Zinc Oxide

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Photocatalytic Activity and Fluorescence of Gold/Zinc Oxide Nanoparticles Formed by Dithiol Linking Ilyas Unlu†, Jason W. Soares‡, Diane M. Steeves‡, James E. Whitten†* †

Department of Chemistry and Center for High-Rate Nanomanufacturing,

University of Massachusetts Lowell, Lowell, Massachusetts 01854, USA ‡

U.S. Army Natick Soldier Research, Development, and Engineering Center, Natick, Massachusetts 01760, USA

*Corresponding author: Professor James E. Whitten Phone: (978) 934-3666 Fax: (978) 934-3013 Email: [email protected] Abstract

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Monolayer-protected gold nanoparticles (AuNPs) with average diameters of 2-4 nm have been covalently attached to zinc oxide nanorods using dithiol ligands. Electron microscopy and Raman spectroscopy show that ozone treatment or annealing at 300oC or 450oC increases the average diameter of the AuNPs to 6, 8 and 14 (±1) nm, respectively, and decomposes the organic layers to various degrees. These treatments locate the AuNPs closer to the nanorods. Heating and subsequent ozone exposure changes the color of the as-prepared nanocomposite powder from blue to purple due to oxidation of the outer layer of the AuNPs, and heating to 300oC changes it to pink due to oxygen desorption. ZnO nanorods have a bimodal photoluminescence spectrum that consists of an ultraviolet excitonic peak and a visible, surface defect-related peak. Ozone treatment and annealing of the nanocomposite decreases the intensities of both peaks due to quenching by the AuNPs, but the visible peak is affected less. The photocatalytic efficiency of the nanocomposites towards oxidative degradation of rhodamine B has been measured and follows the order: 300oC > 450oC > ozone treated ≈ as-prepared ≈ bare ZnO. The greater efficiency of the annealed samples likely arises from decreased electron-hole pair recombination rates.

Keywords: Dithiol, gold, ozone, photocatalysis, photoluminescence, Raman spectroscopy, zinc oxide


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Introduction Metal oxides decorated with metal nanoparticles have potentially important applications such as gas and vapor sensing,1-14 hydrogen storage,15 electro-optics,16 and catalysis.17-20 Methods used to deposit metal nanoparticles on metal oxides include in-situ reduction of metal ions in solution,2 sputter or thermal deposition,21 laser ablation of microparticle aerosols,22 and “solflame synthesis”.23 With respect to catalysis, applications include using inert metal oxides as supports for catalytically active gold nanoparticles for reactions such as the oxidation of carbon monoxide19 and using metal nanoparticles to dope photocatalytically active metal oxides to enhance their photo-oxidative efficiency. Examples of the latter include loading platinum onto the surface of titanium dioxide nanoparticles18 for the photocatalytic degradation of methyl orange and decorating Cu2O cubes with palladium nanoparticles to enhance the photoreductive dechlorination of polychlorinated biphenyls.17 In a previous study, we developed a simple, one-step synthetic procedure to attach gold nanoparticles to zinc oxide nanorods.24 The method consists of stirring a colloidal suspension of zinc oxide nanorods with commercially available, thiol-protected gold nanoparticles (AuNPs) in the presence of a dithiol. Linkage occurs because of the affinity of thiols to both gold and zinc oxide surfaces,25,26 with attachment to the AuNPs occurring via a ligand place exchange reaction. Using this method, we demonstrated that the surface density of AuNPs may be controlled by varying the molar ratios of the reactants. Initial studies were performed using p-terphenyl-4,4’dithiol (TPDT), and the effects of the AuNPs on the UV-vis absorbance and photoluminescence (PL) spectra were investigated, along with measurements of the valence photoelectron spectra of the nanocomposites. In the present study, we have investigated methods of removing the dithiol ligands toward the goal of fabricating stable functional materials, including photocatalysts. Approaches that have


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been explored include ozone treatment and thermal annealing, and the properties of the resulting nanocomposites have been measured. The PL intensities of the ZnO excitonic ultraviolet and surface-related visible emission peaks decrease to differing degrees in intensity following ozone treatment and annealing as the AuNPs make physical and electronic contact with the ZnO nanorods. Possible mechanisms of these changes are discussed. It is also demonstrated that heating the AuNP/dithiol/ZnO nanorods at 300oC results in optimal photocatalytic performance for the oxidative decomposition of rhodamine B dye. Higher temperatures give less efficient photocatalysts, likely due to agglomeration that leads to fewer and larger AuNPs. A reversible color change of the AuNP/dithiol/ZnO nanocomposite powder (from blue to purple) due to ozone treatment and its reversion upon heating is also discussed. This work demonstrates that thermal annealing of easily synthesized, dithiol-linked AuNP/ZnO nanocomposites is a convenient method of fabricating photocatalysts and novel optoelectronic materials.

Experimental Section Nanocomposite Synthesis. 100 mg of ZnO nanorods (Nanocerox, Inc.), with typical lengths and diameters of 100-700 and 50-100 nm, respectively, were dried in a vacuum oven at 200oC for 48 h prior to suspension in 20 mL of toluene via ultrasonication. 20 mg of 1-octanethiolfunctionalized AuNPs (Sigma-Aldrich, Inc.) with an average particle size of 2-4 nm were then added to the ZnO suspension and vigorously stirred for about 20 min. Because the nanorods are polar and not solubilized by surfactant, ultrasonication and vigorous stirring are necessary to keep them suspended. 0.1 mmol of dithiol was dissolved in toluene, added to the suspension and stirred vigorously for about 1 h at room temperature. The final product was vacuum filtered and thoroughly washed with toluene to remove unbound AuNPs, dithiol, any dithiol-aggregated AuNPs, and then dried under vacuum. Control samples of ZnO with just the ligand (e.g.,


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designated “TPDT/ZnO”) were prepared using the same procedure but with omission of the AuNPs. For most of the studies, p-terphenyl-4,4’-dithiol (TPDT) was used. However, some comparative studies were performed with 1,2-ethanedithiol (EDT), tetra(ethylene glycol) dithiol (TEGDT), 1,4-benzenedithiol (BDT), and biphenyl-4,4’-dithiol (BPDT). All were purchased from Sigma-Aldrich.

Ligand Removal. Ozone and thermal annealing were performed to remove the ligands from the nanocomposites. For ozone treatment, powder samples were placed into a UV-ozone cleaner (Novascan PSD-UV) and exposed for 1 h. Alternatively, treatments at various temperatures were performed for 2 h in a GS Lindberg tube furnace under air or flowing nitrogen, as indicated.

Sample Characterization. DSC was carried out using a TA Instruments model Q20 calorimeter. The measurements were run with 1-2 mg of the sample (accurately weighed) in air or nitrogen, as noted, with a heating rate of 25oC min-1. UV-visible spectroscopy (UV-Vis) was performed on RhB solutions using a Perkin Elmer LAMBDA 3 spectrophotometer. The absorbance spectra of liquid samples were obtained in scanning mode between 300 nm and 800 nm, with a scan speed of 480 nm min-1 and slit width of 1 nm. JEOL JSM 7401F field emission scanning and Philips EM 400t transmission electron microscopes (FE-SEM and TEM) were used to image samples before and after ozone and thermal treatments. TEM samples were prepared by drop-casting from toluene suspensions onto carbon-coated copper grids. Imaging was performed at an acceleration voltage of 100 kV. Raman spectroscopy using 785 or 532 nm excitation was carried out with a Bruker SENTERRA Raman microscope with a 2 µm spot size and 1 mW of laser power. PL spectra of powders at room temperature were obtained with a Horiba Jobin Yvon Fluorolog 3 fluorescence spectrometer equipped with a solid sample holder accessory.


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Photocatalytic Measurements. 1.5 mg of rhodamine B (RhB, Sigma Aldrich) was dissolved in 100 ml of deionized water. Identical 1 cm path length quartz cuvettes were filled with 3 ml of this solution. One was used as a control sample and contained no particles. The others were filled with 0.80 ± 0.02 mg of bare ZnO, as-prepared, ozone-treated and annealed nanocomposite samples. The samples in the cuvettes were placed next to each other and mixed using micro-stir bars while they were irradiated with UV light from above. Periodically, irradiation was stopped, and the absorbance of the RhB solution was measured using UV-vis spectroscopy. For irradiation, a nominally 365 nm UV LED array lamp (Model E27-AC from Golden Gadgets, Inc.) was used. The actual wavelength and optical power of the lamp were measured as a broad peak from 380-400 nm and 4.5 mW cm-2, respectively, using an Ocean Optics spectrometer and a photometer.

Results and discussion Differential scanning calorimetry (DSC) was performed to determine the temperature necessary to remove the ligands. Fig. 1 shows DSC for heating TPDT-linked nanocomposites under air and nitrogen atmospheres, along with corresponding scans for ZnO nanorod control samples (i.e., bare nanorods). A scan is also included for TPDT-functionalized ZnO without gold nanoparticles (TPDT/ZnO). As expected, very little heat flow is observed for the ZnO control samples, regardless of the atmosphere. In the case of the TPDT/ZnO control, the peak at ca. 400oC arises from combustion of the TPDT ligands, and the larger peak at ca. 540oC is likely due to transformation of the outer layer of ZnS on the nanorods to ZnO. The ZnS-to-ZnO transition is known to be complete by 700oC.27 Previous studies by our group for thiol adsorption on ZnO surfaces demonstrated that Zn-S bonds form,26 and it is reasonable that decomposition of


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the ligands would result in ZnS formation on the outer layer of the nanorods, which subsequently converts to ZnO at higher temperature. In the case of the AuNP/TPDT/ZnO sample heated in air, the small peak at ca. 280oC is due to combustion of the octanethiol ligands that surround the AuNPs. The large, broad peak between ca. 350 and 550oC is from combustion of the TPDT ligands and ZnS conversion to ZnO. The DSC peaks are no longer resolved, with the presence of the AuNPs causing ZnS-to-ZnO conversion to occur at slightly lower temperatures. One of the initial goals of this work was to use the dithiol ligands to accomplish facile decoration of the nanorods with the AuNPs and then to decompose them with minimal heating and agglomeration of the AuNPs. It was therefore desirable to explore different dithiols, and DSC scans similar to those in Fig. 1 (in air) were performed for biphenyl-4,4’-dithiol (BPDT), 1,4-benzene dithiol (BDT), tetraethylene glycol dithiol (TEGDT), and 1,2-ethanedithiol (EDT) adsorbed on ZnO nanorods. From these experiments, the dithiol decomposition temperatures were 350, 360, 380, and 390oC, respectively. These are slightly lower than the corresponding TPDT value of 400oC. Note that for all of these various ligands, the 280oC DSC peak was present, which substantiates its assignment as associated with octanethiol combustion. Thermal gravimetric measurements (not shown) were attempted, with marginal success due to the lack of well-defined peaks in the spectrum. However, comparison of the mass loss for a sample of TPDT-covered ZnO to that of AuNP/TPDT/ZnO indicates that TPDT and the AuNPs comprise (very approximately) 1.7 and 2.3 wt. % of the sample, respectively, with the balance of the mass due to ZnO. Fig. 2 displays FE-SEM images of the AuNP/TPDT/ZnO samples before and after UV ozone treatment and annealing at 300 and 450oC for 2 h, and Fig. 3 shows corresponding TEM images. The first observation from these data is that the images become sharper after the treatments, and significantly so following annealing at 450oC. The initial poor resolution is due to surface


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charging from negative charge buildup caused by the poorly electrically conducting organic ligands. Ozone treatment and annealing at 300oC partially removes the ligands and completely does so by 450oC, consistent with the DSC results. Energy dispersive spectroscopy (not shown) was also performed as a function of temperature and indicates a decrease in intensity of the carbon peak due to ozone treatment and thermal treatments. The second observation is that the AuNP size increases due to annealing, which occurs due to the thermodynamic driving force of the gold nanoparticles to minimize their surface area and surface free energy. Analysis of the TEM images shows that the average AuNP diameters are 3±1 nm before treatment and 6, 8, and 14 (±1) nm after ozone treatment or annealing at 300 or 450oC, respectively. Fig. 4a displays Raman spectra using 785 nm excitation light of AuNP/BPDT/ZnO samples before and after ozone treatment and annealing in air at 450oC. BPDT is of interest because of its slightly lower decomposition temperature compared to TPDT. In the case of the as-prepared sample, the major peaks arise from aromatic C-C stretches (1578 and 1495 cm-1), =C-H in-plane ring deformations (1278 and 1005 cm-1), =CH out-of-plane ring deformations (808 and 713 cm1

), C-S stretches (269 cm-1) and from aliphatic C-C stretches (e.g., 1078 and 542 cm-1) from the

1-octanethiol ligands surrounding the AuNPs.28 Ozone treatment causes these to decrease in intensity,

with

shifts

and

broadening

occurring

in

some

peaks

due

to

partial

oxidation/decomposition of the ligands. Heating at 450oC results in complete loss of the BPDT and octanethiol ligand Raman features; the peaks at 195, 433 and 575 cm-1 are due to ZnO phonon modes. Fig. 4b shows Raman spectra using 532 nm laser irradiation of AuNP/TPDT/ZnO samples before and after annealing in air at 300oC, with the latter spectrum also acquired using 785 nm laser light. As expected, the spectrum of the as-prepared sample is very similar to that of AuNP/BPDT/ZnO in Fig. 4a, with minor differences in the energies and shapes of the Raman


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peaks due to the different number of phenyl rings. Notably, heating at 300oC causes disappearance of the 1067 cm-1 peak, which arises from straight-chain aliphatic C-C stretches in the octanethiol ligands. Also reduced, but not to the same extent, is the 1583 cm-1 peak that arises from aromatic C-C stretches. These data indicate that annealing in air at 300oC essentially completely decomposes the octanethiol ligands on the AuNPs, but the dithiol ligands are only partially decomposed or desorbed. Decomposition of the 1-octanethiol ligands in air is consistent with the DSC spectra presented in Fig. 1. An unexpected finding in Fig. 4b is that annealing of the AuNP/dithiol/ZnO nanocomposite in air at 300oC leads to fluorescence that dwarfs the Raman peaks when 532 nm excitation light is used. This does not occur for 785 nm laser radiation, or for the as-prepared sample, or if the sample is annealed at 300oC in nitrogen. This will be discussed in more detail shortly. ZnO nanorod powder is white, of course, and remains so after ozone treatment and heating in air or nitrogen. However, AuNP/dithiol/ZnO nanocomposites are blue prior to any treatment. In the course of performing ozone and heating experiments, color changes were observed in the powder samples, as depicted in Fig. 5. Heating of the nanocomposite at 300oC changes its color from blue to pink, likely due to sintering of the AuNPs into slightly larger particles (from 3 to 8 nm, as discussed earlier). Ozone exposure of the annealed sample turns the powder purple, and heating the ozone-exposed powder in air at 300oC causes it to revert to pink. This is in accord with gold nanoparticle aqueous solution experiments by Pacey and colleagues in which they observed a reversible color change upon ozone exposure.29 The authors postulated that ozone exposure results in oxygen adsorption on the nanoparticles that modify their absorbance spectrum; the oxygen desorbs once ozone exposure ceases. In the present work, the experiments were performed on powders, and the oxygen desorbs upon heating. The cycle may be repeated, as we have confirmed by repeating the process at least ten times.


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Zinc oxide is an attractive material from an optoelectronic and chemical sensing point of view because of its bimodal photoluminescence (PL) spectrum.30 The UV emission peak is due to excitonic recombination of an excited electron as it falls from the conduction band to the valence band. The details of origin of the visible PL is still the subject of some debate, but it certainly involves nonradiative electron transfer from the conduction band to surface defectrelated states and subsequent decay to the valence band, resulting in visible emission. Fig. 6 shows PL spectra of the as-prepared, UV-ozone exposed, and 300oC annealed composites linked with TPDT. Ozonation and heating both result in a dramatic decrease in the UV peak and only a minor decrease in the visible one. The decrease in the UV peak is consistent with a decrease in distance between the gold nanoparticles and the ZnO nanorod surface. Zhang et al.31 systematically varied the distance between a ZnO layer and AuNP film by depositing different thicknesses of alumina spacer layers between them. They found that PL increased with AuNP/ZnO separation in the range of 0 to 25 nm. Beyond that, the PL decreased. The distance dependence is due to multiple effects, including the surface plasmon of the AuNPs increasing the incident excitation light field on the ZnO layer and quenching of the ZnO fluorescence by the metal at short distances. In our case, the length of the TPDT ligand is in the range of 1-2 nm, and its decomposition is expected to bring the gold particles closer to the ZnO nanorod surface and, hence, decrease the PL, as observed. The interesting aspect of Fig. 6 is that the decrease is so much greater for the UV peak than the visible one. In our previous study (which did not involve ozone treatment or annealing), we found that adsorbing TPDT on ZnO nanorods (with no AuNPs present) caused the visible peak to decrease in intensity, as observed in general for thiol adsorption on ZnO.26 This is believed to be due to passivation of defects. In the present work, removing the TPDT ligands by ozonation or thermal decomposition enhances the visible PL peak, and this counteracts to some extent the decreased PL intensity that occurs as the AuNP


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particles get closer to the ZnO surface. The overall effect is a decrease in the visible PL that is mitigated by the loss of the TPDT ligands. It should be noted that Hou and Wang32 synthesized AuNP/ZnO composite materials using a different method than in the present work. In their case, the

gold

nanoparticles

were

grown

from

reduction

of

AuCl4-

ions

onto

3-

aminopropyltrimethoxysilane functionalized ZnO nanorods. These authors also observed decreased intensities of both the UV and visible ZnO PL peaks due to the presence of gold nanoparticles. While the fluorescence in Fig. 4b is relatively weak, it is strong enough to overwhelm the Raman spectra following heating at 300oC, when 532 nm light is used. Alexson et al.33 studied Raman scattering of gold- and silver-coated metal oxide nanowires functionalized with benzenethiol as a function of laser wavelength. Interestingly, these authors found that background fluorescence arises from adsorbate-induced surface states of the metal particles and that the emission intensity depends on the choice of laser wavelength. They demonstrated that whether fluorescence overwhelms Raman scattering depends on how efficient the laser is at populating benzenethiol-induced high-lying excited surface states. They found that 785 nm laser light resulted in significantly less fluorescence than for shorter wavelengths. This is the exactly the situation observed in the present work in which heating at 300oC desorbs/decomposes octanethiol ligands but preserves TPDT ligands. It is reasonable, then, to conclude that the fluorescence observed in Fig. 4b originates from the gold particles that make more intimate contact with the TPDT ligands following desorption/decomposition of octanethiol. One of the original motivations for investigating the various treatments was to explore photocatalytic activity of the nanocomposite materials. After ozone treatment and heating at various temperatures, the photo-oxidative activity of the AuNP/DBDT/ZnO nanocomposite was evaluated. Examples of the types of experiments performed are shown in Fig. 7, which contains


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absorbance spectra of 3.0 ml, aqueous 30 µM rhodamine B (RhB) samples following irradiation for 0, 1.0 and 1.5 h with 380-400 nm UV light; one corresponds to a control sample containing just the dye, and the other two contain 0.8 mg of bare ZnO and 450oC annealed AuNP/BPDT/ZnO nanocomposite powder. Note that the different absorbance values for an exposure time of zero are due to differing sample turbidities that cause different degrees of scattering. While this scattering affects the values of the absorbance by artificially increasing them slightly, the relative changes between samples are comparable because the cuvettes contain the same mass of powders. As shown in the figure, no change in absorbance is detected for the RhB sample, while both the ZnO nanorod and nanocomposite samples undergo decreases in absorbance as a function of exposure time, with the 450oC annealed nanocomposite decreasing faster than the bare ZnO sample. Fig. 8 contains a graph of absorbance at 552 nm versus UV irradiation time for bare ZnO nanorods, as-prepared AuNP/BPDT/ZnO, ozone-exposed AuNP/BPDT/ZnO, and AuNP/BPDT/ZnO samples annealed at various temperatures. The photocatalytic performances of the as-prepared and ozone-treated samples are similar to each other but worse than that of bare ZnO nanorods. The 300, 400 and 450oC annealed nanocomposites show substantial enhancement in activity over that of bare ZnO. It should be noted that a control experiment was run on TPDT-covered ZnO nanorods, and the photocatalytic activity was found to be the same, within experimental error, to bare nanorods. This confirms that the gold particles are, indeed responsible for the photocatalytic enhancement of ZnO. The annealed composites were stable with respect to performance and exhibited essentially the same photocatalytic activity, even after storage for several months at room temperature in air. RhB is commonly used for photocatalytic investigations. For example, Khanchandani et al.34 demonstrated photocatalytic enhancement for indium sulfide-coated ZnO nanorods compared to bare ones. The corresponding half-life for the In2S3/ZnO sample in ref. 34 is ca. 40 min for a


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concentration of 0.25 mg/ml of catalyst solution, which is essentially identical to the concentration used in our experiments. The half-life for the 300oC annealed sample (assuming first-order kinetics) is ca. 60 min (see Fig. 8). These numbers illustrate that the performance of the conveniently prepared gold/ZnO photocatalysts produced by thiol linkages is comparable to that of state-of-the-art ZnO photocatalysts. Of course, a variety of other factors are involved, including differing wavelengths, optical power and surface area that make direct comparison difficult. The observed photocatalytic enhancement of ZnO nanorods due to AuNPs is in accord with several other studies related to metal decoration of ZnO. Liu and coworkers35 grew ZnO nanorods on an alloy substrate using aqueous zinc nitrate. Gold islands were then solutiondeposited on the ZnO by immersion in HAuCl4 solution (after activation with tin chloride). The authors observed a 3-to-4 times enhancement in the rate of photocatalytic degradation of methyl orange for the Au/ZnO nanorod array compared to a ZnO nanorod array control. Enhanced photocatalytic decomposition rates were also observed for Ag-decorated ZnO nanoparticles, with the metal adsorbed on the ZnO either by deposition precipitation36 or a one-pot autoclave synthetic method.37 Copper doping of ZnO nanorods using the so-called “chemical bath deposition” method, and studies of their photocatalytic efficiency, have also been performed.38 The mechanism of photocatalytic activity is promotion of electrons from the valence band to the conduction band by light whose energy exceeds the bandgap. The holes in the valence band may oxidize adsorbed molecules. However, the efficiency of the photo-oxidative process is limited by the recombination rate of the electron-hole pairs.39 Other researchers have observed similar gold-induced enhancements in ZnO nanoparticle photocatalytic activity, and the accepted mechanism for the enhancement is transfer of the excited electrons from the ZnO conduction band to the gold nanoparticles.40,41 Trapping of the electrons on the gold particles then slows


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down the recombination process, as discussed by Subramanian et al.42 Our results are consistent with this mechanism. In the present work, the details of the results may be explained as follows. The dithiols block adsorption sites on the ZnO nanorods. Thermal annealing does a better job of removing the ligands than does ozone treatment, with the order of photocatalytic activity being 300oC > 400oC > 450oC > ozone treated ≈ as-prepared ≈ bare ZnO. Annealing at 300oC enhances activity by decomposing the ligands sufficiently (but not completely, as indicated by electron microscopy charging experiments) such that the AuNPs are in closer physical and electronic contact with the ZnO nanorods. The decrease in activity with higher anneal temperatures likely results from AuNP agglomeration that leads to lower surface area contact between gold and ZnO.

Conclusion A variety of procedures exist in the literature for decorating ZnO (and other metal oxides) with metal nanoparticles. The method discussed in this paper is extremely simple and can be used for commercially available ZnO nanoparticles and monolayer-protected AuNPs. It is made possible by the bonding of thiol functional groups to both gold and zinc oxide surfaces. Ozone treatment of the nanocomposite powder is not effective at adequately removing the organic ligands to synthesize optimally photocatalytic nanocomposites. However, heating at around 300oC, which decomposes most of the ligands but minimizes AuNP agglomeration, results in enhanced photocatalytic performance for the oxidation of aqueous RhB compared to bare nanorods or nanocomposite material heated to higher temperatures. Interesting optical effects related to ozonation and annealing have also been observed, and colorimetric sensor applications may be possible. It was also observed that annealing of the AuNP/dithiol/ZnO nanocomposite causes the PL of both the UV and visible emission peaks to decrease due to quenching by the AuNPs. However, it was found that the visible (surface defect related) peak was affected to a


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much lower extent. ZnO has potential optoelectronic applications, including LEDs and diode lasers, and methods of turning the emission spectrum by depositing metals should be considered. It is also noteworthy that gold islands deposited on ZnO nanorods using the methods outlined in the present study may make possible surface enhanced resonance spectroscopy (SERS) on ZnO.


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Acknowledgements 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, under Cooperative Agreement #W911QY-13-2-003. Approved for public release (U15-079).


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References (1) Eranna, G. Nanostructured Metal Oxides and Gas-Sensing Devices, in Metal Oxide Nanostructures as Gas Sensing Devices; CRC Press: Boca Raton, FL, 2012, 41-190. (2) Trung, D.D.; Nguyen, D.H.; Tong, P.V.; Nguyen, V.D.; Dao, T.D.; Chung, H.V.; Nagao, T.; Nguyen, V.H. Effective Decoration of Pd Nanoparticles on the Surface of SnO2 Nanowires for Enhancement of CO Gas-Sensing Performance. J. Hazard. Mater. 2014, 265, 124132. (3) Kukkola, J.; Mohl, M.; Leino, A.-R.; Toth, G.; Wu, M.-C.; Shchukarev, A.; Popov, A.; Mikkola, J.-P.; Lauri, J.; Riihimaeki, M., et al. Inkjet-Printed Gas Sensors: Metal Decorated WO3 Nanoparticles and their Gas Sensing Properties. J. Mater. Chem. 2012, 22, 17878-17886. (4) Zhang, Y.; Xiang, Q.; Xu, J. High Performance Chemical Sensors Constructed by Noble Metal Nanoparticle Decorated ZnO Nanowires. Sensor Lett. 2011, 9, 332-337. (5) Li, X.; Feng, W.; Xiao, Y.; Sun, P.; Hu, X.; Shimanoe, K.; Lu, G.; Yamazoe, N. Hollow Zinc Oxide Microspheres Functionalized by Au Nanoparticles for Gas Sensors. RSC Adv. 2014, 4, 28005-28010. (6) Chang, C.M.; Hon, M.H.; Leu, I.C. Influence of Size and Density of Au Nanoparticles on ZnO Nanorod Arrays for Sensing Reducing Gases. J. Electrochem. Soc. 2013, 160, B170-B176. (7) Rai, P.; Kim, Y.-S.; Song, H.-M.; Song, M.-K., Yu, Y.-T. The Role of Gold Catalyst on the Sensing Behavior of ZnO Nanorods for CO and NO2 Gases. Sens. Act. B 2012, 165, 133-142.


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(8) Liu, X.; Zhang, J.; Wang, L.; Yang, T.; Guo, X.; Shihua, W; Wang, S. 3D Hierarchically Porous ZnO Structures and their Functionalization by Au Nanoparticles for Gas Sensors. J. Mater. Chem. 2011, 21, 349-356. (9) Liu, X.; Zhang, J.; Guo, X.; Wu, S.; Wang, S. Amino Acid-Assisted One-Pot Assembly of Au, Pt Nanoparticles onto One-Dimensional ZnO Microrods. Nanoscale 2010, 2, 1178-1184. (10) Wongrat, E.; Pimpang, P.; Choopun, S. Comparative Study of Ethanol Sensor Based on Gold Nanoparticles: ZnO Nanostructure and Gold – ZnO Nanostructure. Appl. Surf. Sci. 2009, 256, 968-971. (11) Joshi, R.K.; Hu, Q.; Alvi, F.; Joshi, N.; Kumar, A. Au Decorated Zinc Oxide Nanowires for CO Sensing. J. Phys. Chem. C 2009, 113, 16199-16202. (12) Gyorgy, E.; Giannoudakos, A.; Kompitsas, M.; Mihailescu, I.N. Laser Grown Gold Nanoparticles on Zinc Oxide Thin Films for Gas Sensor Applications. J. Optoelect. Adv. Mater. 2008, 10, 536-540. (13) Socol, G.; Axente, E.; Ristoscu, C.; Sima, F.; Popescu, A.; Stefan, N.; Mihailescu, I.N.; Escoubas, L.; Ferreira, J.; Bakalova, S.; et al. Enhanced Gas Sensing of Au Nanocluster-Doped or Coated Zinc Oxide Thin Films. J. Appl. Phys. 2007, 102, 083103/1-083103/6. (14) Snow, A.W.; Ancona, M.G.; Kruppa, W.; Jernigan, G.G.; Foos, E.E.; Park, D. SelfAssembly of Gold Nanoclusters on Micro- and Nanoelectronic Substrates. J. Mater. Chem. 2002, 12, 1222-1230.


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(15) Callini, E.; Pasquini, L.; Piscopiello, E.; Montone, A.; Antisari, M.; Vittori, B.E. Hydrogen Sorption in Pd-Decorated Mg-MgO Core-Shell Nanoparticles. Appl. Phys. Lett. 2009, 94, 221905/1-221905/3. (16) Li, G.P.; Chen, R.; Guo, D.L.; Wong, L.M.; Wang, S.J.; Sun, H.D.; Wu, T. Nanoscale Semiconductor-Insulator-Metal Core/Shell Heterostructures: Facile Synthesis and Light Emission. Nanoscale 2011, 3, 3170-3177. (17) Zahran, E.M.; Bedford, N.M.; Nguyen, M.A.; Chang, Y.-J.; Guiton, B.S.; Naik, R.R.; Bachas, L.G.; Knecht, M.R. Light-Activated Tandem Catalysis Driven by Multicomponent Nanomaterials. J. Am. Chem. Soc. 2014, 136, 32-35. (18) Rosario, A.V.; Pereira, E.C. The Role of Pt Addition on the Photocatalytic Activity of TiO2 Nanoparticles: The Limit Between Doping and Metallization. Appl. Catal. B 2014, 144, 840-845. (19) Tompos, A.; Margitfalvi, J.L.; Szabo, E.G.; Paszti, Z.; Sajo, I.; Radnoczi, G. Role of Modifiers in Multi-Component MgO-Supported Au Catalysts Designed for Preferential CO Oxidation. J. Catal. 2009, 266, 207-217. (20) Zhao, H.; Lu, B.; Xu, J.; Xie, E.; Wang, T.; Xu, Z. Electrospinning-Thermal Treatment Synthesis: A General Strategy to Decorate Highly Porous Nanotubes on Both Internal and External Sidewalls with Metal Oxide/Noble Metal Nanoparticles. Nanoscale 2013, 5, 2835-2839. (21) Purahmed, M.; Stroscio, M.A.; Dutta, M. Strong Enhancement of Near-Band-Edge Photoluminescence of ZnO Nanowires Decorated with Sputtered Metallic Nanoparticles. J. Elect. Mater. 2014, 43, 740-745.


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(22) Nahar, M.; Gallardo, I.F.; Gleason, K.L.; Becker, M.F.; Keto, J.W.; Kovar, D. Metal-onOxide Nanoparticles Produced using Laser Ablation of Microparticle Aerosols. J. Nanopart. Res. 2011, 13, 3455-3464. (23) Feng, Y.; Cho, I.S.; Rao, P.M.; Cai, L.; Zheng, X. Sol-Flame Synthesis: A General Strategy to Decorate Nanowires with Metal Oxide/Noble Metal Nanoparticles. Nano Lett. 2013, 13, 855-860. (24) Im, J; Singh, J.; Soares, J.W.; Steeves, D.M.; Whitten, J.E. Synthesis and Optical Properties of Dithiol-Linked ZnO/Gold Nanoparticle Composites. J. Phys. Chem. C 2011, 115, 10518-10523. (25) Singh, J.; Im, J.; Whitten, J.E.; Soares, J.W.; Steeves, D.M. Encapsulation of Zinc Oxide Nanorods and Nanoparticles. Langmuir, 2009, 25, 9947-9953. (26) Singh, J.; Im, J.; Watters, E.J.; Whitten, J.E.; Soares, J.W.; Steeves, D.M. Thiol Dosing of ZnO Single Crystals and Nanorods: Surface Chemistry and Photoluminescence. Surf. Sci. 2013, 609, 183-189. (27) Rani, G.; Sahare, P.D. Study of the Structural and Morphological Changes During the Phase Transition of ZnS to ZnO. Appl. Phys. A 2014, 116, 831-837. (28) Socrates, G. Infrared and Raman Characteristic Group Frequencies: Tables and Charts, 3rd Ed. John Wiley & Sons: Chichester, 2001. (29) Puckett, S.D.; Heuser, J.A.; Keith, J.D.; Spendel, W.U.; Pacey, G.E. Interaction of Ozone with Gold Nanoparticles. Talanta 2005, 66, 1242-1246.


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(30) Thonke, K.; Feneberg, M. Photoluminescence of ZnO: Basics and Applications, in Handbook of Luminescent Semiconductor Materials, Bergman, L; McHale, J.L., eds; CRC Press: Boca Raton, FL, 2012, pp. 87-124. (31) Zhang, D.; Ushita, H.; Wang, P.; Park, C.; Murakami, R.; Yang, S.; Song, X. Photoluminescence Modulation of ZnO via Coupling with the Surface Plasmon Resonance of Gold Nanoparticles. Appl. Phys. Lett. 2013, 103, 093114/1-093114/5. (32) Hou, X.; Wang, L. Controllable Fabrication and Photocatalysis of ZnO/Au Nanohybrids via Regenerative Ion Exchange and Reduction Cycles. RSC Adv. 2014, 4, 56945-56951. (33) Alexson, D.A.; Badescu, S.C.; Glembocki, O.J.; Prokes, S.M.; Rendell, R.W. Metal Adsorbate Hybridized Electronic States and their Impact on Surface Enhanced Raman Scattering, Chem. Phys. Lett. 2009, 477, 144-149. (34) Khanchandani, S.; Kundu, S.; Patra, A.; Ganguli, A.K. Band Gap Tuning of ZnO/In2S3 Core/Shell Nanorod Arrays for Enhanced Visible-Light-Driven Photocatalysis, J. Phys. Chem. C. 2013, 117, 5558-5567. (35) Liu, X.; Li, Z.; Zhao, W.; Zhao, C.; Yang, J.; Wang, Y. Zinc Oxide Nanorod/Au Composite Arrays and their Enhanced Photocatalytic Properties. J. Colloid Interface Sci. 2014, 432, 170-175. (36) Saoud, K.; Alsoubaihi, R.; Bensaleh, N; Bora, T.; Bertino, M; Dutta, J. Synthesis of Supported Silver Nanospheres on Zinc Oxide Nanorods for Visible Light Photocatalytic Applications. Mater. Res. Bull. 2015, 63, 134-140.


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Figure Captions Fig. 1. Differential scanning calorimetry of ZnO nanorods in air and nitrogen (as indicated in the legends) and scans corresponding to TPDT/ZnO and AuNP/TPDT/ZnO samples. The heating rate was 25oC min-1. Positive values on the ordinate indicate exothermic processes. Fig. 2. FE-SEM images of as-prepared (a), ozone treated (b), 300oC annealed (c and d), and 450oC annealed (e and f) AuNP/TPDT/ZnO nanocomposite samples. Images d and f are at higher magnifications than the others, as indicated by the scale bars. Note that the images become significantly sharper after annealing at 450oC due to complete decomposition of the ligands, whose presence in images a-d leads to surface charging. Fig. 3. TEM images of bare nanorods (a), as-prepared (b), ozone-treated (c), and 450oC annealed (d) AuNP/TPDT/ZnO nanocomposite samples. Higher magnification TEM images of the asprepared, ozone treated and 450oC AuNP/TPDT/ZnO samples are shown in images e, f, and g, respectively. Fig. 4. a) Raman spectra of as-prepared, ozone treated, and 450oC annealed AuNP/BPT/ZnO powder samples using 785 nm excitation light. The ozone treated and annealed spectra are multiplied by five for ease of viewing. b) Raman spectra of as-prepared and 300oC annealed AuNP/TPDT/ZnO samples acquired with 532 nm excitation light. A spectrum of the 300oC annealed sample using 785 nm excitation light is included for comparison. Fig. 5. Photographs of as-prepared AuNP/TPDT/ZnO nanocomposite powder and before and after annealing at 300oC and after subsequent ozone exposure. The as-prepared sample is blue, and heating at 300oC changes it to pink. Subsequent ozone exposure turns this sample purple,


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and heating at 300oC reverts it to pink. The ozone exposure/heating cycle may be repeated multiple times, as we have confirmed for at least ten cycles. Fig. 6. PL spectra of as-prepared, ozone exposed, and 300oC annealed AuNP/TPDT/ZnO nanocomposite powders. The excitation wavelength was 325 nm. Fig. 7. Absorbance spectra of 30µM aqueous RhB solutions before and after UV exposure for 1.0 and 1.5 h. Fig. a) corresponds to a control in which no nanopowders were used; Fig. b) corresponds to bare ZnO nanorods; and Fig. c) corresponds to a nanocomposite annealed at 450oC. Note that the differing initial absorbance values arise because of variations in the turbidities of the solutions. Fig. 8. Plots of 552 nm absorbance values versus UV exposure time for 30 µM RhB solutions containing

bare

ZnO

nanorods,

as-prepared

AuNP/BPDT/ZnO

nanocomposite,

the

nanocomposite after ozone exposure, and the nanocomposite after annealing at 300, 400, and 450oC.


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3 AuNPs/TPDT/ZnO (air) AuNPs/TPDT/ZnO (nitrogen) TPDT/ZnO (air) ZnO (air) ZnO (nitrogen)

2.5

Heat Flow (mW/mg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

2

1.5

1

0.5

0

-0.5 100

200

300

400

500

600

o

Temperature ( C) Fig. 1


25

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Fig. 2


26

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Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 3


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o

b)

575

433

a)

300 C, 532 nm

o

450 C (x5) 258

500

750

1000

1500

1329 1405

1196

1280

671

825

1583

1483

1208

1067 1083

500

-1

Raman Shift (cm )

1012

708 738 250

963

800 838

470

570 654

412

275

338 404 458 538

1578 1250

1495

1005

808

713

542

404 250

As-Prepared 1197 1278

1078

183

Scattering Intensity

Ozone-Treated (x5)

708

504

400

o

300 C, 785 nm

269

Scattering Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

195

Langmuir

As-Prepared, 532 nm 750

1000

1250

1500

-1

Raman Shift (cm )

Fig. 4


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Langmuir

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Fig. 5


29

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3 10

Photoluminescence Intensity (cps)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2.5 10

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5

As-Prepared Ozone 300o C (air)

5

1 104

2 10

5 8000

1.5 10

5

1 10

5

6000

4000

2000

5 10

4

0 350

0 400

400

450

450

500

500

550

600

550

600

Wavelength (nm) Fig. 6


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Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 7


31

Langmuir

3.0

2.5

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.0

1.5

ZnO As-Prepared AuNP/BPDT/ZnO AuNP/BPDT/ZnO + Ozone o

AuNP/BPDT/ZnO + 300 C o

AuNP/BPDT/ZnO + 400 C o

AuNP/BPDT/ZnO + 450 C

1.0 0.0

0.25

0.50

0.75

1.0

1.3

1.5

Exposure Time (h) Fig. 8


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Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents Figure


33

Zinc Oxide

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