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Synthesis and Optical Properties of Dithiol-Linked ZnO/Gold Nanoparticle Composites Jisun Im,† Jagdeep Singh,† Jason W. Soares,‡ Diane M. Steeves,‡ and James E. Whitten*,† †
Department of Chemistry and Centers for Advanced Materials and High-Rate Nanomanufacturing, University of Massachusetts Lowell, Lowell, Massachusetts 01854, United States ‡ U.S. Army Natick Soldier Research, Development, and Engineering Center, Natick, Massachusetts 01760, United States ABSTRACT: Semiconductormetal nanocomposites are being pursued for use as a new generation of light emitters and photovoltaic devices. A convenient method for attaching gold nanoparticles (AuNPs) onto zinc oxide nanorods with variable surface densities is described that consists simply of mixing suspensions of monolayer-protected AuNPs and the nanorods in the presence of a dithiol. One end of the dithiol linker bonds to AuNPs via a ligand place-exchange reaction, and the other end attaches to ZnO via ZnS bonding. The nanocomposites have been characterized by UVvis absorbance, photoluminescence, and Raman spectroscopies. Attachment of the AuNPs affects the ZnO absorbance and photoluminescence (PL) spectra, resulting in blue shifts of the absorbance and UV excitonic emission peaks. Ultraviolet photoelectron spectroscopy has also been performed on the nanocomposite, zinc oxide nanorod, and gold nanoparticle samples, and an energy level diagram has been constructed. The PL and absorbance shifts are ascribed to the BursteinMoss effect in which photogenerated electrons accumulate on nearby gold nanoparticles, transfer to the ZnO conduction band, and cause band-gap widening.
’ INTRODUCTION Semiconductormetal nanocomposite materials are important for various applications, including charge rectification,1,2 photocatalysis,35 photovoltaics,6,7 chemical sensing,8,9 nonlinear optics,1012 and surface-enhanced Raman scattering (SERS).13,14 Examples of such materials include Au/CdSe,15 carbon nanotube/DNA/Au composites,16 and Au/organic nanowires.17 Nanoparticulate zinc oxide is a wide-band-gap semiconductor that exhibits a bimodal photoluminescence spectrum. The ratio of the visible and UV emission intensities depends on morphology, particle size, presence of defects, and surface adsorption. The ability to tailor the optical properties is useful for the fabrication of light emitters18,19 and for authentication and verification applications. To date, the fabrication of ZnOmetal nanocomposites has been realized using sputter18,19 or electrophoretic20 deposition of metals onto ZnO surfaces and by growing metal nanoparticles by chemical reduction3,6,21 or UV irradiation5 of precursors. Other approaches that have been attempted to achieve better control of the process include presynthesis of metal nanoparticles and deposition via calcination4 or attachment of polymer-coated gold particles to ZnO nanowires via bonding of carboxylic acid functional groups of the polymer to the ZnO surface.22 Additionally, 4-aminothiophenol (ATP) monolayers have been used to form ZnO/ATP/Ag and Ag/ATP/ZnO sandwich structures in which one layer is grown on top of an ATP-covered film of the other material.13 In this paper, we report a simple strategy to synthesize ZnO/ gold nanocomposites using dithiol linkers. This is made feasible in accord with recent reports2328 that thiols chemisorb on single r 2011 American Chemical Society
crystal and nanoparticulate zinc oxide surfaces. While dithiols previously have been used to separate two metal surfaces by an organic layer29 and to tether gold nanoparticles to gold surfaces,30 this is the first time that they have been used as linkers to zinc oxide surfaces. It is shown that thiol-protected gold nanoparticles can be tethered to the surface of ZnO nanorods simply by mixing those particles in the presence of dithiol linkers at room temperature. The affinity of thiol functional groups to AuNP and ZnO surfaces allows formation of a monolayer of AuNPs on the ZnO nanorod surface by forming AuS and ZnS bonds. The surface coverage of the gold nanoparticles can be varied by changing the dithiol linker concentration and the AuNP/ZnO ratio. The presence of gold nanoparticles on the ZnO nanorod surface alters the absorbance and photoluminescence spectra of the nanorods. The mechanism for these optical changes is discussed in the context of an energy level diagram obtained using ultraviolet photoelectron spectroscopy.
’ EXPERIMENTAL METHODS Synthetic Procedure. A 200 mg portion of ZnO nanorods (Nanocerox, Inc.), with typical diameters of 50100 nm and lengths ranging from 100 to 700 nm, was dried in a vacuum oven at 200 °C for 2 days prior to reaction. The dried nanorods were then suspended in 40 mL of toluene via sonication, and 40 mg of octanethiol-functionalized gold nanoparticles (OT-AuNPs), Received: March 15, 2011 Revised: May 2, 2011 Published: May 12, 2011 10518
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Figure 1. Synthesis of ZnO nanorods decorated with gold nanoparticles using dithiol linking molecules.
obtained from Aldrich, was added to the ZnO suspension and stirred for 20 min. p-Terphenyl-4,400 -dithiol (59 mg or 0.2 mmol), hereafter referred to as TPDT, purchased from Aldrich, was dissolved in toluene. This was added to the mixture of ZnO nanorods and OT-AuNPs and stirred vigorously for 1 h at room temperature to allow octanethiol ligands of the AuNPs to place-exchange with TPDT linkers. The product was obtained by vacuum filtration and thoroughly washed with toluene to remove unbound OT-AuNPs and dried under vacuum. The quantities of reagents described above correspond to a 1/5 AuNP/ZnO weight ratio and 0.2 mmol of TPDT. These were used to produce the highest coverage of gold nanoparticles. To demonstrate control of the surface coverage of AuNPs, 1/50 and 1/5 weight ratios of AuNPs/ZnO in the presence of 0.034 mmol of TPDT were also used. A control ZnO/TPDT sample was prepared using the same procedure, but without the addition of OT-AuNPs. Sample Characterization. Transmission electron microscopy (TEM) samples were prepared by drop-casting ZnO/ TPDT/AuNPs composite solutions in toluene onto carboncoated copper grids. The measurements were performed using a Philips EM 400t microscope and an accelerating voltage of 100 kV. The UVvis absorbance spectra were obtained with a PerkinElmer Lambda 900 spectrometer. Room-temperature photoluminescence spectra were acquired using a Fluorolog 3 fluorescence spectrometer (Horiba Jobin Yvon) equipped with a solid sample holder accessory. The excitation wavelength used was 325 nm. Micro-Raman spectroscopy was performed using a Bruker SENTERRA Raman microscope equipped with a confocal microscope. The wavelength, spot size, and power of the laser used were 532 nm, 2 μm, and 20 mW, respectively. Ultraviolet photoelectron spectroscopy (UPS) was used to measure the ionization energies of the valence states and the work functions of ZnO nanorods, TPDT, OT-AuNPs, and ZnO/ TPDT/AuNPs composites. The excitation source was a He I lamp (hν = 21.2 eV), and the kinetic energy of the photoelectrons was measured using a concentric hemispherical analyzer,
operating in fixed analyzer transmission (FAT) mode, with a pass energy of 2 eV. Samples of ZnO nanorods and the composites for UPS were prepared by fully covering those powders onto spectrograde carbon adhesive tabs (Electron Microscopy Sciences). The TPDT and OT-AuNP films for UPS were prepared by spincoating the solutions onto cleaned ITO-coated glass.
’ RESULTS AND DISCUSSION Figure 1 illustrates the synthetic preparation of ZnO/TPDT/ AuNP composites that consists of mixing suspensions of octanethiol-protected AuNPs (OT-AuNPs) with a mean core size of 3.8 ( 0.8 nm and ZnO nanorods with diameters of 50100 nm and lengths ranging from 100 to 700 nm, in the presence of TPDT linkers. TEM images of ZnO nanorods and OT-AuNPs are shown in the inset of Figure 1. Attachment of the dithiol to the OT-AuNPs occurs via a ligand place-exchange reaction, with AuS bond formation; the other end of the dithiol molecule attaches to ZnO nanoparticles via ZnS bonding. While various dithiol ligands may be used, an advantage of TPDT over aliphatic dithiols is that its backbone is more rigid. This rigidity prevents the molecule, for example, from potentially bonding to the gold or ZnO surface via both thiol functional groups, as discussed by Tai et al.31 Murray and colleagues32 have studied the kinetics and mechanism of the ligand place-exchange reaction for thiol-protected gold nanoparticles. The major conclusions from this study are that the rate-determining step is penetration by a thiol of the monolayer protecting the gold core and protonation of a bound thiolate, in an associative reaction, and that the rate of the place-exchange reaction decreases with the size of the entering ligand and with the length of the protecting thiols. The authors further concluded that some sites on the gold core are more reactive than others and that some are essentially nonreactive with respect to ligand exchange. Figure 2 shows TEM images of ZnO/TPDT/AuNPs composites. A monolayer of AuNPs was uniformly formed on the ZnO nanorod surface, and the coverage increased with increasing 10519
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Figure 2. TEM images of ZnO/TPDT/AuNPs composites: (A) composite-1.3 (1/50 of AuNPs/ZnO and 0.034 mmol of TPDT), (B) composite-2.5 (1/5 of AuNPs/ZnO and 0.034 mmol of TPDT), and (C) composite-3.4 (1/5 of AuNPs/ZnO and 0.2 mmol of TPDT). The images labeled DF are magnifications of AC, respectively.
Figure 3. (A) UVvis solid-state absorbance spectra of ZnO nanorods (dotted line), OT-AuNPs (dashed line), and the composite-3.4 (solid line) sample. (B) Photoluminescence spectra of films of ZnO nanorods (black), composite-1.3 (blue), composite-2.5 (green), composite-3.4 (red), and ZnO/TPDT (orange). The inset shows expansion of the UV emission region. The excitation wavelength was 325 nm.
AuNP-to-ZnO weight ratio and with increasing dithiol concentration. The surface coverage of AuNPs on the ZnO nanorods was calculated from the TEM images: 1.3 AuNPs/100 nm2 (Figure 2A,D), 2.5 AuNPs/100 nm2 (Figure 2B,E), and 3.4 AuNPs/100 nm2 (Figure 2C,F). The ZnO/TPDT/AuNP composites with surface coverages of 1.3, 2.5, and 3.4 AuNPs per 100 nm2 are designated as “composite-1.300 , “composite-2.500 , and “composite-3.400 , respectively. Figure 3A displays UVvis absorbance spectra of films of ZnO nanorods, OT-AuNPs, and the composite-3.4 sample. The surface plasmon band of the gold nanoparticle film has maximum absorbance at 536 nm, and the ZnO nanorods exhibit a threshold (i.e., band gap) at 386 nm (3.21 eV). The absorbance spectrum of
the composite-3.4 film shows both a UV peak and a broad surface plasmon band. The UV absorption peak is blue shifted slightly compared to ZnO, with an onset at 376 nm (3.30 eV). Figure 3B shows room-temperature photoluminescence (PL) spectra of powdered samples of ZnO nanorods, ZnO nanorods functionalized with TPDT (ZnO/TPDT), and ZnO/TPDT/ AuNP composites with differing AuNP surface coverages. The ZnO nanorods exhibit a doubly peaked emission spectrum, with the UV peak originating from exciton recombination. The visible emission peak is believed to be due to an electronic transition from defect-associated trap states, such as oxygen vacancies, closer to the conduction band edge, to deeply trapped holes near or in the valence band.3335 10520
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The Journal of Physical Chemistry C Attachment of AuNPs to the ZnO surface changes the intensites and energies of the UV and visible emission peaks. The broad visible emission peak centered at 505 nm is dramatically quenched due to AuNP attachment. There are two contributions to this effect: adsorption of the TPDT linker and reabsorption of the emitted visible light by the AuNPs. The first contribution is due to the TPDT adsorbate, which is confirmed by a control experiment included in Figure 3B: the ZnO/TPDT sample shows a 50% reduction in PL intensity of the visible peak, likely due to passivation of surface defects by TPDT adsorption. The decrease in visible emission intensity is accompanied by a 45% increase in intensity of the UV emission peak, without an energy shift. This effect also has been observed in our previous work in which various thiols have been adsorbed on ZnO nanoparticles.36 The nonzero intensity between 400 and 450 nm in the ZnO/ TPDT sample is due to fluorescence of TPDT itself. The second contribution is due to absorption of the emitted fluorescence by the AuNPs. The wavelength of the visible emission peak is in the region of the surface plasmon band of gold nanoparticles, and the gold nanoparticles absorb the emitted visible light from the ZnO nanorods, thereby contributing to quenching of the visible emission peak. For the ZnO/TPDT/AuNP composites, the intensity of the UV emission peak decreases with increasing AuNP surface coverage, as seen by comparison to the ZnO/ TPDT sample. The UV emission peak of the composites also shifts toward shorter wavelengths, relative to ZnO, and becomes narrower; the fwhm of the ZnO and composite-3.4 peaks are ca. 18 and 13 nm, respectively. The observed narrowing of the UV emission peak may be explained as follows. ZnO (wurtzite) with space group C46v (P63mc) is expected to have A1 þ 2B1 þ E1 þ 2E2 optical phonon modes at the Γ point of the Brillouin zone. The two nonpolar E2 modes at low (E2low) and high (E2high) frequencies are Raman-active only, and the B1 modes are silent.37 The polar A1 and E1 modes are infrared and Raman-active and split into transverse optical (TO) and longitudinal optical (LO) modes because of the long-range macroscopic electric field associated with the LO modes. The free exciton transition and its LO phonon replica contribute to the UV emission peak.3840 Narrowing of the UV emission peak arises from suppression of the LO phonon replica, and this is supported by the Raman scattering spectra in Figure 4. For the ZnO nanorods, the E2low, A1(TO), E2high, and E1(LO) peaks occur at 98, 382, 438, and 584 cm1, respectively. The E1(TO) mode appears at 410 cm1, as a shoulder of the E2high peak. The features at 331, 540, and 662 cm1 arise from multiphonon scattering processes.41 In the case of the composite-3.4 sample, the E2low, A1(TO), and E2high phonon modes appear at 108, 406, and 455 cm1, respectively, with the peaks shifted to higher energies compared with those of bare ZnO nanorods. The E1(LO) mode in the Raman spectrum of ZnO is known to be related to lattice ordering,42 and its absence in the case of the composite-3.4 sample is consistent with the observed narrowing in the UV emission peak and suggests that AuNP attachment induces disorder in the ZnO lattice. It is also interesting that the difference in the energy of the E2high mode is 17 cm1. A shift of E2high to higher frequency has been reported to be due to compressive stress,43 again consistent with lattice disorder. The weak peaks at 287 and 306 cm1 correspond to ZnS44 and AuS stretching modes,45 respectively. In addition, the CS stretching and ring stretching of TPDT are observed at 669 and 822 cm1, respectively.
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Figure 4. Raman spectra of ZnO nanorods and composite-3.4. A laser wavelength, spot size, and power of 532 nm, 2 μm, and 20 mW, respectively, were used for the experiment. The intensity of the composite-3.4 spectrum is multiplied by 10 for clarity.
Figure 5 shows He I (hν = 21.2 eV) ultraviolet photoelectron spectroscopy (UPS) data of ZnO nanorods, OT-AuNPs, and the composite-3.4 samples. This experiment measures the ionization energies of the valence states and the work function of the surface, determined by the low energy threshold of the highest occupied molecular orbital (HOMO). On the basis of these measurements, the work function of the OT-AuNP film is 5.1 eV, and the valence band edge (VBE) of the ZnO nanorod film is 7.0 eV below the vacuum level. Combined with the band gap (from Figure 3), the conduction band edge (CBE) is 3.8 eV, relative to the vacuum level. Figure 6 shows the energy band diagram of the zinc oxide nanorods and octanethiol-functionalized AuNPs, constructed from the UPS data. The diagram assumes that the HOMO LUMO gap of the OT-AuNPs is zero, as is the case for functionalized AuNPs with diameters greater than 1.6 nm.46 Upon irradiation, photogenerated electrons promoted to the conduction band of ZnO can either radiatively decay back to the valence band via excitonic recombination (emitting UV light) or transfer to low-lying ZnO defect levels and then radiatively decay to the valence band (emitting visible light). Some of this visible emission may be absorbed by the AuNPs, because its wavelength matches their surface plasmon absorbance band, and this process leads to reduced intensity of the visible PL peak, as observed in Figure 3B. Transfer of electrons from the ZnO conduction band to the AuNP conduction band, on the other hand, leads to nonradiative decay and a reduction in intensity of the UV PL peak. As discussed by Mulvaney and colleagues,47 metal islands (e.g., Ag, Cu, and Au) deposited on ZnO quantum dots undergo Fermi level equilibration due to photoinduced electron accumulation on the metal particles (analogous to the Stark effect in atoms). The blue shift in the UV absorbance and emission peaks, therefore, likely arises from photoexcited electrons in the ZnO conduction band that transfer to the conduction band of the 10521
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Figure 5. UPS of ZnO nanorods, TPDT, OT-AuNPs, and composite-3.4 samples. Panel (A) shows the entire width of the spectra, and (B) shows the threshold regions of the spectra, with the estimated thresholds indicated by arrows. The spectra are plotted with respect to ionization energy, and zero on this scale corresponds to the vacuum level.
distance between the semiconductor and the gold particles to be varied. A fluorescence-based chemical sensor is envisioned in which ZnO and gold nanoparticles are tethered together via a dithiol-functionalized polymer. Swelling of the polymer film by the chemical vapor would change the distance between the photoluminescent metal oxide particle and the metal and change the PL spectrum. Work is underway in our laboratories to explore this possibility.
’ AUTHOR INFORMATION Corresponding Author
*Phone: (978) 934-3666. Fax: (978) 934-3013. E-mail:
[email protected]. Figure 6. Energy band diagram based on UPS data with UV (325 nm) irradiation. Evac, CB, and VB represent the energy levels of the vacuum level, conduction band, and valence band, respectively. The symbols “e-”, “h”, and “Φ” represent electron, hole, and work function, respectively, and hν1 and hν2 correspond to the UV emission and visible emission energies, respectively. The energies are in units of eV.
gold nanoparticles. Fermi level equilibration leads to band-gap widening, because the bottom of the conduction band is now partially filled instead of empty, and is referred to as the BursteinMoss effect.4850 While this has been observed for gold islands deposited directly onto ZnO nanoparticles, the same phenomenon is likely occurring in the present study, because the distance of the OT-AuNPs from the ZnO nanorods is only ca. 10 Å, based on TEM images. The calculated molecular length of TPDT is 17.6 Å,51 and the significantly shorter observed distance suggests that the ligands are tilted on the ZnO nanorod surface.
’ CONCLUSIONS We have demonstrated a simple method for attaching AuNPs to ZnO nanorods and nanoparticles. The surface density of the attached AuNPs is easily controlled by varying the ZnO-toAuNP ratio and the dithiol concentration during the synthesis. A relatively short, rigid dithiol ligand, TPDT, was employed. However, a variety of other dithiols could be used, permitting the
’ ACKNOWLEDGMENT The authors acknowledge the assistance of Dr. Peng Wang at Bruker Optics in obtaining the Raman spectra. This work is supported by U.S. Army Natick Soldier Research, Development, and Engineering Center. ’ REFERENCES (1) Boettcher, S. W.; Strandwitz, N. C.; Schierhorn, M.; Lock, N.; Lonergan, M. C.; Stucky, G. D. Nat. Mater. 2007, 6, 592–596. (2) Bala, S.; Aithal, R. K.; Derosa, P.; Janes, D.; Kuila, D. J. Phys. Chem. C 2010, 114, 20877–20884. (3) Subramanian, V.; Wolf, E. E.; Kamat, P. V. J. Phys. Chem. B 2003, 107, 7479–7485. (4) Zheng, N.; Stucky, G. D. J. Am. Chem. Soc. 2006, 128, 14278–14280. (5) Chiou, J. W.; Ray, S. C.; Tsai, H. M.; Pao, C. W.; Chien, F. Z.; Pong, W. F.; Tsai, M. H.; Wu, J. J.; Tseng, C. H.; Chen, C. H.; Lee, J. F.; Guo, J. H. Appl. Phys. Lett. 2007, 90, 192112. (6) Chen, Z. H.; Tang, Y. B.; Liu, C. P.; Leung, Y. H.; Yuan, G. D.; Chen, L. M.; Wang, Y. Q.; Bello, I.; Zapien, J. A.; Zhang, W. J.; Lee, C. S.; Lee, S. T. J. Phys. Chem. C 2009, 113, 13433–13437. (7) Nishijima, Y.; Ueno, K.; Yokota, Y.; Murakoshi, K.; Misawa, H. J. Phys. Chem. Lett. 2010, 1, 2031–2036. (8) Joshi, R. K.; Hu, Q.; Alvi, F.; Joshi, N.; Kumar, A. J. Phys. Chem. C 2009, 113, 16199–16202. 10522
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