ZnO Coupled Composites: Toward Tunable Cu

Article pubs.acs.org/JPCC

Nanostructured Cu/ZnO Coupled Composites: Toward Tunable Cu Nanoparticle Sizes and Plasmon Absorption Zhi Yi Tan,† Doreen Wei Ying Yong,† Zhihua Zhang,‡ Hong Yee Low,§ Luwei Chen,∥ and Wee Shong Chin*,† †

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 BASF South East Asia, 61 Science Park Road, The Galen, Singapore 117525 § Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602 ∥ Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, Singapore 627833 ‡

S Supporting Information *

ABSTRACT: We report a synthesis method that made use of a dual-glycol system, allowing discrete and size-tunable Cu nanoparticles to be deposited onto ZnO nanorods. Successful reduction of Cu was demonstrated by the distinct plasmon absorption, and we report for the first time a clear linear correlation of Cu plasmon peak position shifts with the variation of Cu particle sizes. Stable and discrete Cu nanoparticles were obtained despite deliberately leaving out conventional polymeric protecting agents such as poly(vinylpyrrolidone). The synthesized material exhibited robustness in ethanol steam reforming that occurred at high reaction temperature of up to 600 °C. Analysis of absorption spectra revealed the relative attenuation of ZnO absorption with increasing amount of Cu nanoparticles deposited, confirming clear electronic interaction between the coupled metallic and oxide components.

1. INTRODUCTION The recent nuclear energy crisis and the exigency to reduce the carbon footprint have resulted in tremendous research effort for an alternative clean energy source. The hydrogenation of carbon dioxide1,2 and utilization of hydrogen gas as clean fuel are both attractive solutions to the energy problem. To achieve these primary goals, however, catalysts are needed for attaining efficient conversion that is scalable for widespread applications. Copper-on-zinc-oxide support (Cu/ZnO), both abundant and low-cost materials, has recently been highlighted as a potential material for sustainable conversion of carbon dioxide to fuel.1−3 The catalyst was used in alcohol-steam reforming,4−10 methanol synthesis from syn-gas,3 and also as cocatalyst in the photocatalytic conversion of carbon dioxide. Despite all of these, nevertheless, there is still a lack of understanding on the influence of the nano- and heterostructures of Cu on catalysis. This is partially due to challenging problems in depositing well-defined Cu nanoparticles with controllable amount and sizes on the support. A recent report has suggested that better catalytic performance could be obtained if there exist strong electronic interactions at the interface of the coupled materials.1 We believe that such requirement asks for a proper synthetic design in which the reduction and deposition of Cu nanoparticles (NPs) could occur directly on a host with efficient kinetics yet minimal capping protection. For better catalytic performance, the deposited NPs must be discretely isolated from each other © XXXX American Chemical Society

and do not suffer significant morphology change through sintering under the harsh reaction conditions. Hence, we report a new method to deposit discrete Cu NPs with tunable sizes and ratios onto ZnO nanorods (NRs). We proposed to use a dual-glycol system; a longer chain glycol such as triethylene glycol (TEG) or diethylene glycol (DEG) was used as the dispersing medium for the host NRs, while a shorter chain ethylene glycol (EG) was used as the reducing agent for Cu2+ ions. The choice of EG allows reduction to occur at lower temperatures, while using solely EG will not produce the desired discrete deposition onto the ZnO host. To demonstrate the robustness of the coupled composite prepared, catalytic ethanol steam reforming that typically operates at high temperatures was studied. Our Cu NPs were found to exhibit good stability and do not sinter under such harsh reaction conditions.

2. EXPERIMENTAL PROCEDURES Chemicals. Zinc acetate dihydrate (>98%), oleylamine (OLA) (70%), DEG (99%), and TEG (99%) were purchased from Aldrich. EG (99.5%) was purchased from Merck. Copper acetate monohydrate (99.9%) was purchased from Alfa Aesar. All chemicals were used without further purification. Received: March 4, 2013 Revised: May 3, 2013

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dx.doi.org/10.1021/jp4021855 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Nanocomposite Synthesis. In a typical synthesis of Cu/ ZnO nanocomposite, ZnO NRs were first prepared using a method adopted from a previous report11 with OLA acting as both the capping and the reducing agent. Zinc acetate (3 mmol) was added to 1.3 mL of OLA in a three-necked flask. The reactants were subsequently degassed at 80 °C for 45 min. Under N2 purging, temperature was increased to 220 °C. During the heating process, the colorless solution turned white upon reaching ∼180 °C. After 15 min of reaction, the mixture was cooled to room temperature and ∼6 mL of ethanol was added to the reaction products. The mixture was centrifuged; the precipitate was isolated and washed with ethanol two more times to ensure complete removal of the reactants or byproducts. ZnO NRs prepared above were redispersed in 20 mL of DEG or TEG by sonication for ∼2 h followed by stirring under room conditions overnight to deposit Cu NPs. EG (2 mL) was added to the ZnO dispersion in a three-necked flask, and the mixture was degassed at room temperature for 5 min before heating to 190 °C. In a separate pot, a certain feed amount of copper acetate monohydrate was dissolved in 1 mL of EG with sonication. The mixture was then added to the ZnO mixture in a dropwise manner over ∼10 min. A further 5 min of aging at 190 °C was allowed before the composite was isolated from the mixture and washed with isopropanol (IPA). Catalytic Activity Measurement. Catalytic activity measurement was carried out in a quartz microreactor equipped with an online GC (Varian CP-3800) for product analysis. Liquid mixture of EtOH/H2O 1:3 (v/v) was fed into a vaporizer (443 K) at 0.005 mL/min with Ar (40 mL/min) as carrier gas; the GHSV was 34 000 h−1. Prior to the reaction, samples were reduced in pure H2 with a flow of 50 mL/min at 300 °C for 0.5 h. The time on stream at each temperature was 1 h. Characterization. UV absorption and reflectance spectra were obtained using a Shimadzu UV-3600 spectrometer. Scanning wavelength ranges from 250 to 800 nm. Analytical grade propanol was used as the reference. Morphology and dimensions of the products were studied using TEM, which was done using Philips CM300 FEGTEM with an acceleration voltage of 300 kV. The surface morphologies of ZnO and Cu/ ZnO nanostructures were studied using a JEOL JSM-6701F, JEOL JSM-6700 field-emission scanning electron microscope (FE-SEM). XRD analyses was done using a Siemens D-5000 diffractometer equipped with Cu Kα radiation (λ = 0.15406 nm).

Figure 1 shows a typical TEM image with distinct Cu NPs deposited over the ZnO NRs dispersed in TEG. This is in

Figure 1. (a) Typical TEM image of coupled Cu NPs/ZnO NRs nanocomposites. (b) HRTEM image showing one copper particle coupled onto ZnO NRs, clearly presenting their respective lattice planes.

contrast with composites produced using traditional techniques such as coprecipitation, where large clusters of Cu particles are randomly embedded in the matrix. We believe that the deposition of distinct and well-dispersed Cu NPs in our case is mainly due to the controlled (dropwise addition) and fast heterogeneous nucleation on the ZnO NRs surface. XRD patterns (Supporting Information Figure S1) correlated well with the reference hexagonal ZnO (JCPDS 36-1451) and metallic cubic Cu crystals (JCPDS 04-0836). The coupling between the two components was investigated by the HRTEM image shown in Figure 1b. An interplanar distance of 0.217 nm was determined from the spherical particle, which corresponds to the [111] lattice of metallic Cu. The rod-like region has an interplanar distance of 0.281 nm that matched well to the [100] lattice planes of ZnO. These distinct lattices became distorted at the interfacial region due to the slight lattice mismatch. Keeping the Zn-to-Cu ratio constant by adjusting the relative amount of ZnO accordingly, we varied the Cu precursor feed volume and obtained particles of increasing average size. Figure 2 presents the TEM analysis for samples prepared with average sizes of 31 ± 8 to 58 ± 14 nm. As previously mentioned, we have deliberately chosen a solvent system that does not provide good capping to Cu NPs, hence driving the high surface energy Cu monomers onto the ZnO support. We proposed a deposition mechanism as shown in Scheme 1. When drops of precursor solution are added to the ZnO NR-dispersed system, nucleation and reduction of Cu (step 2) occurs, forming unstable Cu0 monomers that attach themselves to nearby ZnO NRs (step 3). It is important to note that a slow introduction of precursor is critical in this step to form discrete Cu particles, as this will prevent the Cu0 monomers from self-coagulating. As more and more Cu0 monomers are deposited on the ZnO NRs, the addition of Cu does not induce further nucleation. Instead, the Cu precursors are directly reduced on the deposited Cu particles, promoting the growth of Cu NPs. This allows us to control the size of deposited Cu NPs through varying the precursor feed amount. As shown by typical absorption spectra in Figure 3, distinct shift in the Cu plasmon peak position with variation in their sizes was detected. While the absorption peak of ZnO NRs remains the same at 370 nm, it is clearly seen that the Cu

3. RESULTS AND DISCUSSION As described in the Experimental Procedures section, ZnO NRs were presynthesized using a typical OLA system.11 Subsequently, the washed NRs were redispersed in a polyol solvent system for the deposition of Cu NPs. Polyols such as TEG and EG have previously been employed to synthesize Cu NPs, but protecting agent such as poly(vinylpyrrolidone) (PVP) was always added to prevent coagulation.12−15 However, this protecting layer will create a barrier between the two components in the composite. Hence, we omitted PVP and deliberately made use of the intrinsic coagulation tendency to drive the nucleation of Cu onto the ZnO NRs. We found that Cu NPs of various sizes can be individually deposited on ZnO NRs by varying the reaction parameters such as Cu precursor feed volume. B



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Figure 2. Typical TEM images with the respective size distribution histograms for Cu NPs/ZnO NRs composites prepared from TEG. Experimental feed volume of Cu precursor from left to right: 1, 2, and 4 mL.

Scheme 1. Proposed Deposition Mechanism of Cu NPs on ZnO NRs

for metallic Cu NPs, as shown in Figure 4, to the best of our knowledge, is reported for the first time. Metallic Cu NPs often suffer from surface oxidation even in the presence of polymeric protecting agent such as PVP.18 Because any slight surface oxidation is detrimental to surface plasmon properties, a distinct surface plasmon peak for Cu NPs is not always shown in literature reports. Surface oxidation will result in Cu NPs losing their distinct absorption characteristics in the region below 600 nm. The formation of surface cupric oxide will show up as an absorption peak in the 630 nm region.19,20 Notably, our Cu NPs deposited on ZnO NRs exhibit distinct and tunable plasmon resonance peak in the region below 600

plasmon peak red-shifted from 580 to 593 nm as Cu precursor feed volume increased from 1 to 4 mL. On the basis of Mie theory, surface plasmon resonance in metallic NPs involves dipolar oscillations of the free electrons in the metallic conduction band near the Fermi level. This phenomenon has predicted the linear dependency of plasmon peak position on the average particle size, subject to internal scattering limits. While this linear dependency has been widely tested for other noble metals such as Ag and Au NPs,5,16 it has not been well-studied for Cu NPs due to the difficulties in getting good Cu plasmon absorption as well as the isolation of these NPs for TEM size analysis.5,17 A linear correlation plot C



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nm. In Figure 5, the stability of these Cu NPs toward oxidation was demonstrated even after washing several times in IPA. Samples kept in closed or open system for prolonged periods of time exhibit no sign of surface cupric oxide absorption (Figures 5c−f), although a slight shift (∼10 nm) in the plasmon peak was detected. To demonstrate that such stability is obtained only with the prepared Cu NPs/ZnO composites, we monitored also a physical mixture of presynthesized Cu NPs and ZnO NRs. It was found that the Cu plasmon peak red shifts significantly in this case, even after just two times of washing with IPA (Figure 6). The plasmonic profile is lost after 24 h of standing, shifting to 614 nm and indicating conversion to cupric oxide even in a closed system. We believe the stability exhibited by the composite samples is a result of good electronic coupling between Cu and ZnO. Further analysis using samples of varying Zn-to-Cu ratios reconfirms this postulation (Figure 7). Thus, with increasing Cu feed volume, we found that the intensity of excitonic ZnO peak at 370 nm decreases linearly, although the ZnO content in each sample was kept constant. Physically mixed samples of ZnO NRs and Cu NPs at similar ratios exhibited absorption spectra as shown in Figure 7b. Quantitative analysis of the absorption peak areas (see Supporting Information Figures S4 and S5 for details) yielded a plot of ZnO peak area versus Cu feed amount as shown in Figure 8. The near-linear attenuation of ZnO peak in the coupled samples clearly demonstrated interactions between the two components. Interestingly, Fermi-level equilibration that was reported for metal−semiconductor coupled materials in previous studies1,21 is not observed. Instead, a correlated attenuation of ZnO excitonic peak with increasing Cu content exhibited in Figures 7 and 8 suggested the Moss−Burstein effect;17 that is, the ZnO conduction band is filled with additional electrons from Cu, and hence transition of electrons from the valence to the conduction band becomes less probable. Another plausible explanation for the observation is Stark effect,22 whereby additional charge carriers on the particle surface result in an inhomogeneous charge distribution and thus establish an electric field that modulates the excitonic

Figure 3. Absorption spectra of Cu NPs/ZnO NRs composites prepared from TEG. Cu plasmon maxima at 580, 586, and 593 nm corresponding to 1, 2, and 4 mL of Cu feed volume, respectively, keeping the Zn-to-Cu feed ratio constant at 4:1 (mol/mol).

Figure 4. Plots of average Cu particle size against Cu plasmon absorption for Cu/ZnO composites prepared in TEG and DEG, respectively. (The corresponding TEM images and absorption spectra are given in the Supporting Information Figures S2 and S3.)

Figure 5. Absorption spectra of coupled Cu NPs/ZnO NRs nanocomposites: (a) fresh sample withdrawn directly from reaction pot, (b) after two times of washing, (c) in closed system after 24 h, (d) in closed system after 96 h, (e) in open system after 24 h, and (f) in open system after 96 h. D



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Figure 6. Absorption spectra of Cu NPs and ZnO NRs physically mixed together: (a) fresh Cu NPs sample withdrawn directly from reaction pot, (b) after 2 times of washing, and (c) in closed system after 24 h.

Figure 7. Absorption spectra of (a) Cu NPs/ZnO NRs composite synthesized in DEG at various Zn to Cu feed ratios. (b) Cu NPs and ZnO NRs mixture with increasing amount of Cu added.

composites synthesized using TEG were found to consistently exhibit plasmon maximum red-shifted relative to those synthesized using DEG for each set of Cu NPs of the same average size. It is well known that plasmon resonance can be affected significantly by the environment, in particular, the dielectric constant of the surrounding medium. The frequency of Mie resonance is expected to be inversely proportional to the dielectric constant of the medium according to eq 1:23 ωs ≈ [N /ε0meff ]1/2 ·[2εm + 1 + χ1inter ]−1/2

(1)

where ωs = spectral Mie resonance frequency, N = number of “conduction” electrons in the cluster, ε0 = dielectric constant of vacuum, meff = effective mass of electrons, εm = dielectric constant of the medium, and χinter = susceptibility contribution by interband transition in the metal. The observed relative red shift in TEG system, however, cannot be accounted for by dielectric constant because TEG is known to have a lower εm compared with DEG,24 and thus a blue shift is predicted instead from their relative dielectric constants. To explain this observation, we postulate that the number of conduction electrons (parameter N in eq 1) is affected by the extent of electron transfer between the Cu NPs and the ZnO NRs; this in turn is affected by the different capping ability of TEG and DEG. Comparatively having a more branched structure, TEG is expected to cap less efficiently onto the surfaces of Cu NPs and ZnO NRs. Thus, better electron transfer could occur between the TEG-capped particles,

Figure 8. Plots of ZnO excitonic peak area versus Cu feed amount for (◆) Cu NPs/ZnO NRs composites and (□) Cu NPs mixed with ZnO NRs. Connecting lines are drawn to show visually the general trends.

levels. Such attenuation of absorption was previously proposed for excess electron accumulation on coupled semiconductor materials.21 Either explanation provides insight that the Cu metallic component can alter the electronic properties of the composite and hence is able to bring about synergistic properties that cannot be obtain from simply mixing the two components together. In addition, we also noted yet another interesting observation from the two correlation plots in Figure 4. In these linear plots, E



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Figure 9. (a) Ethanol conversion and yield of H2 at varying temperature on Cu NPs/ZnO NRs catalyst. (b) Selectivity of various products from ethanol steam reforming reaction.

One interesting observation noted from Figure 9b is the relatively high percentage of acetone formed. This species is usually not observed using catalysts prepared with conventional precipitation method. According to the mechanism proposed by Elliott and Pennella,26 the formation of acetone is a multistep process, which we believe is enhanced in the present system with these nanosized composites. Robustness of the catalyst is evident from TEM analysis in Figure 10 where the spent catalysts were analyzed. Although ZnO NRs suffers slight deformation, there is no sign of the deposited Cu particles sintering under the harsh reacting conditions. XRD analysis (see Supporting Information Figure S6) also confirms that the crystals of Cu NPs and ZnO NRs remained unchanged after the catalysis reaction. The estimated Cu NPs

resulting in a lower number of conduction electrons and hence a red shift in the plasmon peak position according to eq 1. Further theoretical work will be needed to test this postulation, however. Lastly, we attempted to test the robustness of our composite catalysts in ethanol steam reforming that typically requires high temperatures. As shown in Figure 9a, high ethanol conversion is attained at temperature above 450 °C and remains at 600 °C. Hydrogen yield of 3.2 mol was obtained at 450 °C and further increased to 4.4 mol at 600 °C. This yield was significantly higher compared with catalysis studies using Cu/ZnO composite prepared from conventional precipitation method.25 Figure 9b showed that very efficient hydrogen conversion can be attained without the production of undesirable carbon monoxide byproduct. F



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with Cu Nanoparticles at the Materials’ Interface in Selective Hydrogenation of CO2 to CH3OH. Angew. Chem., Int. Ed. 2011, 50, 2162−2165. (2) Meunier, F. C. Mixing Copper Nanoparticles and ZnO Nanocrystals: A Route towards Understanding the Hydrogenation of CO2 to Methanol? Angew. Chem., Int. Ed. 2011, 50, 4053−4054. (3) Wu, J.; Saito, M.; Takeuchi, M.; Watanabe, T. The Stability of Cu/ZnO-Based Catalysts in Methanol Synthesis from a CO2-Rich Feed and From a CO-Rich Feed. Appl. Catal., A 2001, 218, 235−240. (4) Waugh, K. C. Methanol Synthesis. Catal. Today 1992, 15, 51−75. (5) Ressler, T.; Kniep, B. L.; Kasatkin, I.; Schlögl, R. The Microstructure of Copper Zinc Oxide Catalysts: Bridging the Materials Gap. Angew. Chem., Int. Ed. 2005, 44, 4704−4707. (6) Kasatkin, I.; Kurr, P.; Kniep, B.; Trunschke, A.; Schlögl, R. Role of Lattice Strain and Defects in Copper Particles on the Activity of Cu/ZnO/Al2O3 Catalysts for Methanol Synthesis. Angew. Chem., Int. Ed. 2007, 46, 7324−7327. (7) Lin, Y.-G.; Hsu, Y.-K.; Chen, S.-Y.; Lin, Y.-K.; Chen, L.-C.; Chen, K.-H. Nanostructured Zinc Oxide Nanorods with Copper Nanoparticles as a Microreformation Catalyst. Angew. Chem., Int. Ed. 2009, 48, 7586−7590. (8) Agrell, J.; Boutonnet, M.; Fierro, J. L. G. Production of Hydrogen From Methanol Over Binary Cu/ZnO Catalysts: Part I. Catalyst Preparation and Characterisation. Appl. Catal., A 2003, 253, 201−211. (9) Pillai, U. R.; Deevi, S. Copper-Zinc Oxide and Ceria Promoted Copper-Zinc Oxide as Highly Active Catalysts for Low Temperature Oxidation of Carbon Monoxide. Appl. Catal., B 2006, 65, 110−117. (10) Müller, S. P.; Kucher, M.; Ohlinger, C.; Kraushaar-Czarnetzki, B. Extrusion of Cu/ZnO Catalysts for The Single-Stage Gas-Phase Processing of Dimethyl Maleate to Tetrahydrofuran. J. Catal. 2003, 218, 419−426. (11) Zhang, Z.; Lu, M.; Xu, H.; Chin, W.-S. Shape-Controlled Synthesis of Zinc Oxide: A Simple Method for The Preparation of Metal Oxide Nanocrystals in Non-Aqueous Medium. Chem.Eur. J. 2007, 13, 632−638. (12) Fievet, F.; Fievet-Vincent, F.; Lagier, J.-P.; Dumont, B.; Figlarz, M. Controlled Nucleation and Growth of Micrometre-Size Copper Particles Prepared By The Polyol Proces. J. Mater. Chem. 1993, 3, 627−632. (13) Huang, H. H.; Yan, F. Q.; Kek, Y. M.; Chew, C. H.; Xu, G. Q.; Ji, W.; Oh, P. S.; Tang, S. H. Synthesis, Characterization, and Nonlinear Optical Properties of Copper Nanoparticles. Langmuir 1997, 13, 172−175. (14) Orel, Z. C.; Matijevic, E.; Goia, D. V. Conversion of Uniform Colloidal Cu2O Spheres to Copper in Polyols. J. Mater. Res. 2003, 18, 1017−1022. (15) Park, B. K.; Jeong, S.; Kim, D.; Moon, J.; Lim, S.; Kim, J. S. Synthesis and Size Control of Monodisperse Copper Nanoparticles by Polyol Method. J. Colloid Interface Sci. 2007, 311, 417−424. (16) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment. J. Phys. Chem. B 2002, 107, 668− 677. (17) Wood, A.; Giersig, M.; Mulvaney, P. Fermi Level Equilibration in Quantum Dot−Metal Nanojunctions. J. Phys. Chem. B 2001, 105, 8810−8815. (18) Jeong, S.; Woo, K.; Kim, D.; Lim, S.; Kim, J. S.; Shin, H.; Xia, Y.; Moon, J. Controlling the Thickness of the Surface Oxide Layer on Cu Nanoparticles for the Fabrication of Conductive Structures by Ink-Jet Printing. Adv. Funct. Mater. 2008, 18, 679−686. (19) Yin, M.; Wu, C. K.; Lou, Y. B.; Burda, C.; Koberstein, J. T.; Zhu, Y. M.; O’Brien, S. Copper Oxide Nanocrystals. J. Am. Chem. Soc. 2005, 127, 9506−9511. (20) Borgohain, K.; Murase, N.; Mahamuni, S. Synthesis and Properties of Cu2O Quantum Particles. J. Appl. Phys. 2002, 92, 1292− 1297. (21) Hoyer, P.; Weller, H. Size-Dependent Redox Potentials of Quantized Zinc Oxide Measured With An Optically Transparent Thin Layer Electrode. Chem. Phys. Lett. 1994, 221, 379−384.

Figure 10. Typical TEM images of the spent catalyst after reaction at 600 °C.

size before and after catalysis reaction was 30.4 and 32.5 nm, respectively, from Debye−Scherrer equation. Such a slight improvement in crystallinity is not unexpected from the hightemperature heating process; nevertheless, it is confirmed that sintering did not occur from both the TEM and XRD analysis.

4. CONCLUSIONS In conclusion, we report here a method designed to deposit discrete Cu NPs of tunable sizes onto ZnO host. The coupled system showed no sign of Cu oxidation and exhibited clear Cu plasmon absorption that red-shifted linearly with increasing particle size. Interesting observations such as the relative attenuation of ZnO absorption and the red-shifting of Cu plasmon resonance in TEG relative to DEG-prepared samples are reported. These observations allowed us to establish the nature of electronic interaction between the coupled Cu metal and the semiconducting ZnO material, which are important for applications in catalytic conversions such as alcohol reforming and CO2 reduction. Finally, we demonstrated that the robust composites can be utilized to effectively catalyze ethanol steam reforming occurring at high temperatures.

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ASSOCIATED CONTENT

S Supporting Information *

Absorption spectra of composites with various Cu to ZnO ratio and the respective peak area analysis, powder XRD measurements, and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 65-6779-1691. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank BASF for funding to support the initial stage of this project. We also thank Ms. Catherine Choong (ICES) for helping with the catalytic analysis.

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REFERENCES

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ZnO Coupled Composites: Toward Tunable Cu

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