural defects in the SiO2 network and it was enhanced considerably by the incorporation of ZnO nanoparticles. We speculate that ZnO nanoparticles either enhanced the PL intensity 545 nm by transferring energy by phonon mediated processes to SiO2 or by stabilizing structural defects responsible for the emission. ACKNOWLEDGMENTS The authors would like to acknowledge S. O. Oluwafemi 共Department of Chemistry, University of Zululand兲 and A. Lombard 共Department of Geology, University of the Free State兲 for XRD measurements. The 325 nm He–Cd laser used was from the Department of Physics at Nelson Mandela Metropolitan University. This study was financially supported by the South African National Research Foundation, and P.H.H. was supported by ARO Grant No. W911-NF-071-0545. 1
FIG. 6. PL emission spectra of 共1兲 SiO2, 共2兲 SiO2 : PbS, 共3兲 ZnO – SiO2 : PbS, 共4兲 ZnO – SiO2, and 共5兲 SiO2 : Pb2+. Note that the PL data for spectra 共1兲 and 共5兲 were not collected at the same time as those of spectra 共2兲 and 共3兲 and therefore the intensity cannot be compared.
JVST A - Vacuum, Surfaces, and Films
S. T. Hayakawa, S. T. Selvan, and M. J. Nogami, J. Lumin. 86–89, 532 共2002兲. 2 M. Nogami, T. Enomoto, and T. J. Hayakawa, J. Lumin. 97, 147 共2002兲. 3 J.-S. Bang, Y. Yang, and P. H. Holloway, J. Chem. Phys. 123, 084709 共2005兲. 4 M. S. Dhlamini, J. J. Terblans, O. M. Ntwaeaborwa, J. M. Ngaruiya, K. T. Hillie, J. R. Botha, and H. C. Swart, J. Lumin. 128, 1997 共2008兲. 5 J.-H. Hong, C.-J. Cong, Z.-G. Zhang, and K.-L. Zhang, J. Phys. Chem. Solids 68, 1359 共2007兲. 6 Y. Uehara, J. Chem. Phys. 51, 4385 共1969兲. 7 A. A. Bol and A. Meijerink, Phys. Status Solidi B 224, 173 共2001兲. 8 J. Lin and K. Baerner, Mater. Lett. 46, 86 共2000兲. 9 Y. Han and H. Zhang, Mater. Lett. 54, 389 共2002兲. 10 T. Inokuma, Y. Kuruta, and S. Hasegawa, J. Lumin. 80, 247 共1990兲. 11 W. H. Green, K. P. Le, J. Grey, T. T. Au, and M. J. Sailor, Science 276, 1826 共1997兲. 12 M. García, M. A. Mondragón, S. Téllez, A. Campero, and V. M. Castano, Mater. Chem. Phys. 41, 15 共1995兲.
