Ultrafast and Efficient Transport of Hot Plasmonic Electrons by

Feb 8, 2016 - *E-mail: [email protected]. .... Insight into plasmonic hot-electron transfer and plasmon molecular drive: new dimensions in energy conv...
0 downloads 0 Views 2MB Size
Download PDF
Subscriber access provided by ORTA DOGU TEKNIK UNIVERSITESI KUTUPHANESI

Communication

Ultrafast and Efficient Transport of Hot Plasmonic Electrons by Graphene for Pt Free, Highly Efficient Visible-Light Responsive Photocatalyst Dinesh Kumar, Ahreum Lee, Taegon Lee, Manho Lim, and Dong-Kwon Lim Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b04764 • Publication Date (Web): 08 Feb 2016 Downloaded from http://pubs.acs.org on February 10, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Nano Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 15

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

Nano Letters

Ultrafast and Efficient Transport of Hot Plasmonic Electrons by Graphene for Pt Free, Highly Efficient Visible-Light Responsive Photocatalyst Dinesh Kumar,† Ahreum Lee,† Taegon Lee,‡ Manho Lim‡ and Dong-Kwon Lim*,† †

KU-KIST Graduate School of Converging Science and Technology, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 136-701, South Korea. ‡ Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan, 609-735 South Korea. ABSTRACT: We report that reduced graphene-coated gold nanoparticles (r-GO-AuNPs) are excellent visible-light–responsive photocatalysts for the photoconversion of CO2 into formic acid (HCOOH). The wavelength-dependent quantum and chemical yields of HCOOH shows a significant contribution of plasmon-induced hot electrons for CO2 photoconversion. Furthermore, the presence and reduced state of the graphene layers are critical parameters for the efficient CO2 photoconversion because of the electron mobility of graphene. With an excellent selectivity toward HCOOH (>90%), the quantum yield of HCOOH using r-GO-AuNPs is 1.52%, superior to that of Pt-coated AuNPs (quantum yield: 1.14%). This indicates that r-GO is a viable alternative to platinum metal. The excellent colloidal stability and photocatalytic stability of r-GO-AuNPs enables CO2 photoconversion under more desirable reaction conditions. These results highlight the role of reduced graphene layers as highly efficient electron acceptors and transporters to facilitate the use of hot electrons for plasmonic photocatalysts. The femtosecond transient spectroscopic analysis also shows 8.7 times higher transport efficiency of hot plasmonic electrons in r-GO-AuNPs compare with AuNPs. KEYWORDS: Plasmonic nanoparticles, Hot electron, Photochemical reaction, Visible light irradiation, CO2 Photoconversion

Nature uses sunlight to recycle CO2 to hydrocarbons. This reduces atmospheric CO2 levels and stores energy at the same time.1 Developing highly efficient artificial photoconversion systems using visible-light-responsive photocatalysts is currently a challenging subject and a field of active research.1– 5 For many years, titania (TiO2) has been investigated for use as a photocatalyst for water splitting. However, titania only responds to UV light because of its wide band gap.6 Consequently, numerous engineering strategies for TiO2 nanomaterials using metals (i.e., Rh, Pd, Pt, Cu, Ru, CdSe, Au, and Ag),6–11 supporting materials (i.e., zeolite, silica, and graphene),12,13 or nanostructures14–18 have been investigated. Among these strategies, designing photocatalysts with plasmonic nanostructures (i.e., gold nanoparticles (AuNPs) and gold nanorods (AuNRs)) is a compelling approach to generate visible-lightresponsive photocatalysts. It is based on the direct transfer mechanism of hot electrons from AuNPs to the photocatalyst or the facilitated generation of electron-hole pairs in the photocatalyst by the enhanced near field on plasmonic nanoparticles.19–22 Recently, both AuNRs covered with Pt nanoparticles (PtNPs)23 and AuNRs covered with a TiO2 layer and PtNPs24 have been suggested as visible-lightresponsive photocatalysts for efficient water splitting.23 Although Pt is the most widely used active catalyst because of its excellent catalytic property,10 its scarcity and high cost prevent its use as a photocatalyst for solar fuels.25 For CO2 photoconversion with visible light, p-type semiconductors and ruthenium complex hybrids26 or carbon nanoparticles doped with Au and/or Pt27 have been investigated to convert CO2 into ACS Paragon Plus Environment

Nano Letters

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

Page 2 of 15

HCOOH using visible light. However, the selectivity to specific hydrocarbons and the conversion efficiency of solar energy to chemical energy are critical issues that need to be addressed properly.26,27 The selectivity is important, because it is strongly associated with chemical yield and cost to obtain pure target product. In addition, HCOOH is an ideal hydrogen storage material for future applications, since HCOOH has a volumetric hydrogen density of 53g of H2 per liter, a low-toxicity and is a liquid under ambient conditions.28 Nonradiative surface plasmon decay involves the generation of hot electrons. These can induce photochemical reactions in the adsorbed molecules on the nanoparticle surface.29-32 The photochemical reactions include the breaking of chemical bonds (H2, D2)29 or water splitting,30-32 involving a multistep reduction–oxidation cycle.24 The intrinsic ultrafast relaxation of hot electrons in the metal (90%), which is an important aspect for practical solar fuel systems. 5,26,48,49 The quantum and chemical yields of HCOOH obtained from the Pt-AuNPs were 1.14% and 1.97%, respectively, and those of HCOOH obtained from the r-GOAuNPs were 1.52% and 2.61%, respectively. These results indicate that the r-GO layer is a promising alternative for Pt, which is currently the most widely used co-catalyst for various photoconversion systems.7 To clearly understand the structural contributions of the plasmonic nanoparticle and the chemical state of graphene on the photocatalytic activities, the CO2 photoconversion efficiencies were investigated with a Xe lamp, providing a visible light source, and an 808 nm laser, providing an NIR light source. As summarized in Figure 2A, AuNPs without a graphene layer showed no noticeable photoproduct when irradiated with the Xe lamp. In contrast, the GO-AuNPs produced little HCOOH (quantum yield: 0.126%, chemical yield: 0.19%). r-GO alone (OD 1.0 at 270 nm, 0.125 mg/mL) showed moderate photocatalytic activity (quantum yield: 0.2%, chemical yield: 0.34%) because of its band gap of 2.4–4.3 eV, which is suitable for water splitting.32,33,50–52 The quantum and chemical yields of r-GO AuNPs for HCOOH were 7.6 times higher than that of r-GO alone. However, when the NIR laser was used, neither AuNRs nor GO-AuNRs showed noticeable HCOOH formation, in spite of the resonant plasmon bands of the AuNRs at 808 nm because of low light energy of NIR compared with visible light. As a control, r-GO-AuNPs were illuminated with NIR and showed no noticeable HCOOH formation; this is due to the nonresonant plasmonic absorption of r-GO-AuNPs at NIR wavelengths (data not shown here). When r-GO-AuNRs are irradiated with an NIR laser, HCOOH was formed with quantum and chemical yields of 0.33% and 0.62%, respectively. r-GO alone did not show any photoproduct because of the low power density of the NIR laser. These results highlight that the high quantum yield of the r-GO-AuNPs originates not only from the plasmonic effects but also from the presence of the graphene layer. The strong dependence of the photocatalytic activity on the reduced state of the graphene layer is indicative of the importance of the high electron acceptance and mobility of graphene.53,54 The effects of the reaction time, pH, and concentration of r-GO-AuNPs were investigated with visible-light illumination. For reaction time studies, we used distilled water and an OD of 1.0 at 540 nm. As the reaction time was increased from 30 min to 3.0 h, the quantum yield increased from 0.29% to 1.52% and the chemical yield increased from 0.48% to 2.61% (Figure 2B). However, after 3.0 h, there was no significant increase in either quantum yield or chemical yield. The photochemical reduction of

ACS Paragon Plus Environment

Nano Letters

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

Page 4 of 15

CO2 also depends on the pH of the reaction medium.55 To determine the optimum pH for efficient CO2 conversion (Figure 2C), both low-pH (4.0 and 5.8) and high-pH (9.0 and 11) solutions were investigated; a 3.0 h reaction time period was used. The quantum yields at pH values of 4.0, 5.8, 9.0, and 11.0 were 1.1%, 1.52%, 1.83%, and 1.33%, respectively, and the chemical yields were 1.86%, 2.61%, 3.06%, and 2.22%, respectively. Mildly basic conditions (pH = 9.0) were found to be the most favorable pH for the conversion of CO2 into HCOOH, indicating that the formation of HCOOH occurs via the HCO3-2 ion.28,55 Figure 2D shows the concentration dependence of the r-GO-AuNPs on the quantum and chemical yields of HCOOH. As the concentration of r-GO-AuNPs was increased from OD 0.5 (at 540 nm) to 1.0, a linear increase in both the yields was observed. When the concentration increased from OD 1.0 to 2.0, the same quantum and chemical yields were observed. However, when the concentration was increased further to an OD of 3.0, both the yields decreased because the r-GOAuNPs formed aggregates in the concentrated solution. Also, spectroscopic analyses show the time-dependent changes in product quantities, as shown in Figure 3. After 1.0 h of irradiation, the gas chromatogram shows the formation of both methanol and HCOOH with relative proportions (area%) of 7.4% and 92.6%, respectively. After 3.0 h of irradiation, the total intensity of the peaks increased, and the relative proportions of methanol and HCOOH were 9.9% and 90.1%. After 5.0 h of reaction, the relative proportion of methanol decreased (6.2%) and that of HCOOH increased (93.8%). This is because of the consumption of methanol, which acts as a hole scavenger to form HCOOH as the reaction times increase (Figure 3A). For Raman analysis, the aliquots (10 µL) of reaction mixtures were placed on a quartz substrate and allowed to dry. Then, the samples were analyzed with 533 nm laser excitation (50 mW). Before the solutions were exposed to light from the Xe lamp, the solutions showed the characteristic Raman peak of the D (1346 cm-1) and G bands (1590 cm-1) of graphene.51,29 Sample solutions illuminated for 1.0, 3.0, and 5.0 h showed characteristic Raman shifts for HCOOH at 625 cm-1 (OCO bending), 920 cm-1 (HOC stretching), 1049 cm-1 (HCO stretching), 1221 cm-1 (CO stretching), 1418 cm-1 (HOC bending) along with 1346 cm-1 (D band), and 1690 cm-1 (G band), respectively (Figure 3B).57 A possible CO2 photoconversion mechanism is a two-step process. That is, the initial photogeneration of hydrogen (H2) by water splitting is followed by the chemical reaction between H2 and CO2 in solution.27,58 To evaluate the feasibility of the second chemical reaction step, we performed CO2 reduction for 3.0 h by purging excess H2 (g) into the CO2-saturated solution. The product HCOONa+, obtained after the removal of the solvent (H2O) and pH adjustment to 12.0 with dilute NaOH solution, was analyzed by FT-IR. As shown in Figure 3C, the FT-IR spectrum (blue line) of the products obtained from the chemical reduction of CO2 showed peaks at 1370 cm-1, 1600 cm-1, and 2858 cm-1, which is a typical FT-IR spectrum of HCOO-Na+, and correspond to the symmetric O=C-O stretching, asymmetric O=C-O stretching, and C-H stretching modes,59 respectively (Figure 3C). The timedependent FT-IR spectrum of CO2 photoconversion products was also the same as that of the chemical reaction product. Finally, the chemical structure of HCOOH was further evaluated by both 1H-NMR and 13 C-NMR. The typical chemical shift of the aldehyde proton (H-C=O) in a 1H-NMR spectrum is 8.01 ppm (Hz). The chemical shift of the carbon in HCOOH is 165 ppm (Hz). The presence of these peaks indicates the successful formation of HCOOH through chemical and photochemical reactions.27 The most desirable light source for CO2 photoconversion is sunlight. To examine the feasibility of using solar energy, we performed the photoconversion reaction with a solar simulator (AM 1.5). The rGO-AuNP solution saturated with CO2 was illuminated with light from the solar simulator (power density = 0.15 W/cm2) for 3.0 h, and then, the quantity of the product was calculated. The quantum and chemical yields were calculated to be 0.17% and 0.3%, respectively (Figure 4A). The yields were lower than those produced using a Xe lamp because of the low power density of the solar simulator (0.15 W/cm2) in comparison with that of the Xe lamp (5.68 W/cm2). Although water is known as a hole scavenger, methanol is a better hole scavenger and hydrocarbon

ACS Paragon Plus Environment

Page 5 of 15

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

Nano Letters

source for HCOOH.50 The yield of HCOOH could be improved by preventing fast electron-hole recombination. To examine this possibility, the effect of the presence of methanol on the yield of HCOOH was investigated. When irradiated in the presence of excess methanol (20%), increased quantum (1.82%) and chemical (3.12%) yields of HCOOH were observed (Figure 4A). The chemical yield of HCOOH obtained from the chemical reaction between excess CO2 and H2(g) in solution for 3.0 h at room temperature was found to be only 0.52%, about five times lower than the typical chemical yield (2.51%) for CO2 photoconversion with r-GO-AuNPs. This occurs because of the absence of additional HCOOH formation pathways such as the oxidation of methanol into HCOOH and other possible photocatalytic pathways.1,60 These require further investigation. The strong photothermal effect of the plasmonic nanoparticles is expected to greatly accelerate the rate of photochemical reactions.29 All the above described reactions were carried out under controlled temperature at 25 °C to exclude thermal effects on the reaction rate. The effect of temperature on CO2 photoconversion using r-GO-AuNPs has been studied by performing the reaction without temperature control for both 10 min and 30 min time periods. Because of the strong photothermal effect of the r-GOAuNPs,45,46 the temperature increased rapidly from 25 °C to 42 °C in 10 min and it further increased to 52 °C after 30 min in visible light irradiation. Temperature directly influenced both the quantum yield and chemical yield, which were 0.64% and 1.02% after 10 min and 1.14% and 1.98% after 30 min, respectively (Figure 4B). For an ideal photocatalytic system, colloidal stability, sustainable photocatalytic activity over reaction time, and the number of reaction cycles are important factors that must be controlled for practical applications. The r-GO-AuNPs particle stability with time was monitored under Xe lamp illumination. The OD of the r-GO-AuNPs at 540 nm was measured every hour for 5 h and remained almost constant, indicating the excellent colloidal stability of the r-GO-AuNPs during the reaction (Figure S4-A). The r-GO-AuNPs were recycled after reaction for 5.0 h and used to perform new batches of the CO2 conversion reaction. The r-GO-AuNPs were recycled five times for the same reaction. As shown the GC chromatogram in Figure S4-B, the CO2 photoconversion activities for the recycled r-GOAuNPs did not change, regardless of the number of times they were recycled. More importantly, no significant formation of by-products was observed in the chromatogram. Also, as shown in the X-ray photoelectron spectroscopy (XPS) spectrum in Figure S4-C and D, the chemical composition of the rGO-AuNPs after five reactions did not change. To fully investigate the plasmonic contributions to the enhanced photocatalytic activity, we performed wavelength-dependent reduction kinetics studies using potassium hexacyanoferrate(III) (500 µM)61 in the presence of r-GO AuNPs (Abs 2.5 at 540 nm) and with three different excitation light sources (450 nm, 520 nm and 633 nm, 100 mW) as shown in Figure 5A. The fastest reduction rate of Fe3+ into Fe2+ was observed when excited with 520 nm wavelength because of peak plasmonic absorption at this wavelength (Figure 5B). We further performed femtosecond transient absorption spectroscopic analysis (λext. = 575 nm) to accurately evaluate the hot-electron transfer process depends on the state of AuNPs.62 As shown in Figure 5C, the transfer of hot electrons into TiO2 nanoparticle induce the intraband absorption of TiO2,62,63 thereby it enabled to investigate the transfer efficiency of hot electrons depends on interface materials. The completion of electron transfer process was observed within 220 fs,62 more importantly, Figure 5D exhibits the higher efficiency of hot electron transfer in rGO-AuNPs compare with GO-AuNPs or AuNPs. The efficiency of hot electron in r-GO-AuNPs is 8.7 times higher than that of AuNPs, but GO-AuNPs only shows 2.8 times higher efficiency compare with naked AuNPs, which is well agreement with experimental results and highlight the role of graphene layer for efficient photocatalytic reactions. In conclusion, we report that reduced graphene-coated gold nanoparticles (r-GO-AuNPs) are excellent photocatalysts for solar fuel applications such as CO2 photoconversion of CO2 into HCOOH. The r-GO-AuNPs showed excellent selectivity for HCOOH (>90%) and higher conversion efficiency

ACS Paragon Plus Environment

Nano Letters

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

Page 6 of 15

than those of Pt-AuNPs. The results indicate that the r-GO layer is a promising alternative material to platinum, which is scarce as well as expensive,25 for photocatalysis. The contribution of the plasmonic effect is significant and the role of the graphene layer as an electron acceptor and transporter is important. In the photoconversion of CO2 with r-GO-AuNPs, methanol acts as a hole scavenger, as shown by the time-dependent GC chromatogram and the results of CO2 photoconversion in the presence of excess methanol. The rate of CO2 conversion could be accelerated with the photothermal effect that occurs during the CO2 photoconversion reaction. The use of a homogeneous, aqueous solution and the excellent photostability of the r-GO-AuNPs enables efficient visible-light–induced photochemical reactions under desirable reaction conditions. These results suggest that reduced graphene layers can play a role as highly efficient electron acceptors and transporters, facilitating the utilization of hot electrons for plasmonic photocatalysts. ASSOCIATED CONTENT Supporting Information. Detailed experimental procedures, additional UV-visible, TEM, XRD, XPS, GC-MS and NMR data. “This material is available free of charge via the Internet at http://pubs.acs.org.”. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. D. Kumar and A. Lee are equally contributed to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF-2013R1A1A1061387) and the KU-KIST research fund. The authors also acknowledge Dr. Youngsoo Kim’s helpful discussion for the kinetic study. REFERENCES

1. 2. 3. 4. 5. 6. 7.

Studt, F.; Sharafutdinov, I.; Abild-Pedersen, F.; Elkjær, C. F.; Hummelshøj, J. S.; Dahl, S.; Chorkendorff, I.; Nørskov, J. K. Nat. Chem. 2014, 6, 320-324. Leung, K.; Nielsen, I. M. B.; Sai, N.; Medforth, C.; Shelnutt, J. A. J. Phy. Chem. A 2010, 114, 10174-10184. Morris, A. J.; Meyer, G. J.; Fujita, E. Acc. Chem. Res. 2009, 42, 1983-1994. Savéant, J.-M. Chem. Rev. 2008, 108, 2348-2378. Roy, S. C.; Varghese, O. K.; Paulose, M.; Grimes, C. A. ACS Nano 2010, 4, 1259-1278. Robert, D. Catal. Today 2007, 122, 20-26. Ni, M.; Leung, M. K.; Leung, D. Y.; Sumathy, K. Renew. Sust. Energy Rev. 2007, 11, 401-425.

ACS Paragon Plus Environment

Page 7 of 15

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

8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40.

Nano Letters

Pu, Y.-C.; Wang, G.; Chang, K.-D.; Ling, Y.; Lin, Y.-K.; Fitzmorris, B. C.; Liu, C.-M.; Lu, X.; Tong, Y.; Zhang, J. Z.; Hsu, Y.-J.; Li, Y. Nano Lett. 2013, 13, 3817-3823. Murdoch, M.; Waterhouse, G. I. N.; Nadeem, M. A.; Metson, J. B.; Keane, M. A.; Howe, R. F.; Llorca, J.; Idriss, H. Nat. Chem. 2011, 3, 489-492. Wang, W.-N.; An, W.-J.; Ramalingam, B.; Mukherjee, S.; Niedzwiedzki, D. M.; Gangopadhyay, S.; Biswas, P. J. Am. Chem. Soc. 2012, 134, 11276-11281. Zhao, G.; Kozuka, H.; Yoko, T. Thin Solid Films 1996, 277, 147-154. Kim, H.-I.; Moon, G.-H.; Monllor-Satoca, D.; Park, Y.; Choi, W. J. Phy. Chem. C 2012, 116, 1535-1543. Wang, W.; Yu, J.; Xiang, Q.; Cheng, B. Appl. Catal. B: Environ. 2012, 119–120, 109-116. Varghese, O. K.; Paulose, M.; LaTempa, T. J.; Grimes, C. A. Nano Lett. 2009, 9, 731-737. Tian, Y.; Tatsuma, T. J. Am. Chem. Soc. 2005, 127, 7632-7637. Gomes Silva, C.; Juarez, R.; Marino, T.; Molinari, R.; Garcia, H. J. Am. Chem. Soc. 2010, 133, 595-602. Wu, F. Plasmonics 2012, 8, 501-508. Zhang, N.; Liu, S.; Fu, X.; Xu, Y. J. J. Phys. Chem. C 2011, 115, 9136-9145. Atwater, H. A.; Polman, A. Nat. Mater. 2010, 9, 205-213. Brongersma, M. L.; Halas, N. J.; Nordlander, P. Nat. Nanotechnol. 2015, 10, 25-34. Linic, S.; Christopher, P.; Ingram, D. B. Nat. Mater. 2011, 10, 911-921. Zheng, B. Y.; Zhao, H.; Manjavacas, A.; McClain, M.; Nordlander, P.; Halas, N. J. Nat. Commun., 2015, 6. Zheng, Z.; Tachikawa, T.; Majima, T. J. Am. Chem. Soc. 2014, 136, 6870-6873. Mubeen, S.; Lee, J.; Singh, N.; Kramer, S.; Stucky, G. D.; Moskovits, M. Nat. Nanotechnol. 2013, 8, 247-251. Marinescu, S. C.; Winkler, J. R.; Gray, H. B. Proc. Natl. Acad. Sci. 2012, 109, 15127-15131. Sato, S.; Arai, T.; Morikawa, T.; Uemura, K.; Suzuki, T. M.; Tanaka, H.; Kajino, T. J. Am. Chem. Soc. 2011, 133, 15240-15243. Cao, L.; Sahu, S.; Anilkumar, P.; Bunker, C. E.; Xu, J.; Fernando, K. A. S.; Wang, P.; Guliants, E. A.; Tackett, K. N.; Sun, Y.-P. J. Am. Chem. Soc. 2011, 133, 4754-4757. Moret, S.; Dyson P. J.; Laurenczy, G. Nat. Commun. 2014, 5, 4017. Mukherjee, S.; Libisch, F.; Large, N.; Neumann, O.; Brown, L. V.; Cheng, J.; Lassiter, J. B.; Carter, E. A.; Nordlander, P.; Halas, N. J. Nano Lett. 2013, 13, 240-247. Robatjazi, H.; Bahauddin, S. M.; Doiron, C.; Thomann, I. Nano Lett. 2015, 15, 6155–6161. Mubeen,S.; Lee, J.; Liu, D.; Stucky, G. D.; Moskovits, M. Nano Lett. 2015, 15, 2132–2136. Shi, Y.; Wang, J.; Wang, C.; Zhai, T.-T.; Bao, W. –J.; Xu, J.-J.; Xia, X. –H.; Chen, H. –Y. J. Am. Chem. Soc. 2015, 137, 7365–7370. Hoggard, A.; Wang, L.-Y.; Ma, L.; Fang, Y.; You, G.; Olson, J.; Liu, Z.; Chang, W.-S.; Ajayan, P. M.; Link, S. ACS Nano 2013, 7, 11209-11217. Chen, J.-H.; Jang, C.; Xiao, S.; Ishigami, M.; Fuhrer, M. S. Nat. Nanotechnol. 2008, 3, 206-209. Yeh, T.-F.; Cihlář, J.; Chang, C.-Y.; Cheng, C.; Teng, H. Mater. Today 2013, 16, 78-84. Yeh, T.-F.; Syu, J.-M.; Cheng, C.; Chang, T.-H.; Teng, H. Adv. Func. Mater. 2010, 20, 22552262. Knight, M. W.; Sobhani, H.; Nordlander, P.; Halas, N. J. Science 2011, 332, 702–704. DuChene, J. S.; Sweeny, B. C.; Johnston-Peck, A. C.; Su, D.; Stach, E. A.; Wei, W. D. Angew. Chem. Int. Ed. 2014, 53, 7887–7891. Knight, M. W.; Wang, Y.; Urban, A. S.; Sobhani, A.; Zheng, B. Y.; Nordlander, P.; Halas, N. J. Nano Lett. 2013, 13, 1687–1692. Clavero, C. Nat. Photon. 2014, 8, 95-103.

ACS Paragon Plus Environment

Nano Letters

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

41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63.

Page 8 of 15

Kim, J.; Son, H.; Cho, D. J.; Geng, B.; Regan, W.; Shi, S.; Kim, K.; Zettl, A.; Shen, Y.-R.; Wang, F. Nano Lett. 2012, 12, 5598-5602. Niu, J.; Shin, Y. J.; Lee, Y.; Ahn, J.-H.; Yang, H. Appl. Phys. Lett. 2012, 100, 061116. Fang, Z.; Liu, Z.; Wang, Y.; Ajayan, P. M.; Nordlander, P.; Halas, N. J. Nano Lett. 2012, 12, 3808-3813. Manjavacas, A.; Liu, J. G.; Kulkarni, V.; Nordlander, P. ACS Nano 2014, 8, 7630-7638. Lim, D.-K.; Barhoumi, A.; Wylie, R. G.; Reznor, G.; Langer, R. S.; Kohane, D. S. Nano Lett. 2013, 13, 4075-4079. Moon, H.; Kumar, D.; Kim, H.; Sim, C.; Chang, J.-H.; Kim, J.-M.; Kim, H.; Lim, D.-K. ACS Nano 2015, 9, 2711-2719. Pathak, P.; Meziani, M. J.; Castillo, L.; Sun, Y.-P. Green Chem. 2005, 7, 667-670. Inoue, T.; Fujishima, A.; Konishi, S.; Honda, K. Nature 1979, 277, 637-638. Usubharatana, P.; McMartin, D.; Veawab, A.; Tontiwachwuthikul, P. Ind. Eng. Chem. Res. 2006, 45, 2558-2568. Xie, G.; Zhang, K.; Guo, B.; Liu, Q.; Fang, L.; Gong, J. R. Adv. Mater. 2013, 25, 3820-3839. Xiang, Q.; Yu, J. J. Phy. Chem. Lett. 2013, 4, 753-759. Li, Q.; Guo, B.; Yu, J.; Ran, J.; Zhang, B.; Yan, H.; Gong, J. R. J. Am. Chem. Soc. 2011, 133, 10878-10884. Wang, X.; Li, X.; Zhang, L.; Yoon, Y.; Weber, P. K.; Wang, H.; Guo, J.; Dai, H. Science 2009, 324, 768-771. Yeh, T. F.; Syu, J. M.; Cheng, C.; Chang, T. H.; Teng, H. Adv. Func. Mater. 2010, 20, 22552262. Jin, F.; Zeng, X.; Liu, J.; Jin, Y.; Wang, L.; Zhong, H.; Yao, G.; Huo, Z. Sci. Rep. 2014, 4. Khorasaninejad, M.; Raeis-Zadeh, S. M.; Jafarlou, S.; Wesolowski, M. J.; Daley, C. R.; Flannery, J. B.; Forrest, J.; Safavi-Naeini, S.; Saini, S. S. Sci. Rep. 2013, 3. Balabin, R. M. J. Phy. Chem. A 2009, 113, 4910-4918. Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341-357. Birer, Ö.; Havenith, M. Annu. Rev. Phys. Chem. 2009, 60, 263-275. Liu, G.; Hoivik, N.; Wang, K.; Jakobsen, H. Sol. Energ. Mater. Sol. C. 2012, 105, 53-68. Sudo, Yukio, S.; Toda, F. Chem. Commun. 1979, 23, 1044-1045. Furube, A.; Du, L.; Hara, K.; Katoh, R.; Tachiya, M. J. Am. Chem. Soc. 2007, 129, 14852-14853. Kim, S.; Park, J.; Lee T,; Lim, M. J. Phys. Chem. B 2012, 166, 6346-6355.

ACS Paragon Plus Environment

Page 9 of 15

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

Nano Letters

Scheme 1. Schematic of the graphene-coated gold nanoparticle for CO2 photoconversion. (A) HR-TEM image. (B) Energy dispersive X-ray mapping image of r-GO-AuNPs (Au (blue), carbon (red)). (C) Schematic representation of hot-electron generation and the role of graphene layer as an efficient electron acceptor and transporter to generate hydrogen and subsequent conversion of CO2 into formic acid (HCOOH) and methanol (CH3OH).

ACS Paragon Plus Environment

Nano Letters

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

Page 10 of 15

Figure 1. Pt-coated AuNPs and the comparison of CO2 photoconversion with r-GO-AuNPs. (A) The HR-TEM image of Pt-AuNPs. (B) UV-visible spectra of r-GO AuNPs (black and red line) and PtAuNPs (blue and green line) before and after Xe-lamp irradiation. (C) Chromatogram of solution after CO2 photoconversion reaction with Xe-lamp irradiation in the presence of r-GO-AuNPs (black line) and Pt-AuNPs (blue line). (D) Mass spectrum of formic acid (HCOOH) and methanol (CH3OH).

ACS Paragon Plus Environment

Page 11 of 15

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

Nano Letters

Figure 2. Various reaction parameters for the quantum and chemical yield of formic acid. (A) Effect of plasmonic nanostructure and reduced state of graphene layer studied with Xe-lamp and NIR laser. (B) Reaction time. (C) The pH effect. (D) Nanoparticles concentration (OD at 540 nm) on the quantum yield (blue bar and line) and chemical yield (red bar and line) investigated with r-GO-AuNPs and visible light (Xe-lamp) (*N/P indicates no product). (The amount of HCOOH was quantified with 1H-NMR and GC analysis). (Data are means ± standard deviations (SD), N = 5).

ACS Paragon Plus Environment

Nano Letters

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

Page 12 of 15

Figure 3. Spectroscopic analysis for the CO2 photoconversion products. (A) Time-dependent gas chromatogram of the solution of CO2 photoconversion with visible light in the presence of r-GOAuNPs. (B) Raman scattering spectrum of reaction mixture (0 h, 1.0 h, 3.0 h and 5.0 h). (C) FT-IR spectrum of the product obtained by reacting CO2 with H2 (g) without light illumination (blue line) and CO2 photoconversion products obtained time-dependently (0 h, 1.0 h, 3.0 h and 5.0 h). (D) 1H-NMR (CDCl3) and 13C-NMR spectrum of products obtained from CO2 photoconversion reaction with r-GOAuNPs (black line) and chemical reaction by H2 (g) purging into CO2 saturated solution without light irradiation (blue line).

ACS Paragon Plus Environment

Page 13 of 15

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

Nano Letters

Figure 4. (A) Quantum yield (blue bar) and chemical yield (red bar) of HCOOH in the presence of rGO-AuNPs with Xe-lamp or r-GO-AuNPs with solar simulator (AM 1.5) or r-GO-AuNPs with Xe-lamp and methanol (20%) or chemical reaction by H2 (g) purging into CO2 saturated solution without light irradiation. (B) Time-dependent quantum yield and chemical yield of HCOOH in the presence of r-GOAuNPs with Xe-lamp without controlling the reaction temperature. (N/A = not available). (The temperature indicates the temperature at 10 min and 30 min). (Data are means ± standard deviations (SD), N = 5).

ACS Paragon Plus Environment

Nano Letters

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

Page 14 of 15

Figure 5. (A) UV-Vis spectra of potassium hexacyanoferrate(III) (500 µM) in the presence of r-GO AuNPs (Abs 2.5 at 540 nm) for excitation wavelength-dependent reduction (Fe3+  Fe2+ by hotplasmonic electrons), (B) Time-dependent decrease of the absorbance of Fe3+ monitored at 420 nm (blue square: 450 nm, green circle: 520 nm, red triangle: 633 nm), (C) Experimental scheme of femtosecond transient absorption spectroscopic analysis (575 nm, 2 µJ), (D) Transient absorption kinetics at 2,978 cm-1 of TiO2 nanoparticle in the presence of AuNPs (blue), GO-AuNPs (red) and r-GO-AuNPs (black).

ACS Paragon Plus Environment

Page 15 of 15

Nano Letters

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

ACS Paragon Plus Environment