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Ag nanoparticle-induced oxidative dimerization of thiophenols: efficiency and mechanism Huanhuan Li, Mengting Si, Lihua Liu, Xiangqian Chu, Shuai Wang, Lei Wan, Rong Yan, Mengtao Sun, and Yingcui Fang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02068 • Publication Date (Web): 01 Sep 2018 Downloaded from http://pubs.acs.org on September 4, 2018

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Ag nanoparticle-induced oxidative dimerization of thiophenols: efficiency and mechanism Huanhuan Lia, Mengting Sia,1, Lihua Liub, Xiangqian Chua, Shuai Wanga, Lei Wanc, Rong Yand, Mengtao Sune*, Yingcui Fanga* a

Department of Vacuum Science and Technology, Hefei University of Technology, Hefei 230009, China b School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China c School of Electrical Engineering and Automation, Hefei University of Technology, Hefei 230009, China d Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China e School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, China. *Corresponding author: [email protected], [email protected] (M. T. Sun)

ABSTRACT: By oxidation of silver nanoparticles (AgNPs) in two ways: thermal oxidation (TO) in molecular oxygen and cool oxidation in oxygen plasma (i.e. oxygen plasma irradiation, OPI), the efficiency and mechanism of visible lights induced selective transformation of 4 - Aminothiophenol (PATP) to 4,4’-dimercaptoazobenzene (DMAB) on the surface of AgNPs was explored. Based on the evolution of surface enhanced Raman scattering (SERS) spectrum of PATP (10-5M in ethanol) with the oxidation time, it can be concluded that OPI could improve the selective transformation efficiency (η) effectively, by 87 times for only 2 s; whereas TO could improve η conditionally, increasing at first and then decreasing gradually to zero. The results imply that silver oxide is not the root cause of the increased η. Combined with the results of SERS of oxygen species on the surface of AgNPs processed by the above-mentioned two ways, superoxide (O2-) and electrophilic oxygen atoms (O-) are suggested to be responsible for this selective transformation. Our study deepens the understanding of the mechanism of plasmonic photocatalysis and the role of silver oxide in selection transformation of organic molecules.

1

Present address: State Key Lab of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China 1

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1 Introduction Silver is unique in its ability to selectively oxidize organic molecules such as ethane, propylene to ethane epoxide and propylene epoxide. 1 Recently, silver nanoparticles (AgNPs) have been reported to induce selective transformations under visible light, 2 due to energy transfer, 3 hot electrons or/and hot holes produced during the decay of the surface plamonic resonance (LSPR) of AgNPs, 4,5 ,6 ,7 and plasmon−exciton co-driven, 8 however, silver oxide was also suggested as the origins. 9 , 10 , 11 By surface-enhanced Raman spectroscopy (SERS), Huang et al.

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studied the

mechanism of LSPR-assisted activation of 3O2 on Au and Ag nanostructures by monitoring the selective oxidation of p-aminothiophenol (PATP) to 4,4’-dimercaptoazobenzene (DMAB) and found that oxygen molecules were activated by accepting hot electrons from metal nanoparticles during the decay of LSPR to form a strongly adsorbed oxygen molecule anion. The anion was then transformed to Au or Ag oxides orhydroxides to oxidize the surface species.

Shilpi et al10 studied the

transformation of propylene to propylene oxide by AgNPs supported on WO3 nanorods and suggested that the synergy between the surfaces of AgNPs and WO3 nanorods facilitates the dissociation of molecular oxygen on the metallic Ag surface to produce silver oxide which transfers its oxygen to the propylene to form propylene oxide selectively. Zhang et al11 made a similar conclusion that oxide and hydroxide formed in the plasmon-catalyzed process play a central role in the selective oxidation of PATP to DMAB. If it is true that oxide or silver oxide is the origin of the selective transformation, we can increase η by producing oxide directly on the surface of AgNPs; and further, the larger the amount of oxide is, the higher the transformation efficiency is. In order to demonstrate this assumption, we fabricate AgNPs by vacuum thermal evaporation and study the selective transformation of PATP to DMAB induced by AgNPs oxidized by two approaches: thermal oxidation (TO) in molecular oxygen and cool oxidation by oxygen plasma irradiation (OPI). It is found OPI could efficiently improve η; however, TO improves η conditionally, depending on oxidation times. η increasing at first and then decreas gradually to zero. According to this evolution, silver oxide as the origin of the selective transformation needs to be reconsidered. 2 Experimental AgNPs were deposited in vacuum by sputtering. The vacuum chamber was pumped to a vacuum degree of 5 ×10-4 Pa by a turbo molecular pump backed by a rotary vane pump. High pure Ar gas (99.999%) was filled in and ignited by radio frequency (rf) discharge to produce Ar+ ions. High pure silver target (99.95%) was sputtered by these Ar+ ions. Silver atoms were deposited on glass substrates to form AgNPs. Such fabricated AgNPs are immobile on the substrate, free of pollution12 2


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and with high SERS sensitivity.13,14 The sputtering time is 20 s (the sample denoted as 20sAgNPs in the following text). First, the influence of the absorbed oxygen on the selective transformation efficiency of PATP to DMAB was explored. Samples of 20sAgNPs right after deposition were stored in nitrogen, oxygen and air ambient at pressure of 10-1Pa for 30 min respectively to saturate the surface of AgNPs with nitrogen, oxygen and air. Then 10 ul PATP in ethanol (10-5 M) were dropped onto the surface of AgNPs. Raman spectra were collected after the samples were dried in the dark. Second, the influence of oxide on the selective transformation efficiency was explored. 20sAgNPs were oxidized by OPI for 2 s with a rf power of 5 W to sustain discharging of molecular oxygen and to avoid sputtering of the atoms on the surface. The sample was kept in oxygen ambient at a pressure of 10-1 Pa for 30 min after OPI. After samples were taken out from the vacuum chamber, η of PATP to DMAB was tested. In contrast, 20sAgNPs were oxidized in molecular oxygen at temperatures of 150, 200 and 250 0C for 30 min and at 250 0C for 5, 30, and 90 min, respectively. After samples were cooled down to room temperature in oxygen ambient for 30 min, η was tested by SERS. Absorption spectra of all samples were taken by a Shimadzu-2500 UV-visible light spectrophotometer. Field emission scanning electron microscopy (FESEM) images were taken by Hitachi SU8020. Raman scattering spectra were taken by HORIBA JOBIN YVON HR Evolution. The excitation power is 2.5 mW if not stated. The excitation wavelength is 532 nm and the collection time is 2 s. η of PATP to DMAB is characterized by the intensity ratio of the peak at 1142 cm-1 to that of 1080 cm -1, as reported by ref.15. 3 Results 3.1 Influence of the absorbed oxygen on the SERS spectra of PATP 20sAgNPs has a mean size of 15 nm according to FESEM images, as shown in figure 1 (a) and its localized surface plasmon resonance peak is around 490 nm (figure 1 (b)). The evolution of SERS spectrum with laser illumination time of PATP on the surface of 20sAgNPs saturated with nitrogen, oxygen and air, is shown in figure 1 (c), curves 1 to 6. The peak at 1080 cm-1 is the typical peak of PATP and the three at 1142, 1388 and 1432 cm-1 are typical peaks of DMAB.16 Curve 1, 3 and 5 are the SERS spectra of the three samples after laser illumination for 2 s (curve 1-stored in nitrogen, 3in air, 5- in oxygen). There is no any characteristic peak of DMAB for the sample stored in nitrogen, while a very small peak at 1142 cm-1 can be seen for the other two samples. η, calculated by the intensity ratio of the peak at 1142 cm-1 to the one at 1080 cm-1, is about 0.05 for the sample exposed 3


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to air and 0.13 for the sample saturated with oxygen. This indicates that without oxygen, no transformation of PATP to DMAB takes place. Curve 2, 4 and 6 are SERS spectra of these samples after laser illumination for 400 s (2-stored in nitrogen, 4-in air, 6- in oxygen). The corresponding η is 0.07, 0.45 and 0.47. η of samples stored in oxygen or air is much larger than that of samples stored in nitrogen. This result demonstrates that molecular oxygen is necessary for the transformation of PATP to DMAB under the illumination of laser with wavelength of 532 nm. The small η of 0.07 is caused by oxygen in air during SERS test which was performed in the air.

Figure 1 (a) The FESEM image of 20sAgNPs. (b) The localized surface plasmon resonance of 20sAgNPs characterized by absorption spectrum. (c) The Surface enhanced Raman scattering (SERS) spectra of 4 - Aminothiophenol (PATP) in ethanol with concentration of 10-5M on the surface of 20sAgNPs stored in nitrogen (curves 1, 2), air (curves 3, 4) and oxygen (curves 5, 6) for 30 min just after deposition. The laser illumination time for curves 1, 3 and 5 is 2 s and for curves 2, 4, 6 is 400 s.

3.2 Influence of silver oxide on the transformation efficiency of PATP to DMAB 3.2.1 Thermal oxidization (TO) in molecular oxygen The SERS spectra of PATP on the surface of 20sAgNPs which were oxidized in molecular oxygen at 150, 200 and 250 0C for 30 min respectively are shown in figures 2 (a)-(c) for laser illumination time of 2, 80 and 400 s. The characteristic peaks of DMAB at 1142, 1388 and 1432 cm-1 can be clearly detected after illumination for only 2 s (a), and their intensities increase with the laser illumination time as shown in figure 2 (b, 80 s) and (c, 400 s). η of PATP to DMAB of each sample is shown at the right side of each panel in figure 2, and the evolution of η with the laser illumination time are shown in figure 3 (red, black dotted lines and red solid line, respectively). 4


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Figure 2 SERS spectra of PATP (10-5M) on the surface of 20sAgNPs oxidized in molecular oxygen at 150, 200 and 250 0C for 30 min, illuminated by a laser of 532 nm for 2 s (a), 80 s (b), and 400 s (c) and these of 20sAgNPs oxidized in molecular oxygen at 250 0C for 5, 30, 90 and 180 min, illuminated by a laser of 532 nm for 2 s (d), 80 s (e), and 400 s (f).

Figure 3 Evolution of η (I1142/I1080) with laser illumination time for 20sAgNPs oxidized in oxygen at 150, 200 0C for 30 min respectively (black, red dot line), at 250 0C for 5, 30, 90 and 180 min respectively (green, red, blue and orange solid line), and after OPI for 2 s (blue dash dotted line), cooled in molecular oxygen for 30 min. The two of 20sAgNPs stored in nitrogen and oxygen ambient for 30 min (black, red dash line respectively) are shown as references. 5


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The following characteristics can be observed based on figure 3: (1) For each sample, η increases with the laser illumination time; (2) η increases with oxidation temperatures from 150 to 200 and 250 0C (blue dotted line, red dotted line and red solid line). Oxidized at 250 0C for 30 min, the sample shows the maximal η, and it is improved by 550 % (from 0.44 to 2.86) after illuminated for 400 s, while by 289 % (from 0.46 to 1.79) for the sample oxidized at 200 0C for 30 min and 206 % (from 0.33 to 1.01) for the sample oxidized at 150 0C for 30 min. (3) When samples are oxidized at 250 0C for 30 min, η increases linearly with laser illumination time (red solid line). Absorption spectra of samples are shown in figure 3 (b). With the increase in oxidization temperature, the absorption intensity decreases gradually. After the sample was oxidized at 250 0C for 30 min in molecular oxygen the LSPR of AgNPs was reduced by more than 1/2, calculated by the peak intensity of the absorption spectrum. The shift of the absorption peak is due to the agglomeration (shifting to 440 nm after oxidized at 150 0C for 30 min ) and coalescence (shifting to 480 and 500 nm after oxidized at 200 0C and 250 0C for 30 min respectively) of the Ag2O capped AgNPs during thermal oxidation. Our former experiments demonstrated that when AgNPs was thermally oxidized in molecular oxygen only one kind of silver oxide, Ag2O was produced, and AgNPs could not be over-oxidized into AgO or Ag2O3 etc.17,18 The formed Ag2O damps the LSPR of AgNPs, resulting in the decrease of the absorption intensity. Thus the absorption spectra indicate that more Ag2O is produced as the oxidation temperature increases. Combined with figures 3 (blue dotted line, red dotted line and red solid line), it can be concluded that the more the content of Ag2O is, the higher the η of PATP to DMAB is. The SERS spectra of sample 20sAgNPs oxidized in molecular oxygen at 250 0C for 5, 30, 90 and 180 min are shown in figures 2 (d)-(f) with the laser illumination time of 2, 80 and 400 s, respectively. The evolutions of η with the laser illumination time for the four samples are shown in figure 3 (green, red, blue and orange solid lines). It can be seen that η increases with the laser illumination time from 2 to 400 s. When the oxidation time is no less than 30 min, η increases linearly with the laser illumination time (figure 3, red, blue and orange lines). Furthermore, the increasing rate depends on the oxidation time. For same period of laser illumination time, η increases with the oxidation time from 5 min (green solid line) to 30 min (red solid line) but decreases sharply when the oxidation time is 90 min (blue solid line), and when the oxidation time is increased to 180 min, no DMAB can be detected. Thus an optimal oxidation time exists for the highest η. It is 30 min. The absorption spectra of the samples are shown in figure 3 (c). The absorption intensity decreases as the oxidation time increases. Oxidation for 30 min, the absorption intensity is reduced by a little more than 1/2 (according to peak intensity); oxidation for 90 min, it is reduced by 1/5, while oxidation for 180 min it decreases to zero, implying that nearly all AgNPs are oxidized into 6


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Ag2O. Combined with figures 2 (d)-(f), it can be concluded that silver oxide is not the origin of the transformation from PATP to DMAB; otherwise, η should increase with oxidation time (i.e. with the increase of the thickness of silver oxide). Even if the SERS intensity decreases when the oxidation time is longer than 30 min due to the damping of LSPR, the ratio of I1142/I1080 should keep increasing at a rate same or larger than that of the sample oxidized for 30 min, but in fact it is much smaller than that of sample oxidized for 30 min (figure 3 (a), blue solid line for sample oxidized for 90 min, and red solid line for sample oxidized for 30 min). 3.2.2 Oxidization in oxygen plasma The evolution of SERS spectrum with the laser illumination time of 20sAgNPs after OPI for 2 s is shown in figure 4 (a), and the absorption spectra of 20sAgNPs before and after OPI for 2 s are shown in figure 4 (b). After OPI for 2 s, the LSPR is nearly completely damped by silver oxide produced during OPI, but it does not mean that the LSPR is completely disappeared,17 in fact it still works.19

Figure 4 SERS spectrum evolution with laser illumination times of 2, 80 and 400 s (black, red and blue line) of PATP (10-5M) on the surface of 20sAgNPs after oxidized by OPI for 2 s (a), absorption spectra of 20sAgNPs before (black solid line) and after (red solid line) OPI for 2 s (b). According to figure 4 (a), after illumination for 2 s, η is 1.74, which is much larger than those of samples oxidized in molecular oxygen at various temperatures and for different times under same period of laser illumination time. But η increases in a much slower rate (figure 3, blue dash-dotted line) than that of sample oxidized in molecular oxygen at 250 0C for 30 min (figure 3, red solid line), for example, illumination for 400 s, η increases from 1.74 to 2.85 and the improvement is only 64 %. 3.3 Discussion We make a summary based on our experiments: (1) The transformation from PATP to DMAB could not happen without oxygen molecules (figure 1) even if the LSPR is strong; (2) The transformation efficiency is low when the content of metal oxide is large while the LSPR is weak 7


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(figure 3 blue and orange line); (3) For AgNPs oxidized in molecular oxygen, an optimal thickness of silver oxide (oxidation for 30 min, figure 3 red line) makes η reach the maximum, and η increases quickest with the laser illumination; (4) For AgNPs oxidized by OPI for 2 s, η is higher than those of samples oxidized by annealing, but it has a weak relation with the laser illumination time (figure 3, blue dotted dash line). Based on these results, oxidation of AgNPs can improve η conditionally. And the process of PATP transformation to DMAB might be different for samples oxidized by TO and by OPI. It is reported that singlet molecular oxygen20 and (O2-)21 are of high reactivity, and could induce selectively chemical transformation. Singlet molecular oxygen has been demonstrated to be produced when triplet molecular oxygen accepts electrons from photo-sensitization process,22or from metal nanoparticles, 23 especially from metal nanoparticles composited with metal oxides, such as AuNPs/TiO2.For example, Saito24suggested that for AuNPs/TiO2, a triplet molecular oxygen accepts a hot electron from the decay of LSPR of AuNPs to form O2-. O2- is then oxidized by a hot hole and turns into singlet molecular oxygen. For Pt-TiO2, Long et al25 demonstrated that under visible light illumination, a kind of oxygen species showing similar chemical and physical properties to singlet O2 was produced. It’s also reported that oxygen vacancy on the surface of metal oxide such as WO326 or BiOCl6 could promote the production of O2- and thus improve the transformation efficiency. On the other hand, electrophilic oxygen (O-), a kind of adsorbed atomic oxygen species with charge, has also been suggested to trigger selective transformation directly.27 In order to demonstrate that O2- and O- are presented on the surface of oxidized AgNPs, SERS spectra of 20sAgNPs oxidized at 250

0

C in molecular oxygen for 30 min (denoted as

20sAgNPs-O2-2500C-30min) and oxidized in oxygen plasma for 2 s (denoted as 20sAgNPs-OPI-2s) were taken. The laser power is 50 mW and the excitation wavelength is 532 nm. The results are shown in figure 5. Five kinds of oxygen species are produced after the sample was oxidized in oxygen plasma (Figures 5 (a) and (b)), while four kinds of species are produced after the sample was oxidized in molecular oxygen (Figures 5 (c) and (d)). According to ref.28, the peaks around 458, 580, 800, and 1090 cm-1 are subsurface oxygen, peroxide oxygen (O22-), absorbed atomic oxygen (O), and superoxide (O2-), respectively. Since the LSPR of 20sAgNPs-OPI-2s (figure 4 (b)) is much weaker than that of sample 20sAgNPs-O2-2500C-30min (figure 3 (b)), the enhancement factor of SERS of 20sAgNPs-OPI-2s is much smaller than that of 20sAgNPs-O2-2500C-30min. Therefore, the true Raman intensities of figure 5 (a) and (b) should be multiplied by a constant larger than 1 to make them comparable to that in figures 5 (c) and (d); but it is impossible to provide an exact value at the 8


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present. However, figure 5 shows the concentration of atomic oxygen (O-) with a peak at 785 cm-1 (figure 5 a) and that of subsurface oxygen with a peak at 458 cm-1 of sample 20sAgNPs-OPI-2s in (figure 5 b) are much larger than the corresponding two of 20sAgNPs-O2-2500C-30min (figure 5 c, with a peak at 800 cm-1, and figure 5 d, with a peak at 458 cm-1). The two kinds of species should play dominant roles in the transformation from PATP to DMAB on the surface of 20sAgNPs-OPI. For sample 20sAgNPs-O2-2500C-30min, the concentrations of O22-(580 cm-1) and O2- (1090 cm-1) are large, however, the absorbed energy of O22- is about two times of that of O2-, thus O22- is difficult to take part in chemical reaction and its influence can be ignored, therefore O2- should be responsible for the transformation.

Figure 5 SERS spectra of 20sAgNPs after OPI for 2 s, in the range of 400-1400 cm -1 (a) and 200-500 cm-1(b), and that after oxidation in molecular oxygen at 250 0C for 30 min in the range of 400-1400 cm -1 (c) and 200-500 cm-1 (d). The peaks at 458, 580, 800, and 1090 cm-1 are subsurface oxygen, peroxide oxygen, absorbed atomic oxygen, and superoxide, respectively. We would like to discuss more about the generation of O2- and O- and their reactions with organic molecules. Molecular oxygen absorbed at the bridge sites or top sites of silver nanoparticles can turn into O2- when it gets an electron from silver nanoparticles, as reported by Wu et al.29 It is well known that the absorption coefficient is small for molecular oxygen on the surface of silver thus the yield of O2- is low. Oxide layer and “subsurface” oxygen were reported to increase the absorption of molecular oxygen on silver surface.30,31 When AgNPs are oxidized in molecular oxygen or in oxygen plasma, silver oxide and subsurface oxygen (figure 5, with a Raman shift of 458 cm-1) are produced.32 Both promote molecular oxygen to be adsorbed on the surface. More O2- will be 9


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produced after the sample was illuminated by laser, thus the transformation efficiency is increased. It is difficult to understand how O- is produced on the surface of silver oxide. We can get a hint of it from the dissociation of molecular oxygen on the surface of TiO2 according to the scan tunneling microscopy (STM) images. STM images show that one way of the dissociation of O2 is through the bridge-bonded oxygen vacancies (BBOV) on the surface of TiO2 , with one O atom healing the BBOV and the other O atom leaving at a neighboring 5-fold-coordinated Ti (Ti 5c ) site as an adatom.33 This adatom turns into O- when it is on the surface of silver or silver oxide after accepting one electron. Since the oxidation speed of AgNPs by OPI is very fast, the silver atoms have little chance to move to equilibrium positions, thus defects are produced largely. These defects facilitate molecular oxygen to dissociate into atomic oxygen to produce O-. High concentration of Owas demonstrated by XPS, a component with a binding energy of 531.5 eV.34,35,36 Such high concentration of O- should increase the transformation of PATP to DMAB. But it is still difficult to understand how O2- and O- react with PATP to produce DMAB. We can also get some hints from what has happened on the surface of TiO2.

37

Ref. 37 reports how H was

subtracted from H2O by O- (Note that PATP lost two H atoms when it was transformed into DMAB). According to ref. 37, when the distance between molecule H2O and O- is suitable, one H of H2O was robbed by O-, producing two OHs. This result clearly demonstrates that O- is much active. It should be able to subtract H from PATP to promote the production of DMAB. It is difficult to get images about the interaction of O2- with organic molecules on the surface of silver oxide from atomic level. However, according to the study by STM, O2 reacts with OH at room temperature, yielding an intermediate hydroperoxyl (HO2) species at the Ti5c site.38 Perhaps O2- could rob H from organic molecules to produce HO2 species and thus to promote the production of DMAB. These results hint that PATP molecules lying on the surface of silver oxide could promote the transformation since only in this way oxygen atoms and O2- can easily take part in the reaction. 4. Conclusions In all, oxidation of AgNPs could improve the transformation efficiency of PATP to DMAB conditionally. By comparison of the evolutions of η with the laser illumination time and by comparison of SERS spectra of AgNPs oxidized in molecular oxygen and in oxygen plasma, the transformation mechanism of PATP to DMAB is explored. We suggest that O2- and O- are responsible for the transformation. In addition, we try to enhance the understanding of the reaction mechanisms of O2- and O- with atomic hydrogen of organic molecules, with the help of what disclosed by STM of molecular oxygen dissociation and reactions with H2O on the surface of TiO2. If flat surfaces of silver oxide with atomic resolution images as TiO2 could be obtained, the whole 10


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process will be observed directly under STM. Acknowledgement The authors acknowledge financial support of Natural Science Foundation of Anhui Province (grant number: 1708085MA11) and National Natural Science Foundation of China (grant number: 11674081), and the Fundamental Research Funds for the Central Universities. References 1

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Wu, Q. M.; Si, M. T.; Fang, Y. C. et al. Localized surface plasmon resonance of Ag nanoparticles damped by Ag2O formed during oxygen plasma irradiation, Nanotechnology 2018, 29, 295702.

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Ushakov, D. B.; Gilmore, K.; Seeberger, P. H. Consecutive oxygen-based oxidations convert amines to a-cyanoepoxides, Chem. Commun. 2014, 50, 12649-12651. 21

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(a)

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N2 - laser-2s Air-laser-2s O2 - laser-2s

(6)

100nm

(b) 0.3

488

20sAgNPs

0.2 0.1

Intensity (a. (a.u.)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

(c)Langmuir

N2- laser-400s Air-laser-400s O2 - laser-400s 1080 1142

1432 1388

0.47

(5)

0.13

(4)

0.45

(3)

0.05

(2)

0.07

(1)

0.00

ACS Paragon Plus Environment 0.0 400 500 600 700 800 400 600 800 1000 1200 1400 1600 1800 Wavelength (nm)

-1

Raman Shift (cm )

Page 15 of 19 9000 laser-2s 6000

3000

1080 1142

1432 1388

O2-250䉝-30min

0.44

O2-200䉝-30min

0.46

O2-150䉝-30min

0.33

0

3000

O2 -250䉝-30min

0.97

O2 -200䉝-30min

0.83

O2 -150䉝-30min

0.71

O2-250 䉝 -30min

0.44

O2-250䉝 -5min

0.24

laser-80s O2 -250䉝 -180min O2-250䉝 -90min

0.12

3000 O2-250䉝 -30min

0.97

(c)

O2 -250䉝-30min

2.86

O2 -200䉝-30min

1.79

O2-150䉝-30min

O2-250䉝 -5min

laser-400s O2 -250 䉝 -180min

0.82

(f )

6000

3000

O2-250 䉝 -90min

0.44

O2-250 䉝 -30min

2.86

ACS Paragon Plus Environment O - 250 䉝 -5min

1.01 0 400 600 800 1000 1200 1400 1600 1800 Raman Shift (cm-1)

(e)

6000

0 laser-400s

3000

0.05

(b)

0

6000

(d)

0 laser-80s

6000

laser-2s 1432 1080 O2-250 䉝 -180min 1142 1388

6000 O -250 䉝 -90min 2

3000

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

9000 (aLangmuir )

1.12 2 0 400 600 800 1000 1200 1400 1600 1800 Raman Shift (cm-1)

N2-30min

3.0

OPI-2s

O2- 250䉝 - 90min

O2- 250䉝 - 30min

O2- 250䉝- 5min

O2- 200䉝 - 30min

O2- 150䉝 - 30min

0.4

Page 16 of 19

(b)

O2 -150䉝 -30min O2 -200䉝 -30min

0.3

488

0.2

O2 -250䉝 -30min 20sAgNPs

0.1

2.0 0.0 0.3 Absorbance

l1142/I1080

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

O2-30min

O2-250䉝䉝 - 180min

(a) Absorbance

Langmuir

1.0


0.0 0

100

200 laser time (s)

300

400

(c)

O2- 250 䉝-5min O2- 250 䉝-30min O2- 250 䉝-90min O2- 250 䉝-180min 20sAgNPs

0.2 0.1 0.0

400

500

600

700

Wavelength (nm)

800

OPI-2s-O2-30min

1500

1432 1142 1388

1080

(a) Langmuir 0.4

20sAgNPs OPI-2s-O2 -30min

(b)

0.3

laser-400s 1 2.85 488 21000 0.2 3 laser-80s 1.92 4 500 0.1 5 ACS Paragon Plus Environment laser-2s 1.74 6 0 0.0 400 500 600 700 7 400 600 800 1000 1200 1400 1600 1800 Raman Shift (cm ) Wavelength (nm) 8

Absorbance

Intensity (a.u.)

Page 2000 17 of 19

-1

800

Intensity (a.u.)

600

1085

laser-180s

Intensity (a.u.)

(a)Langmuir 500

3000 OPI-2s

400

OPI-2s 269

Page 18(b) of 19 458

Intensity (a.u.)

Intensity (a.u.)

785 1 2000 300 laser-80s 2 200 3 1000 4 laser-400s 100 laser-2s 5 486 6 0 0 200 300 400 500 400 600 800 1000 1200 1400 7 Raman Shift (cm-1) Raman Shift (cm-1) 8 (d) (c) 9 5000 O2-250䉝 -㻟㻜㼙㼕㼚 500 O2-250䉝 -㻟㻜㼙㼕㼚㼙㼕㼚㼙㼕㼚 1096 10 579 laser-400s 802 400 114000 458 123000 300 laser-180s 269 13 200 142000 laser-80s 15 laser-400s 100 1000 laser-2s Plus Environment 16 ACS Paragon 17 0 0 200 300 400 500 800 1000 1200 1400 18 400 600 Raman -1 Raman Shift (cm-1) Shift (cm )

Page 19 of 19

NH2

NH2

S

S

OPI: oxygen plasma irradiation TO: thermal oxidation

h

Ag2O

Ag

OPI

Ag

TO

h=TO -250䉝 - 5min h=TO -250䉝 - 30min

3.0

Laser Excitation N N

h=TO -250䉝 - 90min

l1142/I1080

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

Langmuir

2.0 1.0

0.0

S 0

Ag2O

100 200 300 400 Ag ACS(s) Paragon Plus Environment laser time

S

Efficiency and Mechanism - American Chemical Society

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