Enhanced Heterogeneous Nanocatalysis on a Nanoporous Gold Disk

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Functional Nanostructured Materials (including low-D carbon)

10X-enhanced heterogeneous nanocatalysis on nanoporous gold disk array with high-density hot spots Md Masud Parvez Arnob, camille artur, Ibrahim Misbah, Syed Mubeen, and Wei-Chuan Shih ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19914 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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ACS Applied Materials & Interfaces

10X-enhanced heterogeneous nanocatalysis on nanoporous gold disk array with high-density hot spots Md Masud Parvez Arnob1, Camille Artur1, Ibrahim Misbah1, Syed Mubeen2, and Wei-Chuan Shih1,3,4,5* 1. Department of Electrical and Computer Engineering, University of Houston, Houston, TX 77204, USA 2. Department of Chemical and Biochemical Engineering, University of Iowa, Iowa City, IA 52242, USA 3. Department of Biomedical Engineering, University of Houston, Houston, TX 77204, USA 4. Program of Materials Science and Engineering, University of Houston, Houston, TX 77204, USA 5. Department of Chemistry, University of Houston, Houston, TX 77204, USA * Correspondence: [email protected]

Abstract Certain noble metal nanostructures as heterogeneous photocatalysts have drawn significant attention in the recent past due to their unique optical properties which lead to the excitation of localized surface plasmon resonance (LSPR). The LSPR concentrates electromagnetic fields to the surfaces and its relaxation processes can convert photon energy to energetic charge carriers or heat, which can be subsequently harvested to enhance surface catalysis. Here, we report the catalytic performance of a novel plasmonic nanostructure, disk shaped nanoporous gold nanoparticles or simply NPG disks, using a well-tested reduction pathway of Resazurin to Resorufin. We show that the catalytic reaction rate of NPG disks is enhanced by 10-fold upon external light illumination due to the excitation of LSPR. The plasmon enhanced catalytic reaction follows a linear-tosuperlinear transition in the rate dependence on the input light power. In addition, the light input results in a room temperature reaction rate equivalent to that of an ambient temperature of 70 C. Together, the results support that hot charge carriers play the dominant role for the enhancement. Keywords. nanoporous gold in photocatalysis, LSPR, plasmon enhanced photocatalysis, hot electrons.

1. Introduction

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Heterogeneous catalysis, where the catalyst and reactants are in different phase, plays a significant role in chemical conversion, energy production, and pollution mitigation 1. Nowadays many of the commercially important catalytic processes (oxidation of hydrocarbons, oxidation of CO, reduction of NO, etc.) are based on heterogeneous catalysis 2, 3. Traditional heterogeneous catalysts include the late transition metals, such as palladium, platinum, ruthenium, rhodium, and iridium, which are typically supported by a high surface area material (zeolites, alumina, carbon, etc.) to enhance efficiency (the support participates in the reaction) and/or reduce cost 4-6. However, most of these schemes require high temperature and suffer from thermal instability and susceptibility to various “poisoning” issues 6-8. Gold nanostructures of size less than 10 nm, when properly supported on a suitable metal oxide, has exhibited surprisingly high catalytic activity for CO oxidation at low temperature 9. Its catalytic property depends primarily on the size of the particles and the nature of the support 10, 11. The use of some supports, including α-Fe2O3 and TiO2 as well as CeO2, results in highly active catalysts, whereas others, such as Al2O3 and SiO2, showed usually low or no activity 7, 12. To decouple from the dependence on support, support-free Au catalysts have received growing attention 13 14. An effective means to prepare support-free Au catalysts is by selective removal of Ag from an Au-Ag alloy with the product known as nanoporous gold (NPG) material 15, 16. NPG features a sponge-like porous morphology with the characteristic bi-continuous pore/ligament size in the nm length scale (typically on the order of 10 nm). As shown by several groups, NPG can be used for oxidation of CO, alcohols, and sugars with surprisingly high catalytic activity at low temperature 6, 16-18. The origin of the observed catalytic activity of nanoporous gold has been attributed to low-coordinated atoms residing in steps and kinks as active catalytic sites and the role played by residual Ag atoms19,16,20,21. However, the catalytic property of NPG under light illumination condition has not been elucidated. Recently, noble metal nanostructures as catalysts have attracted further attention because of localized surface plasmon resonances (LSPR), where the alternating electromagnetic field of the incident light causes the free electron gas to oscillate. When the incident light frequency matches the LSPR frequency, optical extinction (absorption + scattering) reaches a maximum. The excitation of LSPR is coupled with several phenomena of interest. The incoming electromagnetic field is concentrated on the surface of nanostructures, thereby enhancing the local electric field 22, 23. The LSPR energy can decay via radiative decay or non-radiative decay mechanisms 24. Under the radiative decay mechanism, the LSPR energy gets damped via photon re-radiation leading to enhanced scattering. For the non-radiative decay mechanism, the LSPR energy gives rise to electron-hole pair generations, which are not in thermal equilibrium and have energy between 1 to 3 eV 24. These energetic carriers are called plasmonic hot carriers, which rapidly lose their energy to phonon excitations and thermalization 24. Plasmonic nanoparticles have been extensively used in various applications, e.g., surface enhanced spectroscopy and imaging, bio-sensing, photodetection, photo-thermal therapy, and others 25-27. Plasmon-enhanced and plasmon-driven photocatalysis have been latest additions to this list. Traditional photocatalysts lack the satisfactory visible-light-response and chemical stability 5, 28. Plasmonic photocatalysts, in contrast, can provide high-performance and stable activity over a broadband light source (from UV to nearinfrared (NIR)) 5, 29. In addition, according to the recently proposed hot electron plasmonic catalysis model, specific reaction pathways can selectively be promoted by careful plasmonic substrate design 29. Plasmonic photocatalysis can be classified into two broad categories: indirect and direct photocatalysis. Under indirect photocatalysis, the incident photon energy is transferred from LSPR


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to a nearby semiconductor via resonant energy or direct charge injection, which acts as the reaction center 28. Plasmonic nanostructures have been incorporated with semiconductor oxides (e.g., TiO2, Fe2O3, ZnO, etc.) and others (CdS, CdSe, ZnIn2S4, etc.) as photocatalysts 28, 30, 31. For example, Moskovits et al. exploited the LSPR generated charge carriers of Au nanorods to enhance the catalytic water splitting by platinum and cobalt nanocatalysts 32, 33. Similarly, Awazu et al. investigated the decomposition rate of methylene blue by TiO2 catalyst film and found enhanced catalytic activity due to the plasmon resonance of Ag nanoparticles under near-UV irradiation 34. In contrast, direct photocatalysis features plasmonic nanostructure serving as both the light harvesting unit and the catalytic component 28. Direct photocatalysis has been attributed to either phonon (or photothermal) mechanism or hot carrier mechanism 29, 35. For the phonon/photothermal route, the plasmon population decays into phonon modes and accelerate the reaction dynamics at elevated temperature 29. For the hot carrier mechanism, hot electrons or holes generated due to Landau damping transiently populate the otherwise unpopulated vibronic adsorbate states, thereby converting the energy of the charge carrier along the reaction coordinate 29. Whatever the underlying plasmonic catalysis mechanism, however, experimental observations of direct plasmon enhanced photocatalysis are still limited to Ag and Au nanoparticles. As mentioned previously, NPG has attracted significant interest as support-free heterogeneous catalyst. It furthermore possesses interesting plasmonic properties owing to its bicontinuous three-dimensional network of Au ligaments and pores 36. There has been nevertheless no plasmonic catalytic study reported so far for this nanostructure, which is partly owing to NPG in the form of thin films exhibit weak plasmonic extinction and fundamentally limited plasmon resonance tunability 37. Recently, our group has demonstrated that these limitations can be overcome by patterning NPG film into NPG nanoparticles, a.k.a., NPG disks. Besides stronger plasmonic extinction and wide-range LSPR tuning, NPG disks possess large specific surface area and high-density plasmonic localization near the bi-continuous network of ligaments and nanoscale channels. They are perfect candidate for much enhanced surface catalysis by plasmonic hot carriers, which is the central hypothesis we would like to test in this work. In this paper, we have selected the reduction of Resazurin (RZ, C12H7NO4) to Resorufin (RF, C12H7NO3) as our model system. RZ is an anionic dye which belongs to the phenoxazine core based class of dyes such as Nile blue, Nile red and Amplex red. RZ has been widely used as an oxidation/reduction probe of mitochondrial activity in cell viability assays 35, anti-oxidant activity 36, etc. All its uses rely on the fluorogenic catalytic reduction of the weakly fluorescent blue RZ to the pink RF with much higher fluorescence quantum yield. The UV-vis spectrum of RZ in water is well known 37 with an absorption maximum located at 602 nm whereas the absorption maximum of the reduced form RF is located at around 532 nm 38. RZ reduction to RF does not occur without catalytic action, be it via photocatalysis 37, homogeneous catalysis 35, 36, 39 heterogeneous catalysis on gold nanoparticles 40–43 or electrocatalysis on single walled carbon nanotubes 44. In the rest of the paper, we first characterize the catalytic activity of NPG disks for RZ reduction to RF using absorption spectroscopy. We then monitor direct enhancement of the catalytic reaction rate on NPG disks excited at their LSPR wavelength by in situ Raman spectroscopy. We show that NPG disks are a unique and novel catalytic material. They combine the inherent catalytic properties of nanoporous gold through large surface area and high-density of active sites with the enhanced light-harvesting and energy conversion associated with the diskshape patterning and subsequent LSPR modifications.



2. Methods 2.1. Fabrication of NPG Disks. NPG disks were fabricated utilizing the combination of lithographic patterning and atomic dealloying. As shown in Scheme 1, a film stack consisting of 2 nm Ti, 3 nm Au, and 80 nm Ag70Au30 (atomic percentage) alloy was first deposited onto a glass substrate. A monolayer of polystyrene (PS) beads (300, 460, and 800 nm diameter, different diameter corresponds to different NPG disk size) was then formed on top of the alloy film. Next, the PS beads were shrunk using a timed O2 plasma treatment in order to guarantee the separation between neighboring beads (Scheme 1b). The sample was then etched by ion milling in Ar plasma to transfer the beads pattern into the alloy film (Scheme 1c), after which the PS beads were removed by sonication in water (Scheme 1d). The remaining alloy disks were then de-alloyed in 70% nitric acid (HNO3) at the room temperature for 1 min, followed by a 2-min rinse in de-ionized (DI) water to produce the random array of NPG disks (Scheme 1). Scanning electron microscopy (SEM) images (Scheme 1f to j) show the corresponding nanostructures through the fabrication steps.

Scheme 1. (a-e) illustrate the fabrication process steps to prepare NPG disks: (a) formation of a monolayer of polystyrene (PS) beads on an alloy-coated substrate; (b) O2 plasma shrinkage of the PS beads; (c) Ar sputter etching to form isolated alloy disks; (d) removal of PS beads; (e) formation of NPG disks by dealloying. Figures f to j are the SEM images (~45o viewing angle) taken at each process step.

2.2. Catalytic reduction of resazurin (RZ) to resorufin (RF). The NPG disks substrate was cut into 1 cm × 1 cm pieces. Then a reaction cell was formed by sandwiching a polydimethylsiloxane (PDMS) well, 5 mm diameter and 1 mm height, between the 1 cm × 1 cm NPG disks substrate and a borosilicate coverslip (Corning). 32 µM RZ and 3.2 mM hydroxylamine (NH2OH) solutions were properly mixed and degassed. 20 µL of RZ/ NH2OH solution was then dispensed into the reaction cell and the UV-visible spectra were recorded at appropriate time intervals using a Cary 5000 spectrophotometer. The reaction was carried out at room temperature. 2.3. Reusability of NPG disks substrate. The reusability of NPG disks in catalytic activity was tested for repeated cycles. After each cycle, the NPG disks substrate was successively cleaned in dichloromethane, water, acetone, and ethanol, followed by N2 blow dry and annealing at 65 oC (15 mins). 2.4. Plasmon enhanced catalytic reduction of RZ to RF.


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A reaction cell, 5 mm diameter and 1 mm height, was formed by sandwiching a PDMS well between a 1 cm × 1 cm NPG disks substrate and a borosilicate coverslip. The output of a 785 nm continuous wave laser (3900S, Spectra-Physics) was intentionally defocused and illuminated on the NPG disks substrate over a 2 mm × 1.92 mm area (~20% of the active catalytic area) with a total power of 107 mW. The reduction of RZ to RF was monitored by the time dependent Raman spectra of RZ (1 sec acquisition) using a home-built line-scan Raman microscopy system 38, 39. We note that a single laser source (785 nm, ~107 mW) was used for both the Raman measurements and plasmonic excitation. The intentional defocus was implemented by focusing the laser halfway into the depth of the reaction cell, i.e., 0.5 mm above the NPG disk substrates. Time dependent Raman spectra of RZ and RF were recorded in situ under two different laser illumination conditions: laser off and laser on. Under the laser off condition, the laser was on only during the data acquisition (1 sec). For the laser on condition, the laser was on throughout the experiment. The temperature rise was recorded using an infrared (IR) thermal camera (FLIR A320) in separate experiments. 2.5. Finite difference time domain (FDTD) simulation. FDTD simulation was carried out for 200 nm diameter NPG disks to obtain the electric field (Efield) distribution. The NPG disk model was described in Ref. 39 40. The simulation domain was fixed at 1.5 µm× 1.5 µm× 1.5 µm. Perfectly matched layers (PMLs) were used on all the boundaries. The effect of staircase approximation was addressed by reducing the mesh size around the disk to 2 nm× 2 nm× 2 nm. The substrate effect was included by putting the NPG disk on a 1.5 µm× 1.5 µm× 1 µm slab of 1.52 refractive index. The simulations were performed on a computer with 8 cores with 32 GB memory (RAM). 3. Results and discussion Fig. 1(a) and 1(b) show the SEM images of ~200 and ~550 nm diameter (average) NPG disks, made from ~300 and ~800 nm diameter PS beads, respectively. The average disk thickness was ~70 nm with ligament size between ~10-15 nm. The X-ray photoelectron spectroscopy (XPS) analysis (supporting information) reveals that the residual Ag was ~8%. No oxides (Ag or Au) were observed in the measurements, which is in agreement with the previous XPS studies on NPG dealloyed in HNO3 by us and others 17, 41. We note that Xu et al. reported the existence of metal oxides in the anodic-etched (electrochemical corrosion) NPG structures 17. Fig. 1(c) shows the typical NPG disk array of 550 nm diameter. Fig. 1(d) shows the extinction spectra of NPG disks of different diameters in water. The LSPR peak positions are at ~760 and 1700 nm for ~200 and 550 nm NPG disks, respectively. Fig. 2 shows the calculated E-field distributions at different physical positions (disk top, middle, and bottom surface and central cross-section) of 200 nm NPG disk in water under 785 nm light illumination. The NPG disk supports high density internal E-field localization which are randomly distributed due to the random 3D pore-ligament network.



Fig. 1. SEM image of NPG disks with 200 and 550 nm diameter in (a) and (b), respectively. The scale bars are 100 nm. (c) SEM image of 550 nm diameter NPG disk array. The scale bar is 2 µm. (d) Normalized extinction spectra of 200 and 550 nm NPG disks in water. LSPR peak positions are at 760 and 1700 nm for the 200 and 550 nm NPG disks, respectively.

Fig. 2. Calculated E-field distributions for 200 nm diameter NPG disk (in water) at (a) top, (b) middle, (c) bottom, and (d) central cross-section. The E-field is distributed three dimensionally. The color bar represents the log10 values.


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3.1. Catalytic reduction of RZ to RF in a dark environment. For the catalytic reduction on NPG disks substrates, the reaction occurs via adsorption of the reactants on the Au surface is written as follows:

(1)

, where the S subscript denotes adsorbed species and the ↑ superscripts indicates desorption of products. The general understanding is that Au provides proper reaction sites and required energy landscape for the reduction which otherwise does not occur in solutions. The surface-adsorbed NH2OH, at first, donates electrons to the NPG disk catalytic sites. The electrons are subsequently transferred to RZ for the reduction reaction. The residual Ag in NPG disks might facilitate the adsorption of the reactants to the substrate via atomic O (exists in the both RZ (C12H7NO4) and NH2OH) coupling 18, 42. The NPG disk mediated electron transfer rates are much faster than the direct electron transfer from the NH2OH to RZ 43, 44. Fig. 3 shows the (dark) catalytic activity of NPG disks. In the presence of NH2OH, RZ gets reduced to RF (Fig. 3a) with a corresponding color change from blue to pink. The NH2OH gets oxidized in the catalytic reaction and likely become nitrite or related products 45. Since the rate constant depends on the concentration of NH2OH, we have conducted a series of experiment to identify the concentration range where the rate constant is essentially constant for later experiments.The initial concentrations of RZ and NH2OH are 16 µM and 1.6 mM, respectively. The time-dependent spectra show intensity decrease of the RZ absorption peak (602 nm), and intensity increase of the RF absorption peak (572 nm). The absorption of RZ and RF can be ascribed to the delocalized ππ* transition of the heterocyclic ring 46. An isosbestic point at 582 nm is observed in the absorption spectra, indicating the quantitative conversion of RZ to RF. Inset shows the stock solution of the RZ/NH2OH mixture and the converted RF in the presence of NPG disks. In a control experiment, no color change was observed in the stock solution after 7 days without the presence of NPG disks, thereby validating the critical role of NPG disks. The reduction of RZ fits the pseudo first order reaction well, i.e., ln(C/C0) = −kt, where C is the final concentration of RZ, C0 is the initial concentration of RZ, t is the reaction time, and k is the rate constant. Concentration C is obtained by the intensity of the RZ absorption peak at 602 nm and the calibration curve can be found in the supporting information (Fig. S3). Fig. 3b shows the plot of ln(C/C0) against t and the linear fitting provides k = 0.00189 s-1. The rate constant increases with increasing NH2OH concentration as shown in Fig. 4a, where six different NH2OH concentrations are used: 0.4, 0.8, 1.6, 3.2, 6.4, and 12.8 mM. Higher NH2OH concentration leads to more active sites with electrons, and hence more RZ molecules get converted into RF. However, the continuous increase in NH2OH concentration does not increase the k indefinitely. As shown in Fig. 4a, k is almost constant beyond 1.6 mM NH2OH. The NH2OH dependent variations in k can be understood with the insight in RZ to RF catalytic conversion mechanism mentioned previously 43, 44. The observed plateauing of k with increasing NH OH concentration is attributed to the 2 saturation of surface adsorption. It is to be noted that increasing the RZ concentration beyond 16



µM, when NH2OH concentration is in the plateau region, does not significantly change k, also suggesting the saturation of the available disk surface area by RZ. Fig. 4b shows the reusability of NPG disks in different catalytic cycles. The cleaning methodology after every cycle has been documented in the method section. The constant k value for 6 different repeated cycles demonstrates the excellent reusability of NPG disks substrate in RZ to RF catalytic reduction. We note that the cleaning step was required mainly due to the nonflowing situation of our experimental configuration, which is expected to be significantly alleviated with flow.

Fig. 3. (a) Heterogeneous catalysis by NPG disks (330 nm diameter) to convert 16 µM RZ into RZ in the presence of 1.6 mM NH2OH. Inset shows the color change, blue (RZ) to pink (RF). (b) Plot of ln (C/C0) vs time, t. Linear fitting to the data points provide the rate constant, k.

Fig. 4. (a) Variation of rate constant, k with respect to different NH2OH concentrations. (b) Reusability of NPG disks in different catalytic cycles.


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3.2. Comparison of catalytic reduction of RZ to RF in a dark environment vs. laser illumination Since NPG disks are excellent plasmonic substrates 37,40, it is of great interest to investigate the potential enhancement effect to its catalytic performance. Fig. 5a and 5b depict the time dependent in situ Raman spectra of a mixture of 4 mM RZ and 0.4 M NH2OH incubated on 200 nm diameter NPG disks with or without laser illumination, respectively. The laser wavelength (785 nm) was near the LSPR peak (~ 760 nm) but far from any of the molecular absorption peak. The top and bottom Raman spectra in both figures correspond to bulk RZ and RF, respectively. RZ has a unique Raman peak at 514 cm-1, which is absent in RF. NH2OH has a single weak peak at 902 cm-1, which is overshadowed by the strong RZ/RF Raman spectra. Figure 6a shows that the 514 cm-1 peak intensity (I514) starts to decline and disappears completely at 46 min and 17 min for the laser off and on conditions, respectively. Since the reduction of RZ to RF follows the first order rate constant 45,47, I514 can be fitted by a single exponential function of the form of e-kt, where k is the reactant consumption rate constant and t is the reaction time. The fits yield koff = 0.0011 s-1 and kon = 0.0034 s-1 for the laser off and laser on conditions, respectively. The kon can be written as kon = koff + 0.2 kplasmon, where kplasmon is the plasmon enhanced rate constant. The coefficient 0.2 was due to the laser illuminated area only accounted for 20% of the total area with NPG disks. Thus, kplasmon = 5(kon - koff) = 0.0117 = ~10 koff, i.e., LSPR enhances the catalytic reaction rate by ~10 times (Fig. 6c). Without NPG disks, there was no significant change in I514 under the laser on condition, therefore ruling out the possibility of laser alone can trigger the reaction. To further validate the idea, off-resonance condition was set up using NPG disks of ~550 nm diameter under similar experimental conditions. Fig. 6b shows I514 vs. t for both the laser off and on conditions. The koff = 0.0007 s-1 and kon = 0.0016 s-1 values give kplasmon = 0.003 = ~3 koff. This implies that the plasmonic enhancement for ~550 nm NPG disks is ca. one third of that for the ~200 nm ones (Fig. 6c). As shown in previously in Fig. 1d, the plasmon resonance wavelengths for the ~200 and ~550 nm NPG disks locate at 760 and 1700 nm in water, respectively. Since 760 nm resonance wavelength is better aligned with the incident laser (785 nm), the plasmonic enhancement is larger for the ~200 nm NPG disks. The values of kplasmon are significantly larger compared with similar experiment on graphene oxides catalyst where the values of reaction rates are 0.000238 s-1 47.



Fig. 5. Time dependent in situ Raman spectra of mixture of 4 mM RZ and 0.4 M NH2OH on ~200 nm diameter NPG disks under (a) the laser off (laser illuminated only during the 1 sec measurements) and (b) laser on (continuous laser illumination) conditions.


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Fig. 6. Time dependent intensity (peak height) variations in 514 cm-1 Raman peak for (a) ~200 and (b) ~550 nm diameter NPG disks. (c) kplasmon/koff ratio for two different disk diameters.

In the literature, plasmon-enhanced catalysis has been attributed to local heating due to plasmonic photothermal effect, photoexcitation from enhanced electromagnetic field, and direct injection of hot electrons to molecular adsorbates 29. For example, Chen et al. reported visible light (λ > 400 nm) driven oxidation of HCHO using 23 nm Au nanoparticle/ZrO2 catalyst and the enhancement was primarily attributed to the photothermal effect 22. Similarly, Fasciani et al. reported the catalytic reduction of Dicumyl Peroxide using 12.7 nm Au nanoparticles upon 532 nm laser excitation 48. However, Christopher et al. used Ag nanocubes of ~60 nm edge length supported on α-Al2O3 and 590 nm light (~250 mW/cm2) excitation for ethylene epoxidation and found that plasmon-induced hot carriers to be responsible for ~4 times enhancement in the catalytic reaction rate 1. Mukherjee et al. reported hot-electron-induced photo-dissociation of H2 on small Au nanoparticles (13.7 nm) using 450−1000 nm white light laser source (2400 mW/cm2) 49.



To investigate the origin of the observed plasmon-enhanced catalysis by NPG disks, we have conducted a series of experiments at controlled temperature on a hot plate set at 30, 50, and 70 C. In a separate experiment, we found the steady-state temperature rise on ~200 nm diameter NPG disks under continuous laser illumination to be ~7 to 11 C in air. The temperature rise is thus expected to be less than that when the sample is placed in water as previously confirmed by us 28. In other words, the substrate or reactor temperature cannot exceed ~30-33 C in our laser-on experiments. Fig. 7a shows I514 vs. t at different NPG disks substrate temperature under laser off conditions. For comparison, the curve for laser on condition at ambient temperature is also plotted. It should be mentioned that the external temperature is applied to the whole substrate area while the photo thermal phenomenon is much localized to affect only ~20% of the catalytic substrate area. Even so, the reactant consumption rate constant, k at 30 oC (7 C T rise, ambient is 23 C) is significantly lower than that for the laser on condition. The rate constant for the experiment conducted at 70 C is roughly the same as that of the laser-on condition at ambient temperature. In other words, a Delta T of ~50 C is needed to match the plasmon enhancement. These results suggest that hot carriers played a significant role in the observed plasmon-enhanced catalysis.

Fig. 7. (a) Time dependent intensity (peak height) variations in 514 cm-1 Raman peak for ~200 nm diameter NPG disks under the laser off and on conditions. For the laser off condition, the ambient temperature was set at 30, 50, and 70 oC. (b) Input power dependent k variations. Experimentally observed power dependent kplasmon follows linear to superlinear transition. Calculated rate constant from Arrhenius equation, kthermal doesn’t show superlinear behavior. Inset shows the power dependent temperature rise recorded experimentally.

Fig. 7b shows the experimental input power dependent plasmon-enhanced rate constant, kplasmon for the ~200 nm NPG disks. The transition from linear to superlinear dependence of the rate constant on the input laser power is an experimental signature of the hot carrier-driven chemical process. No other mechanism, such as heating, exhibits this behavior 29,48-50. Fig. 7b also shows the rate constant, kthermal, calculated using the Arrhenius equation (details can be found in the supporting information). The kthermal considers only the temperature variations in the catalytic


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reaction and it does not follow the linear-to-superlinear rate transition. Input power dependent temperature rise is experimentally obtained and shown in Fig. 7b inset. Using the total laser power and the area of NPG sample illuminated (~20% of the total sample area that was 1 cm2), together with the SEM image in Fig. 1, the number of disks illuminated can be estimated and the average power per disk can be calculated. Significant amount of work has been done on gold nanoparticle catalyzed resazurin reduction to resorufin using hydroxylamine as the reducing agent. The resazurin typically adsorbs on the gold surface and is converted to resorufin, which is then released through direct dissociation or surface-assisted dissociation. The rate limiting step is often thought to be the dissociation of product resorufin from the gold surface 45. Following above reasoning, the operating principle for plasmon-enhanced reduction of resazurin to resorufin is summarized simplistically in Fig. 8. Under laser-off conditions, the reduction of adsorbed resazurin molecule on NPG disk is initiated by the presence of hydroxylamine, creating a transient population of resorufin which eventually dissociates releasing the product into the bulk solution. Under laser-on conditions, a great density of hot electrons and hot holes are generated in the NPG following plasmon excitation and decay. The hot-holes can accelerate the dissociation of resorufin possibly by weakening the donor interactions of the gold atoms at the surface with resorufin 43. The hot electrons left in the metal surface can interact with the adsorbed resazurin, further accelerating the reduction rate in the process of restoring the charge neutrality of the NPG. We suggest that this markedly increased production of hot electrons and hot holes following LSP decay is the primary reason for the observed 10x fold increase in room temperature catalytic activity. Since no resorufin was observed in the absence of hydroxylamine under laser-on conditions, the photogenerated charge carriers alone produced pursuant to plasmon decay is thermodynamically not energetic to reduce resazurin molecule.

Fig. 8. The operating principle for reduction of resazurin to resorufin on NPG disks under dark

light-off and light-on (plasmon-enhanced) conditions. 4. Conclusion In conclusion, we have reported NPG disks as novel heterogeneous catalyst. The step and kink atoms at the curved Au ligament surfaces have large number of dangling bonds, which are responsible for its high catalytic activity. The catalytic performance of NPG disks can further be



enhanced by utilizing its external light induced plasmonic properties. The plasmon resonance enhances the catalytic reaction rate by 10 times for 200 nm diameter NPG disks. Upon light illumination, the excited LSPR can provide hot charge carries to enhance the catalytic reaction, as supported by the linear-to-superlinear dependence of the plasmon-enhanced catalytic reaction rate on the input laser power, as well as comparing with reactions at elevated temperature. The plasmon-enhanced catalytic performance of NPG disks has the potential in improving energy efficiency for heterogeneous catalysis by harvesting solar energy. Supporting Information X-ray photoelectron spectroscopy (XPS) analysis, Absorption spectra of resazurin (RZ) and resorufin (RF), RZ absorption peak intensity (A602) vs. RZ concentration (C) calibration. Acknowledgement. Wei-Chuan Shih acknowledges the National Science Foundation (NSF) CAREER Award 1151154, NSF 1605683, and NSF 1643391. References (1) Christopher, P.; Xin, H.; Linic, S., Visible-Light-Enhanced Catalytic Oxidation Reactions on Plasmonic Silver Nanostructures. Nat. Chem. 2011, 3 (6), 467-472. (2) Satterfield, C. N., Heterogeneous Catalysis in Industrial Practice. 1991. (3) Bond, G. C., Heterogeneous Catalysis. 1987. (4) Ding, Y.; Chen, M.; Erlebacher, J., Metallic mesoporous nanocomposites for electrocatalysis. J. Am. Chem. Soc. 2004, 126 (22), 6876-6877. (5) Meng, X.; Liu, L.; Ouyang, S.; Xu, H.; Wang, D.; Zhao, N.; Ye, J., Nanometals for Solar‐to‐Chemical Energy Conversion: From Semiconductor‐Based Photocatalysis to Plasmon‐Mediated Photocatalysis and Photo‐Thermocatalysis. Adv. Mater. 2016, 28 (32), 67816803. (6) Biener, J.; Biener, M. M.; Madix, R. J.; Friend, C. M., Nanoporous Gold: Understanding the Origin of the Reactivity of a 21st Century Catalyst made by Pre-Columbian Technology. ACS Catal. 2015, 5 (11), 6263-6270. (7) Xu, C.; Xu, X.; Su, J.; Ding, Y., Research on Unsupported Nanoporous Gold Catalyst for CO oxidation. J. Catal. 2007, 252 (2), 243-248. (8) Ge, X.; Wang, R.; Liu, P.; Ding, Y., Platinum-Decorated Nanoporous Gold Leaf for Methanol Electrooxidation. Chem. Mater. 2007, 19 (24), 5827-5829. (9) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N., Novel Gold Catalysts for the Oxidation of Carbon Monoxide at a Temperature Far Below 0 C. Chem. Lett. 1987, 16 (2), 405-408. (10) Abad, A.; Concepción, P.; Corma, A.; García, H., A collaborative effect between gold and a support induces the selective oxidation of alcohols. Angew. Chem. ,Int. Ed. 2005, 44 (26), 4066-4069. (11) Valden, M.; Lai, X.; Goodman, D. W., Onset of Catalytic Activity of Gold Clusters on Titania with the Appearance of Nonmetallic Properties. Science 1998, 281 (5383), 1647-1650. (12) Daniel, M.-C.; Astruc, D., Gold Nanoparticles: Assembly, Supramolecular Chemistry, Quantum-Size-Related Properties, and Applications Toward Biology, Catalysis, and Nanotechnology. Chem. Rev. 2004, 104 (1), 293-346.


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Enhanced Heterogeneous Nanocatalysis on a Nanoporous Gold Disk

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