pubs.acs.org/Langmuir © 2009 American Chemical Society
Encapsulation of Zinc Oxide Nanorods and Nanoparticles Jagdeep Singh, Jisun Im, and James E. Whitten* Department of Chemistry and Centers for Advanced Materials and High-Rate Nanomanufacturing, The University of Massachusetts Lowell, Lowell, Massachusetts 01854
Jason W. Soares and Diane M. Steeves* U.S. Army Natick Soldier Research, Development & Engineering Center, Natick, Massachusetts 01760 Received March 29, 2009. Revised Manuscript Received June 13, 2009 A simple method for encapsulating zinc oxide nanoparticles within an organic matrix is described that consists of dispersing them in an ethanolic solution, adding an organothiol, and stirring while heating. Electron microscopy, photoemission, Raman spectroscopy, and thermal gravimetric analyses demonstrate that partial dissolution of the oxide occurs, accompanied by encapsulation within a matrix consisting of a 1:2 zinc/thiol complex. Using this methodology, it is possible to surround ZnO within diverse matrices, including fluorescent ones. The process is demonstrated for 1-dodecanethiol (DDT) and fluorescent 2-naphthalenethiol (NPT). For DDT, ZnO nanorods become surrounded by a layer of the zinc-thiol complex that is greater than 100 A˚ thick. In the case of NPT, significantly greater dissolution of the ZnO occurs, with the encapsulated rods taking on a spherical geometry, as evidenced by electron microscopy.
Introduction Metal oxide nanoparticles, nanotubes, nanorods, nanowires, and whiskers are finding important applications ranging from catalysis to optoelectronics.1 Because of its unique semiconducting and optical properties,2,3 zinc oxide has been used for lightemitting diodes,4-6 photovoltaic devices,7,8 heterogeneous catalysis,2 and chemical sensors.6,9,10 In organic-inorganic hybrid devices, it is often desirable to covalently attach organic molecules to metal oxide surfaces such that electrons and holes may be transported across the oxide-organic molecule interface.11,12 Future hybrid devices may require encapsulation of semiconducting inorganic materials within organic matrices. In one example of how this may be achieved, Fuertes et al.13 recently demonstrated a strategy for confining inorganic materials, such as iron oxides and nickel oxide, within mesoporous carbon capsules. *To whom correspondence should be addressed. Phone: (978) 934-3666 (J.E.W.), (508) 233-4320 (D.M.S.). Fax: (978) 934-3013 (J.E.W.), (508) 2335521 (D.M.S.). E-mail:
[email protected] (J.E.W.), diane.steeves@ us.army.mil (D.M.S). (1) Rodriguez, J. A.; Fernandez-Garcia, M. Synthesis, Properties, and Applications of Oxide Nanomaterials; Wiley-Interscience: Hoboken, NJ, 2008. (2) W€oll, C. Prog. Surf. Sci. 2007, 82, 55–120. (3) Wang, Z. L. ACS Nano 2008, 2, 1987–1992. (4) Dai, L. P.; Deng, H.; Mao, F. Y.; Zang, J. D. J. Mater. Sci.: Mater. Electon. 2008, 19, 727–734. (5) Huang, J.; Xu, Z.; Zhao, S.; Li, Y.; Zhang, F.; Song, L.; Wang, Y.; Xu, X. Solid State Commun. 2007, 142, 417–420. (6) Willander, M.; Zhao, Q. X.; Hu, Q.-H.; Klason, P.; Kuzmin, V.; Al-Hilli, S. M.; Nur, O.; Lozovik, Y. E. Superlattices Microstruct. 2008, 43, 352–361. (7) Hau, S. K.; Yip, H.-L.; Ma, H.; Jen, A. K. Y. Appl. Phys. Lett. 2008, 93, 233304. (8) Ravirajan, P.; Peiro, A. M.; Nazeeruddin, M. K.; Graetzel, M.; Bradley, D. D. C.; Durrant, J. R.; Nelson, J. J. Phys. Chem. B 2006, 110, 7635–7639. (9) Fernandez, M. J.; Fontecha, J. L.; Sayago, I.; Aleixandre, M.; Lozano, J.; Gutierrez, J.; Gracia, I.; Cane, C.; del Carmen Horrillo, M. Sens. Actuators, B 2007, 127, 277–283. (10) Chadwick, A. V.; Russell, N. V.; Whitham, A. R.; Wilson, A. Sens. Actuators, B 1994, 18, 99–102. (11) Jennings, J. R.; Ghicov, A.; Peter, L. M.; Schmuki, P.; Walker, A. B. J. Am. Chem. Soc. 2008, 130, 13364–13372. (12) Gaudiana, R.; Hadjikyriacou, S.; He, J.-A.; Waller, D.; Zhu, Z. J. Macromol. Sci., Part A: Pure Appl. Chem. 2003, 40, 1295–1306. (13) Fuertes, A. B.; Sevilla, M.; Valdes-Solis, T.; Tartaj, P. Chem. Mater. 2007, 19, 5418–5423.
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In the present work, a method of encapsulating zinc oxide nanorods and nanoparticles within an organic matrix is described that simply consists of stirring and heating an ethanolic nanorod/organothiol mixture. This leads to complete encapsulation of the nanorods within a thick organic shell composed of a thiol-Zn-thiol complex. Although previous studies14-20 have shown that alkanethiols adsorb on ZnO surfaces and nanoparticles, the present work is the first to demonstrate encapsulation of ZnO nanoparticles using thiols. Because a large selection of functionalized thiols is available, it is possible to surround ZnO nanoparticles with a variety of chemical functional groups.
Experimental Section Synthetic Procedure. ZnO nanorods (500 mg) (Nanocerox, Inc.), with typical diameters of 50-100 nm and lengths ranging from 100 to 700 nm, were dried overnight in a vacuum oven at 200 °C. This drying step was found to be crucial to the process. The dried nanorods were then suspended by sonication with a cell disruptor in 50 mL of 95% ethanol/5% H2O for 3 min, and 1.25 g of the thiol, either 1-dodecanethiol (DDT) or 2-naphthalenethiol (NPT), was added to the mixture (equating to a 1:2.5 w/w ZnO/ thiol ratio) and sonicated for another 5 min in a water bath. The mixture was then stirred for 1 h at 75 °C. In some comparison experiments, the mixture was not heated but simply stirred at ambient temperature. The pH varied, depending on the thiol and its concentration, but dropped to ca. 4.0 and 3.5 for DDT and NPT, respectively. In the case of DDT, the product was collected (14) Sadik, P. W.; Pearton, S. J.; Norton, D. P.; Lambers, E.; Ren, F. J. Appl. Phys. 2007, 101, 104514. (15) Dvorak, J.; Jirsak, T.; Rodriguez, J. A. Surf. Sci. 2001, 479, 155–168. (16) Pesika, N. S.; Hu, Z.; Stebe, K. J.; Searson, P. C. J. Phys. Chem. B 2002, 106, 6985–6990. (17) Garcia, M. A.; Merino, J. M.; Pinel, E. F.; Quesada, A.; de la Venta, J.; Gonzalez, M. L. R.; Castro, G. R.; Crespo, P.; Llopis, J.; Gonzalez-Calbet, J. M.; Hernando, A. Nano Lett. 2007, 7, 1489–1494. (18) Tsao, M.-W.; Rabolt, J. F.; Sch€onherr, H.; Castner, D. G. Langmuir 2000, 16, 1734–1743. (19) Hedberg, J.; Leygraf, C.; Cimatu, K.; Baldelli, S. J. Phys. Chem. C 2007, 111, 17587–17596. (20) Halevi, B.; Vohs, J. M. J. Phys. Chem. B 2005, 109, 23976–23982.
Published on Web 08/05/2009
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Figure 1. Mg KR XPS of the C 1s, S 2p, O 1s, and Zn 3p regions of unfunctionalized ZnO nanorods (referred to as “control”), DDTfunctionalized ZnO nanorods prepared by stirring the nanorods with DDT at room temperature (referred to as “unheated”), and DDT- and NPT-encapsulated nanorods prepared by heating the ZnO with the thiol. For ease of viewing, the peaks have been normalized to have approximately similar S 2p intensities.
by filtration, washed extensively with 100% ethanol, dried at 110 °C for 10 min, and then stored at room temperature in a desiccator. In the case of NPT, a large fraction of the ZnO dissolved, as opposed to staying suspended, and the solution (which consisted of encapsulated nanoparticles) was decanted and dried. Control nano-ZnO samples were prepared using the same procedure, but without the addition of the thiol, and collected by filtration as previously described. XPS and Sample Characterization. Samples for X-ray photoelectron spectroscopy (XPS) were prepared by suspending 60 mg of the functionalized nano-ZnO in 30 mL of the aqueous ethanol solution, ultrasonicating for 10 min, and pouring the dispersion in a Gooch crucible over a ca. 1 cm2 copper coupon that had been precleaned with hydrochloric acid/distilled water. The solvent was then allowed to evaporate, the samples were dried in air, and vacuum-compatible conductive silver adhesive was used to “paint” conducting paths from the edges of the copper coupon to the sample stub. The sample stub was then introduced into a VG ESCALAB MKII photoelectron spectrometer with a base pressure in the 10-10 Torr range. Mg KR X-rays were used, 9948 DOI: 10.1021/la9010983
and photoelectrons were detected at a takeoff angle of 90°, defined as the angle between the surface plane and the entrance of the focusing lens of the concentric hemispherical analyzer. As will be discussed, it was desirable in one experiment to expose an unfunctionalized nanorod ZnO-coated copper coupon to methanethiol vapor. This was accomplished by leaking methanethiol into the preparation chamber of the XPS instrument, with the chamber temporarily isolated from its diffusion pump. The sample was exposed to the gas at ca. 0.1 Torr for about 7 h, with the pressure measured using an MKS Baratron capacitance pressure gauge. Transmission electron microscope (TEM) samples were prepared by drop-casting ethanolic ZnO solutions onto carboncoated copper grids; the measurements were performed using a Philips EM 400t microscope and an accelerating voltage of 100 kV. Raman spectroscopy was performed using a “LabRam” J-Y spectrometer with a 514.5 nm argon ion laser. Photoluminescence spectra were acquired using a Fluorolog 3 fluorescence spectrometer (Horiba Jobin Yvon, Inc.) equipped with a solid sample holder accessory. Langmuir 2009, 25(17), 9947–9953
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Article Table 1. XPS Peak Binding Energies of DDT- and NPT-Functionalized ZnO Nanorods a
C 1s (eV)
S 2p (eV)
O 1s (eV)
Zn 3p (eV)
Control (ZnO) 285.6 n/a 531.1, 532.6 89.5, 92.0 DDT (unheated)-uncorrected 285.8 163.7 530.5, 531.5 89.7, 92.3 DDT (unheated)-corrected 285.6 163.5 530.3, 531.2 89.5, 92.1 DDT (with heating)-uncorrected 288.6 166.2 535.2 92.4, 95.3 DDT (with heating)-corrected 285.7 163.3 532.3 89.5, 92.4 NPT (with heating)-uncorrected 288.1 166.5, 171.9 535.9 93.0, 96.0 NPT (with heating)-corrected 284.6 163.0, 168.4 532.4 89.5, 92.5 a “Corrected” indicates that the peaks have been corrected for charging by shifting the peaks rigidly such that the Zn 3p3/2 peak agrees with that of the control ZnO sample.
Results and Discussion Figure 1 displays XPS data corresponding to unheated DDT/ ZnO, heated DDT/ZnO, and heated NPT/ZnO preparative procedures; a ZnO control is also included. Several important points should be noted. The first is that the control sample contains a small amount of adventitious carbon at ca. 285.6 eV due to surface contamination, but the carbon peak is significantly more intense after thiol functionalization. The presence of a strong S 2p peak at ca. 163.7 eV, in the case of unheated DDT, indicates that the thiol is covalently attached to the ZnO surface because DDT remains after extensive washing with ethanol. The S 2p envelope consists of unresolved, spin-orbit coupled S 2p3/2 and S 2p1/2 peaks. This spin-orbit coupled doublet is not resolved in thiol/Au SAMs unless the measurements are performed with a monochromatic X-ray source or synchrotron radiation.21 However, the lack of a well-defined shoulder to the high binding energy side of the more intense S 2p3/2 peak suggests that the thiol/ZnO layer is less homogeneous than in the case of thiols assembled on gold.22,23 The observed binding energy of the center of the peak is in agreement with values obtained by Sadik, Norton, and co-workers14 (164 eV) when dodecanethiol was adsorbed on macroscopic oxygen- and zinc-terminated ZnO surfaces and by Rodriguez and colleagues15 (163.5 eV), who exposed ZnO films prepared in ultrahigh vacuum (UHV) to methanethiol gas. It should be noted that protonated sulfur atoms (i.e., “free thiols”) and sulfur atoms bonded to a gold surface normally appear at ca. 164.2 and 162.6 eV, respectively.24 The measured value of 163.7 eV indicates that the proton has dissociated from the thiol to form a covalent bond to the ZnO surface. Assuming that initial state effects in the photoemission process dominate, the higher binding energy for S bonded to ZnO, compared to S bonded to Au, suggests that the sulfur is more oxidized in the former case. For the heated DDT and NPT samples, the observed peak S 2p binding energies are 166.2 and 166.5 eV, respectively; the observed C 1s values are 288.6 and 288.1 eV, respectively. The higher binding energies for the heated DDT and NPT samples, compared with those in the unheated DDT one, are due to incomplete charge compensation from ejected photoelectrons that arises because the organic layer is so thick that it effectively insulates the (semiconducting) ZnO nanorods from their neighbors and from the copper coupon substrate. The weaker Zn 3p and O 1s signals (especially in the case of DDT) from these samples are consistent with them having a sufficiently thick organic layer completely surrounding the ZnO nanorods that these photoelectrons cannot escape elastically. The detection depth of XPS for photoelectrons having energies of 700-1200 eV (as for Zn 3p and (21) Ge, Y.; Weidner, T.; Ahn, H.; Whitten, J. E.; Zharnikov, M. J. Phys. Chem. C 2009, 113, 4575–4583. (22) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152–7167. (23) Ahn, H.; Kim, M.; Sandman, D. J.; Whitten, J. E. Langmuir 2003, 19, 5303– 5310. (24) Singh, J.; Whitten, J. E. J. Phys. Chem. C 2008, 112, 19088–19096.
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O 1s) is 50-100 A˚.25 The almost complete absence of O 1s and Zn 3p signals indicates that, in the case of the heated DDT sample, the ZnO nanorods are surrounded by an S- and C-containing organic layer that is at least this thick. Table 1 displays the observed peak binding energies for the control ZnO sample, the DDT-functionalized sample prepared without heating, the DDT-functionalized (encapsulated) sample prepared by heating, and the NPT-functionalized (encapsulated) sample prepared by heating. As discussed above, significant charging occurs for the encapsulated DDT and NPT samples. This is evidenced by shifts in the Zn 3p3/2 and Zn 3p1/2 spin-orbit coupled peaks to higher binding energies by ca. 3 eV compared with the ZnO control sample. Zn 2p spectra (not shown) were also acquired, and the shifts in those peaks agree with those in the Zn 3p spectra. A very minimal amount of charging (ca. 0.2 eV) occurs in the case of the unheated DDT sample. To first order, it is possible to correct the peaks for charging by shifting them all by the same amount. The results of this correction are included in the table. An important conclusion from Table 1 is that the S 2p peaks from the corrected spectra all occur at a similar binding energy of 163.0-163.5 eV. In the case of the NPT-ZnO sample, the O 1s signal mainly originates from a fraction of the NPT molecules (estimated as about 1/4) that contain oxidized sulfur, present as sulfonate groups and consistent with the higher binding energy S 2p peak appearing at a corrected value (Table 1) of 168.4 eV. This value agrees with the experimentally determined value for oxidized thiol groups in 3-mercaptopropyltrimethoxysilane layers.24 The slightly larger Zn 3p signal for the NPT sample compared with the DDT one indicates that the NPT sample has a thinner encapsulating layer. The O 1s XPS spectra contain additional information related to bonding of DDT to the nanorod surface. Peak fitting of the O1s spectra of the control and DDT unheated samples shows that they have two components, one centered at ca. 532.5 eV and another at ca. 531 eV. The higher and lower binding energy components are, respectively, due to surface hydroxyl groups and oxygen atoms attached to zinc.14,17 As shown in Figure 1, chemisorption of DDT to the surface of the ZnO nanorods causes the oxygen signal attributed to the hydroxyl groups to decrease relative to that of the oxide, indicating that thiols are replacing surface hydroxyl groups and forming Zn-S bonds. This behavior is consistent with the thiols protonating and displacing adsorbed hydroxyl groups, as known to occur for Brønsted acids reacting with hydroxylated metal oxide surfaces.26 Measurement of the S/Zn atomic ratios from XPS data (corrected for sensitivity factors) yields values of 0.13(0.01 and 1.8 ( 0.2 for the unheated and heated DDT samples, respectively. The corresponding C/S atomic ratio for the heated DDT sample is (25) Watts, J. F.; Wolstenholme, J. An Introduction to Surface Analysis by XPS and AES; John Wiley & Sons: Chichester, U.K., 2003. (26) Barteau, M. A. Chem. Rev. 1996, 96, 1413–1430.
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Figure 2. TEM images of (a) unfunctionalized ZnO nanorods, (b) DDT-functionalized nanorods prepared by stirring a ZnO/DDT mixture at room temperature (unheated), and (c) nanorods encapsulated within a DDT-Zn-DDT matrix (prepared by heating a ZnO/DDT mixture). (d) An example of a highly magnified TEM image of the encapsulated ZnO nanorods, with the arrow pointing to an eroded region of the nanorod.
11.8 ( 1.2, and only a small amount of oxygen (O/Zn atomic ratio= 0.40 ( 0.04) is present. The C/S ratio is consistent with that of DDT, with each molecule containing 12 carbon atoms and 1 sulfur. To confirm that the large S/Zn is not due to physisorbed DDT multilayers, the sample was heated at 150 °C in UHV for 2 h, and this ratio did not change. In the case of the NPT heated sample, the S/Zn and C/S atomic ratios are 2.3 ( 0.2 and 9.3 ( 0.9, respectively. Because an NPT molecule contains 10 carbon atoms and 1 sulfur atom, the C/S is consistent with the organic layer containing this molecule or a derivative of it. For both the DDT and the NPT heated samples, the S/Zn ratio is 2. This strongly suggests that the organic layer that surrounds the ZnO core in the DDT and NPT heated samples consists of a 1:2 Zn/thiol complex (e.g., R-S-Zn-S-R), where R is dodecane or naphthalene. An experiment (not shown) was also performed in which a nanorod ZnO sample on a copper coupon was extensively dosed in vacuum at room temperature with methanethiol gas using an exposure of 2.3 109 L. This enormous exposure was used to ensure saturation of the near-surface region interrogated by XPS. After dosing and being allowed to sit in UHV overnight, XPS was performed. The presence of a strong S 2p signal was confirmed, with a S/Zn atomic ratio of 0.14 ( 0.01, as observed for the unheated DDT sample prepared by adsorption from solution. Because methanethiol is small enough to penetrate between the nanoparticles on the sample surface and is not expected to form multilayers at room temperature, it is concluded that a monolayer of a thiol on this particular type of nano-ZnO surface leads to a S/Zn ratio of ca. 0.14 ( 0.01 and that heating nano-ZnO in the presence of DDT or NPT leads to the thick, encapsulating layer. These conclusions are supported by transmission electron microscopy (TEM) images shown in Figure 2. Minimal differences 9950 DOI: 10.1021/la9010983
are seen between an unfunctionalized nano-ZnO sample (Figure 2a) and the unheated DDT sample (Figure 2b). The latter results in a monolayer of DDT, as previously discussed. However, dramatic differences are observed between these and the DDT heated sample (Figure 2c) in which the ZnO nanorods are completely encapsulated by an organothiol-containing layer. The situation is analogous to the flesh around a bone, with the ZnO being the bone and the organothiol-Zn complex layer serving as the flesh. In fact, close examination of Figure 2c shows the darker ZnO rods encased within the lighter organic layer, which is several hundred angstroms thick. Although it is difficult exactly to discern the diameters of the encapsulated nanorods, examination of the TEM images indicates that the average diameter is similar for both the control ZnO and the encapsulated samples. However, it must be noted that the nanorods are polydisperse and that the encapsulated rods that are most easily discerned are the larger ones. Figure 2d shows expansion of several encapsulated nanorods and their surrounding matrix. The inhomogeneous, patchy appearance of the encapsulated ZnO rods suggests that regions of the nanorods have been eroded during the formation of the encapsulating layer. It is, therefore, concluded that portions of the ZnO nanorods are inhomogeneously eroded during the encapsulation process. Figure 3 shows a TEM image of spherical encapsulated particles that result from heating the ZnO nanorods with NPT. In this case, the nanorods have dissolved to a much greater extent due to the lower pH of the NPT/ZnO solution. The diameters of the particles range from 50 to 150 nm in this particular image, but diameters as large as 500 nm are observed. It is impossible from the TEM images to discern the encapsulating layer from the ZnO cores. In the case of the heated NPT/ZnO mixture, the existence of Langmuir 2009, 25(17), 9947–9953
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Figure 3. TEM image of nanospheres encapsulated within an NPT-Zn-NPT matrix (prepared by heating a ZnO/NPT mixture).
spherical, encapsulated particles suggests that the ZnO nanorods have dissolved to a much greater extent than in the case of the heated DDT/ZnO mixture, with concomitant decreases in diameter and length. The remaining ZnO core then becomes encapsulated within an organic matrix that minimizes its volume by assuming spherical geometry. As a whole, these results indicate that mixing and heating DDT and NPT with suspended ZnO nanorods results in dissolution of a portion of the nanorods, with the effect especially pronounced in the case of NPT. Because thiols are weak acids, there are two possible reactions to consider: Figure 4. Illustration of the dissolution process that results in
ZnOðsÞ þ 2Hþ f Zn2þ þ H2 O
ð1Þ
ZnO-OHðsÞ þ RSH f ZnO-SRðsÞ þ H2 O
ð2Þ
Note that in eq 2, the hydroxyl groups and thiol molecules are bonded to zinc atoms on the zinc oxide lattice and not to oxygen atoms. Equation 1 leads to direct dissolution of the metal oxide, which is known to occur at pH = 3.5.27 Equation 2 leads to the formation of a Zn-thiol surface complex from a hydroxylated site. This surface complex facilitates dissolution of the zinc oxide and proceeds through transfer from the solid phase to the solution, as observed for metal oxides reacting with oxalic acid.27 This results in the formation of a dissolved Zn-SRþ complex, which apparently reacts with another thiol molecule in solution to form a 1:2 zinc/thiol complex, with liberation of a proton. The ZnO(s) surface is then available to react with thiol molecules to repeat the cycle. Heating, of course, increases the rate of this process. The greater extent of dissolution of ZnO when NPT is used, compared with DDT, is a result of eq 1, consistent with pKa values28 of 6.6 and 10.6, respectively, for NPT and DDT. An illustration of the dissolution process for the DDT-treated sample that results in encapsulation is shown in Figure 4. A rough calculation may be made of the available nanorod surface area by assuming that the average ZnO nanorod is a solid cylinder with a radius and length of 44 and 371 nm, respectively (estimated by examining TEM images and taking the average dimensions of many particles). Assuming these dimensions and a bulk density29 of 5.6 g/cm3, the available surface area of the ZnO (27) Rodenas, L. A. G.; Blesa, M. A.; Morando, P. J. J. Solid State Chem. 2008, 181, 2350–2358. (28) The pKa values are from SciFinder “predicted properties” database, calculated using ACD/Laboratories Software v8.14 for Solaris. (29) Lin, S.-S.; Huang, J.-L.; Lii, D.-F. Surf. Coat. Technol. 2004, 176, 173–181.
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encapsulation of ZnO nanorods within a layer consisting of a 1:2 Zn/DDT complex. Greater dissolution of the nanorods occurs in the case of the NPT heated sample than depicted in the illustration.
is 9.1 m2/g. Hedberg et al.19 have measured the saturation packing density of a monolayer of octadecanethiol on an oxidized zinc electrode as 6.7 10-9 mol/cm2. Combining this value with the estimated surface area indicates a maximum thiol adsorption of 6.110-4 mol/g of ZnO. The 1:2.5 w/w ZnO/thiol ratio used in our experiments corresponds to a factor of approximately 20 more thiol than this value, consistent with having adequate thiol to react with surface Zn atoms, dissolve a portion of the surface, and form a thick encapsulating layer. Figure 5 displays thermal gravimetric analysis (TGA) spectra of the ZnO control, DDT unheated sample, DDT heated, and NPT heated samples. As expected, the nano-ZnO control (curve a, Figure 5) exhibits negligible weight loss upon heating to 700 °C. The DDT unheated sample (curve b, Figure 5) exhibits a ca. 6% weight loss in the temperature range of 210-370 °C. In contrast, the DDT sample prepared with heating (curve d, Figure 5) exhibits a ca. 40% weight loss in the temperature range of 250400 °C. This very large weight loss is consistent with both the XPS and the TEM results and confirms that the sample contains a very high proportion of organic material. The normal boiling point (BP) of DDT is 266-283 °C. The higher desorption/decomposition temperatures observed for the DDT-functionalized samples are consistent with strongly bound (i.e., chemisorbed) DDT. In the case of the heated DDT sample, the greater than 283 °C decomposition temperature suggests that the organic layer surrounding the ZnO nanorods has a higher molecular weight than that of DDT, consistent with the formation of the proposed thiol-Zn -thiol complex that surrounds the ZnO nanorods. This is also consistent with the XPS results discussed earlier in which the sample was heated at 150 °C for 2 h in UHV without exhibiting a decrease in the S/Zn atomic ratio. Under these DOI: 10.1021/la9010983
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Figure 5. Thermal gravimetric analysis of (a) an unfunctionalized ZnO nanorod control sample, (b) a ZnO/DDT sample prepared by stirring at room temperature (unheated), (c) a ZnO/NPT sample prepared by stirring/heating, and (d) a ZnO/DDT sample prepared by stirring/heating. For all scans, the heating rate was 20 °C/min, and the measurements were performed in a nitrogen atmosphere.
Figure 6. Raman spectroscopy of powdered specimens of the unfunctionalized ZnO nanorod control (dashed line) and the encapsulated DDT-ZnO nanorods (solid line), prepared by heating the ZnO/DDT ethanolic mixture. A laser wavelength, spot size, and power of 514.5 nm, 1-2 μm, and 5 mW, respectively, were used for the experiment.
conditions, the BP of DDT would be significantly less than 150 °C at 10-10 Torr, as can easily be shown using the ClausiusClapeyron equation. Also included in Figure 5 is the TGA scan corresponding to Figure 3 in which ZnO is encapsulated with the NPT-Zn-NPT complex (curve c, Figure 5). In this case, the weight loss is 31%. This weight loss indicates that the spherical particles must consist of ZnO cores, because particles consisting exclusively of the NPT-Zn-NPT complex would give an even greater weight loss; a control TGA experiment of pure NPT yields a 100% weight loss. The decomposition temperature is significantly greater than that of the DDT-functionalized/encapsulated samples, exceeding 520 °C. 9952 DOI: 10.1021/la9010983
Figure 7. (a) Photoluminescence spectra of powdered specimens of ZnO nanorods (dashed curve) and nanorods encapsulated by the Zn/DDT complex, prepared by heating (solid curve). (b) Photoluminescence spectra of ZnO nanorods (nano-ZnO), nanorods encapsulated by the Zn/NPT complex (NPT-ZnO, intensity divided by 10), neat NPT (NPT, intensity divided by 10), and a mixture of naphthalene and ZnO (naphthalene/ZnO mixture). The excitation wavelength for all of these scans was 325 nm.
Figure 6 shows the low-energy portion of the Raman spectrum of the DDT-encapsulated ZnO nanorod sample (i.e., heated DDT) and an unfunctionalized control. The higher-energy region (not shown) contains bands consistent with dodecanethiol, except that the thiol S-H stretch30 expected at ca. 2565 cm-1 is completely absent. The strong band at 438 cm-1 present in both the control and the encapsulated sample is due to a phonon mode of ZnO.31 The bands at 650 and 725 cm-1, which appear exclusively in the DDT-encapsulated sample, are attributed to C-S stretches and CH2 rocking modes of the -CH2-S- functional groups, respectively.30 The peak at 272 cm-1 is tentatively assigned to the S-Zn-S vibration, based on theoretical and (30) Socrates, G. Infrared and Raman Characteristic Group Frequencies: Tables and Charts; John Wiley & Sons: Chichester, U.K., 2001. (31) Alim, K. A.; Fonoberov, V. A.; Shamsa, M.; Balandin, A. A. J. Appl. Phys. 2005, 97, 124313.
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experimental studies of zinc-cysteine complexes.32 It is noteworthy that this peak is absent in the Raman spectrum of a DDT/ ZnO sample prepared at room temperature (i.e., an unheated sample), indicating that it is not due to Zn-S vibrations. Figure 7a shows photoluminescence (PL) spectra of an unfunctionalized nano-ZnO sample and a heated DDT-ZnO sample. The powdered samples were reheated under vacuum at 110 °C for 10 min immediately prior to analysis to remove any surface moisture adsorbed during storage. The nano-ZnO shows a doubly peaked emission spectrum. The ultraviolet peak (UV) corresponds to an exciton emission band, whereas the visible peak is believed to be due to an electronic transition from a level close to the conduction band edge to a defect-associated trap state, such as an oxygen vacancy.33,34 For the ZnO nanorod sample encapsulated by the 1:2 Zn/DDT complex, both the visible and the UV peaks decrease relative to the control. There are various factors that may affect the ratio of the intensities of the visible and UV peaks. These include humidity, surface modification,34,35 and particle size,33 with the latter affecting the surface area-to-volume ratio. Elegant vacuum dosing experiments by Idriss and Barteau36 have measured the effect of various adsorbates on the photoluminescence intensities of the UV and visible emission peaks of ZnO powders at various temperatures. At room temperature, oxygen, hydrogen, and carbon monoxide decrease the intensities of both peaks. In contrast, methanol and formic acid increase the intensities. Although we have not yet explored the effect of surface temperature, and work is still in progress in our laboratory to understand how a thiol monolayer affects the photoluminescence spectrum, the major reason for the difference in PL intensities between the encapsulated ZnO and the control in Figure 7a is likely a dilution effect because 40% (by mass) of the sample is nonphotoluminescent. Figure 7b shows a spectrum of ZnO encapsulated within the 1:2 Zn/naphthalenethiol complex layer, with the emission feature centered at ca. 410 nm. The figure also includes an unfunctionalized nano-ZnO sample, a nano-ZnO/naphthalene (no thiol group) mixture sample, and a sample of 2-naphthalenethiol (neat). The naphthalene/ZnO mixture sample was prepared using the described synthetic procedure (with heating at 75 °C), but naphthalene instead of 2-naphthalenethiol was used. The similarity of the naphthalene/ZnO mixture and the unfunctionalized nano-ZnO samples confirms that the thiol group is needed to achieve ZnO (32) Foley, S.; Enescu, M. Vib. Spectrosc. 2007, 44, 256–265. (33) van Dijken, A.; Meulenkamp, E. A.; Vanmaekelbergh, D.; Meijerink, A. J. Phys. Chem. B 2007, 104, 1715–1723. (34) Norberg, N. S.; Gamelin, D. R. J. Phys. Chem. B 2005, 109, 20810–10816. (35) Marczak, R.; Werner, F.; Gnichwitz, J.-F.; Hirsch, A.; Guldi, D. M.; Peukert, W. J. Phys. Chem. C 2009, 113, 4669–4678. (36) Idriss, H.; Barteau, M. A. J. Phys. Chem. 1992, 96, 3382–3388.
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Article
functionalization and encapsulation. Neat NPT shows a main emission peak at 400 nm and a shoulder at ca. 385 nm. An interesting observation from Figure 7b is that the NPT-encapsulated zinc oxide (NPT-ZnO) emission spectrum is red shifted, with the main peak shifting to ca. 410 nm compared to the neat NPT spectrum. This indicates that bonding of NPT to the nanoZnO surface and/or NPT bound to zinc atoms in the surrounding matrix (formed via S-Zn-S bonds) affects the fluorescence spectrum of NPT. It should be noted that the variation in the ratio of the intensities of the UV and visible photoluminescence peaks for the nano-ZnO controls in Figure 7a,b is due to sample processing, with slightly different drying conditions used. Because surface contamination, moisture, and presence of adsorbates can all influence the inherent photoluminescence of the nano-ZnO, consistency of all processing conditions is necessary if one wishes to make comparisons between samples sets. However, comparison within a single sample set is valid if the control and surfacemodified samples are handled identically, which was the case for sample sets in Figure 7a,b. Therefore, the nano-ZnO control data in both sample sets are accurate for each respective set, and any alterations in the PL due to surface modification has been reproduced for the respective sample sets under identical processing conditions.
Conclusions This paper demonstrates a previously unreported method of encapsulating zinc oxide nanoparticles and nanorods within an organic matrix consisting of a 1:2 Zn/thiol complex. The thickness and morphology of the encapsulating layer is controllable by the choice of thiol and preparation conditions. This method may be useful in future photovoltaic applications in which one wishes to surround ZnO nanorods and whiskers with light-absorbing molecules, which could be achieved by using thiol-terminated dye molecules. Work continues in our laboratories to understand the effects of monolayer thiol adsorption and encapsulation on the photoluminescence properties of nanoscale zinc oxide. Acknowledgment. The authors acknowledge Dr. Peter Stenhouse and Dr. Joel Carlson from the U.S. Army Natick Soldier Research, Development & Engineering Center for their assistance with characterization techniques. The authors also acknowledge Dr. Vasil Pajcini of Evans Analytical Group (EAG) for his help with the Raman spectroscopy measurements. A portion of this work was supported by the U.S. Army Natick Soldier Research, Development & Engineering Center under Contract No. W911NF07-D-0001-0335 TCN 08047. This document has been approved for unlimited distribution (PAO No. U09-093).
DOI: 10.1021/la9010983
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