The pH-Controlled Plasmon-Assisted Surface Photocatalysis …n03.iphy.ac.cn/Article/2011 J. Phys. Chem. C 115 9629-9636.pdf · 17 vibrational mode is the NdN stretching mode of DMAB

Published: April 28, 2011

r 2011 American Chemical Society 9629 dx.doi.org/10.1021/jp201002v | J. Phys. Chem. C 2011, 115, 9629–9636

ARTICLE

pubs.acs.org/JPCC

The pH-Controlled Plasmon-Assisted Surface PhotocatalysisReaction of 4-Aminothiophenol to p,p0-Dimercaptoazobenzeneon Au, Ag, and Cu ColloidsMengtao Sun,*,† Yingzhou Huang,§,† Lixin Xia,*,‡ Xiaowei Chen,|| and Hongxing Xu†,^

†Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences,P.O. Box 603-146, Beijing 100190, People's Republic of China‡College of Chemistry, Liaoning University, Shenyang 110036, People's Republic of China§Department of Applied Physics, Chongqing University, Chongqing 400044, People's Republic of China

)Departamento de Ciencia de los Materiales, Ingeniería Metal�urgicay Química Inorg�anica, Facultadde Ciencias,Universidad de C�adiz (Campus Río San Pedro), E-11510 Puerto Real (Cadiz), Spain^Division of Solid State Physics, Lund University, Lund 22100, Sweden

ABSTRACT:

In this paper, we report experimentally and theoretically a surface photocatalysis reaction of 4-aminothiophenol (PATP) to p,p0-dimercaptoazobenzene (DMAB) on Au, Ag, and Cu colloids. Surface enhanced Raman scattering (SERS) spectra of PATP on AuandCu colloids are significantly different from the normal Raman spectrum of PATP powder. Quantum chemical calculations revealthat PATP on Au and Cu colloids is converted to DMAB by a surface photocatalysis reaction, and all the strongly enhanced Ramanpeaks are the symmetric Ag vibrational mode by surface plasmon. The pH value effects on surface photocatalysis reaction were alsoinvestigated experimentally. It is found that plasmon-assisted surface photocatalysis reaction can be efficiently controlled by differentpH values. The possibility of protonation of PATP adsorbed on Au and Ag nanoparticles at pH 3 is investigated theoretically. Themolecular mechanism is proposed for controlling surface photocatalysis reaction by pH values.

I. INTRODUCTION

It has become an important scientific area for the interaction oflaser radiation with molecules absorbed on metal surfaces.Initially the effect in this area focused on the phenomena ofsurface-enhanced Raman scattering (SERS).1�5 These experi-ments showed that the apparent cross section for Ramanscattering of a molecule absorbed on a rough surface of silverenhanced bymany orders of magnitude above its gas phase value.There are two widely accepted enhancement mechanisms, whichare electromagnetic field enhancement and chemical enhance-ment (charge transfer between molecules and metal).4,6�10

Using SERS, recently surface photocatalysis reactions have beeninvestigated theoretically and experimentally. Recently, we haveproved the photochemical conversion of 4-aminothiophenol(PATP) to p,p0-dimercaptoazobenzene (DMAB) occurs duringSERS experiments with enhancing Ag colloids.11 The SERS spectra

of a mixture of PATP with Ag colloids showed good agreementwith theoretically predicted spectra of DMAB and were signifi-cantly different from PATP spectra. This interpretation was furtherproven by surface mass spectrometry measurements.12 The massspectra clearly confirmed the presence of DMAB in the sample.12

All of the Raman peaks are assigned as the symmetric Ag vibrationalmodes, which are strongly enhanced by surface plasmon.11,13 Bymeans of time-dependent SERS, Canpean and co-workers alsoclaimed catalytic coupling reaction PATP on polystyrene spheresdecorated with gold nanoparticle.14 It is also confirmed experi-mentally that 4-nitrobenzenethiol can be converted to DMAB bysurface photochemical reaction in Cu sol.15 Recently, it is found

Received: January 30, 2011Revised: April 2, 2011

9630 dx.doi.org/10.1021/jp201002v |J. Phys. Chem. C 2011, 115, 9629–9636

The Journal of Physical Chemistry C ARTICLE

that gold nanopartices are a new class of visible light photocatalystsfor synthesis of fine organic chemicals.16�18

Recent reports show an increasing trend in applying SERS-based pH sensing probes in cellular and tissue studies,19�21

where SERS nanosensors act as a pH meter. Using SERS-basedpH sensing, in this paper, we studied pH-controlled surfacephotocatalysis reaction of 4-aminothiophenol (PATP) to p,p0-dimercaptoazobenzene (DMAB) on Au and Ag colloids. First,in the case of near neutral pH value, the surface photocatalysisreaction of PATP toDMAB onAu andCu colloids is investigatedexperimentally and confirmed by quantum chemical calculations.Second is the pH-controlled surface photocatalysis reaction ofPATP to DMAB on Au and Ag colloids. Lastly, the conclusionsare derived that in the alkaline environment, such surfacephotocatalysis reaction also occurs, while the acidic conditiondoes not favor this kind of photocatalysis reaction.

2. EXPERIMENTAL SECTION

The SERS active Au, Ag, and Cu colloids were synthesized,according to refs 22, 23, and 24, respectively. The scanningelectron microscopy (SEM) images were obtained using a fieldemission (FE) microscope (Sirion, FEI) operating at an accel-erating voltage of 10 kV. The SEM images (see Figure 1) showthat most Au nanoparticles, Ag nanoparticles, and Cu micro-meter-sized particles are hollow spheres with an average dia-meters of 25 nm, 80 nm, and 5 μm, respectively.

PATP was purchased from Aldrich Chemical Co. and usedwithout further treatment or purification. The solution of PATP inethanol was introduced into capillary sample cells for normalRaman measurement. The solution of PATP in aqueous Au andCu sols with 10�4 M concentrations was introduced into capillarysample cells for SERS measurement. The SERS spectra were

recorded by a Renishaw inVia Raman system equipped with anintegral microscope (LEICA, DMLM). The 632.8 nm radiationfrom the He�Ne laser was used as an excitation source. In ourRaman experiment, the laser power used on the SERS sample waslimited to 2 mW with a 50� objective. The appropriate holo-graphic notch filter was set in the spectroscopy. The holographicgrating (1800 grooves/mm) and the slit allowed for a spectraresolution of 1 cm�1. The spectrometer was measured to bee0.2 cm�1. Raman scattering was detected using a Peltier-cooledCCD detector (576� 384 pixels). For comparison, the pH valueeffects on SERS spectra were also measured experimentally, whereHCl and NaOHwere used to control pH = 3 and 10, respectively.

3. THEORETICAL CALCULATIONS

All of the quantum chemical calculations were done with theGaussian 09 suite.25 The ground state geometry of PATP (seeFigure 2a) was optimizedwith density functional theory (DFT),26

PW91PW91 functional27 and 6-311þg(2d,p) functional. Withthe optimized ground state geometry, the normal Raman scatter-ing (NRS) spectrum of PATP was calculated at the same level oftheory. The molecular structure of DMAB can be seen fromFigure 2b. The models of PATP�Au5 complex, PATP�Ag5complex, PATP�Cu5 complex, Au5�DMAB-Au5 junction andAu5�DMAB�Au5 (see Figure 2c�f) were employed to study theelectronic structures and SERS spectra. The ground-state geome-tries were optimized using DFT, PW91PW91 functional, 6-31G-(D) basis set for C, N, S, and H and LANL2DZ basis set28 for Au,Ag, and Cu. With the optimized ground state geometries, theSERS spectra were simulated with the same functional andbasis set. The calculated Raman frequencies were scaled to0.99. With the optimized ground state geometries, optical absorp-tion spectra of PATP�Au5 complex, PATP�Ag5 complex,

Figure 1. (a) SEM images of the Au nanoparticles, (b) Ag nanoparticles, and (c, d) Cu micrometer-sized hollow particles.



Au5�DMAB�Au5, and Cu5�DMAB�Cu5 junctions were cal-culated using the time-dependent DFT (TD-DFT) method29

with LC-PW91PW91 functional30and the same basis set. It isnoted that the long-range (LC) related density functional30 wasemployed in the optical absorption calculations. The SERS spectraof protonated PATP adsorbed on Au and Ag nanoparticles inacidic condition are also calculated at the same level of theory.

To study the local surface plasmon resonance effect on SERS,the near field distribution of the electromagnetic field is investi-gated with the finite difference time domain (FDTD) method,31

using FDTD solutions software.32 The diameter of Au nanopar-ticle is 20 nm, and the nanogap of nanoparticle dimer is 3 nm. Theelectromagnetic field enhancement is calculated in the center ofnanogap. The permittivity of Au at 632.8 nmwas taken from ref 33.

4. RESULTS AND DISCUSSION

We first measured the NRS spectrum of PATP in ethanol,which can be seen in Figure 3a. The simulated NRS spectrum of

PATP can be seen in Figure 3b. Experimental and theoreticalNRS spectra of PATP revealed that there are five Raman peaks.Before the SERS experiment, we simulated the Raman spectra ofPATP adsorbed on Au and Cu nanoparticles, where the Au5 andCu5 clusters were used in the quantum chemical calculations.Panels c and d of Figure 3 indicate that the SERS spectra of PATPon Au and Cu sols should be similar with those of theisolated PATP.

The experimental SERS spectra (see Figure 3e,f) present thatthey are significantly different from the NRS spectrum of isolatedPATP. There are three strongly enhanced SERS peaks around1135, 1384, and 1428 cm1 in Au sol (and 1150, 1380, and1422 cm�1 in Cu sol). The small differences in Raman peaks(several nanometers) for the same vibrational modes in differentsols result in different substrate effect. Our previous experimentalreports and theoretical calculations revealed that theyshould be symmetric Ag12, Ag16, and Ag17 vibrational modes ofDMAB.11 Especially, Ag17 vibrational mode is the �NdN�stretching mode of DMAB. Therefore our experimental results

Figure 2. Molecular structures of (a) PATP, (b) DMAB, (c) PATP�Au5 complex, (d) PATP�Cu5 complex, (e) Au5�DMAB�Au5 junction, and(f) Cu5�DMAB�Cu5 junction. The symbols of atoms can be seen from parts c and d.



demonstrate that surface photocatalysis reaction of PATP toDMAB can occur in Au and Cu sols. Note that PATP cannot beconverted to DMAB on flat Au film by surface photocatalysisreaction.13

To further confirm that the experimental SERS spectra inpanels e and f of Figure 3 are the SERS spectra of DMABproduced from PATP by surface photocatalsis reaction, wecalculated the SERS spectra of Au5�DMAB�Au5 and

Figure 3. (a)NRS spectrum of PATP, (b) calculatedNRS spectrum of PATP, (c) calculated PATP�Au5 complex, (d) calculated PATP�Cu5 complex,(e) SERS spectrum of DMAB in Au sol, (f) SERS spectrum of DMAB in Cu sol, (g) calculated SERS spectrum of DMAB in Au sol, and (h) calculatedSERS spectrum of DMAB in Cu sol.



Cu5�DMAB�Cu5 junctions, respectively. It is found that thesethree Raman peaks that appeared in SERS experiment are also, asexpected, strongly enhanced. The symmetric Ag12, Ag16, and

Ag17 vibrational mode for SERS of DMAB in Au and Cu sol canbe seen from Figure 4, and Ag17 is the �NdN� stretchingmode. The above-mentioned theoretical simulations stronglysupport experimental results.

Figure 4. (a) Ag12 of DMAB in Au5�DMAB�Au5 junction, (b) Ag12 of DMAB in Cu5�DMAB�Cu5 junction, (c) Ag16 of DMAB inAu5�DMAB�Au5 junction, (d) Ag16 of DMAB in Cu5�DMAB�Cu5 junction, (e) Ag17 of DMAB in Au5�DMAB�Au5 junction, and (f) Ag17 ofDMAB in Cu5�DMAB�Cu5 junction.

Table 1. Electronic Transition Energies of Au5�DMAB�Au5and Cu5�DMAB�Cu5 Junctions, Where f Is OscillatorStrength

Au5�DMAB�Au5 junction Cu5�DMAB�Cu5 junction

nm f nm f

S1 447.53 0.0000 441.13 0.0000

S2 360.87 0.0056 374.82 0.0089

S3 360.85 0.0000 374.82 0.0000

S4 347.05 0.0000 353.61 0.0000

S5 346.93 0.0843 352.90 0.8101

S6 330.60 1.6262 333.25 1.7629

S7 318.98 0.0000 328.56 0.0000

S8 315.84 0.2045 328.52 0.0036

S9 307.74 0.0000 320.97 0.0000

S10 307.74 0.0002 319.89 0.0637

Table 2. Calculated Static Polarizabilities of PATP, Au5�DMAB�Au5 Junction and Cu5�DMAB�Cu5 Junction

xx yy zz

PATP 153.377 100.904 63.505

Au5�DMAB�Au5 junction 1221.991 573.792 430.347

Cu5�DMAB�Cu5 junction 1108.686 511.572 413.491

Figure 5. The distribution of the local electromagnetic field enhance-ment at 632.8 nm in junctions of Au nanoparticle dimer (diameter20 nm) with a nanogap of 3 nm. A color bar in logarithmic scale forelectric enhancement is shown.



The electronic transitions of Au5�DMAB�Au5 and Cu5�DMAB�Cu5 junctions were studied with the TD-DFT method.The calculated results are listed in Table 1, which shows that theyare the SERS spectra, not surface enhanced resonance Ramanscattering (SERRS) spectra, since S1 values are 447 and 441 nm,respectively. Theoretical results also exhibit that 0.373 and 0.021 etransferred to Au and Cu clusters, respectively, due to theinteraction between molecule and metals. The charge transfer(between molecule and metal) results in the increase of the

polarizabilities (see Table 2), which leads to the stronglyenhancement of SERS spectra.

To study the local surface plasmon resonance effect on SERS,the near field distribution of an electromagnetic field is investigatedwith FDTD. Figure 5 shows that the SPR is localized in thenanogap and the strongest enhancement is at the center of the gap.The electromagnetic enhancement is |M|2 = 3.3� 104, where |M| =|Elocal/Ein|, and Elocal and Ein are local and incident electric fields,respectively. The SERS enhancement is |M|4 = 1.0 � 109.

Figure 6. (a) SERS spectrum of DMAB in Au sol (pH = 10), (b) SERS spectrum of DMAB in Ag sol (pH = 10), (c) SERS spectrum of PATP in Au sol(pH = 3), (d) SERS spectrum of PATP in Ag sol (pH = 3), (e) calculated SERS spectrum of PATP�Au5 complex, (f) calculated SERS spectrum ofPATP�Ag5 complex, (g) calculated SERS spectrum of protonated PATP�Au5 complex, and (h) calculated SERS spectrum of protonated PATP�Ag5complex.



The pH value dependent SERS spectra were also measured.For pH = 10 in Au and Ag sol, we found that the Raman peaks forAg12, Ag16, and Ag17 vibrational modes are strongly enhanced(Figure 6a,b), which confirmed experimentally that the surfacephotocatalysis reaction of DMAB produced from PATP oc-curred. For pH = 3 in Au and Ag sol (Figure 6c,d), we foundthat the Raman peaks for Ag12, Ag16, and Ag17 vibrational modesare absent, and by comparing with Figure 6e,f, they are similar tothe simulated SERS spectra of PATP adsorbed on Au and Agclusters. This means that under the condition of pH = 3, thesurface photocatalysis reaction cannot happen. It is derived thatthe Hþ and OH� in the sols can control surface photocatalysisreaction. To check whether the Raman spectra of PATP in Auand Ag sols in acidic condition is resonant Raman or not,electronic transitions of PATP adsorbed on Au and Ag clusterswere calculated with quantum chemical method. The calculated

results (see Table 3) revealed that they are not resonant Ramanscattering.

The possibility of protonation of PATP adsorbed on Au andAg nanoparticles at pH = 3 is investigated theoretically (seeFigure 6g,h). Figure 6g revealed that there are two additionalstrongly enhanced Raman peaks at 1134 and 1489 cm�1 forprotonation of PATP adsorbed on Au nanoparticles (theirvibrational modes can be seen from Figure 7a,b), which do notappear in Figure 6c. So PATP adsorbed on Au nanoparticles wasnot protonated for pH = 3. Figure 6h revealed that there are fouradditional strongly enhanced Raman peaks at 1039, 1132, 1483,and 1627 cm�1 for protonation of PATP adsorbed on Agnanoparticles (their vibrational modes can be seen in panelsc�f of Figures 7), which do not appear in Figure 6d. So, PATPadsorbed on Ag nanoparticles was not protonated for pH = 3.

The molecular mechanism is proposed for controlling surfacephotocatalysis reaction by pH. In the neutral and alkalinecondition, the PATP can be oxidized to DMAB on supportedAu and Ag nanoparticles by surface plasmon resonance effectinduced by laser. In the neutral and alkaline conditions, PATPfirst can be easily deprotonated, then the deprotonated PATP isoxidized to DMAB. While in the acidic condition, PATP is hardto be oxidized, because of abundant Hþ in solution.

5. CONCLUSION

The pH-controlled plasmon-assisted surface photocatalysisreaction of PATP to DMAB on Au, Ag, and Cu colloids has beeninvestigated experimentally and theoretically. It is found thatplasmon-assisted surface photocatalysis reaction can be effi-ciently controlled by different pH values. When pH = 3, thesurface photocatalysis reaction cannot occur; however underneutral and alkaline conditions (pH = 7 and 10), such a reactiondoes take place. By comparison of the experimental and calcu-lated results, it is revealed that PATP was not protonated in Auand Ag sols, when pH = 3.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected] (M. T. Sun), [email protected] (L. Xia).

’ACKNOWLEDGMENT

This work was supported by the National Natural ScienceFoundation of China (Grants 10874234, 90923003, and20703064), the National Basic Research Project of China(Grant 2009CB930701). Lixin Xia thanks the Innovative TeamProject of Department of Education of Liaoning Province, China(LT2010042), the Liaoning Natural Science Foundation, China(20092016), the Shenyang Natural Science Foundation, China(F10-230-4-00 and 090082), and the Liaoning University 211-Projects of the third period. Xiaowei Chen is grateful to theRam�on y Cajal program from the Ministry of Science andInnovation of Spain.

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Table 3. Electronic Transition Energies of PATP�Au5 andPATP�Ag5 Complexes, Where f Is the Oscillator Strength

PATP�Au5 complex PATP�Ag5 complex

nm f nm f

S1 356.69 0.0118 387.31 0.0050

S2 344.65 0.0791 357.94 0.8404

S3 334.35 0.0995 316.21 0.0001

S4 312.37 0.0577 292.39 0.8575

S5 302.72 0.0012 287.59 0.0344

S6 297.14 0.0002 286.07 0.0555

Figure 7. (a, b) Two calculated strongly enhanced vibrational modes ofprotonated PATP adsorbed on Au nanoparticle in Figure 6g (c�f) Fourcalculated strongly enhanced vibrational modes of protonated PATPadsorbed on Ag nanoparticle in Figure 6h.



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The pH-Controlled Plasmon-Assisted Surface Photocatalysis …n03.iphy.ac.cn/Article/2011 J. Phys. Chem. C 115 9629-9636.pdf · 17 vibrational mode is the NdN stretching mode of DMAB