Plasmon-Driven Diazo Coupling Reactions of p-Nitroaniline via −NH2

Article pubs.acs.org/JPCC

Plasmon-Driven Diazo Coupling Reactions of p‑Nitroaniline via −NH2 or −NO2 in Atmosphere Environment Qianqian Ding,†,⊥ Maodu Chen,†,⊥ Yurui Fang,† Zhenglong Zhang,§ and Mengtao Sun*,‡,∥ †

Key Laboratory of Materials Modification by Laser, Electron, and Ion Beams (Ministry of Education), School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116024, China ‡ Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, China § School of Physics and Information Technology, Shaanxi Normal University, Xi’an 710062, China ∥ Beijing National Laboratory for Condensed Matter Physics, Beijing Key Laboratory for Nanomaterials and Nanodevices, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ABSTRACT: Plasmon-driven diazo coupling reaction of pnitroaniline (PNA), containing both an amine group (−NH2) and a nitro group (−NO2), in ambient atmosphere environments remains unknown in experiments, on both metal nanoparticles and graphene-mediated metal nanostructures. Using the surface-enhanced Raman scattering (SERS) technique, we presented the surface coupling reactions of PNA under atmospheric environment, adsorbed on the Ag nanoparticle (AgNP) arrays, graphene-coated Ag nanoparticle (G/AgNPs) hybrids, and graphene-covered Ag nanowire (G/ AgNW) composites, respectively. It was found that PNAs were converted into 4,4′-diaminoazobenzene (DAAB) through a selective reduction reaction, rather than oxidized into 4,4′dinitroazobenzene (DNAB). These results may be useful for understanding the reaction mechanism for plasmon-driven chemical reactions of the molecules containing both amine groups and nitro groups under ambient conditions.

1. INTRODUCTION The strong interaction of light and the electrons on the metal nanoparticles’ surface can concentrate the electromagnetic field into nanoscale domains, due to the collective excitation of the electrons on the metal surface, known as surface plasmons (SPs). SPs decay either radiatively by re-emitted photons1 or nonradiatively through the generation of hot electrons,2 which can trigger chemical reactions of the molecules adsorbed on the surface of metal nanoparticles.3−7 Since the discovery in 2010, plasmon-driven surface catalytic reactions on nanoscale metal surfaces, which refer to the oxidation reactions of aromatic amine compounds and the reduction reactions of aromatic nitro compounds, have been extensively studied both experimentally8−12 and theoretically.13,14 Importantly, these reactions can be monitored in situ by surface-enhanced Raman scattering (SERS) and tip-enhanced Raman spectroscopy (TERS),15−21 thus noble metal nanostructures act as photocatalysts and signal enhancers simultaneously. Recently, the combination of graphene or its derivatives with metal nanomaterials, which form graphene−metal hybrid structures, has been successfully utilized in plasmon-driven chemical reactions22−26 due to their marked advantages, such as high durability and good catalytic activity. Particularly, ultrafast transient absorption spectroscopy and SERS spectroscopy have revealed that the graphene−Ag nanowire (G/AgNW) hybrid © XXXX American Chemical Society

structures can significantly enhance the photocatalytic efficiency of surface reduction reactions, resulting from strong plasmon− exciton coupling in G/AgNW hybrids.27 On the other hand, the reaction surroundings, for example, atmosphere conditions, electrochemical environments, icy environments, and highvacuum conditions, can strongly affect the selectivity and stability of plasmon-driven surface catalytic reactions.28,29 The selectivity of plasmon-driven surface catalytic reactions in various environments is an interesting issue in the field of plasmon-related photochemistry. p-Nitroaniline (PNA), with both nitro (−NO2) and amine (−NH2) groups, is the best candidate for studying the selectivity for plasmon-driven chemical reactions under different environments. Recently, it has been reported that PNA (see Figure 1a) can be selectively converted to 4,4′-diaminoazobenzene (DAAB, see Figure 1b) on a roughed Ag electrode in an aqueous environment.30 Subsequently, Zhao et al. proposed theoretically that PNA will be dimerized to 4,4′-dinitroazobenzene (DNAB, see Figure 1c) on the Ag10 cluster by an oxidative coupling, when exposed in the air.31 However, the plasmon-driven chemical reactions of the molecules containing both the amine group and the nitro Received: December 14, 2016 Revised: February 18, 2017 Published: February 18, 2017 A

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centrifugation. Finally, the fresh AgNWs were deposited on the SiO2/Si substrate by the spin-coating process. 2.2. Preparation of G/AgNPs and G/AgNW Hybrid Structure. Large-scale monolayer graphene was synthesized on copper (Cu) foils using chemical vapor deposition (CVD).39 The detailed fabrication process was shown in our previous work.27 The hybrid G/AgNP (G/AgNW) structure can be prepared via the PMMA-assisted transfer method. First, PMMA thin film was spin-coated (3000 rpm, 1 min) on the graphene/ Cu foil. Then, the Cu foil was etched away in a ferric chloride (FeCl3) solution (0.5 M) for 4 h. Subsequently, the floating PMMA/graphene film was soaked in deionized water four times and transferred onto the substrate with AgNPs (AgNWs). At Last, the PMMA/G/AgNP (AgNW) substrate was soaked in acetone solution to remove PMMA and rinsed by ethanol solution and deionized water in sequence. Note that, in order to thoroughly remove PMMA, the sample was annealed in a gas mixture of Ar and H2 for 2 h at 400 °C. 2.3. Characterization and Raman Measurements. The surface morphologies of the AgNP array, G/AgNP hybrid, and G/AgNW composite were obtained on a Hitachi S4800 scanning electron microscope (SEM). An UV−visible−nearinfrared spectrometer (Ocean Optics, Maya 2000-Pro, Dunedin, FL) was used to measure the transmission spectra of AgNPs and the G/AgNP hybrid. The PNA, DAAB, and DNAB powder were purchased from Aldrich Chemical Co. and Beijing Kaida Co., respectively. Using 20 mM, 2 mM, 0.2 mM, and 0.02 mM PNA ethanol solution as the probe molecules, the surface catalytic reactions on the individual AgNPs, hybrid G/ AgNPs, and G/AgNW were performed at room temperature on a Renishaw inVia Raman microscope spectrometer. The excitation wavelengths are 532 and 632.8 nm, equipped with 1800 grooves/mm and 1200 grooves/mm grating, respectively. The Raman signal was collected by a 100 × 0.85 NA objective lens. Every Raman spectrum was recorded with an accumulation time of 10 s.

Figure 1. Molecular structures of (a) PNA, (b) DAAB, and (c) DNAB.

group, such as PNA, on metal nanoparticles in an atmospheric environment have not been reported experimentally. On the other hand, the oxidation of the amine group (p-aminothiophenol, PATP) and reduction of the nitro group (4nitrothiophenol, 4-NBT) on graphene-covered metal nanostructures have been investigated, respectively.22,27 The influence of graphene on the plasmon-catalyzed reaction can be interpreted with strong metal−graphene coupling interaction. Owing to the strong plasmon−exciton coupling, the metal−graphene hybrid platform increases the plasmon-toelectron conversion efficiency and induces a significant accumulation of high-density hot electrons, thereby enhancing the efficiency of surface catalytic reactions in comparison with that assisted by a single plasmon.27 Moreover, graphene can provide chemical enhancement for SERS detection32 and protect Ag nanostructures from rapid oxidation.33 As a consequence, the graphene-based SERS (G-SERS) materials have been widely used in SERS measurement and plasmon− exciton codriven chemical reactions.22−27,33−37 Untillnow, such hybrid material has not been exploited to investigate the plasmon-driven chemical reactions of the molecules containing both the amine group and the nitro group. Therefore, we study not only the surface chemical reactions of PNA on AgNPs but also that on graphene-coated Ag nanostructures. Herein, we found that PNA can be selectively converted into DAAB on AgNPs in the air through plasmon-driven reduction reactions, monitored by in situ SERS spectroscopy, not as theoretically expected to DNAB. Furthermore, the same reactions occur when PNA is adsorbed on monolayer graphene-covered AgNPs and a single AgNW. This finding could provide more possibilities for the plasmon-driven reaction mechanism of PNA molecules.

3. RESULTS AND DISCUSSION 3.1. Characterization of SERS and Graphene-Mediated SERS Substrates. The typical morphology of annealed AgNPs on the SiO2/Si substrate is shown in Figure 2a. The average diameter (D̅ ) and the standard deviation (SD) of AgNPs are 29 and 11 nm, respectively, which were calculated with D̅ =

∑i niDi ∑i ni

and SD =

∑i ni(D̅ − Di)2 /∑i ni , where ni is

the numbers of AgNPs with diameter Di. The graphene border in Figure 2b clearly displays that the AgNPs are covered with graphene. Furthermore, the transmission spectra in Figure 2c demonstrate the local surface plasmon resonance (SPR) peaks of individual AgNPs and the G/AgNP hybrid are 412 and 441 nm, respectively. Compared with that of AgNPs, the SPR peak of the G/AgNP hybrid red shifts and widens. This phenomenon results from the interaction between graphene and AgNPs: dielectric constant of the surroundings changes, and the energy transfers from AgNPs to graphene. Besides, the single AgNW in Figure 2d, with a diameter of around 140 nm, is veiled with graphene (G/AgNW composite), which is another hybrid nanostructure acting as a nanocatalyst for chemical reactions of PNA. 3.2. Selective Reduction Reactions of PNA on the AgNPs in the Air. In this study, we first investigated the surface catalytic reactions of PNA in the atmosphere circumstances, using AgNPs as a catalyst as well as a SERS-active

2. EXPERIMENTAL SECTION 2.1. Preparation of AgNPs and AgNWs. In order to fabricate AgNPs, a layer of silver film (10 nm thick) was deposited on the SiO2/Si wafer by a thermal evaporation. The vacuum degree was approximately 8.6 × 10−5 Pa, and the deposition rate was set to 0.06−0.08 Å/s. Subsequently, the prepared Ag film/SiO2/Si was annealed in the gaseous mixture Ar:H2 for 2 h at 400 °C, forming AgNPs. On the other hand, monocrystalline AgNWs were synthesized by a chemical method.38 Typically, 167 mg of poly(vinylpyrrolidone) (PVP) was added to 15 mL of ethylene glycol (EG) solution under continuous magnetic stirring. Then, 0.2 mL (1.5 M) of silver nitrate solution (AgNO3) was added. The mixed solution was heated at 70 °C for 30 min in an oil bath and kept to 150 °C for 90 min. The residual EG and PVP can be removed by B

DOI: 10.1021/acs.jpcc.6b12589 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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where the band at 1350 cm−1 is attributed to the two symmetric −NO2 stretching modes. The detailed quantitative analysis of molecular spectra of DAAB and DNAB has been reported in electrochemical SERS for catalysis in liquid environment.30 These results provide more direct evidence that PNAs are transformed into DAAB on the AgNPs in atmospheric environment via a reductive coupling reaction, which is the same reaction as that in an electrochemical environment,40 rather than an oxidation reaction to DNAB. To verify the reliability of plasmon-driven reduction reactions of PNA-to-DAAB, we performed the SERS mapping of PNA (2 mM) adsorbed on AgNPs. Color-coded Raman intensity two-dimensional (2D) SERS mapping in the wavenumber range from 1000 to 1700 cm−1 was conducted under 532 nm laser excitation with a laser power of 0.01 mW, just as shown in Figure 4a. It is obviously observed that the reduction reactions can all occur on the selected rectangular region (shown in Figure 4b), although the intensity of each spectrum is not very uniform. To quantitatively analyze the dimerization, the Raman mapping of the intensity at 1399 cm−1, the main characteristic peak of product DAAB, is presented in Figure 4b. The mapping of brightness is proportional to the signal intensity at 1399 cm−1, namely, generating more product in the bright area. 3.3. Selective Reduction Reactions of PNA on the Surface of G/AgNPs Hybrids under Ambient Conditions. The types of surface catalytic reactions of PNA in ambient air is further confirmed, when we apply the G/AgNP hybrid system for the chemical reactions. Figure 5a (top) presents the Raman spectrum of G/AgNP hybrid material, where two distinct Raman bands at 1582 and 2661 cm−1 are observed (see the blue dotted lines), known as the G and 2D peak of graphene. The intensity ratio of I(2D)/I(G) is approximately 2.01, implying high-quality and monolayer graphene on the AgNPs. Then, the surface catalytic reactions of PNA (2 mM) on the G/ AgNP hybrid were monitored using SERS technique, as shown in Figure 5a (bottom). The emergence of new Raman bands at 1127, 1188 cm−1 and 1396, 1443 cm−1 is attributed to the C−N (−NN−) and −NN− stretching vibrations of DAAB, respectively. From the SERS signal, it can be found that PNAs on the G/AgNPs are dimerized into the DAAB molecule under atmospheric environment through the same reaction path. In addition, Figure 5b exhibits the effect of molecular concentration of reactant PNA on the reductive coupling reactions. It is obvious to see that 0.2 mM is approximately the detectable low molecular concentration for the reactions on this hybrid nanostructure. From the intensity of product DAAB perspective (see the green dotted lines), 2 mM is a better reaction concentration for the conversion. The results indicate that the conversion of PNA to DAAB requires appropriate reactant concentration. In order to exclude contingency, we measured the SERS spectra of PNA (2 mM) at five different collecting points (Figure 5c), which display good reproducibility of the hybrid G/AgNP substrate. 3.4. Hot Electron-Induced Reduction Reactions of PNA on G/AgNW Composites at Atmospheric Environment. The hybridization of graphene and Ag nanoparticles is for the coupling between exciton with local surface plasmon resonance (LSPR), while the hybridization of graphene and Ag nanowire is for the coupling between exciton with propagating surface plasmon polaritons (PSPP). The PSPP has been applied to the measurement for remote excitation SERS and remote excitation surface catalysis reaction.41−44 The advantage of this

Figure 2. SEM images of (a) AgNPs and (b) G/AgNPs hybrid. (c) Transmission spectra of AgNPs and the G/AgNP hybrid, respectively. (d) SEM image of the G/AgNW composite.

substrate. Figure 3a shows the normal Raman spectrum of PNA powder. It is clear that its Raman spectrum is characterized by

Figure 3. (a) Normal Raman spectrum of PNA powder. (b) SERS spectrum of PNA on the AgNPs. SERS spectra of (c) DAAB and (d) DNAB on the AgNPs.

the intense Raman peaks centered at 1282 and 1314 cm−1, all assigned to the mixed vibrations of C−NH2 stretching and −NO2 stretching. When it adsorbed on the surface of AgNP arrays, the SERS spectrum of PNA (2 mM) in Figure 3b is significantly different from the normal Raman spectrum of PNA solid. Two intense Raman bands at 1399 and 1444 cm−1 are attributed to the −NN− stretching vibrations of DAAB molecules.40 More importantly, the SERS profile in Figure 3b is almost identical to the SERS spectrum of DAAB (2 mM) on the AgNPs (Figure 3c) and is apparently different from the SERS spectrum of DNAB (2 mM) on the AgNPs in Figure 3d, C

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Figure 4. (a) Color-coded SERS intensity of 2D mapping (step size: 1 μm, mapping area: 8 × 8 = 64 points) of PNA. (b) Brightness-dependent intensity at 1399 cm−1 from the measured rectangular region.

Figure 5. (a) Raman spectrum of G/AgNPs (top) and the SERS spectrum of PNA (2 mM) on the G/AgNP substrate (bottom). (b) SERS spectra of molecular concentration-dependent chemical reactions of PNA on the G/AgNPs hybrid. (c) SERS signal of PNA from five different collecting points on the G/AgNPs substrate. All spectra were measured in ambient air, using a 532 nm excitation source with a laser power of 0.14 mW.

0.14 mW), the intensities of the Raman peak at 1285 cm−1 decrease, while Raman intensities of the bands at 1400 and 1443 cm−1 (marked with asterisk sign) increase, implying a gradual conversion of PNAs to DAAB. After continuous laser illumination for 720 s, the characteristic band of PNA almost completely disappears, and the SERS spectra are dominated by the Raman peaks of DAAB. The phenomenon demonstrates almost all PNA molecules have been converted into DAAB. Besides, we conducted another control experiment under 632.8 nm laser excitation. The dimerization process can be clearly observed in Figure 6d. However, the intensity of the characteristic peak of reactant PNA (marked by up triangle sign) is much stronger than those of product DAAB (marked with asterisk sign) even after irradiation for 720 s, meaning a low yield of target product excited at 632.8 nm laser. To quantitative analysis of the reaction process, we calculated the apparent rate constants of the reduction reactions under 532 and 632.8 nm excitation, respectively. According to the equality45

novel method has been discussed in our previous work.41−44 Very recently, it has been found that the graphene-coated AgNW composites can produce high-density and long-lifetime hot electrons,27 which can be used as a reliable analytical platform to observe the hot electron-induced surface chemical reactions. In the future, the coupling between exciton with PSPP will be a better way to study the remote excitation SERS and the remote excitation surface catalysis reaction. Herein, we applied this hybrid nanostructure to investigate the chemical reactions of PNA. Note that the following catalytic reactions were all performed under atmospheric environments. As shown in Figure 6a, the SERS spectrum of PNA on the G/AgNW hybrid (top) is almost identical to that of DAAB on the same substrate (bottom), which provides the solid evidence that PNAs have been dimerized into DAAB. However, the chemical reactions of DAAB molecules on the G/AgNW hybrid have not occurred, for any spectral changes in the series of SERS spectra (Figure 6b). That implies that the DAAB molecule has a stable structure, as reported by Cui et al.40 The dynamic process of the dimerization of PNA-to-DAAB can be investigated by using in situ time-dependent SERS technique, as presented in Figure 6c. Initially, it is clearly seen that the characteristic band of PNA at 1285 cm−1 (marked by up triangle sign), due to ν(−NO2), dominates in the SERS spectrum. With increasing the laser irradiation time (532 nm,

ISERS,P ISERS,R

= kt(k ∝ ak 2)

(1)

where ISERS,P and ISERS,R represent the intensities of Raman peak at 1443 cm−1 of the product DAAB and 1285 cm−1 of the reactant PNA, respectively. The apparent reaction rate, denoted D

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controlled reaction channel leading to DAAB, and (c) the plasmonic hot electrons giving rise to exclusive DAAB. According to our experimental results, the third one is the mechanism. Without assistance from plasmon excitation, there is no chemical reaction for PNA powder. With the plasmon for molecules adsorbed on the SERS substrate, the reaction occurred. In this reduction reaction, hot electrons not only provide 8 electrons for producing each DAAB molecule 8e

PNA + PNA → DAAB

(2)

but also provide kinetic energy to overcome the reaction barrier. Holze investigated SERS spectroscopy of PNA on silver and gold electrodes.47 It was proposed that PNA adsorbed on metal electrodes with a perpendicular orientation via the nitro group, not via the amino group. The plasmonic hot electrons on the SERS surface are closer to the nitro group, which results in hot electrons captured by PNA, and then generating DAAB. Note that our experimental results disagreed with previously published theoretical predictions in ref 31. In the theoretical calculations for plasmon-driven oxidation reaction in atmosphere environment, the model is based on the assumption: the hot electrons (generated from plasmon decay) first cause the singlet O2 adsorbed on the substrate to be triplet excited state O2, and then the triplet O2 adsorbed on the substrate catalyzes the oxidation reaction.31 Recent experimental reports revealed that O2 plays an active role in the case of photoinduced azocoupling reaction but is not required under plasmonic conditions in argon environment.44 Their study suggested that plasmon-induced hot electrons provide the necessary activation energy for the azo-coupling reaction without the need for O2.48 Moreover, by using a silver-coated tip in the transmission-mode AFM-TERS in the vacuum chamber, Kumar et al. also reported nanoscale Raman mapping for photocatalytic reaction of PATP to DMAB.49 It has also reported plasmon-driven oxidation reaction of PATP to DMAB in high vacuum STM-TERS.50 The above two experiments in high vacuum condition demonstrate that the azo-coupling reaction from the amino group does not need O2. So, it is necessary to further provide reasonable interpretation for plasmon-driven oxidation reaction. Our results provide reasonable experimental data for the further theoretical investigation in the future.

Figure 6. (a) SERS spectra of PNA (2 mM) (top) and DAAB (2 mM) (bottom). (b) Series of SERS spectra of DAAB (2 mM) against time. Reaction dynamics of PNAs transformed into DAAB using excitation sources of (c) 532 nm (0.14 mW) and (d) 632.8 nm (0.29 mW).

by k, is proportional to the second-order reaction rate constant k2. Figure 7a and 7b illustrates the corresponding plot of I(1443

Figure 7. Reaction rate constants for the selective reductive reactions of PNAs to DAAB excited at (a) 532 nm and (b) 632.8 nm. Error bars indicate the average and standard deviation from the experiments performed in quintuplicate. The error in the apparent rate constant is the standard errors of fitting slope.

4. CONCLUSIONS In summary, we designed and synthesized AgNPs, G/AgNP hybrids, and G/AgNW composites, which all acted as the bifunctional platforms for investigating the surface chemical reactions of PNA. Using the SERS spectroscopy, the selective dimerization reactions of PNA into DAAB were first witnessed on three different substrates in atmosphere conditions, which are the plasmon-driven reductive coupling reactions. Besides, the SERS mapping of PNA on AgNP arrays indicates the conversion of PNA to DAAB can occur easily. Furthermore, the comparative kinetics results on G/AgNW composites demonstrate that the conversion efficiency of PNA-to-DAAB is strongly affected by the laser wavelength. These direct observations of PNA-to-DAAB in the air on metal nanoparticles and graphene−metal hybrid nanostructures may prompt the understanding of the reaction mechanism of plasmon-driven chemical reactions of molecules similar to PNA.

cm−1)/I(1285 cm−1) against collecting time t, under the excitation source of 532 and 632.8 nm, respectively. Compared with the k value under 632.8 nm illumination, the apparent rate constants of the catalytic reactions excited at 532 nm laser are obviously larger, giving k532 = (1.3 ± 0.1) × 10−2 s−1 and k632.8 = (2.7 ± 0.2) × 10−4 s−1, respectively. The phenomenon is presumably associated with the effect of excitation wavelength on the efficiency of the generation of hot electrons.46 On the other hand, it reveals that the hot electron-induced reduction reactions of PNA-to-DAAB on the G/AgNW hybrids require proper laser wavelength and irradiation time. There are three kinds of reaction mechanisms for generating DAAB: (a) the reaction channel leading to DAAB thermodynamically more favorable than that to DNAB, (b) kinetically E

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M. T. Sun). ORCID

Mengtao Sun: 0000-0002-8153-2679 Author Contributions

⊥ Q.D. and M.C. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos: 11374353, 11374045, 91436102, 11504224, and 11474141), National Basic Research Program of China (Grant number 2016YFA02008000), the program for New Century Excellent Talents in University (Grant No: NCET-12-0077), and the program of Liaoning Key Laboratory of Semiconductor Light Emitting and Photocatalytic Materials.

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