Carrier Diffusion - The Main Contribution to Size-Dependent

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Carrier Diffusion - The Main Contribution to Size-Dependent Photocatalytic Activity of Colloidal Gold Nanoparticles Ziliang Mao, Hnubci Vang, Anthony Garcia, Anargul Tohti, Benjamin J. Stokes, and Son C. Nguyen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00390 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 1, 2019

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Carrier Diffusion - The Main Contribution to SizeDependent Photocatalytic Activity of Colloidal Gold Nanoparticles Ziliang Mao, Hnubci Vang, Anthony Garcia, Anargul Tohti, Benjamin J. Stokes, and Son C. Nguyen* Department of Chemistry and Chemical Biology, University of California Merced, 5200 North Lake Road, Merced, California 95343, United States

ABSTRACT: Harnessing hot carriers from photoexcited metallic nanoparticles for catalysis is very challenging because these carriers have extremely short lifetimes. Here, we demonstrate that smaller particles have higher surface-to-volume ratios that allow hot carriers to diffuse to particle surfaces with a higher probability and thereby exhibit higher photocatalytic activities as quantified by quantum yields. The measured photocatalytic activities for photoinduced etching of gold nanospheres by FeCl3 and the previously unreported aqueous hydrogenation of styrene using sodium borohydride under interband excitation show perfect dependence on the reciprocal of particle size. The size dependent photocatalytic activity for photoinduced etching of gold nanospheres by FeCl3 under plasmon excitation, however, slightly deviates from this scaling law and may be influenced by other factors such as the surface field enhancement effect. This scaling law is expected to apply to other nanomaterial-based photocatalysts that rely on hot carrier

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diffusion to a surface for catalysis. Future design of nanomaterials for the harnessing of hot carriers for catalysis should take this scaling law into account.

KEYWORDS: Photocatalysis, gold nanoparticles, size-dependence, interband, plasmon, hot carriers, styrene hydrogenation, etching Introduction Noble metal nanoparticles have recently received a lot of interest for photocatalysis of various chemical reactions.1-9 These particles generate hot carriers through the optical excitation of their localized surface plasmon resonances or interband transitions. In many demonstrations, these hot carriers have been implicated as driving catalytic reactions via electron (or hole) transfer mechanisms.3, 8 In the course of catalysis, these nascent carriers are scattered inside the particles, then reach the particle surface and activate chemical transformation of adsorbed reactants before decaying within a few picoseconds.10 Theoretical studies have estimated that the scattering mean free path of hot carriers in gold and silver nanoparticles can cover tens of nanometers,11-12 and carriers closer to the particle surface than the mean free path can participate in photocatalysis.13 We hypothesize that small particles will allow hot carriers to move to the surface with higher probability and show higher catalytic activity than larger particles. The influence of gold nanoparticle (GNP) size on catalytic activity has been reported and with somewhat conflicting results. For non-photoexcited (thermally-driven or dark) reactions, it is generally thought that the particle catalytic activity is proportional to the total surface area of the GNPs whereas the chemical reactivity per unit surface area generally increases as the GNP size decreases, probably due to the increased surface roughness or the increased number of undercoordinated reactive sites on the surface.14-16 However, in some reactions, groups have reported


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contradictory results17 or optimal sizes for highest catalytic activities.18-19 As for photocatalytic activity (i.e., catalytic activity under photoexcitation of the catalyst), most of the size-dependent studies have been conducted on GNPs supported by metal oxides, such as TiO2, Al2O3, or ZnO, whereby photoexcitation of the metal oxides generates hot carriers while the GNPs act as electron reservoirs.20-23 The Schottky barrier between the metal oxide and the GNPs helps prevent electron backflow, thereby facilitating charge separation. Only a few studies have invoked direct photoexcitation of GNPs.23-25 Regardless of the photoexcitation pathway, smaller particles were generally reported to exhibit higher photocatalytic efficiency,20-21,

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although a few studies

indicate the opposite case,22 and the mechanism is not clear. Possible reasons for these conflicting results, besides the above-mentioned surface roughness and number of under-coordinated surface reaction sites, include differences between specific reaction mechanisms, the species that are interacting with the nanoparticles, the size and shape uniformity of the embedded GNPs, the influence of the perimeter or interface of GNPs and metal oxide supports,26 and the fact that the photocatalytic activities were not quantified in a uniform way. Although substrate-supported systems may afford better charge separation, the overall reaction mechanism is much more complicated than that of bare/colloidal particles. As for the means of quantifying photocatalytic activity, while particles of a certain size may exhibit overall high or low photocatalytic reaction rates (or yields), what offers a fair comparison is the photocatalytic activity per absorbed photon, i.e., the quantum yield. For metal oxide supported GNPs, the strong scattering of light by these catalysts prohibits accurate determination of quantum yields. Thus, we selected colloidal gold nanoparticles with uniform size and shape to evaluate size-dependence by eliminating any influence of supporting or heterojunction materials and light scattering effects.27


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In this work, we systematically evaluated photocatalytic activities of colloidal gold nanoparticles in model inorganic and organic reactions (Figure 1A) by correlating quantum yields with particle size. We’ve also studied the corresponding etching of gold nanoparticles in the dark in parallel for comparison to the photocatalytic reactions. The photocatalysts were prepared with uniform spherical shape and narrow size distribution (Figure 1 B and C, also see Supporting Information Figure S1 and S2, and Table S1).28 By sampling particle sizes ranging from 10 to 42 nm in diameter, a clear relationship between particle size and quantum yield has been established for two representative excitations: the 5d-to-6sp interband transition (using a monochromatic light source centered at 405 or 440 nm) and the localized surface plasmon resonance (530 nm). By using relatively small particles, the light scattering effect is also minimized.27 Results and Discussions The first reaction we studied was the photoinduced oxidative etching of gold nanospheres (GNSs) with FeCl3 (Figure 1A, left), which is a self-catalyzed reaction in which GNSs play the role of both catalyst and reactant. This reaction is a good model for evaluating photocatalytic activity because the role of each carrier is well established: hot electrons reduce Fe3+ and hot holes oxidize the gold nanoparticles.2 This etching reaction happens very slowly under dark conditions, and can therefore be systematically compared to photocatalyzed reactions to provide more insight to the photocatalytic mechanism.


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Figure 1. (A) Two model reactions catalyzed by photoexcited colloidal gold nanoparticles: photooxidative etching of gold nanoparticles by Fe3+, and aqueous hydrogenation of styrene using NaBH4. (B) Representative TEM image of the 34.0 nm GNSs used in this study. (C) The size distribution of the 34.0 nm GNSs. The red solid line is the Gaussian fitting of the distribution. The average size and standard deviation are labelled in the figure. TEM images and size distributions of GNSs of other sizes are provided in the Supporting Information. The quantum yield of the etching reaction can be determined by monitoring GNSs’ plasmon absorption band intensity through UV-Vis spectroscopy since the molar extinction coefficient of a GNS at the plasmon peak is proportional to its volume (and thus the amount of gold).29 Tracking the size of the GNSs at various time-points by dynamic light scattering or transmission electron microscopy is possible but will require tremendous amount of measurements and is not practical in this study due to the large amount of data collected. The concentrations of the reactants were chosen so that the reaction resembles a pseudo-zero-order reaction in the early time scale (Figure S3), although the overall reaction rate has been characterized as first order of Fe3+ concentration.2 Thus, the photocatalytic activity (quantum yield) can be directly quantified through the reaction


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rate (see Supporting Information for details). Figure 2A, C, and E show the spectral evolution over time for dark, 530 nm, and 440 nm irradiation conditions, respectively. To establish a comparison of reactivities for GNSs of different sizes, the photocatalyzed reaction activities were quantified using quantum yields (the number of molecules converted by one absorbed photon), whereas those of the dark reactions were quantified by turnover frequency (TOF, the number of molecules converted per unit time by one GNS, see Supporting Information for details). Consistent with previous reports,2, 5, 9, 30 etching occurs more rapidly via interband excitation than via plasmon excitation, which in turn reacts several orders of magnitude more rapidly than under dark conditions. A simple explanation for this phenomenon is that interband excitation generates hot carriers more efficiently and these carriers have a longer lifetime.2, 10 This observed wavelength dependence stands in contrast to other studied reactions wherein plasmon excitation resulted in higher catalytic efficiency,25 which implies that specific reaction mechanisms and energetics (such as activation energies) may play an important role in determining photocatalytic activities.6, 31


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Figure 2. Representative time-dependent absorption spectra of the etching reactions of 27.6 nm gold nanoparticles by Fe3+ under dark (A), plasmon excitation (C, 530 nm, 92.7 mW), and interband excitation (E, 440 nm, 13.7mW) at accumulated irradiation times, along with the size dependence of the turnover frequency or quantum yields (B, D, F for dark, 530 nm, and 440 nm, respectively). The red solid lines in B, D, and F are fittings to the size-dependence data (open circles) using power functions (blue legends). Based on our hypothesis that hot carrier diffusion to the surface of the nanoparticles is the main contributing factor to the photocatalytic activities, we propose that the size-dependent quantum yields for the photoreactions would be proportional to the reciprocal of the particle diameter, which reflects the probability of carriers diffusing from the inside to the surface of the particle since the reciprocal of particle diameter is, for a sphere, proportional to the surface-to-volume ratio. To test


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whether this hypothesis could be invoked to explain the size-dependent photocatalytic activity, the quantum yields (Table S2 and S3) were fitted with a power function of the particle diameter. Indeed, the quantum yields under interband excitation are fitted well to a power function of particle diameter (x-0.967, where x represents the particle diameter) that is close to the reciprocal of the diameter with a high coefficient of determination (R2 = 0.95, Figure 2F). However, the fitting for the plasmonic excitation condition (x-0.823, R2 = 0.89) slightly deviates from the x-1 relation (Figure 2D). The close fitting of the interband data (Figure 2F) to x-1 indicates that the quantum yield is proportional to the reciprocal of the particle diameter. Thus, our afore-mentioned hypothesis could be invoked to explain the size-dependent quantum yield observed under interband excitation. The deviation from the x-1 relation for the plasmonic excitation data (Figure 2D), however, points towards other possible contributing factors to the size dependence observed. It is well known that under plasmonic excitation condition, the electric field at the nanoparticle surface tends to be enhanced.13, 32-34 This enhancement effect is generally thought to be stronger for smaller particles or particles with shaper features,32-33 but can be more complicated due to the influence of material, shape, surrounding medium, and other electromagnetic effects.34-35 Deeb et al.34 systematically examined the size-dependent field enhancement effect of gold nanospheres in the size range of 10 nm to 90 nm. Through both electrodynamic simulations and single-nanoparticle photochemical imaging, they characterized the size tunability of the plasmonic near-field of gold nanospheres. They found that the surface enhancement effect increases as the size increases from 10 nm to 50 nm and decreases when the size continues increasing beyond 50 nm. They attributed the “volcano” trend to the interplay of radiative damping, surface damping, and electrodynamic retardation. Based on this study, the surface field enhancement in the range of 10-40 nm as was in our study would increase as the size increases, and thus lead to more photo-absorption and more


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hot carrier generation closer to the surface for relatively larger particles and contribute to relatively higher photocatalytic activity due to the shorter diffusion distance for the hot carriers to reach the surface. This could explain the slight deviation of our fitting (x-0.823) to the plasmonic data from x-1, because it could be a convolution of the hot carrier diffusion effect and the field enhancement effect. To unravel exactly how much the carrier diffusion contribution is under plasmonic excitation requires more studies about the field enhancement effect as well as other possible contributing factors as is mentioned later in the article. This is out of the scope of this work but can be a direction for future work. For the dark reactions, the TOF is proportional to the second power of the GNS diameter (Figure 2B). This observation supports the argument that the dark reaction rate for an individual particle is approximately proportional to the particle surface area.36-38 The TOFs normalized by the surface area of the GNSs were also compared (Figure 3 and Table S5) and show that the TOF per unit surface area does not change much as the GNS size changes.

Figure 3. Turnover frequency (TOF) of the etching of GNSs under dark conditions normalized to the surface area of the GNSs of different sizes.


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To investigate whether the same trend exist for organic reactions, aqueous hydrogenation of styrene using NaBH4 was chosen as a model organic reaction (Figure 1A, right). The hydrogenation of C=C double bonds is critically significant to pharmaceutical, petrochemical, and food industries. The catalytic hydrogenation of styrene is generally carried out using hydrogen gas as the source of hydrogen atoms at elevated pressure or temperature in organic solvents.39-40 Huang and coworkers recently reported the hydrogenation of styrene using formic acid as a hydrogen atom source and ZrO2-supported GNPs as the catalyst, but the reaction rate was low.41 The use of formic acid in our system also proceeded too slowly for convenient kinetic analysis. However, we found sodium borohydride to be a competent source of H atoms for GNS-catalyzed aqueous hydrogenation of styrene (a reaction that has not previously been reported). Styrene has a very strong absorption at 246 nm, while the hydrogenation product (ethylbenzene, Figure S5-7) has a very weak absorption near this region (Figure S4), thus the kinetic studies can be confidently performed by monitoring the 246 nm peak via UV-Vis spectroscopy. A control photoreaction without GNSs showed that there is a negligible amount of ethylbenzene formed (Figure S8). Transmission electron microscopy (TEM) showed that the GNSs were preserved during the reaction (Figure S9). Comparably, the absorbance of the GNSs (~525 nm) in Figures 4A do not change during the reaction, indicating the stability of the catalyst. Recycling the catalysts through eight reactions showed that the photocatalyst maintains its activity well (Figure 4C). As with the photoinduced etching of GNSs, the concentrations of the reactants were chosen so that the hydrogenation reaction resembles a pseudo-zero-order reaction (Figure S10). Figures 4A shows the time-dependent evolution of the styrene absorption peak under interband excitation. The observed quantum yields can be fitted very well to a power function of particle sizes (Figure 4B), again reflecting the probability of carriers diffusing from the inside to the surface


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of the particle. Unlike the etching experiment, the hydrogenation of styrene proceeds extremely slowly under plasmon excitation and dark conditions. The reaction under plasmon excitation is only slightly faster than under dark conditions (Figure S5 and S11). Therefore, the size dependence study only probed interband excitation. The influence of dark reactions (Figure S11) and possible side reactions, such as the slow free-radical-initiated addition polymerization of styrene to form polystyrene,42can be ignored for the case of interband excitation (Figure S12). Again, this reaction represents another case in which interband excitation of metallic nanoparticle catalysts is more efficient in catalyzing reactions.

Figure 4. (A) Representative time-dependent absorption spectra for the styrene hydrogenation reactions under interband excitation (405 nm, 415 mW) using 11.2 nm gold nanospheres, along with (B) the size dependence of the quantum yields. The red solid line in B shows the nonlinear fitting of the size-dependence data (open circles) using power functions (blue legends). (C) Recyclability of the GNSs for the hydrogenation of styrene using 32.8 nm GNSs as example. The reaction time for each test was ten minutes. Based on these results, two possible catalytic mechanisms for the photoinduced hydrogenation of styrene using NaBH4 are proposed. Hydrogen atoms attach to the surface of the GNSs, and styrene molecules adsorb to the surface of GNSs.37, 43-44 Hydrogen atom attachment need not be photocatalyzed nor rate-limiting, as previously reported hydrogenations of nitrophenol using


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sodium borohydride and gold nanoparticles proceed very fast under non-photoexcited conditions.37 This is further confirmed by the control experiment wherein a hydrogen species abstractor (TEMPO)41, 45 was added to abstract hydrogen atoms from the GNS surface—only a trace amount of ethylbenzene was formed under interband excitation (Figure S13). Upon photoexcitation, the hot carriers generated inside the GNSs migrate to the surface of the GNSs to facilitate the hydrogenation of styrene to ethylbenzene through two possible mechanisms: (1) the hot electrons directly activate the surface hydrogen species to attack the double bond of the styrene to form ethylbenzene in a mechanism similar to the Horiuti-Polanyi mechanism,46 but accelerated by the hot electrons, or (2) the hot electrons first transfer to the adsorbed styrene molecules and excite the non-aromatic double bonds to the π* antibonding orbital, and then return back to the GNSs, leaving the non-aromatic double bonds vibrationally activated and eventually leading to π bond cleavage.25, 47 The two carbon atoms of the non-aromatic double bond could then react with hydrogen atoms (either present on the GNS surface, or through an outer sphere mechanism) to form ethylbenzene. Similar mechanisms have been proposed before where transient exchange of hot carriers between nanoparticles and adsorbates leads to vibrational energy deposition to the adsorbates and eventually lead to chemical transformations.25, 47-50 Therefore, the role of a hot carrier could be facilitation of the transfer of the surface hydrogen species to styrene, or activation of the non-aromatic double bond of styrene, instead of directly participating in a redox reaction similar to the photoinduced etching of GNSs using Fe3+. Similar to the GNS etching reactions, smaller particles show higher photocatalytic activities in the hydrogenation of styrene. It should be mentioned that the increase in temperature of the GNSs and their local environment after each single photon absorption is very small under our experimental conditions (continuous light source, not pulsed laser),2,

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thus the photothermal enhancement of reaction rate can be


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ignored. The accumulated temperature rise of the entire reaction solution is also very small (~1 C) during the reaction. A general photocatalytic mechanism for GNSs can be postulated based on all these results and discussions. Under light irradiation, hot carriers are generated inside the particles. They then diffuse to the surface of the GNSs through multiple scatterings and catalyze chemical transformation of the reactant molecules adsorbed on the particle surfaces (Scheme 1). The probability of carriers reaching the surface of a spherical nanoparticle for catalysis is proportional to the ratio of the surface area over the volume of the nanoparticles. This ratio is proportional to the reciprocal of the particle diameter. This, we propose, explains the relationship that was observed between GNS’s photocatalytic activities and particle sizes. This relationship also suggests that the diffusion of hot carriers to the surface is the main contributing factor to the sizedependent photocatalytic activities observed. Whereas this relationship can be invoked for interband excitation, the case for plasmon excitation is more complicated and could involve other potential contributing factors such as the surface field enhancement effect34 and deserve further study.

Scheme 1. Proposed mechanism for photocatalysis of gold nanoparticles. The probability of carriers diffusing to the surface is higher for smaller particles, resulting in higher photocatalytic activities.


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Previous theoretical studies predicted that bigger particles generate hot carriers more efficiently while their energy state distributes closer to the Fermi level (more lower energy carriers),52 while smaller particles generate hot carriers with much broader energy distribution (more higher energy carriers).53 These predictions align with our observations. However, estimating the energy of hot carriers and correlating it to experimental activity are still challenging because the energy thresholds necessary for hot carriers to catalyze the two model reactions are still unknown. In this study, our photoreaction data are fitted well to the reciprocal of particle size, indicating that carrier diffusion is probably the most reasonable explanation for the size dependence trend. We rule out the possibility that smaller particles’ lower packing of surface ligands (such as CTAB) and thus larger space for reactants to diffuse and adsorb to the surface is responsible for the observed size dependence trend. Under our experimental condition, each GNS absorbs one photon every several microseconds (Supporting Information), while the hot carriers have a lifetime of only several picoseconds.10 Therefore, each GNS spends most of its time in the non-photoexcited state during which the reactants can diffuse and adsorb to the particle surface. Due to this long resting state and the high surface coverage (vide infra), reactants are always ready at the surface when hot carriers reach the surface for photocatalysis. Therefore, the diffusion of reactants to the particle surface should not be the rate-limiting step. We also rule out the possibility that the density of reactants on the particle surface may change when we change particle size, and eventually interfere with our interpretation of the observed size dependence trend. For the etching experiment, the TOF per surface area under dark condition does not change when the size of the GNSs changes (Figure 3). Considering that the concentration of Fe3+ is much higher (≥ 106 times) than that of the GNSs, the surface coverage of Fe3+ on GNSs should be constant across different sizes of samples.54 Similarly,


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for the hydrogenation of styrene, the concentration of styrene is more than one million times that of the GNSs, therefore, the density of the surface adsorbed styrene on the GNSs can be treated as a constant as well when moving from one particle size to another.54 Conclusion In conclusion, the size dependence of the photocatalytic activities of GNSs has been studied systematically in the photoinduced etching of GNSs using Fe3+ and in the photocatalyzed hydrogenation of styrene using sodium borohydride. Two possible reaction mechanisms were proposed for the hydrogenation of styrene. The photocatalyzed reaction rate decreases as the GNS size increases and is largely proportional to the reciprocal of the particle size, with the trend under plasmonic excitation slightly deviating from this trend. The TOF of the etching of GNSs under non-photoexcited conditions increases as the second power of the GNS diameter, suggesting that the TOF is proportional to the total surface area of the GNSs. The present results not only strengthen the widely held hypothesis that catalytic activities originate from hot carriers, but also establishes a scaling law for predicting activities. We expect that this scaling law will be applicable to other colloidal metal nanoparticle catalysts where the hot carriers need to diffuse to the particle surface for catalysis. This law provides some guidance for improving efficiency of harvesting hot carriers, and ultimately the overall efficiency of nanoparticle photocatalysis.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Materials and methods, control experiment data, reaction rate and quantum yield calculations, Figure S1-S13, Tables S1-S5 (PDF).


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AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions Z.M. and S.C.N. initiated the research. Z.M., A.G. and H.V. developed and performed the sizedependence study and recyclability tests using UV-Vis spectroscopy, Z.M. and A.T. performed the GC-MS analysis. Z.M. performed TEM characterization of the GNS particles, analyzed the data and wrote the main text of the manuscript with S.C.N. through contributions from all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was funded by the University of California, Merced (UC Merced). We thank the Imaging and Microscopy Facility of UC Merced for the use of the JEOL JEM-2010 TEM. A.G. acknowledges research fellowships from UC LEADS (funded by the University of California Office of the President) and the Merced Nanomaterials Center for Energy and Sensing (funded by NASA grant no. NNX15AQ01A). ABBREVIATIONS GC-MS, gas chromatography – mass spectrometry; GNP, gold nanoparticle; GNS, gold nanosphere; QY, quantum yield; TEM, transmission electron microscope; TEMPO, 2,2,6,6Tetramethyl-1-piperidinyloxy; TOF, turnover frequency; UV-Vis, ultra violet – visible spectroscopy.


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33. Chavez, S.; Aslam, U.; Linic, S., Design Principles for Directing Energy and Energetic Charge Flow in Multicomponent Plasmonic Nanostructures. ACS Energy Lett. 2018, 3, 1590-1596. 34. Deeb, C.; Zhou, X.; Plain, J.; Wiederrecht, G. P.; Bachelot, R.; Russell, M.; Jain, P. K., Size Dependence of the Plasmonic near-Field Measured Via Single-Nanoparticle Photoimaging. J. Phys. Chem. C 2013, 117, 10669-10676. 35. Schatz, G. C., Theoretical-Studies of Surface Enhanced Raman-Scattering. Acc. Chem. Res. 1984, 17, 370-376. 36. Mei, Y.; Sharma, G.; Lu, Y.; Ballauff, M.; Drechsler, M.; Irrgang, T.; Kempe, R., High Catalytic Activity of Platinum Nanoparticles Immobilized on Spherical Polyelectrolyte Brushes. Langmuir 2005, 21, 12229-12234. 37. Herves, P.; Perez-Lorenzo, M.; Liz-Marzan, L. M.; Dzubiella, J.; Lu, Y.; Ballauff, M., Catalysis by Metallic Nanoparticles in Aqueous Solution: Model Reactions. Chem. Soc. Rev. 2012, 41, 5577-5587. 38. Carregal-Romero, S.; Perez-Juste, J.; Herves, P.; Liz-Marzan, L. M.; Mulvaney, P., Colloidal Gold-Catalyzed Reduction of Ferrocyanate (Iii) by Borohydride Ions: A Model System for Redox Catalysis. Langmuir 2010, 26, 1271-1277. 39. Corvaisier, F.; Schuurman, Y.; Fecant, A.; Thomazeau, C.; Raybaud, P.; Toulhoat, H.; Farrusseng, D., Periodic Trends in the Selective Hydrogenation of Styrene over Silica Supported Metal Catalysts. J. Catal. 2013, 307, 352-361. 40. Li, J. W.; Bin, Z.; Chen, Y.; Zhang, J. K.; Yang, H. M.; Zhang, J. W.; Lu, X. L.; Li, G. C.; Qin, Y., Styrene Hydrogenation Performance of Pt Nanoparticles with Controlled Size Prepared by Atomic Layer Deposition. Catal. Sci. Technol. 2015, 5, 4218-4223. 41. Huang, Y. M.; Liu, Z.; Gao, G. P.; Xiao, Q.; Martens, W.; Du, A. J.; Sarina, S.; Guo, C.; Zhu, H. Y., Visible Light-Driven Selective Hydrogenation of Unsaturated Aromatics in an Aqueous Solution by Direct Photocatalysis of Au Nanoparticles. Catal. Sci. Technol. 2018, 8, 726734. 42. Goldfinger, G.; Skeist, I.; Mark, H., On the Mechanism of Inhibition of Styrene Polymerization. J. Phys. Chem. 1943, 47, 578-587. 43. Guella, G.; Patton, B.; Miotello, A., Kinetic Features of the Platinum Catalyzed Hydrolysis of Sodium Borohydride from B-11 Nmr Measurements. J. Phys. Chem. C 2007, 111, 1874418750. 44. Liu, B. H.; Li, Z. P., A Review: Hydrogen Generation from Borohydride Hydrolysis Reaction. J. Power Sources 2009, 187, 527-534. 45. Roth, J. P.; Yoder, J. C.; Won, T. J.; Mayer, J. M., Application of the Marcus Cross Relation to Hydrogen Atom Transfer Reactions. Science 2001, 294, 2524-2526. 46. Zhao, X. J.; Zhao, Y.; Fu, G.; Zheng, N. F., Origin of the Facet Dependence in the Hydrogenation Catalysis of Olefins: Experiment and Theory. Chem. Commun. 2015, 51, 1201612019. 47. Christopher, P.; Xin, H. L.; Linic, S., Visible-Light-Enhanced Catalytic Oxidation Reactions on Plasmonic Silver Nanostructures. Nat. Chem. 2011, 3, 467-472. 48. Rao, V. G.; Aslam, U.; Linic, S., Chemical Requirement for Extracting Energetic Charge Carriers from Plasmonic Metal Nanoparticles to Perform Electron-Transfer Reactions. J. Am. Chem. Soc. 2019, 141, 643-647. 49. Christopher, P.; Xin, H. L.; Marimuthu, A.; Linic, S., Singular Characteristics and Unique Chemical Bond Activation Mechanisms of Photocatalytic Reactions on Plasmonic Nanostructures. Nat. Mater. 2012, 11, 1044-1050.


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50. Kale, M. J.; Avanesian, T.; Christopher, P., Direct Photocatalysis by Plasmonic Nanostructures. ACS Catal. 2014, 4, 116-128. 51. Nguyen, S. C.; Zhang, Q.; Manthiram, K.; Ye, X. C.; Lomont, J. P.; Harris, C. B.; Weller, H.; Alivisatos, A. P., Study of Heat Transfer Dynamics from Gold Nanorods to the Environment Via Time-Resolved Infrared Spectroscopy. ACS Nano 2016, 10, 2144-2151. 52. Manjavacas, A.; Liu, J. G.; Kulkarni, V.; Nordlander, P., Plasmon-Induced Hot Carriers in Metallic Nanoparticles. ACS Nano 2014, 8, 7630-7638. 53. Govorov, A. O.; Zhang, H.; Gun'ko, Y. K., Theory of Photoinjection of Hot Plasmonic Carriers from Metal Nanostructures into Semiconductors and Surface Molecules. J. Phys. Chem. C 2013, 117, 16616-16631. 54. Davis, M. E.; Davis, R. J., Heterogeneous Catalysis. In Fundamentals of Chemical Reaction Engineering, 1 ed.; McGraw-Hill: New York, NY, 2003; pp 133-177.

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Figure 1. (A) Two model reactions catalyzed by photoexcited colloidal gold nanoparticles: photo-oxidative etching of gold nanoparticles by Fe3+, and aqueous hydrogenation of styrene using NaBH4. (B) Representative TEM image of the 34.0 nm GNSs used in this study. (C) The size distribution of the 34.0 nm GNSs. The red solid line is the Gaussian fitting of the distribution. The average size and standard deviation are labelled in the figure. TEM images and size distributions of GNSs of other sizes are provided in the Supporting Information. 84x70mm (300 x 300 DPI)


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Figure 2. Representative time-dependent absorption spectra of the etching reactions of 27.6 nm gold nanoparticles by Fe3+ under dark (A), plasmon excitation (C, 530 nm, 92.7 mW), and interband excitation (E, 440 nm, 13.7mW) at accumulated irradiation times, along with the size dependence of the turnover frequency or quantum yields (B, D, F for dark, 530 nm, and 440 nm, respectively). The red solid lines in B, D, and F are fittings to the size-dependence data (open circles) using power functions (blue legends). 84x121mm (300 x 300 DPI)


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Figure 3. Turnover frequency (TOF) of the etching of GNSs under dark conditions normalized to the surface area of the GNSs of different sizes. 68x52mm (300 x 300 DPI)


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Figure 4. (A) Representative time-dependent absorption spectra for the styrene hydrogenation reactions under interband excitation (405 nm, 415 mW) using 11.2 nm gold nanospheres, along with (B) the size dependence of the quantum yields. The red solid line in B shows the nonlinear fitting of the size-dependence data (open circles) using power functions (blue legends). (C) Recyclability of the GNSs for the hydrogenation of styrene using 32.8 nm GNSs as example. The reaction time for each test was ten minutes. 152x43mm (300 x 300 DPI)


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Scheme 1. Proposed mechanism for photocatalysis of gold nanoparticles. The probability of carriers diffusing to the surface is higher for smaller particles, resulting in higher photocatalytic activities. 84x34mm (300 x 300 DPI)


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TOC graphic 67x44mm (300 x 300 DPI)


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Carrier Diffusion - The Main Contribution to Size-Dependent

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