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Observation of Surface Coverage-Dependent SurfaceEnhanced Raman Scattering and Kinetic Behavior of Methylene Blue Adsorbed on Silver Oxide Nanocrystals Hsiang-Ling Chen, Zen-Hung Yang, and Szetsen Lee Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02213 • Publication Date (Web): 20 Sep 2016 Downloaded from http://pubs.acs.org on September 24, 2016

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Observation of Surface Coverage-Dependent Surface-Enhanced Raman Scattering and Kinetic Behavior of Methylene Blue Adsorbed on Silver Oxide Nanocrystals Hsiang-Ling Chen, Zen-Hung Yang, and Szetsen Lee* Department of Chemistry, Chung Yuan Christian University, Jhongli District, Taoyuan City, 32023, Taiwan

ABSTRACT We report the study of surface-enhanced Raman scattering and photocatalytic reactions of methylene blue (MB) adsorbed on cubic silver oxide nanocrystals (c-Ag2O). The observed SERS activities of MB on c-Ag2O are surface coverage-dependent. According to the SERS selection rules, the strong intensities of the C–S–C and C–N–C bending modes imply that the MB molecules are adsorbed on c-Ag2O surface with a near-perpendicular position. On the other hand, if these modes are weak, MB assumes a near-parallel position to c-Ag2O surface. Under the “perpendicular” condition, the observed photocatalytic reaction rate of MB on c-Ag2O surfaces is first-order at high surface coverage. While under the “parallel” condition, the observed rate is zero-order at low surface coverage. The SERS intensity trends and the kinetic behavior of MB on c-Ag2O are correlated.

KEYWORDS: SERS, cubic Ag2O, methylene blue, surface coverage

ACS Paragon Plus Environment

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* Corresponding author: Tel: 886-3-2653308; Fax: 886-3-2653399; E-mail address: [email protected] (Szetsen Lee)


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1. INTRODUCTION One of the important applications of nanocrystals is surface-enhanced Raman scattering (SERS), which employs nanocrystals as substrates to enhance Raman cross-section of adsorbed molecules by orders of magnitude. SERS activity is highly dependent on the substrate, where excitation of the localized surface plasmons (LSPs) enhances the Raman scattering signals of adsorbed molecules. The high surface selectivity and sensitivity achieved by SERS on interfacial systems are inaccessible by normal Raman technique. In general, there are two primary mechanisms describing the SERS effects.1–3 The electromagnetic (EM) mechanism can be ascribed to the enhancement of the EM fields near the plasmon resonances of metal substrates. Typically, coinage metals such as Ag, Au, and Cu exhibit strong Raman intensity enhancement of orders of magnitude. The chemical enhancement (CE) mechanism involves the formation of new electronic states due to charge-transfer (CT) or adsorbate–substrate bonding interactions. There have been some theories developed to interpret the contributions of the CE mechanism to the SERS effect.4–7 The measured SERS spectra usually exhibit the combination of both EM and CE effects. In solar energy utilization, direct photocatalysis using nanocrystals has been a hot research topic. The nanocrystals can act as the light absorber and the catalytic center. Nanocrystals such as TiO2,8 ZnO,9 and Ag2O10–12 have been widely used as plasmonic catalysts in photodegradation reactions of environmental pollutants. Ag2O is of particular


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interests in plasmonic applications. Firstly, Ag2O is available in a variety of shapes and sizes through hydrothermal synthetic routes.10–12 Secondly, the band gap value of Ag2O is reported to be around 1.46 eV.13 It is in the visible-near infrared light range, which favors many visible light and sunlight-harnessing applications. The SERS of methylene blue (MB) on Ag surfaces has been investigated to some extent.14–20 On the other hand, the study of Raman spectra of MB on Ag2O surfaces has been less focused. Almost all of the published works about MB and Ag2O were related to photocatalytic reactions.10–12 The study of cyanine dyes adsorbed on Ag2O colloids suggests that it is the complexes of the small Ag ion clusters with the adsorbed dye molecules that contributes to the SERS activity and enhancement.21 CT effect plays a major role in the SERS observed on Ag2O surface. The observed SERS effect on Ag2O surface may as well be contributed from both Ag2O and the partially formed metallic Ag on the surface. It is well-known that Ag2O is photosensitive.6 Photo-generated electrons provided from photon-absorbing Ag2O are received by Ag+ ions to form metallic Ag. Therefore, it is expected that there are some Ag covering the Ag2O surface. Therefore the observed SERS effect on Ag2O involves both EM and CE mechanisms. In this work, we have observed SERS spectra of MB adsorbed on the cubic silver oxide (c-Ag2O) surface. The observed SERS activities of MB on c-Ag2O are surface coverage-dependent. It is confirmed by investigating the kinetic behavior of the


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photocatalytic degradation reactions of MB on c-Ag2O surfaces. Experimental and theoretical studies have demonstrated that both surface coverage of the Raman active molecule and Ag or Au nanoparticle concentration can influence the observed SERS frequencies and intensity patterns.22–26 More importantly, Brolo et al.22 have pointed out that the SERS intensities showed a direct proportionality to the surface concentration at low coverage. As the surface coverage value approached one monolayer, the SERS intensity decreased, and the trend became inverse. The properties of MB on c-Ag2O surfaces, such as SERS activities, adsorption orientations, surface coverage, and photocatalytic degradation rates will be correlated.

2. EXPERIMENTAL Silver nitrate (AgNO3, Aencore Chemical, 99.8%), ammonium nitrate (NH4NO3, Acros Organics, 99%), sodium hydroxide (NaOH, Showa Chemical, 97%), and methylene blue (MB, Acros Organics, 95%) were used as received. Ultrapure distilled and deionized water was used for preparing solutions. Similar to the preparation procedures by Lyu et al.,27,28 c-Ag2O nanocrystals were synthesized by mixing various concentration ratios of AgNO3, NH4NO3, and NaOH solutions. Field emission scanning electron microscopy (FESEM, JEOL JSM7600F, operating voltage = 10 kV) was used to inspect the surface morphology of c-Ag2O. X-ray diffraction


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(XRD) using a Panalytical X’Pert Pro(MRD) PW3040/60 diffractometer, in Bragg-Brentano geometry, with Cu radiation (Kα; λ= 0.154 nm) operated at 40 kV/40mA and a detector (Panalytical X-celerator) was carried out to characterize c-Ag2O. Please refer to Supporting Information for the SEM micrographs and XRD spectrum of synthesized c-Ag2O nanocrystals. SERS spectra of MB on c-Ag2O were recorded with the excitation line at 632.8 nm using a He–Ne laser. The Raman scattering was focused on a scanning monochromator (Jobin Yvon, 600 lines/mm, blazed for 330 nm). The slit width was fixed at 0.01 mm. The signal was amplified by a TE-cooled CCD (Jobin Yvon, 1024 × 256 pixels). The exposure time was varied from 5 s to 20 s to optimize the signal-to-noise ratio. The Raman signal was an average taken from measurements at five different spots in each sample. The spectral resolution was 1 cm−1. Laser power at the output was 25 mW and diameter of focused laser beam spot on the substrate was approximately 10 µm. Raman shifts were calibrated using the silicon (Si) reference peak at 521 cm−1. The intensity and linewidth of the Si peak were also monitored constantly, to ensure the stability of the laser power. In SERS experiments, the concentration of MB solution used was fixed at 0.01 mM and that for c-Ag2O was 0.1 mg/ml or 0.5 mg/ml. In photocatalytic degradation reaction experiments, a tungsten-halogen lamp (150 W) with a cutoff filter (λ > 400 nm) was used as a visible light source. The prepared c-Ag2O


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nanocrystals were dispersed in MB solutions (from 2 to 10 ppm) and sonicated before being transferred to a quartz cuvette. After visible-light irradiation for a period of time, the reaction solution was centrifuged for measuring the concentration of MB solutions with a UV/Vis spectrophotometer (V-675, Jasco, Japan). The intensity variation of the 664 nm absorption peak of MB was monitored.

3. RESULTS AND DISCUSSION 3.1. SERS observation Figure 1 shows the SERS spectra of MB on c-Ag2O. In the upper two SERS spectra, the MB solution concentration was fixed (0.01 mM) and [Ag2O] was 0.5 mg/ml or 0.1 mg/ml. The bottom trace of spectrum represents the bulk MB solution (1 mM). The variation of the relative SERS intensity ratios between 458 cm-1 to 1635 cm-1 and 493 cm-1 to 1635 cm-1 are related to different adsorption orientations of MB on c-Ag2O. The 1635 cm-1 band is assigned as the ring C-C stretching mode (νring(C=C)).17,18,29 According to the study of MB adsorbed on Ag surface by Zhong et al.,15 when the δ(C–S–C) peak at 458 cm-1 and the δ(C–N–C) peak at 493 cm-1 are strong, it implies that the MB molecules stand on the substrate surface with a nearly vertical orientation. In contrast, if those peaks are weak, MB lies on the substrate surface with a nearly flat orientation..


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24000

δ(C–S–C)

νring(C=C)

δ(C–N–C)

20000

MB on c-Ag2O (0.5 mg/ml)

16000

Intensity (counts)

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νasym(C–N) 12000

8000

MB on c-Ag2O (0.1 mg/ml) 4000

1 mM MB(aq) 0 300

400

500

600

1400

1500

1600

1700

1800

-1

Raman shift (cm )

Figure 1. SERS spectra of MB on c-Ag2O nanocrystals. The peak intensity ratios between 458 cm-1 to 1635 cm-1 and 493 cm-1 to 1635 cm-1 show the effect of MB surface coverage and orientation on c-Ag2O.

The relation between SERS effect and MB adsorption orientations can be explained by the SERS selection rules.30,31 The vibrational modes of molecules with a higher perpendicular polarizability component with respect to the substrate surface will have greater enhancement. Thus, for MB with aromatic ring moieties standing perpendicularly to the substrate surface, the δ(C–S–C) and δ(C–N–C) bands increase their relative intensities by surface effect. These two vibrational modes whose dipole moment derivatives possess components normal to the


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substrate surfaces are preferentially enhanced. If MB lies flat or tilted with an angle on the surface, the projection of the dipole moment on the surface normal must be smaller and therefore have weaker enhancement. The vibrational modes of the 1396 cm-1 band (νasym(C–N)) and 1635 cm-1 band (νring(C=C)) has the dipole moment derivative dominantly parallel to the substrate surfaces. It is insensitive to the adsorption orientation. Therefore, we can say that the upper trace of SERS spectrum ([c-Ag2O] = 0.5 mg/ml) in Figure 1 belongs to horizontally oriented MB. The middle trace of SERS spectrum ([c-Ag2O] = 0.1 mg/ml) is contributed from the perpendicularly oriented MB. The intensity ratios of vibrational modes with different [c-Ag2O] are presented in Table 1. The SERS intensities from low to high coverage conditions show direct proportionality to the relative MB surface concentrations. According to the study by Brolo et al.,22 the “high coverage” condition in this work is probably still less than one monolayer.

Traditional SERS studies involving Ag substrates more or less contain a native oxide layer. However, most of the contribution of SERS intensity comes from Ag (electromagnetic mechanism, EM) and a little comes from oxide layer (chemical/charge transfer mechanism, CM). EM and CM contributions can be different by orders of magnitude. In this work, the observed SERS effect on Ag2O surface may as well be contributed from both Ag2O and the partially formed metallic Ag on the surface. The EM enhancement from Ag helps to boost


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SERS intensities.

Table 1. Relative intensity ratios of vibrational modes of MB with different [c-Ag2O]. δ(C–S–C)/νring(C=C)

δ(C–N–C)/νring(C=C)

νasym(C–N)/νring(C=C)

0.5

0.62 ± 0.09

0.77 ± 0.10

0.33 ± 0.05

0.1

1.11 ± 0.16

1.77 ± 0.26

0.33 ± 0.05

[c-Ag2O] (mg/ml)

Vibrational wavenumbers: δ(C–S–C): 458 cm-1; δ(C–N–C): 493 cm-1; νasym (C–N): 1396 cm-1; νring (C=C): 1635 cm-1.

3.2. Surface coverage and kinetic behavior The results of typical studies for photocatalytic reaction experiments can be interpreted by the Langmuir–Hinshelwood model,32,33

υ=

d ( C Cmax ) dθ kKC = −kθ = =− dt dt 1 + KC

(1)

with the rate constant k. The rate υ is proportional to the surface coverage θ which is proportional to adsorbate concentration C divided by maximum adsorption capacity Cmax. K is the adsorption equilibrium constant. Integration of equation (1) gives

C  ln   + K ( C − C0 ) = −kCmax Kt  C0 


(2)

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or

−

C  C ln   + 0 kCmax K  C0  kCmax 1

 C  1 C  1 C   1 −  = − ln   +  1 −  = t k1  C0  k0  C0   C0 

(3)

C0 is the initial concentration. At the left-hand side of equation (3), it shows that the first term is contributed from the first-order kinetics with rate constant k1 = kCmax K . The second term is related to the zero-order kinetics with rate constant k0 =

kCmax . Usually, for a C0

Langmuir-Hinshelwood mechanism, first-order kinetics represents the low coverage range (KC > 1). Typically, first-order kinetics is appropriate for the low concentration range up to few ppm and the experimental data are usually well fitted by this kinetic model.34–38 Figure 2 illustrates the UV-visible absorption spectra of MB degraded using c-Ag2O nanocrystals as photocatalysts. The photocatalytic activity of c-Ag2O in degrading MB was evaluated under visible light irradiation by monitoring the decrease in absorption at 664 nm. By changing the initial concentrations of MB from 2 to 10 ppm (Figure 3(a)), the relative absorption intensity (C/C0) versus irradiation time data trends of MB degradation appear to be linear using c-Ag2O (0.1 mg/ ml) as a catalyst. It is an indication of zero-order kinetics.39 The combination of MB concentrations (2 to 10 ppm) with low c-Ag2O concentration (0.1 mg/ml) shows zero-order kinetics behavior, which indicates high coverage of MB on c-Ag2O surfaces. For the Langmuir-Hinshelwood model, zero-order kinetics dominates the high coverage and saturation region.


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MB+c-Ag2O 0.3

absorbance (Arb. uint)

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0(min)

0.2

10 0.1

0.0

200

300

400

500

600

700

800

wavelength(nm)

Figure 2. Photocatalytic degradation reactions of MB + c-Ag2O solutions monitored by UV-visible absorption spectroscopy. Initial MB concentration is 2ppm.


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1.0

10 ppm 0.8

0.6

C/C0

8 ppm 4 ppm

0.4

2 ppm

6 ppm

0.2

(a)

MB + c-Ag2O (0.1 mg/ml)

0.0 0

5

10

15

20

25

30

Time (min)

1.0

MB + c-Ag2O (0.5 mg/ml)

0.8

0.6

C/Co

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0.4

6 ppm

0.2

(b)

10 ppm

4 ppm 2 ppm

8 ppm

0.0 -2

0

2

4

6

8

10

12

14

16

18

Time (min)

Figure 3. Plots of relative absorption intensities of MB versus irradiation time with fitted curves. MB concentration range: 2 ppm to 10 ppm. c-Ag2O catalyst concentration: (a) 0.1 mg/mL and (b) 0.5 mg/mol.


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On the other hand, we have performed photocatalytic degradation experiments by using a higher c-Ag2O concentration (0.5 mg/ml) with MB concentrations kept the same. The photocatalytic degradation rate behavior of MB on c-Ag2O surface becomes first-order (Figure 3(b)). For the Langmuir-Hinshelwood model, first-order kinetics dominates the low coverage region. Figure 4 shows the plot of MB degradation rate constants versus initial MB concentrations. There are two trends: zero-order and first-order, which correspond to the conditions of high surface coverage (low c-Ag2O concentration) and low surface coverage (high c-Ag2O concentration), respectively. The degradation rate constants of MB range from 0.0128 to 0.0625 mol L-1 min-1 with the zero-order fitting (k0) and from 0.0610 to 0.5860 min-1 with the first-order fitting (k1). It shows that k1, but not k0, is strongly sensitive to the initial concentration C0 .As indicated in equation (3),

k1 ∝ K =

1  1  1  Cmax  − 1  − 1 =  C0  θ  C0  C0 

k0 ∝

1 C0

for the same initial concentration C0, k1 is strongly surface coverage dependent.


(4)

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0.6 -1

k1(min ) -1

0.4

-1

0.5

-1

Rate constant (mol L min or min )

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0.3

0.2

-1

0.1

-1

k0(mol L min )

0.0 2

4

6

8

10

Initial MB concentration (ppm)

Figure 4. Plot of degradation rate constants versus initial MB concentrations. Rate constants k0 (zero-order) and k1 (first-order) correspond to the conditions of high surface coverage and low surface coverage, respectively.

The SERS model reasonably matches with the surface coverage of MB adsorbed on c-Ag2O surface. At high surface coverage conditions ([c-Ag2O] = 0.1 mg/ml), MB molecules are standing nearly vertically on the c-Ag2O surface, allowing more MB molecules to be packed on the c-Ag2O surface. While at low surface coverage conditions ([c-Ag2O] = 0.5 mg/ml), MB molecules are lying almost parallel to the c-Ag2O surface and less number of MB molecules can occupy on the c-Ag2O surface.


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4. CONCLUSIONS The MB adsorption orientations on c-Ag2O by SERS observation has been correlated with the photocatalytic degradation rate behavior of MB adsorbed on c-Ag2O. The relative SERS intensity ratios between δ(C–S–C) to νring (C=C) and δ(C–N–C) to νring (C=C) are related to different adsorption orientations of MB on c-Ag2O. The strong relative intensities imply that the MB molecules are adsorbed on the c-Ag2O surface with a near-perpendicular orientation. However, if the relative intensities of these modes are weak, MB adopts a near-parallel orientation to the c-Ag2O surface. As verified with photocatalytic reaction experiments, the vertical orientation condition corresponds to high surface coverage of MB on c-Ag2O and first-order kinetic behavior. The zero-order kinetic behavior of MB on c-Ag2O surfaces at low surface coverage is related to parallel orientation condition.

Supporting Information Please refer to Supporting Information for the SEM micrographs and XRD spectrum of synthesized c-Ag2O nanocrystals.

ACKNOWLEDGEMENTS The authors would like to acknowledge the support from the Ministry of Science and Technology of Taiwan (NSC 103–2632–M-033–001–MY3 and MOST


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104-2119-M-033-003).

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Press. 2014, p. 824.


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Figure Caption: Figure 1. SERS spectra of MB on c-Ag2O nanocrystals. The peak intensity ratios between 458 cm-1 to 1635 cm-1 and 493 cm-1 to 1635 cm-1 show the effect of MB surface coverage and orientation on c-Ag2O.

Figure 2. Photocatalytic degradation reactions of MB + c-Ag2O solutions monitored by UV-visible absorption spectroscopy. Initial MB concentration is 2ppm.

Figure 3. Plots of relative absorption intensities of MB versus irradiation time with fitted curves. MB concentration range: 2 ppm to 10 ppm. c-Ag2O catalyst concentration: (a) 0.1 mg/mL and (b) 0.5 mg/mol.

Figure 4. Plot of degradation rate constants versus initial MB concentrations. Rate constants k0 (zero-order) and k1 (first-order) correspond to the conditions of high surface coverage and low surface coverage, respectively.


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