Article pubs.acs.org/crystal
Synthesis of ZnO Nanoparticles with Controlled Shapes, Sizes, Aggregations, and Surface Complex Compounds for Tuning or Switching the Photoluminescence Jianhui Zhang,*,† Baodan Zhao,† Zhongda Pan,† Min Gu,† and Alex Punnoose‡ †
National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, China Department of Physics, Boise State University, Boise, Idaho 83725, United States
‡
S Supporting Information *
ABSTRACT: The electronic energy transfer (EET) usually induces the fluorescence self-quenching, but it has been positively used here to tune and/or switch the photoluminescence (PL) of ZnO nanoparticles (NPs). Monodisperse ZnO nanospheres, rods, tripods, and clusters with tunable sizes have been synthesized to reproducibly and finely control the NP aggregation because EET is sensitive to the interparticle separation. The complex reactions between these NPs and their dispersion media have been used to further control the EET for tuning the ZnO PL. By changing the NP concentrations, shapes, and/or the cluster sizes, the band-edge UV PL of the ZnO NPs dispersed in alcohol or water is modified in both intensity and peak position, and new blue emissions with tunable intensity around 418, 435, and 468 nm are induced. As confirmed by the X-ray diffraction patterns and the infrared, PL, absorption, and Raman spectra, the ZnO NPs made here can slowly react with ethanol to form a new composite ZnO−(C2H5OH)n, which changes the EET between NPs and leads to strong blue PL around 435 nm. By simply using different dispersion media (such as ethanol or water) to modify the surface complex compounds of ZnO NPs, the 435 nm blue PL can be turned on or off.
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INTRODUCTION With high exciton binding energy (60 meV), wide bandgap (3.34 eV), and advantages of nontoxicity, chemical stability, and cheapness, ZnO has been extensively investigated for its broad applications such as room-temperature UV lasers,1 lightemitting diodes,2 solar cells,3 sensors,4 and biolabeling.5 However, as a luminescent material, ZnO nanoparticles (NPs) are greatly limited by the following factors. (1) Their luminescence intensity is relatively weak in comparison to that of the strong luminescent materials such as CdSe and CdTe.6 (2) They tend to aggregate or undergo Ostwald ripening7 because of their high surface energy, resulting in unstable luminescence. (3) Their emission wavelength range is narrow and usually limited in the band-edge related UV and defectrelated green/yellow regions.8 (4) Although the other visible luminescence types such as blue, orange, and red light can also be induced in ZnO NPs by introducing the defects and/or impurities, they are usually unstable and irreproducible because the defects or impurities generally vary with the experimental conditions and circumstances.9 Therefore, increasing the intensity, wavelength range, and repeatability of the luminescence is crucial for practical applications of ZnO NPs. It is well-known that an excited donor chromophore can transfer energy to a ground state acceptor chromophore through nonradiative dipole−dipole coupling when their separation is in the range of 1−10 nm.5,10 Besides the © XXXX American Chemical Society
separation, the electronic energy transfer (EET) between chromophores is also sensitive to their dispersion medium.11 Similarly, the EET also occurs between neighboring nanocrystals with interparticle separations of 0.5−10 nm.12 For luminescent materials, EET usually leads to the fluorescence self-quenching and should be avoided. However, its nature of redistributing the electronic energy among NPs can be used to tune the emission bands or create new emission bands. To control the EET for obtaining stable and strong ZnO luminescence in a broad wavelength range, here the monodisperse ZnO nanospheres, rods, tripods, and clusters with tunable sizes have been successfully synthesized by developing a microwave-assisted hydrolysis method and a unique water/PVP/1-pentanol system to reproducibly and finely control the NP aggregation. Furthermore, the high surface activity of these NPs allows us to modify their surface using the dispersion media to further control the EET and the ZnO luminescence.
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EXPERIMENTAL SECTION
Materials and Characterization. All the AR-grade reagents were used as received. The morphologies of the samples were examined Received: November 21, 2014 Revised: May 8, 2015
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Figure 1. Typical TEM images of ZnO nanospheres (a), rods (b), and tripods (c), the corresponding PL spectra of their water-dispersions are shown in d, e, and f, respectively. heated to 180 °C at a heating rate of ∼75 °C/min and maintained at 180 ± 1 °C for 6 min under magnetic stirring. The resulting product was washed with ethanol 3 times by centrifugation and ultrasonication. Twenty minutes of microwave heating at 150 °C, 45 min of microwave heating at 145 °C, and 1 h of microwave heating at 135 °C were used for the synthesis of the monodisperse spheric clusters of 100, 150, and 300 nm, respectively.
using a JEOL JEM-2100HR TEM with an accelerating potential of 200 kV. The hydrodynamic size distributions were measured using a Malvern Zetasizer NanoZS. With quartz cells of 1 cm path length, a UV−vis spectrophotometer (UV 1800) and a fluorophototmeter (F97XP) were used for the measurements of the absorption and PL spectra at room temperature, respectively. All the dispersions of NPs except for the defined ones were freshly prepared for the above measurements. The 325 nm line of a 150 W xenon lamp was used as the excitation source. Room temperature Raman and infrared (IR) absorption spectra were collected using a Jobin-Yvon spectrometer (T64000) and a far-field Fourier transform (FT) IR spectrometer (NEXUS870, USA), respectively. Powder X-ray diffraction (PXRD) patterns were recorded using a X-ray diffractometer (Rigaku D/maxRA, Japan) with Cu Kα radiation (k = 1.5418 Å). Synthesis of ZnO Nanospheres and Rods. A total of 0.5 g of Zn(Ac)2·2H2O was completely dissolved in diethylene glycol (DEG, 100 mL) at 180 °C. After cooling down, 0.8 mL (for nanospheres) or 1.0 mL (for nanorods) of deionized water was added and the mixture was stirred for 10 min. Then the mixture was microwave-heated to 180 °C at a heating rate of ∼75 °C/min, and maintained at 180 ± 1 °C for 6 min under magnetic stirring. The resulting product was washed with ethanol 3 times by centrifugation and ultrasonication. Synthesis of ZnO Nanotripods. In a typical synthesis, 2.4 mL of aqueous Zn(NO3)2·6H2O (0.25 M) was added into 60 mL of 1pentanol solution of PVP (Mn = 58 000, 2.4 g) and the mixture was stirred for 30 min, followed by 2.4 mL of 0.5 M NaOH·H2O in methanol. After 2 h of stirring, the resulting product was washed with ethanol 3 times by centrifugation and ultrasonication. Synthesis of Monodisperse ZnO Spheric Clusters. A total of 0.5 g of Zn(Ac)2·2H2O was completely dissolved in DEG (100 mL) at 180 °C. After cooling down, 0.2 mL of deionized water was added and the mixture was stirred for 10 min. Then the mixture was microwave-
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RESULTS AND DISCUSSION By taking advantage of the rapid and uniform microwave heating, the homogeneous and controllable nucleation of ZnO was realized, and the monodisperse ZnO nanospheres (11 ± 2 nm, Figure 1a) and rods (∼21 ± 2 × 70 ± 18 nm, Figure 1b) were successfully synthesized. In all cases, 200 particles were measured to get the average size. The dominant particle shape can be easily changed from sphere to rod by simply modifying the amount of water from 0.8 to 1.0 mL. With the unique water/polyvinylpyrrolidone (PVP)/1-pentanol system, ZnO nanotripods with uniform diameter (Figure 1c) were also successfully made. XRD patterns show that all the above ZnO samples made here have the same pure wurtzite crystal phase (P63mc, a = 0.322 nm, c = 0.521 nm, Supporting Figure 1, Supporting Information). The main PL peak of the water dispersion of ZnO nanospheres (0.2 mM) locates around 360 nm (Figure 1d). With the increase of the NP concentration from 0.2 to 50 mM, the UV PL gradually red shifts from 360 to 398 nm, and increases in intensity to a maximum at 25 mM, and then decreases. When the NP concentration is increased to 5 mM, two new blue emissions around 418 and 468 nm appear B
DOI: 10.1021/cg5017017 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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fine control of the NP aggregations and to further confirm the EET induced by the NP aggregations, monodisperse spherical ZnO clusters with tunable size in a wide range of 60−300 nm were synthesized using a microwave-assisted forced hydrolysis method. Compared with the previously reported microwaveassisted route,13 in our method, the following advantages are highlighted. (1) The tedious seeding procedure is avoided. (2) The cluster yield is greatly increased up to ∼1000 fold. (3) As shown in Figure 3 and Supporting Figure 3, the clusters can be
and increase gradually in intensity with further increasing the NP concentration. The PL of the water dispersions of ZnO nano rods (Figure 1e) and tripods (Figure 1f) shows a similar variation trend with the increase of the NP concentration. The red-shift of the UV PL and the appearance of the new blue PL induced by the increase of the NP concentration can be ascribed to the EET between NPs because the absorption edge positions of the corresponding absorption spectra of all the dispersions at different concentrations remain unchanged (Supporting Figure 2).12 By assuming that the ZnO NPs are monodisperse dense spheres, the average center to center separation Lcc of the dispersed NPs is estimated by using the formula, ⎛ 1.33πr 3ρ ⎞1/3 Lcc ≅ ⎜ ⎟ ≅ 6.57r(c M)−1/3 ⎝ c MM ⎠
Here r is the NP radius, ρ is the bulk density of ZnO (5.6 g/ cm3), cM is the NP mass molarity, and M is the molar mass. Even in the thickest dispersion (50 mM) used here, the calculated inter-NP separation is still very large (17.8r), and far beyond the range of the dipole−dipole coupling. Therefore, the NPs might aggregate into clusters and lead to the EET even in the highly diluted dispersions due to their high surface energy. To confirm the hypothesis, dynamic light scattering (DLS) has been used to measure the hydrodynamic size distributions of the water dispersions of ZnO nanospheres. As expected, three types of size distributions (Figure 2) already appear at the
Figure 3. Typical TEM images and the corresponding PL spectra of the monodisperse ZnO spherical clusters of 60 ± 8 nm (a−b), 100 ± 8 nm (c−d), and 150 ± 10 nm (e−f) dispersed in ethanol.
tuned and enlarged in size up to 300 nm by simply changing the reaction temperature while keeping their fine monodispersity. The lower reaction temperature is used, the larger clusters are obtained. This can be explained by the observation that the initial nuclei formed at lower temperature are less than that formed at higher temperature. (4) Finally and importantly, the clusters of nanocrystals or free-standing nanocrystals can be selectively synthesized by simply controlling the water amount. Here the following reaction mechanism is proposed. When the water amount is very small, the ions formed in the reaction solution are not enough for the as-formed NPs to adsorb to generate a stable electrical double layer to repel each other. Therefore, the as-formed ZnO NPs tend to attach themselves together to form clusters to reduce their high surface energy. When the ions in the reaction solution are increased to a critical value by increasing the water amount, the as-formed NPs can adsorb ions enough to repel each other, resulting in the freestanding nanocrystals. Consistent with the above mechanism, the surface charge density of the ZnO nanocrystals (Figure 1a) dispersed in water is indeed higher than that of the ZnO clusters (Figure 3a) dispersed in water (Supporting Figure 4). Like the above nanospheres, the water dispersions of the clusters composed of nanocrystals of ∼7 nm (Figure 3a) show a similar PL-variation trend with the NP concentrations (Supporting Figure 5). But the UV PL reaches its maximum
Figure 2. Size distributions of the ZnO nanospheres dispersed in water at different concentrations, detected by DLS.
lowest NP concentration (0.2 mM) used here. The smallest size distribution is around 60 nm, the dominant size distribution locates around 200 nm, and the biggest size distribution is around 5540 nm. All these sizes are far bigger than the actual size (11 ± 2 nm) measured by TEM (Figure 1a) of the ZnO nanospheres. With increasing the NP concentration, the dominant size distributions broaden and shift to a bigger size, showing that the aggregates or clusters of the nanospheres already appear in the highly diluted dispersion and increase in size with increasing the NP concentration. The above results indicate that one can control the EET by modifying the aggregations and thus the PL of the dispersed NPs. In fact, as shown in Figure 1, with changing the NP shape from sphere to rod, and to tripod, the intensity ratio of the UV PL to the new blue PL around 418 nm greatly reduces, and this reduction increases with the increase of the NP concentration. Apparently, the ZnO nanospheres, rods, and tripods should aggregate differently in solution due to their different shapes, which will change the EET and the resultant PL. To realize the C
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Crystal Growth & Design intensity at 10 mM instead of 25 mM. This can be ascribed to the smaller and more-uniform NP aggregates of the monodisperse clusters, as revealed by the size distributions (Supporting Figure 6) monitored by DLS. To further narrow the size distribution for better and reproducible control of the NP aggregation, the clusters were dispersed in ethanol, which usually disperses ZnO NPs well. As expected, the size distribution is further narrowed successfully (Supporting Figure 6). Besides the similar PL evolutions with the NP concentrations, the clusters dispersed in ethanol show a new strong blue PL around 435 nm (Figure 3b) in comparison to that dispersed in water. This new blue PL induced by ethanol is much stronger than the band-edge UV PL, and continuously increases with the increase of the storage time of the clusters dispersed in ethanol. After storage for 8 months, the 435 nm blue PL of the ethanol dispersion of 60 nm ZnO clusters (0.4 mM) is enhanced up to ∼30 fold in intensity (Supporting Figure 7). This not only shows the unexpected high emission efficiency of the 435 nm PL but also implies the possible contribution of the ZnO−ethanol interaction to the 435 nm PL. Additionally, the band edge UV PL and the size distribution of the ethanol dispersion of 60 nm ZnO clusters (10 mM) was unvaried, showing the ZnO clusters dispersed in ethanol are very stable. With increasing the average diameter from 60 to 100, and 150 nm (Figure 3c−f), the band edge UV emissions of the clusters dispersed in ethanol at all concentrations red shift and increase in intensity, and reach their maximum intensity at 10, 5, and 2 mM respectively. Meanwhile, all the new blue emissions also increase in intensity but keep the band position unvaried. It should be noted that the PL of the ethanol dispersions of the NPs with size of >150 nm is not discussed here because these dispersions are not stable due to the rapid sedimentation of NPs. The above results clearly show that the ZnO PL can be finely tuned by modifying the NP aggregations via adjusting the cluster sizes. As in the aforesaid water dispersions, in the freshly prepared ethanol dispersions, the absorption spectra of the clusters at different concentrations maintain the absorption edge position unvaried (Supporting Figure 8) in spite of the variations of the PL with the NP concentrations, showing that the variations of the UV PL mainly arise from the EET between NPs.12 The increase of the cluster size intensifies the aggregation and the resultant EET, thus leading to the bigger variations of the UV PL. However, after storage for 4 months, the ethanol dispersions of ZnO clusters show a new absorption band around 394 nm (Figure 4). With increasing the storage time to 8 months, this new absorption band increases in intensity, and another new absorption band around 373 appears. As shown in Figure 4, the new absorption bands match well with the excitation peaks (of the excitation spectra recorded at 435 nm) in position. These results show that ethanol molecules slowly react with ZnO NPs to form a new composite with a bandgap of ∼3.31 eV (evaluated by the main absorption band corresponding to the main excitation peak) and lead to the 435 nm blue PL. The composite concentration of the fresh prepared ethanol dispersions is too low to be detected initially by the absorption spectrum, but will gradually increase with the increase of the reaction time (i.e., storage time), resulting in the enhancement of the 435 nm PL discussed above. To further confirm the formation of the new composite, the clusters (60 nm, 0.4 mM) dispersed in ethanol or water were dried on silicon substrates at room temperature for measuring
Figure 4. Absorption and excitation (recorded at 435 nm) spectra of freshly prepared ethanol dispersions of 60 nm ZnO clusters (0.4 mM) before and after being stored for 4 and 8 months. The excitation spectra before and after being stored are very similar in shape, and only the one before being stored for 8 months is shown for clarity.
the XRD patterns, and the Raman and infrared (IR) absorption spectra. As shown in Figure 5a, the XRD patterns of the dried
Figure 5. XRD patterns (a), and Raman (b) and IR (c) absorption spectra of absolute ethanol, dried ZnO clusters (60 nm) separated from ethanol or water dispersions.
clusters separated from the ethanol dispersions are identical to that of the dried clusters separated from the water dispersions in shape, position, and full width at half-maximum except for the smaller intensity, implying that ethanol molecules do not enter the crystal lattice of the ZnO NPs, but react to the NP surface and weaken the intensity of the XRD patterns. Both the dried clusters separated from the ethanol and water dispersions show a Raman band around 438 cm−1 (arising from the E2 phonons of ZnO14,15), indicating the same wurtzite structure of D
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435 nm PL keeps position unvaried and independent of the particle concentration. As shown in Figure 3b,d,f, the 435 nm PL will vanish when the particle concentration increases to a critical point, which can be ascribed to the EET (between NPs) enhanced by the reduction of the interparticle separation. One should be able to modify the 435 nm PL by simply changing the dispersion medium since it arises from the reaction product between ZnO NPs and ethanol. As expected, the strong 435 nm PL of the ethanol dispersions of ZnO NPs can be turned off by replacing ethanol with water as the dispersion medium. Once some water is added into the ethanol dispersion, the 435 nm blue PL will gradually weaken with increasing time (Supporting Figure 10). To further investigate the influence of the dispersion medium on the 435 nm PL, the clusters (60 nm) were also dispersed in other alcohols. All the clusters dispersed in ethanol, methanol, and 1,3-propanediol show the 435 nm PL, and a similar PL-variation trend with the NP concentrations. However, the 435 nm PL of the clusters dispersed in 1,3-propanediol is much weaker (Supporting Figure 11). No 435 nm PL is observed in the DEG dispersions that show totally different PL-variation trend with the NP concentrations. These results clearly show that the 435 nm PL is very sensitive to the position and number of hydroxyl groups of alcohol. In fact, the different molecular structures such as the hydrocarbon chain of alcohol and the proton of water have different influences on the reactivity of hydroxyl group, which will lead to a different product and PL. Therefore, using the other dispersion media such as water and DEG instead of ethanol can change the composites of ZnO NPs and ethanol molecules, thus modifying the 435 nm PL. Finally, to eliminate the influences of the surface oxygen vacancies on the 435 nm blue PL, the ethanol dispersions (0.4 mM) with the strong 435 nm PL were bubbled with oxygen or nitrogen for 40 min, and no obvious influences on the 435 nm PL were observed, showing that the 435 nm blue PL observed here has no detectable relationship with the surface oxygen vacancies and is different from the previously reported blue PL induced by the defects19 or the polymer coating.20
the ZnO NPs (Figure 5b), which is consistent with the above XRD patterns. This Raman band is weaker in the clusters separated from the ethanol dispersions compared with the clusters separated from the water dispersions, suggesting that ethanol molecules interact with the NP surface and weaken the crystal quality of the ZnO NPs.14,15 As shown in Figure 5c, in the dried clusters separated from the water dispersions, a strong absorption band (∼453 cm−1) arising from the Zn−O stretching of ZnO16−18 is observed, also indicating the formation of a ZnO phase. The absorption bands around 1551 and 3361 cm−1 can be assigned to the O−H bending and stretching of the adsorbed water molecules, respectively. For the dried clusters separated from the ethanol dispersions, the absorption bands of the Zn−O stretching and the O−H bending and stretching are also observed, but their positions comparatively shift to ∼464, ∼1586, and ∼3397 cm−1, respectively. Here the O−H bending and stretching bands should arise from the absorbed ethanol molecules rather than water molecules because of the appearance of the new absorption band of the C−H stretching (∼2920 cm−1) of ethanol molecules. The position shifts of the absorption bands of the Zn−O and O−H bonds clearly indicate that the hydroxyl groups (OH) of water and ethanol molecules interact with ZnO NPs differently. Furthermore, compared with pure ethanol, in the dried clusters separated from the ethanol dispersions, the center position of the C−H stretching band remains unvaried, but the center positions of both the O−H bending and stretching bands apparently shift. These results show that ethanol molecules interact with ZnO NPs by their hydroxyl groups rather than their hydrocarbon chains and produce a new composite ZnO-(C2H5OH)n on the surface of the ZnO NPs. Compared with the water dispersions (Figure 1d,e, Supporting Figure 5), in the ethanol dispersions (Figure 3b, Supporting Figure 9), the red-shift and intensity enhancement of the UV PL of the ZnO clusters, especially of the ZnO nanospheres and rods that have a larger surface area to react with ethanol molecules, become small and even vanish in the concentration range of 0−5 mM where the 435 nm blue PL usually appears. These phenomena imply that the new composite formed on the NP surface weakens the direct coupling effect between NPs, and some electronic energy is transferred to the new composite. According to the above results, a luminescent mechanism is proposed here. As shown in Scheme 1, the 435 nm PL arises from the radiative recombination of the electrons and holes in the conduction and valence bands of the new composite. The electron−hole pairs are induced by the photoexcitation of the new composite itself, or transferred from the photoexcited ZnO core. Therefore, the
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CONCLUSION In summary, the EET between NPs has been modified to tune and switch the PL of the dispersions of ZnO NPs by adjusting the growth, aggregation, and/or surface complex reaction of NPs. The following important results were achieved. (1) By developing a microwave-assisted hydrolysis method and a unique water/PVP/1-pentanol system, monodisperse ZnO nanospheres, rods, tripods, and spherical clusters with tunable sizes were successfully synthesized at a relatively large scale for controlling the particle aggregations. Significantly, the freestanding nanocrystals or the clusters of nanocrystals with tunable sizes can be selectively synthesized by simply controlling the water amount and reaction temperature of the microwave-assisted reaction. (2) The band-edge UV PL with tunable intensity of ZnO was greatly shifted from 360 to 398 nm by enhancing the NP aggregations via increasing the NP concentration from 0.2 to 50 mM. Meanwhile, the new blue emissions with tunable intensity around 418, 435, and 468 nm were induced. (3) The PL of ZnO dispersions can also be tuned by changing the NP aggregations via modifying the particle shape, for example, changing the NP shape from sphere to rod weakens the UV PL but enhances the blue PL around 418 nm. (4) With the monodisperse clusters, the NP aggregations and the PL can be reproducibly and finely tuned
Scheme 1. Conceptual Diagram Illustrating the Luminescent Mechanism for the 435 nm Blue PL
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(6) Steckel, J. S.; Zimmer, J. P.; Coe-Sullivan, S.; Stott, N. E.; Bulović, V.; Bawendi, M. G. Blue Luminescence from (CdS)ZnS Core−Shell Nanocrystals. Angew. Chem., Int. Ed. 2004, 43, 2154−2158. (7) Liu, D.-P.; Li, G.-D.; Su, Y.; Chen, J.-S. Highly Luminescent ZnO Nanocrystals Stabilized by Ionic-Liquid Components. Angew. Chem., Int. Ed. 2006, 45, 7370−7373. (8) Yang, B.; Feng, P.; Kumar, A.; Katiyar, R. S.; Achermann, M. Structural and Optical Properties of N-doped ZnO Nanorod Arrays. J. Phys. D: Appl. Phys. 2009, 42, 195402. (9) Willander, M.; Nur, O.; Sadaf, J. R.; Qadir, M. I.; Zaman, S.; Zainelabdin, A.; Bano, N.; Hussain, I. Luminescence from Zinc Oxide Nanostructures and Polymers and their Hybrid Devices. Mater. 2010, 3, 2643−2667. (10) Helms, V. Principles of Computational Cell Biology; Wiley-VCH: Weinheim, 2008; p 202. (11) Iozzi, M. F.; Mennucci, B.; Tomasi, J.; Cammi, R. Excitation Energy Transfer (EET) between Molecules in Condensed Matter: A Novel Application of the Polarizable Continuum Model (PCM). J. Chem. Phys. 2004, 120, 7029−7040. (12) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocrystal Assembles. Annu. Rev. Mater. Sci. 2000, 30, 545−610. (13) Hu, X.; Gong, J.; Zhang, L.; Yu, J. C. Continuous Size Tuning of Monodisperse ZnO Colloidal Nanocrystal Clusters by a MicrowavePolyol Process and Their Application for Humidity Sensing. Adv. Mater. 2008, 20, 4845−4850. (14) Güell, F.; Ossó, J. O.; Goñic, A. R.; Cornet, A.; Morante, J. R. Synthesis and optical spectroscopy of ZnO nanowires. Superlattices Microstruct. 2009, 45, 271−276. (15) Zhang, R.; Yin, P.-G.; Wang, N.; Guo, L. Photoluminescence and Raman scattering of ZnO nanorods. Solid State Sci. 2009, 11, 865−869. (16) Ambrožič, G.; Djerdj, I.; Škapin, S. D.; Ž igona, M.; Orel, Z. C. The Double Role of p-toluenesulfonic Acid in the Formation of ZnO Particles With Different Morphologies. CrystEngComm 2010, 12, 1862−1868. (17) Maensiria, S.; Laokula, P.; Promarak, V. Synthesis and Optical Properties of Nanocrystalline ZnO Powders by a Simple Method using Zinc Acetate Dihydrate and Poly(vinyl pyrrolidone). J. Cryst. Growth 2006, 289, 102−106. (18) Pouchert, C. J. The Aldrich Library of Infrared Spectra; Aldrich Chemical Company, Inc.: Milwaukee, WI, 1981; p 66. (19) Zeng, H.; Duan, G.; Li, Y.; Yang, S.; Xu, X.; Cai, W. Blue Luminescence of ZnO Nanoparticles Based on Non-Equilibrium Processes: Defect Origins and Emission Controls. Adv. Funct. Mater. 2010, 20, 561−572. (20) Xiong, H.-M. Photoluminescent ZnO Nanoparticles Modified by Polymers. J. Mater. Chem. 2010, 20, 4251−4262.
by simply modifying the cluster sizes. (5) As shown by the IR, PL and absorption spectra, the 435 nm blue PL of the ZnO NPs dispersed in ethanol arises from the complex reaction between the ZnO NPs and ethanol, which forms a new composite ZnO−(C2H5OH)n with a bandgap of ∼3.31 eV and changes the EET between ZnO NPs. (6) Because of the high surface activity, the ZnO NPs made here can coordinate with a variety of dispersion media including water, ethanol, methanol, 1,3-propanediol, and DEG. Therefore, the 435 nm blue PL can be tuned or switched by simply changing the dispersion medium, which opens a new approach for ZnO applications. (7) The 435 nm blue PL of the ZnO clusters dispersed in ethanol increases in intensity up to ∼30 fold with time and then remains stable even after 8 months; this highly stable and strong PL makes ZnO clusters one of the best luminescence materials.
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ASSOCIATED CONTENT
S Supporting Information *
Figure S1 (XRD patterns of ZnO nanospheres, rods, and tripods), Figures S2, 8 (absorption spectra of water or ethanol dispersions of ZnO nanospheres, rods, tripods, and clusters of 60, 100, and 150 nm), Figure S3, typical TEM image of ZnO clusters of 300 nm, Figure S4, zeta potential of the water dispersions of ZnO nanospheres (10 nm) and clusters (60 nm) at different concentrations, Figures S5, 7, 9−11 (PL spectra of water and/or alcohol dispersions of ZnO nanoclusters (60 nm), spheres, and rods), Figure 6 (size distributions of 60 nm ZnO nanoclusters dispersed in water and ethanol). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/cg5017017.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Basic Research Programs of China (Nos. 2012CB932304, 2011CB922102, 2011CB933400, and 2012CB93400) and program for new Century Excellent Talents at Nanjing University, and Project No. 61264008 of NSFC. At Boise State University, this work was supported in part by NSF CBET 1134468, NSF EAGER DMR-1137419, and ARO W911NF-09-1-0051 grants.
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