Photocatalytic Method for the Simultaneous Synthesis and

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Photocatalytic Method for the Simultaneous Synthesis and Immobilization of Ag Nanoparticles onto Solid Substrates Italo Azevedo Costa, Ayessa Pires Maciel, Maria José A. Sales, Luis Miguel Ramirez Rivera, Maria A.G. Soler, Marcelo A. Pereira-da-Silva, Sanclayton G C Moreira, and Leonardo Giordano Paterno J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07081 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 9, 2018

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The Journal of Physical Chemistry

Photocatalytic Method for the Simultaneous Synthesis and Immobilization of Ag Nanoparticles onto Solid Substrates

Ítalo A. Costa1, Ayessa P. Maciel1, Maria José A. Sales1, Luis Miguel R. Rivera2, Maria A. G. Soler2, Marcelo A. Pereira-da-Silva3,4, Sanclayton G. C. Moreira5 and Leonardo G. Paterno1,*

1

Laboratório de Pesquisa em Polímeros, Instituto de Química, Universidade de Brasília, 70904-

970, Brasília-DF, Brazil 2

Instituto de Física, Universidade de Brasília, 70910-900, Brasília-DF, Brazil

3

Instituto de Física de São Carlos, IFSC, Universidade de São Paulo, 13560-9700, São Carlos-

SP, Brazil 4

Centro Universitário Central Paulista – UNICEP, 13563-470, São Carlos-SP, Brazil

5

Instituto de Ciências Exatas e Naturais (ICEN), Universidade Federal do Pará, 66075-900,

Belém-PA, Brazil

*author to whom correspondence should be addressed: L. G. Paterno, [email protected]

ACS Paragon Plus Environment

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Abstract A photocatalytic method based on layer-by-layer photocatalyst films made of titanium dioxide nanoparticles and graphene oxide, TiO2NP/GO, is proposed for performing simultaneously the synthesis and immobilization of spherical Ag nanoparticles (AgNP) using diluted AgNO3 solutions and UV irradiation (254 nm). The novelty provided by this method is that the amount and aggregation extent of AgNP can be controlled by the number of TiO2NP/GO bilayers and the composition of the outermost layer in the photocatalyst film. When the outermost layer is made of GO, more AgNP are deposited because GO serves as an anchoring harbor for Ag+ ions and a venue to the transportation of photoexcited electrons for the subsequent photo reduction to Ag0. The photodeposition follows a first order kinetics, while in the equilibrium it can be fitted by the Langmuir isotherm model for which surface coverage as high as 60% is attained after 70 min. of UV irradiation. The TiO2NP/GO/AgNP substrates show surface-enhanced resonant Raman scattering (SERRS) activity for nickel(II) phthalocyanine-tetrasulfonic acid tetrasodium salt (NiTsPc). The SERRS activity is ascribed to a combination of local electromagnetic field enhancement due to the plasmon resonance of AgNP, and proper excitation of samples in resonance with the Q-band of NiTsPc.

1. Introduction Silver nanoparticles (AgNP) of different sizes and shapes are undoubtedly the most widespread nanotechnology product nowadays. Besides the old century use in X-ray and photographic films, AgNP are now successfully commercialized as bactericide agents because of their high toxicity to microorganisms.1 Other interesting features such as strong localized surface plasmon resonance (LSPR) effect2 and enhanced electrocatalytic activity3 span the use of AgNP


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in highly sensitive optical4 and electrochemical sensors.5 In order to harness the full potential of AgNP in novel solid state devices, the development of new methods for the controlled immobilization of AgNP onto solid substrates is highly desirable. Electrochemical deposition methods are quite effective for performing, simultaneously, the synthesis and immobilization of AgNP. Nonetheless, they are limited to electrical conductive substrates.6 Alternatively, the synthesis/immobilization of AgNP onto solid supports, even including no conducting ones, can be accomplished by using previously prepared water- or organic solvent-based AgNP sols followed by widespread thin film deposition techniques, such as casting, spin coating, dip coating, layer-by-layer, and so on.7-9 In fact, the use of AgNP based inks has made it possible the construction of electrodes via doctor blade and inkjet printing on both rigid and flexible substrates.10,11 However, different strategies have been developed to conduct synthesis and immobilization of AgNP in a single step, which will save time and reduce production costs. For example, Amiri et al.12 have reported a method for the simultaneous synthesis and deposition of Ag and Cu dendrites onto Zn foils by simple surface redox reactions at optimized conditions and their use in plasmonic enhanced dye-sensitized solar cells (DSSC). With the same goal, Suresh et al.13 have proposed the RF magnetron sputtering using Ag/Nb2O5 targets for the deposition of AgNP and Nb2O5 passivation thin layers onto TiO2 nanoporous photoanodes. The DSSC built with the AgNP modified TiO2 photoanodes had the energy conversion efficiency increased to about 43%. Fu et al.14 have developed a method in which AgNP are simultaneously synthesized and immobilized onto glassy carbon electrodes after dipping the electrodes sequentially into AgNO3, sodium borohydride, and graphene oxide (GO) solutions. Each deposition cycle takes


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about 3 min. and the modified electrode shows excellent electrocatalytic activity towards H2O2 electrochemical reduction. Another elegant way of performing both processes at once is by means of the photocatalytic deposition. The photocatalytic deposition relies on a solid photocatalyst framework that is exposed to a metal precursor solution and irradiated by a luminous source of appropriate wavelength.15-18 Although the chasing for photocatalysts that work in the visible range never ends, TiO2, which is the most effective photocatalyst so far, works properly only in the UV range.19 Indeed, the scientific literature since the 1970's provides many examples of photocatalytic formation and/or immobilization of AgNP onto TiO2 nanoparticles (TiO2NP).20-23 In the present contribution, we report on a photocatalytic method based on TiO2NP/GO films assembled layer-by-layer onto glass slides that is capable of performing in a single step the synthesis and immobilization of AgNP. The AgNP are formed by the photocatalytic action of TiO2NP/GO films when they are exposed to UV irradiation (254 nm) while immersed into diluted ethanol/water AgNO3 solutions. The main advantage of the present method compared to previous ones is that it enables controlling the amount and aggregation extent of AgNP by simply varying the number of TiO2NP/GO bilayers and the composition of the outermost layer in the photocatalyst film. Along with TiO2NP, GO with its 2-D molecular architecture and different oxygenated groups serves as a suitable anchoring harbor for Ag+ ions. Moreover, photogenerated electrons from UV-excited TiO2NP are transported across the GO structure, which thus facilitates subsequent photo reduction of anchored Ag+ into Ag0. Indeed, graphenes exhibit high electronic mobility that improves charge separation and mitigates recombination of the photogenerated electron-hole pairs.24,25 Kinetics and thermodynamic parameters of the photocatalytic deposition are determined with UV-vis absorption spectroscopy by monitoring the


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absorption of the AgNP LSPR band. The microstructural features of films are assessed by attenuated total reflectance Fourier transform infrared (ATR-FTIR) and micro Raman spectroscopies, scanning electron microscopy coupled to energy dispersive X-ray spectroscopy (SEM/EDS), and atomic force microscopy (AFM). The SERRS activity of AgNP films is evaluated with a model probe, namely nickel(II) phthalocyanine-tetrasulfonic acid tetrasodium salt (NiTSPc).

2. Experimental Section 2.1. Materials AgNO3 99%, titanium(IV) isopropoxide 97%, graphite flakes (> 100 mesh), poly(sodium 4-styrenesulfonate) (PSS, Mw 70,000 g mol-1), poly(diallyldimethylammonium chloride) (PDAC, Mw 450,000 g mol-1), and NiTSPc were all purchased from Sigma-Aldrich Brazil and used as received. Analytical grade ethyl alcohol, potassium permanganate, sodium nitrate, sulfuric acid 98%, hydrochloric acid 36%, nitric acid 65%, hydrazine hydrate 25%, hydrogen peroxide 30%, and ammonium hydroxide 30% were all purchased from Vetec Brazil and used without additional purification. Quartz and optical glass slides (1 x 10 x 25 mm) were used as solid substrates for the photocatalyst film and AgNP photodeposition. The water used in all experimental procedures was of ultra pure type (18 Mohm.cm), provided by a Milli-Q MilliPore purification system.

2.2. Preparation of TiO2NP and GO The TiO2NP aqueous colloidal suspension was prepared by acid hydrolysis of titanium(IV) isopropoxide with nitric acid 0.1 mol L-1, as described elsewhere.26 The obtained


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milky suspension was centrifuged at 10,000 rpm for 30 min. to remove eventual aggregates and its concentration was determined afterwards by gravimetric analysis. GO was obtained in a twostep procedure, starting with the oxidation of graphite with potassium permanganate and sulfuric acid via the Hummers' method27 followed by ultrasonic exfoliation of the as-obtained graphitic oxide into individual GO sheets. The exfoliation was carried out by ultrasonic stirring (Branson, Sonifier 450; 90 min. in pulsed mode, 5s/5s on/off cycles; 315 W) of graphitic oxide (~ 1 g) suspended in 100 mL of ammonium hydroxide solution, pH = 10. During the entire treatment, the suspension was kept in an ice bath. The obtained suspension was centrifuged at 10,000 rpm for 30 min. to remove eventual aggregates and, then, its final concentration (in g L-1) was estimated by gravimetric analysis.

2.3 Layer-by-layer (LbL) deposition of the photocatalyst film Quartz and glass slides used as substrates to support the photocatalyst film and AgNP were sequentially cleaned in piranha solution (H2SO4:H2O2, 3:1, v/v) followed by RCA solution (H2O/NH4OH/H2O2, 5:1:1, v/v) prior to the film assembly. The following solutions/suspensions were used in LbL depositions: PDAC (1 g L-1, pH = 8), PSS (1.0 g L-1, pH = 6.8), TiO2NP (10 g L-1, pH = 1.5), and GO (0.2 g L-1, pH = 10). The LbL deposition was performed by hand and at room temperature (~ 25oC). The deposition cycle consisted on alternate immersions of the cleaned substrate into cationic (PDAC or TiO2NP) and anionic (PSS or GO) suspensions. The immersion step was fixed in 5 min. because it was sufficient for the adsorption to reach equilibrium. After every immersion, the substrate + film was rinsed in ultra pure water and dried with compressed air flow. All substrates were firstly primed with a PDAC/PSS bilayer in order to increase the number of adsorption sites and enhance the film adhesion. The photocatalyst


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films were made with different number “n” of (TiO2NP/GO) bilayers, varying from 1 to 10. Exclusively for the ATR-FTIR analysis, a film with 50 bilayers was deposited to improve the spectra. In order to ascertain the role played by the outermost layer on the photocatalytic deposition, tests were conducted with photocatalyst films with n =10 and ended by either TiO2NP or GO layers. Labeling and description of all tested samples are provided in Table 1.

Table 1. Labeling and description of film samples. Label

Description

T10

Plain photocatalyst film containing ten (TiO2NP/GO) bilayers ended by TiO2NP.

G3

Plain photocatalyst film containing three (TiO2NP/GO) bilayers ended by GO.

G10

Plain photocatalyst film containing ten (TiO2NP/GO) bilayers ended by GO.

G50

Plain photocatalyst film containing ten (TiO2NP/GO) bilayers ended by GO.

T10-Ag-07

T10 film coated with AgNP, after 7 min. of photoreduction

G10-Ag-07

G10 film coated with AgNP, after 7 min. of photoreduction.

G10-Ag-70

G10 film coated with AgNP, after 70 min. of photoreduction.

2.4 Photocatalytic deposition of AgNP The photocatalyst film was immersed into a 10 mL Petri dish containing 5 mL of water/ethanol (1/1, v:v) solution with different concentrations of AgNO3 (0.5, 1.0 and 5.0 mmol L-1). After a conditioning step of 5 min., the whole set (photocatalyst film in the Petri dish with AgNO3 solution) was exposed for different periods of time to UV irradiation (254 nm, 16 W, irradiation time: 1 min to 70 min.). The UV source assembled in a home-made chamber along with a schematic of the experimental setup is pictured in the supporting information (Figure S1).


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At the end of a defined UV treatment time, the photocatalyst film coated with AgNP was removed from the AgNO3 solution, rinsed with ultrapure water and left dry in room air. For most of samples, the AgNP formation was limited to the photocatalyst surface. Nonetheless, eventually it could be seen some large metallic silver aggregates in the supernatant AgNO3 solution for longer UV treatment periods. A control experiment carried out with a plain glass substrate in the same experimental setup did not form AgNP at all. The description of AgNP coated samples is also given in Table 1.

2.5 Structural and morphological characterizations of nanomaterials and films Zeta potential and hydrodynamic diameter of nanomaterials (in aqueous suspensions) were determined with the Malvern Zetasizer ZS90 equipment. The electronic structure of nanomaterials, photocatalyst assembly and kinetics and equilibrium parameters of AgNP formation were investigated by UV-vis spectroscopy (Varian Cary 5000; range: 200-800 nm with 10 nm s-1 scan rate and 0.05 nm resolution, with quartz cuvettes of 10 mm optical path and two polished windows). Characterization of structural features were complemented by attenuated total reflectance Fourier transform infrared (ATR-FTIR) (Nicolet Nexus 470, 0.125 cm-1 resolution, 64 scans) and micro Raman (Jobin Yvon T6400, excitation in 514.5 nm and 633 nm, 0.1 cm-1 resolution, laser intensity 2.97 mW) spectroscopies and X-ray diffractometry (Bruker D8 Focus, resolution 0.05o and rate 0.5o min-1). Morphology of film samples was investigated by SEM-EDS (Jeol JSM 7001F) and tapping mode AFM (Bruker Dimenson ICON; silicon tips, cantilever spring constant 40 N m-1). Film thicknesses were determined by AFM according to the procedure developed by Lobo et al.28


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2.6 Evaluation of the SERRS activity An aqueous stock solution of NiTsPc was prepared at 1 mg L-1 and diluted afterwards, when necessary. A 10 micro liter aliquot of the NiTsPc solution was dripped onto the surface of plain and AgNP-coated photocatalyst films and left dry in a vacuum desiccator. Raman spectra were recorded from 150 cm-1 to 2200 cm-1, with 514.5 nm and 633 nm laser excitation at acquisition time of 5 s and laser intensity of 2.97 mW. The choice for two different laser lines was intended to probe the effect of AgNP alone and in resonance with the NiTsPc Q-band as well. Some spectra were registered with an optical filter to limit the Raman signal up to 30,000 arbitrary units.

3. Results and Discussion 3.1 Fabrication and structure of the photocatalysts films The structural features of individual nanomaterials were first assessed by X-ray diffractometry and TEM (supporting information, Figure S2), zeta potential, and hydrodynamic diameter measurements. The TiO2NP sample is predominantly composed of the anatase phase with particles size determined by the 101 diffraction plane and the Scherrer formula as 9.6 nm (Figure S2a and S2b). The colloid TiO2NP sample exhibits a positive zeta potential of + 38.7±1.7 mV and hydrodynamic diameter of 43.6±0.9 nm. The cationic structure of TiO2NP is ascribed to the protonation reaction that occurs with surface hydroxyl groups as follows: ≡TiO2-OH + H+ → ≡TiO2-OH2+. Exfoliation of graphite into individual GO sheets is confirmed by the substantial change in the 002 diffraction peak (Figure S2c). In pristine graphite, this peak is very sharp and located at 2θ = 26.6o while it gives an interplanar distance of 0.335 nm. However, it broadens in graphitic oxide and almost disappears in GO. The GO diffractogram additionally exhibits a


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rather broad peak in 2θ = 10.55o, which gives an interplanar distance of 0.894 nm and confirms the exfoliation. The colloidal GO sample exhibits a negative zeta potential equals to -57.3±0.5 mV, which confirms its anionic structure. Since the intensive ultrasonic action used for exfoliation may cause the break of GO sheets in different sizes, the resulting sample exhibits a distribution of hydrodynamic diameters equal to 22, 48, and 104 nm. The colloidal suspensions and their respective photocatalysts films assembled via LbL were characterized by UV-vis absorption spectroscopy and obtained data are provided in Figure 1. As one observes in Figure 1a, the UV-vis spectra of nanomaterials in the colloidal state show electronic transitions solely in the UV range. The absorption bands are ascribed as follows: TiO2NP: 2p O→3d Ti(IV), 234 nm;19 GO: pi→pi*, 230 nm and n→pi*, 300 nm.29 The films UV-vis spectra, Figure 1b, show a single and broader band with the maximum absorbance located at 235 nm accompanied by a subtle absorption close to 300 nm. The graphic in the inset of Figure 1b shows that the absorbance at 235 nm scales linearly with the number n of deposition cycles or bilayers (n=1 to 10), with a linear regression fit equals to: abs = 0.1417 + 0.11682 n, with r2=0.99787. This linear dependence implies that the mass of adsorbed TiO2NP and GO is the same in every deposition cycle. This behavior is typical of electrostatic interaction accounted by the charge overcompensation mechanism as described by Schlenoff.30


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Figure 1. UV-vis absorption spectra of (a) TiO2NP and GO aqueous suspensions and (b) photocatalyst film made with increasing number n of (TiO2NP/GO) bilayers, with inset showing the dependence of film absorbance on the number n; (c) Raman and (d) ATR-FTIR spectra of TiO2NP, GO, G10, and G50 photocatalyst films.


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The Raman spectra of nanomaterials acquired at 514.5 nm displayed in Figure 1c confirm the successful preparation of TiO2NP and GO. The Raman spectrum of TiO2NP exhibits the main Raman modes ascribed to the anatase phase, including Eg at 148 cm-1 and 634 cm-1, B1g/A1g overlapped in 503 cm-1, and a last B1g mode in 399 cm-1.31 The Raman spectrum of GO has a doublet signal ascribed to D (A1g) and G (E2g) bands.32 While the G band is regarded to a symmetry allowed transition mode and common to all graphitic structures, the D band manifests only in defective graphenes that contain significant amounts of sp3 carbons.32 The spectrum of the photocatalyst film, G10, is composed by the main signals of individual TiO2NP and GO. However, two main differences between the spectra of individual nanomaterials and the film should be concerned. First, it is observed that in the film the D band is red-shifted 3 cm-1 while the G band is blue-shifted 7 cm-1. These energy shifts are associated to compressive (blue-shift) and tensile (red-shift) strains of the GO lattice as caused by the intercalation of GO sheets with TiO2NP and the strong interaction between them.33 Besides that, it is clearly noted that the relative ratio between the intensities of the D and G bands (ID/IG) increases from 0.75 for plain GO to 0.96 for the G10 film. The value of ID/IG is inversely proportional to the average size of sp2 domains and can be used to estimate the graphite oxidation.34 In the case of GO reduction to reduced graphene oxide (RGO), the ID/IG increases, which means that new sp2 domains are formed, but in smaller sizes, so that, in the average, the size of sp2 domains decreases upon reduction. Therefore, it is believed that GO gets slightly reduced to RGO by electron transfer from TiO2NP, which has already been observed by Williams.35 This hypothesis is confirmed further by an estimative of the GO crystallite size (La) as performed with the general equation (equation 1) developed by Cançado et al., as follows:36 = 2.410

. . (1)


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In equation 1, λL = 514.5 nm stands for the excitation wavelength for Raman spectra acquisition. For plain GO, equation 1 gives La = 24.1 nm, while for the film, La = 17.8 nm. Therefore, the crystallite size decreases in the film, which is due to the partial reduction of GO as discussed earlier. The peaks in 800 cm-1 and 1050 cm-1 are ascribed to the glass substrate. Finally, the ATR-FTIR spectra provided in Figure 1d confirm the identity of the individual nanomaterials in the G50 film besides the strong interaction between them. For example, the C-O stretching in plain GO appears at 1058 cm-1, but it is shifted to 1041 cm-1 in the film; the O-Ti-O stretching in plain TiO2NP is split into asymmetric and symmetric modes at 555 cm-1 and 445 cm-1, respectively, but they are shifted to 534 cm-1 and 437 cm-1 in the film. Still in the spectrum of TiO2NP, it is observed the presence of N-O and C=O stretching. They are regarded as sub products of the acid hydrolysis of Ti(IV) isopropoxide.

3.2 Kinetics and thermodynamics of the AgNP photocatalytic deposition process The photocatalytic deposition of AgNP onto the photocatalyst films was monitored (ex-situ) by UV-vis spectroscopy. In order to ascertain the influence of the photocatalyst film nano architecture on the amount of AgNP formed, the photocatalytic deposition was carried out with photocatalyst films ended by either TiO2NP (T10-Ag-07) or GO (G10-Ag-07) layers and with different number n of deposition cycles, at fixed AgNO3 concentration (5 mmol L-1) and UV irradiation time (7 min.). The UV-vis spectroscopy data for this investigation are presented in Figure 2. In Figure 2a, UV-vis spectra show a broadened band above 500 nm after films were irradiated with UV light in the presence of the AgNO3 solution. This band is ascribed to the LSPR effect typical of AgNP. For the T10-Ag-07 film, the maximum wavelength of the LSPR band is seen at 554 nm, whereas for the G10-Ag-07 film, the maximum is at 584 nm. The


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broadened and red-shifted features of the LSPR band are due to aggregation of AgNP.37 In addition, it is observed that the amount of AgNP is greater onto the photocatalyst film ended by GO (G10-Ag-07), probably because the hydroxyl, ether, and carbonyl groups in GO serve as anchoring sites for Ag+ ions and transportation venue for photoexcited electrons that reduce Ag+ to Ag0.38 Since the G10 photocatalyst film was found the most efficient in terms of the amount of AgNP formed, it was used henceforth as the reference for the investigation. As seen in Figure 2b, the absorbance due to AgNP is greater in films with more TiO2NP/GO bilayers. The spectrum evolves from a narrower band peaking at 432 nm (n = 1) to a broader and red-shifted one peaking at ~ 550 nm (n = 3 to 7). When n = 10, the spectrum is even broader, but the maximum absorbance is seen at 500 nm. In Figure 2c, the graphic compares the absorbance of the bare photocatalyst film (always ended by GO) and that coated by AgNP as a function of the number n of TiO2NP/GO bilayers. As pointed out before, the absorbance of the photocatalyst film increases linearly with n (see Figure 1b). The absorbance due to AgNP increases more or less in the same way. Therefore, for a fixed deposition condition (AgNO3 concentration and UV irradiation time), the amount of AgNP scales with the number n of TiO2NP/GO bilayers.


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Figure 2. Photodeposition of AgNP at fixed AgNO3 concentration (5 mmol L-1) and UV irradiation time (7 min) onto photocatalyst films (a) ended by different materials, (b) with different number of (TiO2NP/GO) bilayers and (c) AgNP absorbance at 450 nm versus the number n of TiO2NP/GO bilayers.


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In order to access the kinetics of deposition, UV-vis spectra were registered at different time intervals of UV irradiation (photodeposition time) for different initial AgNO3 concentrations, with photocatalyst films with 3 and 10 (TiO2NP/GO) bilayers, G3 and G10, respectively. As seen in Figure 3, the spectra are more symmetrical and narrower at the initial stages of the deposition, but they rapidly broaden because of agglomeration of AgNP. Evidently, the amount of deposited AgNP is greater onto the G10 film, which is made with more TiO2NP/GO bilayers, Figure 3b.

Figure 3. UV-vis spectra of AgNP photodeposition performed onto photocatalyst films (a) G3 and (b) G10 registered in different time intervals, and respective (c) kinetics isotherms, all attained with [AgNO3]0 = 5 mmol L-1. (d) Equilibrium isotherm (photodeposition time = 6000 s)


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for the photodeposition performed onto the G10 film with different initial AgNO3 concentrations.

The kinetics isotherms shown in Figure 3c exhibit an asymptotic profile, which could be fitted with a first order kinetic equation (equation 2), as follows: () = (∞)1 −

!" #

$ (2)

in which A(t) is the absorbance in arbitrary units at any photodeposition time t (in seconds, s), A(∞) is the absorbance at infinite time and kobs is the observed rate constant (in seconds-1, s-1). The obtained parameters are collected in Table 2 for different initial AgNO3 concentrations.

Table 2. Kinetics parameters and surface coverage fraction of AgNP photodeposition onto aG3 and G10 photocatalyst films. [AgNO3] (mmol L-1)

A(∞)

kobs (s-1) x 10-4

r2

A/Amáx x 100% (%)

0.5

1.51

1.4

0.9925

37

1

1.70

5.1

0.9920

43.4

5

2.12

5.4

0.9940

62.2

5a

0.97

4.9

0.9854

-

According to data collected in Table 2, kobs does not vary significantly with the AgNO3 concentration and they are all very close to 5 x 10-4 s-1, except for the lowest AgNO3 concentration, 0.5 mmol L-1 (Table 2, first row), in which desorption of Ag+ ions and dissolution of AgNP compete with the photoreduction of Ag+ and formation of AgNP. It is also worth


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mentioning that the rate constant does not change with n (compare 3rd and 4th rows in column 3), confirming the photocatalyst role played by the TiO2NP/GO film. Figure 3d displays an equilibrium isotherm of the photocatalytic deposition performed with the G10 photocatalyst film. In that experiment, the G10 film was exposed to a fixed UV irradiation equal to 6000 s, which is the equilibrium time determined in the kinetics experiments. For each AgNO3 concentration tested, replicas of the G10 photocatalyst film deposited under identical conditions were employed. The experimental data in Figure 3d are fitted by the Langmuir model using equation 3, %=

&'

=

( ) (3) 1 + ( )

in which θ = A/Amax is the surface coverage fraction by AgNP while C is the concentration of AgNO3 solution (in mol L-1) and KL is the Langmuir equilibrium constant (in arbitrary units). The adjustment of experimental data to equation 3 is quite good (r2 = 0.988). Parameters extracted from it are as follows: Amax = 3.48 and KL ≈ 782. The standard free-energy of reaction, ∆G0, is estimated after applying the fundamental equation, ∆G0 = -RTlnKL, with R = 8.31 J.mol1

.K-1 and T = 298.15 K. The value found is -16.5 kJ.mol-1, which is typical of physisorption

processes.

3.3. Microstructure and morphology of immobilized AgNP Figure 4 provides digital pictures of samples, which show that the bare glass substrate as well as the G10 film are highly transparent, while those coated by AgNP (G10-Ag-07 and G10Ag-70) are brownish and become darker for a longer UV irradiation time. In fact, longer UV treatments could eventually lead to a mirror-like surface. Nonetheless, visual inspection of samples prepared and stored for a year retained the same appearance, with no evidence of


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darkening or cracking. Figure 5 displays AFM topography images and cross-section profiles of samples G10, G10-Ag-07 and G10-Ag-70. The G10 sample (Figures 5a and b) exhibits spherical particles of ~10 nm in diameter. This size is comparable to that estimated by X-ray diffractometry and TEM (Figure S2, supporting information) of the TiO2NP powder and colloid samples. Although the very last layer of this film is made of GO, the profile traced by the AFM tip senses the globular morphology of the TiO2NP underneath layer. After the sample is submitted to different UV irradiation periods, its topography features larger and more numerous spherical particles, which are in fact AgNP (as confirmed by SEM-EDS, see below). For the shortest photodeposition time (sample G10-Ag-07, Figures 5c and 5d), the film topography is mostly composed by individual particles with diameters of about 40 nm and few aggregates. For the longest photodeposition time (sample G10-Ag-70, Figures 5e and 5f), particles are larger, with diameters ranging from 100 nm up to 300 nm. This observation is consistent with the broadened LSPR bands seen in the UV-vis spectra shown before. These morphologies were also revealed by SEM micrographs provided in the supporting information (Figure S3). Table 3 collects morphological parameters obtained by AFM and microstructural composition determined by SEM-EDS. Accordingly, the film thickness and surface roughness increase slightly from G10 to G10-Ag-07. However, for a longer photodeposition period, film thickness and surface roughness show a 3-fold increase (G10-Ag-70). This trend is accompanied by the increase on the Ag/Ti atomic relative ratio given in the fourth column of Table 3.


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Table 3. Morphological parameters determined by aAFM of G10 film submitted to AgNP photodeposition and respective Ag/Ti atomic ratio (average of three distinct areas) determined by b

SEM-EDS.

Photodeposition time (min.)

Film Thicknessa (nm)

Roughnessa (nm)

Ag/Ti atomic ratiob

0

41.5

10.5

0

7

42.6

11.7

0.13±0.05

70

122.3

38.4

4.22±0.07

Figure 4. Digital photos of samples: (a) bare glass substrate, (b) G10, (c) G10-Ag-07, and (d) G10-Ag-70.


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Figure 5. AFM characterization of samples. Topography images (5 µm x 5 µm) and cross-section profiles of the G10 (a,b), G10-Ag-07 (c,d), and G10-Ag-70 (e,f). All photodepositions performed with [AgNO3]0 = 5 mmol L-1. The horizontal white lines in the topography images indicate where the profiles were traced.

Some important features of the photocatalytic method described herein are worth to mentioning. First, its capability of controlling the amount and aggregation extent of AgNP by simply varying the number of TiO2NP/GO bilayers is of technological relevance and since both, the AgNO3 concentration and UV irradiation time can be held constant, its practical


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implementation should be much simpler. The LbL deposition is known to be influenced neither by shape nor composition of the supporting substrate. Therefore, the method can be applied to coat any type of substrate with AgNP. As a second remark, the mechanism of AgNP formation observed here is, to some extent, similar to that reported in other investigations. According to Mohamed et al.39 and Piwonski et al.40, the silver ions in solution are attached to and get reduced by electrons stored in TiO2NP. As the UV irradiation time elapses, the small Ag(0) nuclei grow to form AgNP that finally coalesce and transform into bigger particles. As we have seen by SEM-EDS, AFM topography images, and cross-section profiles of films (please refer to Table 3 and Figure 5), the amount and size of AgNP increases with the UV irradiation time, which can be explained by the same mechanism described above. Moreover, kinetics of AgNP photodeposition onto TiO2NP/GO films is of first order. A similar behavior is reported for the photodeposition performed with plain TiO2NP supported in films40 and fibers.41 In addition, rate constants determined here by steady-state UV-vis spectroscopy are in the order of 5 x 10-4 s-1. Values measured by transient UV-vis spectroscopy and photodeposition performed with TiO2NP in aqueous suspension were in the order of 8 to 10 s-1.39 The method developed here is much faster, probably because of the presence of GO, which anchors more silver ions and also inhibits the electron-hole pair recombination. The role played by the photocatalyst end layer observed here has also been reported by other authors. Since silver ions are positively charged, they are attracted more to anionic surfaces, which is the case of carboxylate and alkoxide groups in GO, and repelled by cationic surfaces, like protonated TiO2NP. Indeed, Othani et al.42 have observed that the quantum yield for the photochemical reduction of silver ions at the surface of TiO2NP decreases considerably as the pH of the reaction mixture is decreased below the TiO2NP


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isoelectric point, situation in which they become protonated. However, it is essential for the formation of AgNP that silver ions adsorb onto TiO2NP to receive the photogenerated electrons.

3.4 Study of the SERS/SERRS effect The potential use of the substrates containing low and high amounts of photodeposited AgNP in SERS applications was evaluated with a model dye, NiTsPc. Initially, the UV-vis spectra of AgNP substrates and the free dye (in solution) are compared in Figure 6. The two excitation wavelengths (514.5 nm and 633 nm) employed for Raman spectra acquisition are highlighted in the same figure. The spectrum of NiTsPc exhibits a strong absorption band peaking at 623 nm that is accompanied by a shoulder at 655 nm. They are ascribed to the Q-band of monomeric and dimeric forms of NiTsPc, respectively.43 Because the LSPR band of the photodeposited AgNP is broad, its tail-ends overlap with the main absorptions of NiTsPc. Therefore, it becomes important to know which excitation wavelength is more suitable for acquiring Raman spectra of this dye adsorbed onto AgNP substrates.


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Figure 6. UV-vis spectra of NiTsPc solution (0.5 g L-1), G10-Ag-07, and G10-Ag-70 films, as indicated. Vertical dashed lines indicate the excitation wavelengths for Raman spectra acquisition.

Figure 7 provides the Raman spectra of NiTsPc adsorbed onto G10, G10-Ag-07 and G10-Ag70 registered under different laser excitations, 514.4 nm and 633 nm, with the intention to probe the effect of AgNP alone and in resonance with the NiTsPc Q-band, respectively. When the samples are excited with 514.5 nm, Figure 7a, the Raman spectra are dominated by the A1g (1337 cm-1) and E2g (1554 cm-1) symmetry modes of GO from the underneath photocatalyst film. Indeed, these signals are greatly enhanced owing to the SERS effect caused by the charge transfer between AgNP and GO. On the other hand, when the samples are excited with 633 nm, the spectra are much better resolved, even when no AgNP is present. This is likely due to the resonant condition or Raman resonant scattering (RRS), since this excitation wavelength


24

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coincides with the Q-band of NiTsPc, as indicated in Figure 6. As reported elsewhere, the RRS spectrum of ZnTsPc, whose structure is quite similar to that of NiTsPc, receives an important contribution from the non-totally symmetric ag vibration modes.44 In the present case, those modes are represented by the macrocycle breathing deformation at 753 cm-1 and C=C stretching in the pyrrole (1341 cm-1) and benzene rings (1562 cm-1) (highlighted by asterisks in the bottom spectrum of Figure 7b).45 Additionally, the spectra become enhanced when the samples contain AgNP. As expected, the greatest enhancement is observed with the sample containing more AgNP (in the Figure 7b, G10-Ag-70). It is noteworthy to mentioning that the spectra in Figure 7b (middle and bottom part) were registered with an optical filter after the sample, otherwise CCD was saturated. That is why the intensities are lower than those reported in Figure 7a. This care was not necessary with sample G10, since it had no AgNP and measured intensities were not so high. Therefore, a surface-enhanced resonant Raman scattering (SERRS) phenomena is observed here. Since it was not observed any shift in these band positions (Figure S4, supporting information), it is suggested that most of the Raman enhancement arises on the resonant condition plus the increase of the local electromagnetic field as caused by the LSPR of AgNP. The LSPR contribution is made possible because of the aggregation of AgNP, which broadens their visible spectra beyond 600 nm. As pointed out by Shiohara et al.46, SERS/SERRS efficiency is improved when nanoparticles are partially aggregated or when they show truncated morphologies. The junctions between nanoparticles and sharp edges are like hotspots, where large field enhancements occur. That explains why G10-Ag-70 shows the best SERRS performance.


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Figure 7. Raman spectra of NiTsPc adsorbed onto G10, G10-Ag-07, and G10-Ag-70 films, as indicated, acquired with (a) 514.5 nm and (b) 633 nm laser excitation. Asterisks at the bottom spectrum in part b indicate the macrocycle breathing (753 cm-1), and C=C stretching in the pyrrole (1341 cm-1) and benzene ring (1562 cm-1) of NiTsPc.

4. Conclusions The photocatalytic deposition method developed for the simultaneous synthesis and immobilization of silver nanoparticles (AgNP) was made possible by using layer-by-layer assembled photocatalyst films built with titanium dioxide nanoparticles and graphene oxide bilayers, TiO2NP/GO. The amount and aggregation extent of AgNP are controlled by the number


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of TiO2NP/GO bilayers and the composition of the outermost layer in the photocatalyst film. The presence of GO favors both the anchorage of more Ag+ ions and formation of greater amounts of AgNP. The photodeposition performed in this way follows a first order kinetics process; it is spontaneous and attains surface coverages by AgNP as high as 60% in about 70 min. of UV irradiation. The as-obtained TiO2NP/GO/AgNP substrates enhance the Raman spectrum of nickel(II) phthalocyanine-tetrasulfonic acid tetrasodium salt (NiTsPc) by a combination of local electromagnetic field enhancement due to AgNP plus excitation in resonance with the Q-band of NiTsPc. The method is capable of fabricating SERS/SERRS active substrates in a reproducible manner with the amount and aggregation of AgNP being tuned by the photocatalyst film nano architecture.

Supporting Information UV chamber and schematic experimental set-up for the AgNP photodeposition (Figure S1). Xray diffractograms and TEM images of TiO2NP and GO (Figure S2). SEM images and EDS spectra of films (Figure S3). SERRS spectra of NiTsPc adsorbed onto G10, G10-Ag-07, and G10-Ag-70 films (Figure S4).

5. Acknowledgements The authors acknowledge the financial support of Brazilian funding agencies CNPq (process n.308038/2012-6), CAPES/Pró-Amazônia (process n. 3333-047/2012), FAP-DF (process n. 0193.000829/2015), and FINEP (process n. 01/13/0470/00). Authors also thank Dr. D. T. Balogh (IFSC-USP) for performing ATR-FTIR measurements.


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6. References

(1) Wei, L.; Lu, J.; Xu, H.; Patel, A.; Chen, Z. S.; Chen, G. Silver Nanoparticles: Synthesis, Properties, and Therapeutic applications. Drug Discov. Today 2015, 20, 595–601.

(2) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer-Verlag: Berlin, Germany, 1995.

(3) Bansal, V.; Li, V.; O’Mullane; A. P.; Bhargava, S. K. Shape Dependent Electrocatalytic Behaviour of Silver Nanoparticles. Cryst. Eng. Comm. 2010, 12, 4280–4286.

(4) Mayer, K. M.; Hafner, J. H. Localized Surface Plasmon Resonance Sensors. Chem. Rev. 2011, 111, 3828-3857.

(5) Li, H.; Xu, D. K. Silver Nanoparticles as Labels for Applications in Bioassays. Trends Anal. Chem. 2014, 62, 67-73.

(6) Penner, R. M. Mesoscopic Metal Particles and Wires by Electrodeposition. J. Phys. Chem. B 2002, 106, 3339-3353.

(7) Liz-Marzan, L. M. Tailoring Surface Plasmons through the Morphology and Assembly of Metal Nanoparticles. Langmuir 2006, 22, 32-41.


28

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


(8) Dubas, S. T.; Kumlangdudsana, P.; Potiyaraj, P. Layer-by-Layer Deposition of Antimicrobial Silver Nanoparticles on Textile Fibers. Colloids Surf. A 2006, 289, 105-109.

(9) Epifani, M.; Giannini, C.; Tapfer, L.; Vasanelli, L. Sol-Gel Synthesis and Characterization of Ag and Au Nanoparticles in SiO2, TiO2, and ZrO2 Thin Films. J. Am. Chem. Soc. 2000, 83, 2385-2393.

(10) De Guzman, N.; Ramos, M.; Balela, M. D. Improvements in the Electroless Deposition of Ag Nanowires in Hot Ethylene Glycol for Resistive Touchscreen Device. Mater. Res. Bull. 2018, 106, 446-454.

(11) Gokhale, P.; Mitra, D.; Sowade, E.; Mitra, K. Y.; Gomes, H. L.; Ramon, E.; Al-Hamry, A.; Kanoun, O.; Baumann, R. R. Controlling the Crack Formation in Inkjet Printed Silver Nanoparticle Thin-Films for High Resolution Patterning Using Intense Pulsed Light Treatment. Nanotechnol. 2017, 28, 495301.

(12) Amiri, O.; Salavati-Niasari, M.; Mir, N.; Beshkar, F.; Saadat, M.; Ansari, F. Plasmonic Enhancement of Dye-Sensitized Solar Cells by Using Au-decorated Ag Dendrites as a Morphology-Engineered. Renew. Energy 2018, 125, 590-598.

(13) Suresh, S.; Unni, G. E.; Satyanarayana, M.; Nair, A. S.; Pillai, V. P. M. Silver Nanoparticles-Incorporated Nb2O5 Surface Passivation Layer for Efficiency Enhancement in Dye-Sensitized Solar Cells. J. Colloid Interface Sci. 2018, 524, 236-244.


29


Page 30 of 36

(14) Fu, L.; Wang, A. W.; Su, W. T.; Zheng, Y. H.; Liu, Z. A Rapid Electrochemical Sensor Fabricated Using Silver Ions and Graphene Oxide. Ionics 2018, 24, 2821-2827.

(15) Sakamoto, M.; Fujistuka, M.; Majima, T. Light as a Construction Tool of Metal Nanoparticles: Synthesis and Mechanism. J. Photochem. Photobiol. C: Photochem. Rev. 2009, 10, 33–56.

(16) Cozzoli, P. D.; Comparelli, R.; Fanizza, E.; Curri, M. L.; Agostiano, A.; Laub, D. Photocatalytic Synthesis of Silver Nanoparticles Stabilized by TiO2 Nanorods: A Semiconductor/Metal Nanocomposite in Homogeneous Nonpolar Solution. J. Am. Chem. Soc. 2004, 126, 3868–3879.

(17) Dinh, C.-T.; Nguyen, T.-D.; Kleitz, F.; Do, T.-O. A New Route to Size and Population Control of Silver Clusters on Colloidal TiO2 Nanocrystals. ACS Appl. Mater. Interfaces 2011, 3, 2228–2234.

(18) Carbone, L.; Jakab, A.; Khalavka, Y.; Sonnichsen, C., Light-Controlled One-Sided Growth of Large Plasmonic Gold Domains on Quantum Rods Observed on the Single Particle Level. Nano Lett. 2009, 9, 3710–3714.

(19) Chen, X.; Mao, S. S. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications. Chem. Rev. 2007, 107, 2891-2959.


30

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


(20) Fleischauer, P. D.; Kan, H. K. A.; Shepherd, J. R. Quantum Yields of Silver Ion Reduction on Titanium Dioxide and Zinc Oxide Single Crystals. J. Am. Chem. Soc. 1972, 94, 283–285.

(21) Ada, H.; Yonezawa, Y.; Hino, M.; Anemura, H. Photoreduction of Silver Ion on the Surface of Titanium Dioxide Single Crystals. J. Chem. Soc., Faraday Trans. 1982, 78, 2677-2684.

(22) Chan, S. C.; Barteau, M. A. Physico-Chemical Effects on the Scale-Up of Ag Photodeposition on TiO2 Nanoparticles. Top. Catal. 2011, 54, 378–389.

(23) Wolosiuk, A.; Tognalli, N. G.; Martínez, E. D.; Granada, M.; Fuertes, M. C.; Troiani, H.; Bilmes, S. A.; Fainstein, A.; Soler-Illia, G. J. A. A. Silver Nanoparticle-Mesoporous Oxide Nanocomposite Thin Films: A Platform for Spatially Homogeneous SERS-Active Substrates with Enhanced Stability. ACS Appl. Mater. Interfaces 2014, 6, 5263−5272.

(24) Chowdhury, S.; Balasubramanian, R. Graphene/Semiconductor Nanocomposites (GSNs) for Heterogeneous Photocatalytic Decolorization of Waste Waters Contaminated with Synthetic Dyes: A Review. App. Catal., B 2014, 160–161, 307–324.

(25) Takai, A.; Kamat, P. V. Capture, Store, and Discharge. Shuttling Photogenerated Electrons across TiO2-Silver Interface. ACS Nano 2011, 5, 7369-7376.


31


Page 32 of 36

(26) Patrocínio, A. O. T.; Paterno, L. G.; Murakami Iha, N. Y. Layer-by-Layer TiO2 Films as Efficient Blocking Layers in Dye-Sensitized Solar Cells. J. Photochem. Photobiol., A 2009, 205, 23–27.

(27) Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J Am Chem Soc. 1958, 80, 1339.

(28) Lobo, R. F. M.; Pereira-da-Silva, M. A.; Raposo, M.; Faria, R. M.; Oliveira, O. N. Jr. In Situ Thickness Measurements of Ultra-Thin Multilayer Polymer Films by Atomic Force Microscopy. Nanotechnol. 1999, 10, 389-393.

(29) Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Processable Aqueous Dispersions of Graphene Nanosheets. Nat. Nanotechnol. 2008, 3, 101–105.

(30) Schelenoff, J. B.; Ly, H.; Li, M. Charge and Mass Balance in Polyelectrolyte Multilayers. J. Am. Chem. Soc. 1998, 120, 7626-7634.

(31) Swamy, V.; Kuznetsov, A.; Dubrovinsky, L. S.; Caruso, R. A.; Shchukin, D. G.; Muddle, B. C. Finite-Size and Pressure Effects on the Raman Spectrum of Nanocrystalline Anatase TiO2. Phys. Rev. B 2005, 71, 184302/1.

(32) Dresselhaus, M. S.; Jorio, A.; Hofmann, M.; Dresselhaus, G.; Saito, R. Perspectives on Carbon Nanotubes and Graphene Raman Spectroscopy. Nano Lett. 2010, 10, 751–758.


32

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


(33) Huang, M.; Yan, H.; Chen, C.; Song, D.; Heinz, T. F.; Hone, J. Phonon Softening and Crystallographic Orientation of Strained Graphene Studied by Raman Spectroscopy. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 7304-7308.

(34) Tuinstra, F.; Koenig, J. L. Raman Spectrum of Graphite. J. Chem. Phys. 1970, 53, 11261130.

(35) Williams, G.; Seger, B.; Kamat, P. V. TiO2-Graphene Nanocomposites. UV-Assisted Photocatalytic Reduction of Graphene Oxide. ACS Nano 2008, 2, 1487–1491.

(36) Cançado, L. G.; Takai, K.; Enoki, T.; Endo, M.; Kim, Y. A.; Mizusaki, H.; Jorio, A.; Coelho, L. N.; Magalhães-Paniago, R.; Pimenta, M. A. General Equation for the Determination of the Crystallite Size La of Nanographite by Raman Spectroscopy. Appl. Phys. Let. 2006, 88, 163106-1-3.

(37) Noguez, C. Surface Plasmons on Metal Nanoparticles: The Influence of Shape and Physical Environment. J. Phys. Chem. C 2007, 111, 3806-3819.

(38) Lightcap, I. V.; Kosel, T. H.; Kamat, P. V. Anchoring Semiconductor and Metal Nanoparticles on a Two-Dimensional Catalyst Mat. Storing and Shuttling Electrons with Reduced Graphene Oxide. Nano Lett. 2010, 10, 577-583.


33


Page 34 of 36

(39) Mohamed, H. H.; Dillert, R.; Bahnemann, D. W. Growth and Reactivity of Silver Nanoparticles on the Surface of TiO2: A Stopped-Flow Study. J. Phys. Chem. C 2011, 115, 12163–12172.

(40) Piwonski, I.; Kadziola, K.; Kisielewska, A.; Soliwoda, K.; Wolszczak, M.; Lisowska, K.; Wronska, N.; Felczak, A. The Effect of the Deposition Parameters on Size, Distribution and Antimicrobial Properties of Photoinduced Silver Nanoparticles on Titania Coatings. App. Surf. Sci. 2011, 257, 7076–7082.

(41) Lim, S. K.; Lee, S. K.; Hwang, S. H.; Kim, H. Photocatalytic Deposition of Silver Nanoparticles onto Organic/Inorganic Composite Nanofibers. Macromol. Mater. Eng. 2006, 291, 1265–1270.

(42) Ohtani, B.; Okugawa, Y.; Nishimoto, S.; Kagiya, T. Photocatalytic Activity of TiO2 Powders Suspended in Aqueous Silver Nitrate Solution. Correlation with pH-Dependent Surface Structures. J. Phys. Chem. 1987, 91, 3550-3555.

(43) Gouterman, M. Spectra of Phorphyrins. J. Mol. Spectr. 1961, 6, 138-163

(44) Silva, J. C. O.; Sousa, M. H.; Tourinho, F. A.; Rubim, J. C. Raman Spectroscopic Investigation of Maghemite Ferrofluids Modified by the Adsorption of Zinc Tetrasulfonated Phthalocyanine. Langmuir 2002, 18, 5511-5515.


34

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(45) Silverstein, R. M.; Webster, F. X.; Kiemle, D. J. Spectrometric identification of organic compounds; John Wiley and Sons: New Jersey, USA, 2005.

(46) Shiohara, A.; Wang, Y.; Liz-Marzán, L. M. Recent Approaches Toward Creation of Hot Spots for SERS Detection. J. Photochem. Photobiol. C 2014, 21, 2–25.


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Photocatalytic Method for the Simultaneous Synthesis and

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