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Multifunctional FeO-Au Nanoparticles with Different Shapes: Enhanced Catalysis, Photothermal Effects, and Magnetic Recyclability George K. Larsen, Will Farr, and Simona E. Hunyadi Murph J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03733 • Publication Date (Web): 08 Jun 2016 Downloaded from http://pubs.acs.org on June 10, 2016

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The Journal of Physical Chemistry

Multifunctional Fe2O3-Au Nanoparticles with Different Shapes: Enhanced Catalysis, Photothermal Effects, and Magnetic Recyclability George K. Larsen,1 Will Farr,1 and Simona E. Hunyadi Murph1,2*

1

National Security Directorate, Savannah River National Laboratory, Aiken, SC 29808, U.S.A.

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Department of Physics and Astronomy, University of Georgia, Athens, Georgia 30602

*Corresponding author, email address: [email protected], Phone: 1-803-646-6761

ACS Paragon Plus Environment

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ABSTRACT: We investigate Au-decorated Fe2O3 nanoparticle catalysts, Fe2O3-Au, where the supporting Fe2O3 nanoparticles are of different shapes: spheres, rings, and tubes. The decoration procedure for the Fe2O3-Au nanoparticles is identical for each shape, and is analogous to the synthesis of pure Au nanoparticles (AuNPs). These similarities allows for direct comparison between the different shapes and the pure AuNPs. The morphological, optical, and magnetic characterizations reveal that the Fe2O3-Au nanoparticles are hybrid structures exhibiting both plasmonic and magnetic properties. The different shape Fe2O3-Au nanoparticles and the AuNPs are evaluated for their ability to catalytically reduce 4-nitrophenol. Remarkably, it is found that Fe2O3-Au nanoparticles are more efficient catalysts than AuNPs because they can achieve the same, or better, catalytic reaction rates using significantly smaller quantities of Au, which is the catalytically-active material. Taking into account the Au-loadings, the Fe2O3 rings and tubes are superior to the Fe2O3 spheres as catalytic supports due to their γ-Fe2O3 crystal phase. It is also shown that the Fe2O3-Au nanoparticles have the additional benefit for catalysis in that they can be recovered and reused via magnetic collection. Furthermore, the Fe2O3-Au nanoparticles and AuNPs are found to efficiently transduce heat from light through plasmonic absorbance, and this phenomenon is exploited to demonstrate the photothermal catalytic reduction of 4-nitrophenol.


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1. Introduction Since ancient times, gold has been a highly desirable material, and thus, large volumes are considered to be very valuable. But why are we so captivated by gold? It’s rarity, aesthetic natural beauty and unique physical and chemical properties, particularly resistance to oxidation and corrosion, make gold a fascinating and valuable metal for many applications.

Recent

research has demonstrated that exceedingly small volumes of gold can also have significant value due to the new and intriguing properties that emerge for nanoscale gold.1, 2 For example, bulk gold does not display catalytic properties, while gold nanoparticles (AuNPs) have been shown to be very efficient catalysts.3 In fact, gold-based catalysis is a very active area of research, and AuNP catalysts have been shown to be capable of low temperature CO oxidation4, hydrogen production through water-gas shift reaction5, 6, and the reduction of aromatic nitro compounds.7 The interest in AuNP for catalysis is further heightened by the plasmonic behavior of nanoscale gold, which can be used to transduce heat from light absorption8, enhance the efficiency of CO catalysis9, and can enable the room temperature disassociation of H2 molecules.10 In spite of the small volumes of individual AuNPs used in catalysis, industrial scale processes will still require large amounts of gold, which is expensive and could be an obstacle to general use. One way to reduce costs associated with gold-based catalysis is to create a new and cheaper material that has the same properties as gold. Unfortunately, this material has yet to be discovered. However, while this is not a viable option today, one could create a composite nanostructure that has similar catalytic and plasmonic properties as AuNPs, by simply combining a smaller amount of the precious metal, gold, with another material. Blending gold with a lessexpensive and widely abundant material is a fairly simple solution, but it is also a very


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interesting one because it allows one to create a multifunctional hybrid nanostructure with the physicochemical properties of two or more materials.

Such multifunctional catalysts are

attractive for catalytic applications. For example, Au nanoparticles combined with partially reduced graphene oxide serve as highly modifiable and efficient catalysts for nitroaromatic reduction.11

Or, by combining Pd and CeO2 in a hollow core-shell nanoarchitecture,

aggregation- and leaching-resistant and reusable catalysts can be created.12 From a cost and implementation standpoint, an optimal material to combine with gold would be widely available, inexpensive, and non-toxic. An ideal material would also expand the functionality of the nanoparticle by contributing additional properties not typically seen in gold. Iron(III) oxide, Fe2O3, is one such candidate that fulfills these criteria. Iron oxide has been used successfully as contrast agent for detection and diagnostic therapy in magnetic resonance imaging (MRI) applications and in a variety of bioseparations and immunoasays.13-15 Fe2O3 is also an abundant and well-known semiconductor that exhibits photocatalytic properties as a single material, and additionally, it also exhibits magnetic properties: the maghemite phase, γFe2O3, is ferrimagnetic, and the hematite phase, α-Fe2O3, is weakly ferromagnetic. Maghemite and hematite nanostructures are also of interest due to their size and shape dependent properties and applications in information storage fields, magnetic sensors, magnetic refrigeration, ferrofluids, among others.16,

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Notably, the combination of gold with Fe2O3 could yield

catalytically-active nanoparticles that can be made less expensive than pure gold, exhibit plasmonic properties, and interact with external magnetic fields. The benefits of the combination of iron oxide and gold for catalysis have not escaped researchers, and previous studies have found that gold-iron oxide nanoparticles offer the catalytic capabilities of AuNPs, as well as the benefit of magnetic separation for recyclability.18 While some research work has been done in


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this area, many reports focus only on Fe3O4 or require differing or complicated synthesis and purification methods, making comparison across structures difficult.19-22 Moreover, the catalytic behavior of anisotropic Fe2O3-Au nanoparticles with controllable shape, size and composition as well as photothermal catalytic properties of Fe2O3-Au remain unexplored. In this report we describe the synthesis and properties of Au-decorated Fe2O3 nanoparticle catalysts, Fe2O3-Au, where the supporting Fe2O3 nanoparticles are of different shapes: spheres, rings, and tubes. The synthesis procedure of the Fe2O3-Au nanoparticles is identical for each of the shapes, and is also analogous to the synthesis of pure AuNPs. These similarities allows for the direct comparison between the different shapes and between the pure AuNPs.

The

morphological, structural, optical, and magnetic characterizations reveal that the Fe2O3-Au nanoparticles are hybrid structures exhibiting both plasmonic and magnetic properties. The specific properties of the different Fe2O3 shapes determine the AuNP loading content, which in turn affects the manifestation of gold-based properties in these hybrid structures. To assess the catalytic properties of the different shape Fe2O3-Au nanoparticles, the reduction of 4-nitrophenol is chosen as the model reaction, and these are compared to those of the pure AuNPs fabricated in an identical manner. Remarkably, it is found that the Fe2O3-Au nanoparticles are more efficient catalysts than the AuNPs, even with a much lower loading of gold, which is the catalyticallyactive material. Furthermore, the multifunctional Fe2O3-Au nanoparticles can be magnetically collected and recycled, and they are found to efficiently transduce heat from light through plasmonic absorbance. This phenomenon is exploited to demonstrate the photothermal catalytic reduction of 4-nitrophenol. 2. Experimental 2.1. Catalyst Preparation


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Chloroauric acid trihydrate (HAuCl4*3H2O), trisodium citrate, and Fe2O3 spheres were purchased from Sigma Aldrich. The Fe2O3 rings and tubes were purchased from MK Impex Corporation. All chemicals were used as received. All glassware was cleaned with aqua regia and thoroughly rinsed with deionized water prior use. Gold nanospheres were prepared by a citrate reduction approach described elsewhere 23, 24. With this synthesis, an aqueous solution of 2.5 × 10-4 M HAuCl4 was heated to boiling and one percent by weight sodium citrate solution was subsequently added. The boiling was continued until the solution turned ruby red, indicating the formation of gold nanoparticles. Fe2O3-Au hybrid nanoparticles were prepared by a similar procedure by reducing the same amount of Au3+ ions in the presence of 2.2 × 10-4 M Fe2O3 nanoparticles. The solutions of Au nanospheres and hybrid Fe2O3-Au were centrifuged twice at 8000 rpm for 5 minutes and redispersed in deionized (DI) water to remove excess reactants. 2.2. Nanoparticle Characterization The morphologies of the nanoparticles were monitored by scanning electron microscopy (SEM, Hitachi SU8200). The nanoparticle size distribution and zeta potential were obtained using dynamic light scattering measurements (DLS, Brookhaven Instruments, NanoBrook Omni).

The chemical compositions were analyzed using inductively coupled plasma mass

spectrometry (ICP-MS, Agilent 7500s) and by energy dispersive X-ray analysis (EDX, Oxford Instruments).

The crystalline properties of the nanoparticles were characterized by X-ray

diffraction (XRD, PANalytical X'pert Pro). The optical properties of the nanoparticle solutions were measured using UV-visible-near infrared spectroscopy (UV-Vis-NIR, Tec5 MultiSpec). The magnetic properties of the samples were characterized by a vibrating sample magnetometer at room temperature (VSM, Lake Shore Cryotronics).


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2.3. Catalysis Experiment The catalytic experiments were conducted inside 3 mL quartz cuvettes by combining the following: 1.5 mL DI water; 200 µL nanocatalyst solution; 150 µL 2 mM nitrophenol solution; and 200 µL freshly prepared 0.06 M NaBH4 solution. After adding the NaBH4 to the other reactants, the solution immediately turns yellow and the catalytic reaction commences. The catalytic reaction is monitored by in-situ UV-vis spectroscopy, where the absorption peak of nitrophenol at wavelength λ = 400 nm is used to quantify the concentration. The catalytic reaction proceeds until the peak at λ = 400 nm disappears. 2.4. Photothermal Catalysis Experiment The photothermal catalysis experimental setup consists of a laser with an adjustable output and a wavelength λ = 532 nm (Del Mar Photonics, DMPV-532-1, beam diameter focused to ~20 µm), where the beam path is directed onto the top surface of the solution contained in the methacrylate cuvette, as described above. The cuvette is placed in a cradle that allows for in-situ UV-vis measurements. These data are synchronized with the bulk solution temperature data, which is obtained from an infrared thermocouple (Omega Engineering, OS801-HT). The data are logged using a custom LabVIEW program.

The photothermal catalysis experiment is

conducted as described in section 2.3, except that higher concentrations of nanoparticles are used to increase the photothermal heating effect. Additionally, the nanoparticle solutions are heated with the laser at a specific intensity for 12 minutes prior to adding nitrophenol and NaBH4, and then the reactant solution is illuminated at that same intensity throughout the catalytic experiment. 3. Results and Discussion 3.1. Nanoparticle Characterization


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Seven different particle types (AuNPs; Fe2O3 spheres, rings, and tubes; and Fe2O3-Au spheres, rings, and tubes) have been acquired using the methods described above, and the morphologies of these structures have been investigated using scanning electron microscopy (SEM). The Fe2O3 nanoparticles appear as well-defined spheres, rings, and tubes, respectively (Figures 1a 1c). The average morphological parameters are listed in Table 1. The spheres are the smallest of these structures with an outer diameter of douter = 46 ± 5 nm. The rings and the tubes are similar morphologically, but the rings have larger inner and outer diameters and a shorter length. After the Au treatment, smaller bright, rounded particles appear on the exposed surfaces of the Fe2O3 spheres, rings, and tubes (Figures 1d – 1f), and these are attributed to Au deposition. This type of morphology is often referred to as “decorated” nanoparticles.25 In this case the surface of the supporting particle, Fe2O3, is adorned with smaller, isolated Au nanoparticles.

No

agglomeration of Au nanoparticles is observed on any Fe2O3 supports. Au nanospheres remain on the Fe2O3 surfaces even after several steps of washing, centrifugation and re-dispersion in DI water. This suggests that the interfacial energy and adhesion energy between NPs and their support is favorable. As shown in Table 1, the sizes of the decorating Au nanoparticles, dAu ≈ 20 nm, are essentially the same for all Fe2O3 structures and are the same as the separately fabricated AuNPs. Thus, by exploiting one of the simplest nanoparticle synthesis procedure, specifically citrate reduction approach, we are able to engineer the Fe2O3 surfaces with uniform and monodisperse Au spherical nanoparticles. The sizes and shapes of the Au nanoparticles produced through this approach are independent of the presence or absence of support in the reaction pot. Meaning that, all Au nanoparticles produced in the reaction are spherical in shape with an average size of 19-20 nm, as depicted in the SEM images. The shapes of the Fe2O3 supports used in the reaction do


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not influence the size nor the shape of the Au nanospheres produced in the reaction pot. Moreover, the initial morphology of Fe2O3 (spheres, rings, tubes) is preserved during the reaction procedure. The wall structures of both Fe2O3 tubes and rings are not changed during the Au decoration suggesting that these supports are stable even at high temperatures (100 °C). In the case of tubes and rings, Au nanosphere decoration is observed mostly on the outside “wall” of the tubes and spheres. While Au decoration was easily observed inside the rings, it is certainly possible that Au nanospheres are formed inside the tubes, too. However, it is difficult to image Au nanoparticles inside the tubes by SEM studies. The compositions of the different structures have been investigated using inductively coupled plasma mass spectrometry (ICP-MS) and energy dispersive X-ray analysis (EDX). ICP-MS provides quantitative information regarding absolute concentrations of the elements of interest, and the Fe2O3-Au spheres are found to have Fe and Au concentrations of ρFe = 500 ppb and ρAu = 320 ppb. In comparison, the pure Au nanoparticles have a higher concentration of Au, ρAu = 1000 ppb. EDX mappings describing the distribution of Au, Fe, and O in the Fe2O3-Au nanoparticles are shown in Figure 2 (only Fe2O3-Au rings EDX mapping data shown here), and these confirm Fe2O3 shape and local distribution. The smaller spherical nanoparticles are also confirmed as Au. A notable difference in the amount of Au nanoparticles “attached” to Fe2O3 supports was observed in the semi-quantitative results obtained by EDX. Specifically, the relative ratios of Fe/Au for the Fe2O3-Au spheres, rings, and tubes are found to be 9 ± 1, 49 ± 7, and 55 ± 7, respectively. Thus, in spite of the identical preparation (Au decoration) methods, the Au loading depends on the morphology of the Fe2O3 support. The Fe2O3 spheres have the highest Au loading with about 5× and 6× higher Fe/Au ratios respectively than the rings and tubes, which are the most similar morphologically.


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It is somewhat surprising that the Fe2O3 spheres have the highest loading of Au since they are the smallest structure and have the greatest curvature. However, the differences observed in our studies could be attributed to a combination of effects, including (a) different preparation procedures of the Fe2O3 supports, (b) different surface charges of the Fe2O3 support particles, (c) different aggregation effects before surface engineering, or (d) different crystallinity of the support. The Fe2O3 used in our study were provided by different suppliers and prepared by different synthetic approaches. Specifically, Fe2O3 rings and tubes were prepared by a double anion-assisted (phosphate and sulfate ions) hydrothermal approach. The preparation procedures are based on the procedure described by Yan and Sun’s group.26 On the other hand, Fe2O3 nanospheres were prepared by burning iron metal in a reactive gas. These differences in the preparation procedure of different shaped Fe2O3 leads to differences in the amount of Au deposited on their surfaces. Thus, it is not surprising that the rings and tubes have similar Au loading onto supports since both of these nanostructures were prepared in similar ways. The dynamic properties of the nanoparticles in aqueous solution are investigated using dynamic light scattering (DLS). Figure 3a shows the average hydrodynamic diameter in deionized water of the different sized populations. In all cases, the hydrodynamic sizes are greater than those measured by SEM –as expected, and in some cases, significantly greater, which indicates aggregation in solution. It is also observed that the Au deposition on the Fe2O3 supports induces different effects depending on the structure: the hydrodynamic diameter remains approximately the same for the spheres, increases for the rings, and decreases for the tubes. Zeta potential measurements were also performed using phase analysis light-scattering to monitor the surface charge before and after the surface engineering experiments, and the results


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are presented in Figure 3b. The effective surface charge of the Fe2O3 nanoparticles before decoration with Au nanoparticles in water is negative for all supports. The negative charge is attributed to the electric potential formed at the solid support-liquid interface due to the presence of phosphate, sulfate ions, and/or hydroxide ions adsorbed on the surface. In comparing the surface charge between Fe2O3 tubes and rings, the zeta potential has similar values of around -38 ± 5 mV and -31 ±5 mV, respectively. In this particular case, the negative surface charge is due to the presence of phosphate and sulfate ions used during hydrothermal preparation procedure. Small differences recorded between tubes and rings are due to the higher affinity shown of the phosphate ions than sulfate ions to the Fe2O3 surface, and the preparation procedure requires a higher amount of phosphate ions for preparation of elongated Fe2O3 tubes compared with the rings. In comparing the spheres with the tubes and rings, the effective surface charge of the Fe2O3 spheres is much smaller, -11 mV. This is because the spherical Fe2O3 nanoparticles are produced by burning iron metal in the absence of surfactants. The source of negative surface charge in this case is probably due to hydroxide ions adsorbed on the surface, specifically the deprotonated –Fe-OH species present on the nanoparticle’s surface. The presence of surfactants on the nanoparticle’s surfaces is beneficial as it prevents aggregation of nanoparticles or in some cases can be used to decorate and load other molecules inside their structure. A more negative surface charge, would suggest that the nanoparticles are less prone to aggregation, which is confirmed in our study by the DLS particle size analysis. The aqueous solutions of Fe2O3 tubes have the highest negative surface charge while the “smallest” hydrodynamic radius. A similar trend was observed for the Fe2O3 spherical nanoparticles; a smaller negative surface charge leads to a larger agglomeration of nanoparticles.


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As expected, when the Fe2O3 surfaces are decorated with Au spheres, their surface charge changes significantly. Surface engineering of the Fe2O3 with Au spheres leads to a significant decrease of the overall surface charge for rings and tubes. This could be attributed to the removal of the remaining phosphate and sulfate ions that are adsorbed on the original Fe2O3 surfaces. The surface engineering of Fe2O3 with Au takes place at an elevated temperature, 100 °C, which could favor this process and cause the adsorbed phosphate/sulphate ions to be destabilized and removed from the Fe2O3 surfaces. In the case of Fe2O3 spheres, the surface charge becomes more negative after Au decoration, changing from -11 mV to -16 mV. It is believed that the incoming citrate ions are being adsorbed on either the Fe2O3 and/or Au nanoparticles. As shown in Figure 4, X-ray diffraction reveals that the Fe2O3 spheres are in the maghemite phase (JCPDS 00-025-1402) with no preferred orientation, while the rings and tubes are in the hematite phase (JCPDS 00-033-0664) with no preferred orientation. Thus, it is also a possibility that different crystal structures play a role in the Au loading. “Anchoring” of Au particles at the step edges of different crystal facets of Fe2O3 may influence the nucleation process due to potential formation of mobile Au species across the oxide surface. Different oxide support could stabilize specific atomic configurations, charge states, or electronic properties of the Au nucleation centers, which could result in distinct chemical behavior. While the specific growth mechanism of the Au nanoparticles on the Fe2O3 supports likely arises from several different factors, the analogous Turkevich synthesis for AuNPs is wellunderstood and progresses through 4 steps:27, 28 (1) Au precursors are partially reduced and form small clusters; (2) the clusters coalesce to form seed particles (radii >1.5 nm), and the remaining gold ions become attached to the seed particles as co-ions; the final steps consist of the (3) slow and (4) fast reduction of the remaining ionic gold, where the reduced gold monomers grow


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exclusively on top of the seed particles' surfaces until the precursor is fully consumed.29 The first two steps dictate the total number AuNPs obtained, while the last two steps only determine the final sizes of the AuNPs. In drawing a parallel with synthesis on the different Fe2O3 supports, it can be assumed that steps 3 and 4 progress similarly for all structures since the AuNPs and the decorating Au nanoparticles all have the same sizes. The differences in the Au loading can therefore be attributed to the differing abilities of the Fe2O3 surfaces to reduce the Au precursors and their promotion of coalescence. The surface chemistries of the different supports should play a primary role in the reduction of the precursors and formation of the initial Au clusters. The surfaces properties of the tubes and rings are determined by their synthesis and functionalization with sulfate and phosphate ions. Treating Fe2O3 surfaces with sulfate ions has been shown to create Lewis acid (electron acceptor) sites,30 as well as prevents the adsorption of chloride ions to Fe2O3 surfaces.31 On the other hand, the hydroxylated surface of the spherical particles could promote the precursor reduction, as the exchange of chloride and hydroxide from AuCl4- to AuCl3(OH)- is observed prior to the reduction to metallic gold.27 These observations suggest how the differing surface chemistries could play a role in the initial Au cluster loading. Finally, the coalescence of the initial clusters could be enhanced in the spheres since these structures show greater aggregation in solution compared to the rings and the tubes, as described above. The UV-visible spectroscopy (UV-vis) measurements of the AuNPs, Fe2O3, and Fe2O3-Au nanoparticles are shown in Figure 5. The different shapes of Fe2O3 nanoparticles have different scattering properties as observed in their respective absorbance spectra. These different optical properties are easily observed by naked eye in noting the different colors of the nanoparticle solutions (Figure 5c). Figure 5c show several glass vials containing identical initial amounts of


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iron oxide nanoparticles (spheres, tubes and rings), and the reaction product obtained by Au decoration. A substantial color change was observed in most cases upon Au deposition. This is due to the plasmon band for the new formed Au nanoparticle. This phenomenon can be easily monitored by UV-vis. After the Au treatment, the spectra of the Fe2O3 nanoparticles significantly change demonstrating AuNPs formation on the Fe2O3 supports. Most notably, in all of the structures a peak emerges at λ ≈ 520 nm, which matches the localized surface plasmon resonance (LSPR) of the pure AuNPs. UV-vis spectra of the nanocomposite materials differ not just from the Au metallic colloids but also from the individual Fe2O3 nanoparticles. Fe2O3 nanoparticles absorb across the UV-vis region with distinctive peaks occurring below 650 nm; these peaks are highly dependent on the Fe2O3 shape and size. The plasmonic properties of the Fe2O3-Au nanoparticles are evident in their optical properties demonstrating that these materials are nanocomposite structures and not simply mixtures of individual nanoparticles. In order to confirm the magnetic properties of the hybrid Fe2O3-Au nanoparticles, the nanoparticle solutions have been characterized by a vibrating sample magnetometer (VSM) at room temperature. All of the samples exhibit hysteresis, which is consistent with ferromagnetic behavior (Figure 6a).

The measurements show that the Fe2O3-Au tubes have the highest

saturation magnetization at ms ≈ 20 emu/g, with the spheres and rings having ms ≈ 15 emu/g and ms ≈ 13 emu/g, respectively (Figure 6b). This data suggests that the nanostructures can be easily manipulated and separated from their media by the use of a magnetic field, and demonstrates that the Fe2O3-Au nanoparticles are true multifunctional structures exhibiting properties of both constituent materials (e.g., plasmonic and magnetic). 3.2. Catalysis


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3.2.1. Catalytic Reduction of 4-nitrophenol. The reduction of 4-nitrophenol is employed here as a model reaction to evaluate the catalytic properties of the different Fe2O3-Au nanoparticles. Experiments were performed in the presence and absence of catalyst to evaluate the reaction’s effectiveness. Without a catalyst, sodium borohydride, NaBH4, does not react with 4-nitrophenol. However, in the presence of a catalyst, such as Au, 4-nitrophenol is rapidly reduced to 4aminophenol by NaBH4.32, 33 This conversion reaction is easily monitored by in-situ UV-vis by observing the decay of the strong 4-nitrophenol peak at λ = 400 nm and the increase in the 4aminophenol peak at λ = 300 nm.34 Figure 7a shows representative spectra of such a decay experiment, as measured by UV-vis. Control experiments (NaBH4 without a catalyst, catalysts without NaBH4, and pure Fe2O3 particles with NaBH4) showed no appreciable degradation of 4nitrophenol (data not shown). On the other hand, the AuNPs and the Fe2O3-Au spheres, rings, and tubes are all able to catalytically reduce 4-nitrophenol to 4-aminophenol. This is shown in Figure 7b, which plots ln(A/A0) versus time, t, where A0 is the initial absorbance and A is the absorbance at time t of the 4-nitrophenol peak.

Interestingly, the catalytic decay depends

strongly on the type, shape and composition of structure, with the Fe2O3-Au spheres reducing 4nitrophenol the fastest, followed by the AuNPs, Fe2O3-Au rings, and then the Fe2O3-Au tubes. This is a notable result as it shows that the Fe2O3-Au spheres are more catalytically active than the AuNPs, even though they have 3× less gold (as demonstrated by ICP-MS data). In order to make a more quantitative comparison, first order reaction kinetics are assumed since the concentration of NaBH4 is significantly higher than that of nitrophenol and can

be

considered as a constant during the catalytic reaction. The initial linear portion of the ln(A/A0) versus t is fit to obtain a decay rate,35 and these results are presented in Figure 7c. As synthesized, the Fe2O3-Au spheres are the most catalytically active structure that is investigated


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in this report. However, it is important to note that the Au loadings on the Fe2O3-Au rings and tubes are significantly smaller than Au loading on the Fe2O3 spheres (recall, Fe/Au for the Fe2O3Au spheres, rings, and tubes are found to be 9 ± 1, 49 ± 7, and 55 ± 7, respectively). Taking into account the differing amounts of Au (which shows great catalytic efficiency for this reaction), the Fe2O3-Au rings and tubes are comparable in their catalytic efficiency. Specifically, Fe2O3Au rings and tubes are more than twice as efficient as the spheres for the amount of Au loaded onto the Fe2O3 support. In other words, the Fe2O3-Au rings and tubes are better catalysts than the Fe2O3-Au spheres, when normalized to the amount of Au on the support. As shown in Figure 7d, the Fe2O3-Au spheres remain superior catalysts to the AuNPs for increasing concentrations of nanoparticles. The change in the reaction rate, k, with increasing concentrations of nanoparticles deviates from linear behavior and appears to approach a limiting value for the given reaction conditions, and this saturation effect is seen in the catalytic reduction of nitrophenol using both the AuNPs and the Fe2O3-Au spheres. The Langmuir-Hinshelwood model predicts that k should follow a linear relationship with the total surface area of the catalysts in the solution.21 Thus, the catalytic reactions in these cases follow more complicated reaction kinetics. In all of the cases presented here, Fe2O3-Au nanoparticles are superior catalysts compared to the pure AuNPs, not only by having greater efficiencies but also by using significantly much smaller quantities of gold. The enhanced catalytic efficiency of Fe2O3-supported Au is an interesting result. Previous studies have found that the reduction of 4-nitrophenol by Au can be enhanced by graphene oxide and Fe3O4 supports, which is attributed to an increase in adsorption or to an electronic junction effect, respectively.36,

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For the Fe2O3 nanoparticles investigated

here, the supported and unsupported Au nanoparticles are prepared in the same manner and have


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approximately the same diameters and morphologies. Thus, the enhanced efficiency in these nanoparticles is most likely a direct effect of the close coupling between Fe2O3 and Au, and not from any differences in the Au itself. That is, the Fe2O3 support plays an active role in the catalytic process.

The observation that the γ-Fe2O3 rings and tubes have similar catalytic

efficiencies and are twice as efficient as the α-Fe2O3 spheres for the same amount of Au reinforces this assertion. The greater efficiency of Au/γ-Fe2O3 over Au/α-Fe2O3 has recently been observed for catalytic CO oxidation and was attributed to the more easily reduced/oxidized (Fe2+↔Fe3+) surface of γ-Fe2O3.38, 39 Similarly, since the catalytic reduction of 4-nitrophenol is accomplished through mediated multi-electron transfer from borohydride anions through the catalyst to the adsorbed 4-nitrophenol molecules,40 it could be expected, given the discussion above, that Fe2O3 assists this transfer in general and that the more facile reduction-oxidation mechanism of γ-Fe2O3 enhances this process. Alternatively, the enhancement mechanism could potentially arise from improved co-localization of reactants due to hydrogen spillover and 4nitrophenol adsorption.

Surface hydrogen species transferred from borohydride to the Au

surface have the potential to spillover onto Fe2O3 because it is reducible, especially γ-Fe2O3.41, 42 Fe2O3 has also been found to strongly adsorb nitrophenol from aqueous solutions.43 The interaction between surface adsorbed 4-nitrophenol and surface hydrogen is widely considered the rate-determining step,42 and thus, these phenomena could also contribute significantly to the catalytic process by increasing the likelihood of contact between the reactants. 3.2.2. Magnetic Recyclability. An advantage of using the hybrid Fe2O3-Au nanoparticles for catalysis is that they are multifunctional structures, exhibiting the properties of both materials. Fe2O3 materials have magnetic properties, which allow for external manipulation using magnetic fields. One way this can be utilized is through magnetic collection and recycling of the catalysts


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after a reaction is complete. Figures 8a – 8e are photographs illustrating the magnetic collection and redispersion of Fe2O3-Au spheres. By placing a cuvette containing the hybrid nanoparticle solution in a static magnetic field, all of the magnetic Fe2O3-Au nanoparticles are collected within less than one minute. Then, the initial solvent solution can be drawn off and be replaced by a new one. This collection and redispersion method was used to demonstrate the feasibility of magnetic recycling of the hybrid Fe2O3-Au spheres for catalytic 4-nitrophenol reductions. As shown in Figure 9, the Fe2O3-Au spheres can be collected and successfully reused and recycled with almost complete reproducibility for 5 times before a small change in the catalytic efficiency of 4-nitrophenol reduction is observed. This is not surprising since the amount of catalysts used in these catalytic reactions is extremely small, on the order of hundred ppb as confirmed by ICPMS studies. The minor decrease in catalytic efficiency after 5 cycles is attributed to incomplete collection

of

the

nanoparticles

after

each

magnetic

collection

and

numerous

dispersion/redispersion and container transferring sequences. Scaling up of this catalytic process is expected to eliminate these issues and is currently underway. SEM analyses confirm that no significant changes in the morphologies occur after catalysis (data not shown). The catalytic stability of the Fe2O3-Au catalysts over several cycles is a critical result since these structures have the ability to be magnetically collected and recycled. In comparison, recovery of pure Au nanoparticles after catalysis poses a significant recovery challenge for real world applications, requiring filtration or centrifugation which may not be feasible on larger scales. In spite of this difficulty, the catalytic stability of Au nanoparticles has been investigated previously. For citrate-stabilized and bare Au nanoparticles in aqueous solutions, it has been found that the catalytic efficiency drops remarkably after one or two cycles.36, 44, 45 This loss of efficiency with cycling has been attributed to incomplete collection during centrifugation or to


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irreversible aggregation. Therefore, the ease of collection and reusability of the hybrid Fe2O3-Au nanoparticles offers additional advantages over the use of pure Au nanoparticles for catalysis. 3.2.3. Photothermal Catalysis. Light absorption in nanoparticles is readily dissipated as heat. Due to their large absorbance cross-sections, plasmonic nanoparticles can generate a significant amount of heat and increase temperatures in their vicinities.46

If a sufficient number of

nanoparticles are present, the temperature fields overlap and create a substantial global temperature rise.47 Photothermal heating results using laser irradiation (λ = 532 nm, CW, 1.2 W, focused to 20 µm spot size, ~3.8 × 105 W/cm2) for larger ppb concentrations of hybrid Fe2O3-Au nanoparticles are presented in Figure 10.

For all nanoparticle solutions, a significant

temperature rise in the bulk solution is observed after the laser is turned on at time, t = 120 s. In general, the temperature increases exponentially and approaches an equilibrium value until the laser is turned off at t = 1120 s. These temperature profiles are consistent with heat diffusion in a dissipative medium.48 As shown in Figure 10a, the AuNPs and the Fe2O3-Au spheres, heat the surrounding bulk aqueous solution to equal maximum temperatures, Tmax ≈ 65 ºC, while the pure Fe2O3 spheres heat to Tmax ≈ 50 ºC; the DI water used as control shows no temperature increase. As shown in Figure 10b, the Fe2O3-Au spheres heat the bulk solution to slightly higher temperatures than the rings and tubes. It is important to note that the magnitude of absorbance at the laser wavelength, λ ≈ 532 nm (Figure 5b) does not directly relate to Tmax in these nanoparticles because the absorbance spectra include contributions from both scattering and absorbance. Scattering does not directly lead to photothermal heating, and scattering is expected to be more significant in the larger nanoparticles, the rings and tubes. It is important to note the temperature rises described above are the bulk temperatures of the solution, and it is possible that the temperatures near the


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surface of the irradiated nanoparticles are much higher.49 This is illustrated in Figure 10c, which plots the mass loss of the Fe2O3-Au solutions during the photothermal experiments. The increase in mass losses relative to background evaporation suggests the generation of steam (which was observed in our studies) and higher temperatures on the nanoparticle surfaces.50 The photothermal properties of the multifunctional Fe2O3-Au nanoparticles can be harnessed to increase temperatures using light, and thereby increase the catalytic reaction rate.27 Thus, 4nitrophenol reduction reactions were performed using the AuNPs and the Fe2O3-Au spheres and varying laser intensities.

These materials were selected in our studies due to their higher

catalytic efficiency at room temperature. The bulk temperature and the UV-vis spectra of the reaction solution were monitored in-situ during the photothermal catalytic reactions, and these data were compiled to create Arrhenius plots (Figure 11). As can be seen in the plots, the reaction rates increase with increasing laser intensities/solution temperatures for both, as expected. Linear fits of the Arrhenius data result in measured activation energies EA = 106 ± 7 kJ/mol and EA = 180 ± 10 kJ/mol for the AuNPs and the Fe2O3-Au spheres, respectively. These values are significantly higher than activation energies observed in other Au catalyzed 4nitrophenol reductions, which are generally found to have EA = 50 kJ/mol or less.34 The higher values of EA observed here is interesting, but not necessarily surprising. The measured temperature is that of the bulk solution, and it is likely that the local temperatures are greater on the surfaces of the nanoparticles where the heat is transduced and the catalytic reaction occurs, as described above. Thus, the change in catalytic reaction rate could be occurring over a greater temperature scale than is indicated by the measurements. Furthermore, the increase in temperature corresponds with an increase in irradiation intensity, and it is possible the photochemical effects could also contribute to the greater increase reaction rates with


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“temperature.” The difference between the activation energies of the AuNPs and the Fe2O3-Au spheres is interesting, though, since local temperature effects should be similar between the two particle types.

Thus, this difference could be related to the temperature or photochemical

dependence of the reaction rate and adsorption constants. 4. Conclusions In conclusion, hybrid Fe2O3-Au spheres, rings, and tubes have been synthesized using a straightforward method. It has been found that the Au loading on these structures depends on the morphology and the hydrodynamic properties of the Fe2O3 support.

The decorating Au

nanoparticles on the spheres, rings, and tubes are all similar morphologically, and are similar to separately fabricated pure Au nanoparticles. These similarities allow for direct comparison between the catalytic efficiencies of the different structures for 4-nitrophenol reduction, and it is determined that the Fe2O3 supports improve the catalytic performance, especially γ-Fe2O3. In general, the Fe2O3-Au nanoparticles are multifunctional, exhibiting plasmonic and catalytic properties of Au and magnetic properties of Fe2O3. These multifunctional properties have been exploited to create structures that are more economical and active than pure Au nanoparticle catalysts for 4-nitrophenol reduction, can also be recovered and recycled magnetically, and can transduce light to heat to increase reaction rates. The hybrid Fe2O3-Au nanoparticles offer a platform to further investigate the photothermal catalytic process, as well as explore new application, such as sensing, hyperthermia treatment, environmental implications, medical imaging applications including magnetic resonance imaging contrast agents, tracking and drug deliveries, among others. It is expected that these novel multifunctional nanostructures can also find real world applications due to their lower cost and greater efficiency than Au catalysts. Acknowledgements


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The financial support of this work was provided by Department of Energy DOE- Laboratory Directed Research & Development (LDRD) Strategic Initiative Program. We thank Dr. Robert Lascola, Mr. Henry Sessions, and Mr. Charles Shick for providing their time and expertise to assist us with our experiments. We also thank Mr. Weijie Huang for his assistance with the magnetic measurements.

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FIGURES:

(a)

(b)

200 nm

(d)

(c)

200 nm

(e)

200 nm

(f)

100 nm

100 nm

200 nm

200 nm

100 nm

200 nm

Figure 1. SEM micrographs of the (a) Fe2O3 spheres, (b) Fe2O3 rings, (c) Fe2O3 tubes, (d) Fe2O3Au spheres, (e) Fe2O3-Au rings, and (f) Fe2O3-Au tubes. The insets in (d)-(f) show zoomed in images at different magnifications of the same particle types. Note that the size of the scale bar decreases from (d) – (f).


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(a)

(b)

(c)

(d)

Figure 2. (a) EDX mappings respectively showing the distribution of (b) – (d) Au, Fe, and O in the Fe2O3-Au rings.


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Particle Size (nm)

600

(a)

400

200

0

-40

Zeta Potential (mV)

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


(b)

-35 -30 -25 -20 -15 -10 -5 0

Figure 3. (a) Hydrodynamic particle sizes and (b) zeta potentials of the different nanoparticles as measured by dynamic light scattering.


29


20

30

♦

40

50

60

70

80

♦ Hematite ♣ Maghemite

♦ ♦ ♦

Normalized 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

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♦

♦

♦

♦

♦♦

Tubes ♦ ♦ ♦ ♦

♦♦ ♦ ♦

Rings ♣

♣ ♣

♣ ♣ ♣

♣

♣ ♣

Spheres 20

30

40

50

60

70

80

Diffraction Angle, 2θ (°)

Figure 4. XRD spectra of the Fe2O3 spheres, rings, and tubes. The peak attributions are denoted by the symbols.


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i 1.0

(b)

Fe2O3 spheres

Absorbance (a.u.)

Fe2O3 rings

0.8

Fe2O3 tubes 0.6

0.4

0.2

0.0

ii

iii

iv

1.0

(a) Absorbance (a.u.)

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


400

500

600

700

800

Wavelength (nm)

900

1000

(c)

Au nanoparticles Fe2O3-Au spheres

0.8

Fe2O3-Au rings Fe2O3-Au tubes

0.6

0.4

v vi vii

0.2

0.0

400

500

600

700

800

900

i: AuNPs ii: Fe2O3 spheres iii: Fe2O3 tubes iv: Fe2O3 rings v: Fe2O3-Au spheres vi: Fe2O3-Au tubes vii: Fe2O3-Au rings

1000

Wavelength (nm)

Figure 5. UV-vis spectra of the (a) Fe2O3 and (b) AuNPS and Fe2O3-Au spheres, rings, and tubes. (c) Photograph of the different nanoparticle solutions.


31


5

(a)

Magnetization (emu/g)

4 3 2 1 0

Fe2O3-Au

-1

Spheres Rings Tubes

-2 -3 -4 -5 -150

-100

-50

0

50

100

150

Applied Magnetic Field (Oe) 20


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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(b)

10

0 Fe2O3-Au Spheres Rings Tubes

-10

-20 -10000

-5000

0

5000

10000

Applied Magnetic Field (Oe)

Figure 6. (a) Narrow and (b) and wide range plots of the hysteresis loops of the Fe2O3-Au nanoparticles.


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1.4

(a)

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10

1.2 1.0 0.8 0.6 0.4 0.2 0.0 250

300

350

400

450

500

AuNPs Fe2O3-Au

0.0

Reaction Time (min.)

Fe2O3-Au spheres

-0.5

(b)

Spheres Rings Tubes

-1.0

ln(A/A0)

1.6

Absorbance (a.u.)

-1.5 -2.0 -2.5 -3.0 0

550

5

10

15

20

25

30

35

40

Time, t (min)

Wavelength (nm) -0.010

(c) Fe2O3-Au tubes

(d)

-0.008

Fe2O3-Au rings

k (s-1)

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


Fe2O3-Au spheres

-0.001

-0.002 -1

k (s )

Figure 7.

-0.004 AuNPs Fe2O3-Au spheres

-0.002

AuNPs 0.000

-0.006

-0.003

1x

2x

4x

8x

Nanoparticle Concentration

(a) Representative 4-nitrophenol reduction spectra obtained during a Fe2O3-Au

spheres catalytic decay experiment. (b) Plot of the change in the 4-nitrophenol peak intensity versus time during the catalytic reduction experiments using the AuNPs and the Fe2O3-Au spheres, rings, and tubes. (c) The catalytic decay constants for the the AuNPs and the Fe2O3-Au spheres, rings, and tubes. (d) Catalytic decay constants versus nanoparticle concentration for the AuNPs and the Fe2O3-Au spheres.


33


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

(a)

Figure 8.

(b)

(c)

(d)

Page 34 of 51

(e)

Photographs illustrating magnetic manipulation of the (a) suspended Fe2O3-Au

spheres. (b) - (c) An external magnet placed near a cuvette will attract all of the nanoparticles and collect them on the sides within 1 minute. (d) - (e) After removing the magnet, the collected Fe2O3-Au spheres can be redispersed.


34

Page 35 of 51

1 Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5

0 -1

ln(A/A0)

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 -3 -4 -5 -6

0

5

10

15

20

25

30

Time (min)

Figure 9. Plot of the change in the 4-nitrophenol peak intensity versus time during the magnetic collection and recycling experiment using the Fe2O3-Au spheres.


35

Fe2O3

(a)

Fe2O3-Au Au DI H2O

60

40

Laser on

20

0

250

500

Laser off

750 1000 1250 1500 1750 2000

Time (s)

80

0.06 Spheres Rings Tubes

60

40

20

Page 36 of 51

Fe2O3-Au

(b)

Laser on

0

(c)

Fe2O3-Au

Mass Loss (g)

80

Solution Temperature (°C)

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

Solution Temperature (°C)


Spheres Rings Tubes

0.04

0.02 Laser on Laser off

Laser off

200 400 600 800 100012001400160018002000

Time (s)

0.00

0

200 400 600 800 100012001400160018002000

Time (s)

Figure 10. (a) Temperature traces of the solutions containing the AuNPs, Fe2O3-Au and Fe2O3 spheres, and deionized water under laser irradiation. (b) Temperature traces and (c) solution mass loss comparing the photothermal heating effects of the Fe2O3-Au spheres, rings, and tubes.


36

Page 37 of 51

-3.5 AuNPs -4.0

ln(-k )

-4.5

-5.0

-5.5

(a) -6.0

0.00320

0.00324

0.00328

0.00332

1/T (K-1) -3.0 Fe2O3-Au spheres -3.5 -4.0

ln(-k )

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


-4.5 -5.0 -5.5

(b) 0.00326

0.00328

0.00330

0.00332

-1

1/T (K )

Figure 11. Arrhenius style plots of the photothermally induced temperature-rise versus the catalytic reaction constant for the (a) AuNPs and the (b) Fe2O3-Au spheres


37


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

Page 38 of 51

TABLES: Table 1. Average morphological parameters of the AuNPs, Fe2O3, and Fe2O3-Au nanoparticles (Note: Error bars represent standard error)

Fe2O3

Au Outer

Inner

Diameter (nm)

Diameter (nm)

Diameter (nm)

Length (nm)

AuNPs

20 ± 1

NA

NA

NA

Fe2O3-Au spheres

20 ± 1

48 ± 2

NA

NA

Fe2O3-Au rings

19 ± 1

171 ± 4

75 ± 4

117 ± 6

Fe2O3-Au tubes

19 ± 0.5

78 ± 2

59 ± 2

247 ± 8

Fe2O3 spheres

NA

46 ± 5

NA

NA

Fe2O3 rings

NA

169 ± 3

76 ± 3

117 ± 3

Fe2O3 tubes

NA

81 ± 3

61 ± 2

251 ± 3


38

Page 39 of 51

ASSOCIATED CONTENT Supporting Information. SEM images of the Fe2O3-Au spheres after catalysis. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Corresponding Author: [email protected]. Funding Sources The financial support of this work was provided by Department of Energy DOE- Laboratory Directed Research & Development (LDRD) Strategic Initiative Program.

TOC Graphic:

Reaction Time (min.) 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10

Fe2O3-Au spheres

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 250

300

350

400

450

Wavelength (nm)

500

AuNPs Spheres Rings Tubes

0.0 -0.5 -1.0

ln(A/A0)

1.6

Absorbance (a.u.)

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


-1.5 -2.0 -2.5 -3.0

550

0

5

10

15

20

25

30

35

40

Time, t (min)


39


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

Page 40 of 51

FIGURES:

(a)

(b)

200 nm

(d)

(c)

200 nm

(e)

200 nm

(f)

100 nm

100 nm

200 nm

200 nm

100 nm

200 nm

Figure 1. SEM micrographs of the (a) Fe2O3 spheres, (b) Fe2O3 rings, (c) Fe2O3 tubes, (d) Fe2O3Au spheres, (e) Fe2O3-Au rings, and (f) Fe2O3-Au tubes. The insets in (d)-(f) show zoomed in images of the same particle types.


Page 41 of 51

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


(a)

(b)

(c)

(d)

Figure 2. (a) EDX mappings respectively showing the distribution of (b) – (d) Au, Fe, and O in the Fe2O3-Au rings.



Particle Size (nm)

600

(a)

400

200

0

-40

Zeta Potential (mV)

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

-35

(b)

-30 -25 -20 -15 -10 -5 0

Figure 3. (a) Hydrodynamic particle sizes and (b) zeta potentials of the different nanoparticles as measured by dynamic light scattering.


Page 42 of 51

Page 43 of 51

20

30 

40

50

60

70

80

 Hematite  Maghemite

 

Normalized 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
















Tubes 







Rings

 





  





 

Spheres 20

30

40

50

60

70

80

Diffraction Angle, 2()

Figure 4. XRD spectra of the Fe2O3 spheres, rings, and tubes. The peak attributions are denoted by the symbols.



1.0

Absorbance (a.u.)

Fe2O3 rings Fe2O3 tubes

0.6 0.4 0.2

400

500

600

700

800

Wavelength (nm)

900

1000

ii

iii

iv

Au nanoparticles Fe2O3-Au spheres

(b)

Fe2O3 spheres

0.8

0.0

i

1.0

(a) Absorbance (a.u.)

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

Page 44 of 51

0.8

(c)

Fe2O3-Au rings Fe2O3-Au tubes

0.6 0.4

v vi vii

0.2 0.0

400

500

600

700

800

900

i: ii: iii: iv: v: vi: vii:

AuNPs Fe2O3 spheres Fe2O3 tubes Fe2O3 rings Fe2O3-Au spheres Fe2O3-Au tubes Fe2O3-Au rings

1000

Wavelength (nm)

Figure 5. UV-vis spectra of the (a) Fe2O3 and (b) AuNPS and Fe2O3-Au spheres, rings, and tubes. (c) Photograph of the different nanoparticle solutions.


Page 45 of 51

5


4

(a)

3 2 1 0

Fe2O3-Au

-1

Spheres Rings Tubes

-2 -3 -4 -5 -150

-100

-50

0

50

100

150

Applied Magnetic Field (Oe) 20


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


(b)

10 0 Fe2O3-Au Spheres Rings Tubes

-10 -20 -10000

-5000

0

5000

10000

Applied Magnetic Field (Oe)

Figure 6. (a) Narrow and (b) and wide range plots of the hysteresis loops of the Fe 2O3-Au nanoparticles.



1.4

(a)

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10

1.2 1.0 0.8 0.6 0.4 0.2 0.0 250

300

350

400

450

500

AuNPs Fe2O3-Au

0.0

Reaction Time (min.)

Fe2O3-Au spheres

-0.5

(b)

Spheres Rings Tubes

-1.0

ln(A/A0)

1.6

Absorbance (a.u.)

-1.5 -2.0 -2.5 -3.0 0

550

5

10

15

20

25

30

35

40

Time, t (min)

Wavelength (nm) -0.010

(c)

Fe2O3-Au tubes

(d) -0.008

Fe2O3-Au rings

-0.006

k (s-1)

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

Page 46 of 51

Fe2O3-Au spheres

-0.002

AuNPs 0.000

-0.001

-0.002 -1

k (s )

Figure 7.

-0.004

-0.003

0.000

AuNPs Fe2O3-Au spheres 1x

2x

3x

4x

Nanoparticle Concentration

(a) Representative 4-nitrophenol reduction spectra obtained during a Fe2O3-Au

spheres catalytic decay experiment. (b) Plot of the change in the 4-nitrophenol peak intensity versus time during the catalytic reduction experiments using the AuNPs and the Fe 2O3-Au spheres, rings, and tubes. (c) The catalytic decay constants for the the AuNPs and the Fe2O3-Au spheres, rings, and tubes. (d) Catalytic decay constants versus nanoparticle concentration for the AuNPs and the Fe2O3-Au spheres.


Page 47 of 51

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


(a)

Figure 8.

(b)

(c)

(d)

(e)

Photographs illustrating magnetic manipulation of the (a) suspended Fe 2O3-Au

spheres. (b) - (c) An external magnet placed near a cuvette will attract all of the nanoparticles and collect them on the sides within 1 minute. (d) - (e) After removing the magnet, the collected Fe2O3-Au spheres can be redispersed.



1

Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5

0 -1

ln(A/A0)

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

Page 48 of 51

-2 -3 -4 -5 -6

0

5

10

15

20

25

30

Time (min) Figure 9. Plot of the change in the 4-nitrophenol peak intensity versus time during the magnetic collection and recycling experiment using the Fe2O3-Au spheres.


Fe2O3

(a)

Fe2O3-Au Au DI H2O

60

40

Laser on

20

0

250

500

Laser off

750 1000 1250 1500 1750 2000

Time (s)

80

0.06

Fe2O3-Au

(b)

Spheres Rings Tubes

60

40

20

Laser on

0

Laser off

200 400 600 800 100012001400160018002000

Time (s)

Fe2O3-Au

Mass Loss (g)

80

Solution Temperature (C)

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


Solution Temperature (C)

Page 49 of 51

(c)

Spheres Rings Tubes

0.04

0.02 Laser on Laser off

0.00

0

200 400 600 800 100012001400160018002000

Time (s)

Figure 10. (a) Temperature traces of the solutions containing the AuNPs, Fe 2O3-Au and Fe2O3 spheres, and deionized water under laser irradiation. (b) Temperature traces and (c) solution mass loss comparing the photothermal heating effects of the Fe2O3-Au spheres, rings, and tubes.



-3.5 AuNPs -4.0

ln(-k )

-4.5 -5.0 -5.5 -6.0

(a) 0.00320

0.00324

0.00328

0.00332

1/T (K-1) -3.0 Fe2O3-Au spheres -3.5 -4.0

ln(-k )

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

-4.5 -5.0 -5.5

(b) 0.00326

0.00328

0.00330

0.00332

-1

1/T (K )

Figure 11. Arrhenius style plots of the photothermally induced temperature-rise versus the catalytic reaction constant for the (a) AuNPs and the (b) Fe2O3-Au spheres


Page 50 of 51

Page 51 of 51

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 1. Average morphological parameters of the AuNPs, Fe2O3, and Fe2O3-Au nanoparticles (Note: Error bars represent standard error)

Au

Fe2O3 Outer

Inner

Diameter (nm)

Diameter (nm)

Diameter (nm)

Length (nm)

AuNPs

20 ± 1

NA

NA

NA

Fe2O3-Au spheres

20 ± 1

48 ± 2

NA

NA

Fe2O3-Au rings

19 ± 1

171 ± 4

75 ± 4

117 ± 6

Fe2O3-Au tubes

19 ± 0.5

78 ± 2

59 ± 2

247 ± 8

Fe2O3 spheres

NA

46 ± 5

NA

NA

Fe2O3 rings

NA

169 ± 3

76 ± 3

117 ± 3

Fe2O3 tubes

NA

81 ± 3

61 ± 2

251 ± 3


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