Preparation of gold nanoparticle dispersed TiO2-polymer composite film by a combined layer-by-layer and photocatalytic deposition method

Accepted Manuscript Title: Preparation of Gold Nanoparticle Dispersed TiO2 -polymer composite film by a Combined Layer-by-Layer and Photocatalytic Deposition Method Author: Sayaka Yanagida Yukihiko Kosakai Atsuo Yasumori PII: DOI: Reference:

S0927-7757(14)00429-4 http://dx.doi.org/doi:10.1016/j.colsurfa.2014.04.057 COLSUA 19196

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

30-1-2014 18-4-2014 25-4-2014

Please cite this article as: S. Yanagida, Y. Kosakai, A. Yasumori, Preparation of Gold Nanoparticle Dispersed TiO2 -polymer composite film by a Combined Layer-by-Layer and Photocatalytic Deposition Method, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2014), http://dx.doi.org/10.1016/j.colsurfa.2014.04.057 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Highlights Au nanoparticles dispersed on TiO2/polyelectrolyte composite films were prepared.

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Multilayer composite film was prepared using layer-by-layer method.

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Au particle size was controlled by adding 3-mercapto-1-propanesulfonate.

The composite films exhibited LSPR sensitivity to refractive index of immersing

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solvents.

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Preparation of Gold Nanoparticle Dispersed TiO2-polymer composite film by a Combined

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Sayaka Yanagida*12, Yukihiko Kosakai1, Atsuo Yasumori12

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Layer-by-Layer and Photocatalytic Deposition Method

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Department of Material Science and Technology, Tokyo University of Science, 6-3-1 Niijuku,

Katsushika-ku, Tokyo, 125-8585, Japan 2

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Photocatalysis International Research Center, Research Institute for Science and Technology, Tokyo

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e-mail address

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University of Science, 2641 Yamazaki, Noda-shi, Chiba 278-8510, Japan

Sayaka Yanagida: [email protected] Yukihiko Kosakai: [email protected] Atsuo Yasumori: [email protected]

*Corresponding author Sayaka Yanagida e-mail: [email protected]

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Department of Material Science and Technology, Tokyo University of Science, 6-3-1 Niijuku,

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Katsushika-ku, Tokyo, 125-8585, Japan Tel.: +81-3-5876-1420

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Fax: +8-3-5876-1639

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Abstract

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Gold nanoparticles (AuNPs) can be generated by irradiating a TiO2 photocatalyst with UV, a

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process known as photocatalytic deposition. Since the AuNPs nucleate and grow only on UV

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irradiated TiO2 surfaces, their numbers cannot be increased by multiple stacking. In this study, multilayer composites of polysodium 4-stylene sulfonate (PSS), TiO2, and AuNPs were prepared by

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a combined electrostatic layer-by-layer and photocatalytic deposition method. To acquire a

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negatively charged AuNP surface, 3-mercapto-1-propanesulfonate (Na-MPS) was added to the HAuCl4 solution used in the photocatalytic deposition process. Under these conditions, positively

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charged TiO2 particles were electrostatically deposited on MPS-modified AuNPs, enabling good

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dispersion of AuNPs throughout the composite thin films. In the absence of Na-MPS, TiO2 particles

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were sparsely deposited on AuNP surfaces, and AuNPs grew through repetition of photocatalytic deposition. In addition, the size of the photocatalytically deposited AuNPs was decreased by increasing the amount of Na-MPS in the HAuCl4 solution. Four-layer samples were observed under a scanning transmission electron microscope. The AuNP size was reduced from 80 nm to 10 nm in the presence of Na-MPS. The composite thin films exhibited localized surface plasmon resonance sensitivity to the refractive index of the solvents around the films.

Keywords

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gold nanoparticle, layer-by-layer method, photo catalytic deposition, Localized surface plasmon

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resonance (LSPR)

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1. Introduction

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Gold nanoparticles (AuNPs) absorb visible light of specific wavelength (450–700 nm) by

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resonance between electrons in the gold nanoparticle and incident light. The resonance wavelength

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depends on the particle size and shape, interparticle distance, and refractive index of the surrounding medium [1-4]. This phenomenon, known as localized surface plasmon resonance (LSPR), has been

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widely investigated and applied in optics [5], sensing [6,7], imaging [8], photocatalysis [9,10], and

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energy conversion [11]. In solid-state LSPR applications, the dispersed AuNPs should be immobilized on the substrate because the LSPR spectrum of AuNPs is sensitive to NP aggregation.

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AuNP immobilization techniques include lithography [12,13], self-assembly [14], sputtering [15],

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spin coating with polymer [16], thermal evaporation and annealing [17,18], photocatalytic reaction

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[19,20], and anchoring by thiol [21] or amino groups [22]. The layer-by-layer (LBL) method is a multilayer film fabrication method that exploits the

interaction (especially electrostatic) between chemical species in solution. When a surface-charged substrate is immersed in a solution containing oppositely charged species (such as colloids, polyelectrolytes, or molecules), the dispersible or soluble species adsorb on the substrate surface and reverse the charge on the substrate surface. Alternate deposition of positively and negatively charged species on the substrate yields a multilayered thin film. The LBL method has also been used to immobilize AuNPs on substrates [23-29]. For this

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purpose, the LBL method based on electrostatic interaction requires charged colloidal AuNPs in

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solution. Colloidal AuNPs are typically prepared by reducing solubilized HAuCl4 and simultaneously or subsequently treating the products with protective reagent [23,24,26,28,29]. LBL

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deposition of charged AuNPs, polyelectrolyte, or dispersed nanosheets [29] reportedly yields

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composite thin films. In these studies, the size of the Au particles ranged from 2 nm to 10 nm.

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Although such small AuNPs are easily synthesized from HAuCl4 solution and disperse well, their LSPR peak intensity is much lower than that of large AuNPs. Moreover, the shape of the LSPR

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spectrum frequently alters throughout the layer deposition process because the AuNPs are

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sufficiently closely spaced to undergo plasmon coupling. However, immobilized AuNPs can also be

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prepared by a photocatalytic deposition method. When UV light is irradiated onto a photocatalyst

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(such as TiO2 or ZnO) immersed in HAuCl4 solution, the [AuCl4]− complex is photocatalytically reduced and AuNPs are formed on the photocatalyst surface. During this process, the AuNPs nucleate and grow only on the UV-irradiated TiO2 surface. Although this method readily yields large particles, simultaneous control of the size and number of precipitated AuNPs is problematic. Multilayered structures prepared by repeated cycles of photocatalyst layer deposition and AuNPs precipitation by photocatalytic reaction are considered to effectively increase the number of AuNPs on the substrate while maintaining their size. In this study, AuNPs and TiO2 composite thin films with high LSPR peak intensity were

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prepared by LBL deposition of TiO2 and polyelectrolyte, alternated with AuNP deposition by

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photocatalytic reaction. TiO2 nanoparticles are positively charged in a low pH solution and can be interacted with polysodium 4-stylene sulfonate (PSS), a polyanion frequently used in the LBL

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method [30]. However, bare AuNPs precipitated on the TiO2 surface by photocatalytic deposition

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lack sufficient positive surface charge [31, 32] to electrostatically interact with negatively charged

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PSS in subsequent electrostatic depositions. Consequently, very little PSS and TiO2 adheres to the predeposited AuNPs. Figure 1(a) models the growth of a composite film comprising PSS, TiO2, and

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AuNPs. The AuNP surface remains externally exposed after PSS and TiO2 deposition. During

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subsequent photocatalytic depositions, the AuNPs enlarge. Since the photogenerated electrons in the

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TiO2 conduction band move toward surface-precipitated AuNPs [33, 34], [AuCl4]− complexes in the

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Au precursor solution should be reduced to Au0 on the previously deposited AuNPs. In this study, we increased the Au surface charge by adding a surface modification reagent, sodium 3-mercapto-1-propanesulfonate (Na-MPS), to the photocatalytic deposition process. Na-MPS is expected to bond to the photocatalytically deposited AuNP surface by its thiol group and add a negative charge to the Au surface by electric dissociation of its sulfo group. Figure 1 (b) models the growth of a composite film comprising PSS, TiO2, and MPS-modified AuNPs (MPS-AuNPs). Although the negatively charged MPS-AuNPs are electrostatically repulsive to the PSS (which is also negatively charged), they attract positively charged TiO2, which deposits on MPS-AuNPs to

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form a TiO2 layer. This layer effectively blocks the diffusion of Au3+ complexes toward previously

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deposited AuNPs. Consequently, AuNPs nucleate on the upper TiO2 layer instead of accreting with predeposited AuNPs and with AuNPs effectively dispersed in the TiO2/PSS thin film. Furthermore,

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the size of the AuNPs in the homogeneous AuNPs synthesis can be controlled by simultaneous

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modification with thiols, as reported by Brust et al. [35] and Yonezawa et al. [36]. Surface modified

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thiols may similarly restrict AuNP growth during photocatalytic deposition; however, surface modification by protective reagents such as thiols has yet to be trialed in AuNP photocatalytic

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deposition. In this study, the effects of Na-MPS on the multilayer deposition, the size of the

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photocatalytically deposited AuNPs, and the LSPR properties of the composite thin films are

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investigated in detail. We present the first trial of AuNP surface modification and size control by

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addition of organic molecules to the photodeposition process.

2. Experimental procedure

2.1. Starting materials

Aqueous titanium dioxide suspension (STS-02, 30 mass%, Ishihara Sangyo Kaisha, Ltd), PSS (average Mw ca. 70,000, 30 mass% aqueous solution, Aldrich Corp.), Na-MPS (98.7%, Wako

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Pure Chemical Industries, Ltd.) and Au standard solution (HAuCl4 in 1 mol/L HCl, 1000 mg/L as

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Au) were used as received. The LSPR properties were investigated using reagent grade solvents;

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2.2. Preparation of PSS-TiO2-AuNPs composite thin films

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namely, methanol (99.8%), ethanol (99.5%), 2-propanol (99.7%), and toluene (99.0%).

Vitreous silica glass substrates ((25 × 50 × 0.5) mm3) were cleaned as follows. The

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substrates were immersed in a 1:1 solution of MeOH and concentrated HCl at room temperature

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(R.T.). After 30 min, the substrates were rinsed with distilled water and immersed in concentrated

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H2SO4 solution at R.T. for a further 30 min. After washing with distilled water, the substrates were

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ultrasonicated in distilled water for 10 min, dried at 120 °C, and used for LBL deposition. The TiO2 suspension was diluted with aqueous HCl solution, then adjusted to 0.5 g/L (pH = 1.3). PSS 7 g/L solution (pH = 1.3) was similarly prepared with HCl aqueous solution. The substrates were immersed in the PSS solution at R.T. for 20 min, rinsed twice with HCl aqueous solution (pH = 1.3), and dried at 120 °C for 10 min. Next, the PSS-coated substrates were immersed in the TiO2 0.5 g/L suspension at R.T. for 20 min, rinsed twice with HCl aqueous solution (pH = 1.3), and dried at 120 °C for 10 min. One side of the substrates was wiped with a paper soaked in ethanol and dried at R.T. The solutions used in the photocatalytic deposition of AuNPs were prepared as follows: 1 ml of

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Au standard solution was added to 19 ml of a specified concentration of Na-MPS aqueous solution.

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The concentration of Na-MPS solutions was varied because the MPS to Au ratio should considerably affect the surface charge and size of the photocatalytically deposited AuNPs. To investigate this

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effect, the Au concentration was fixed at 2.5 × 10−4 mol/L, and the molar ratio of MPS/Au was

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varied as 0.17, 0.34, 0.68, 1.71, and 3.41. The obtained solution was mixed with 2 ml of ethanol

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(sacrificial reagent for photocatalytic Au deposition) and used as Au precursor solution in the photocatalytic deposition process. During photodeposition, the Au precursor solution was poured

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into a Petri dish (diameter φ = 78 mm) and cooled in an ice bath at 0 °C. The PSS- and TiO2-coated

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substrates were immersed in the Au precursor solution and UV light was irradiated from above the

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Petri dish. The UV source was a fluorescent UV lamp (black light lamp, FL15BLB; Toshiba Corp.).

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The light intensity, measured by a UV radiometer (UVX radiometer; UVP LLC.) placed at the solution surface, was 1 mW/cm2 at 365 nm. Following the photocatalytic deposition, substrates were washed with pH = 1.3 HCl aqueous solution and dried at 120 °C for 10 min. The PSS/TiO2 LBL deposition was repeatedly alternated with Au photocatalytic deposition to yield (PSS/TiO2/Au)n composite films, where n is the number of deposition cycles (n = 1–4). To remove the PSS and MPS from the composite thin films, the films were heated at 500 °C for 2 h. (TiO2/Au)n thin films were similarly obtained.

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3. Characterization

UV–Vis spectra of the samples were measured by a UV–Vis Spectrophotometer (UV-2400PC;

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Shimazu Corp.). The crystalline phase and crystallite size were evaluated using an X-ray

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diffractometer (XRD, XRD-6100; Shimazu Corp.) operated at 40 kV and 30 mA of monochromatic

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CuKα radiation. Microstructure of the AuNPs in the composites was observed using a scanning transmission electron microscope (STEM, HD-2300C; Hitachi Ltd.). In preparation for STEM

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observation, the composite thin films were peeled from the glass substrate by a scalpel and set on a

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microgrid. The surface chemical compositions of the composite thin films were measured by an

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X-ray photoelectron spectrometer (XPS, JPS-9010MC, JEOL) with a Mg Ka X-lay line (46.950 eV).

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The LSPR properties of the samples were investigated from changes in the UV–Vis absorption spectra as samples were immersed in various solvents with different refractive indexes: distilled water (nD20 = 1.333), methanol (nD20 = 1.329), ethanol (nD20 = 1.362), 2-propanol (nD20 = 1.381), and toluene (nD20 = 1.497). The surfaces of the samples were cleaned by UV/ozone treatment for 15 min at R.T. using a photo surface processor equipped with a low-pressure Hg lamp (PL16-110, Sen Light Corp.) The samples were immersed in the solvents for 5 min and their UV–Vis spectra were measured. After measurement, the samples were dried in vacuo at 100 °C for 30 min and repeatedly used for spectrum measurements in other solvents.

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4. Results and discussion

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4.1. UV–Vis spectra of the (PSS/TiO2/Au)n films

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Figure 2 shows the UV–Vis absorption spectra of the (PSS/TiO2/Au)n (n = 1–4) composite thin films. The UV absorbance of all samples increased with increasing repeats of TiO2 deposition,

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and the LSPR peak (~540 nm) attributable to AuNPs was enhanced with repeated Au deposition.

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When Na-MPS was excluded from the Au precursor solution, the LSPR peak wavelength was

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gradually red-shifted as more layers were deposited (Fig. 2 (a)). Because the LSPR spectra of larger

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Au particles peak at longer wavelengths [1], this result indicates that the Au particle size increased with increasing number of repetitions. On the other hand, addition of Na-MPS to the Au precursor solution appeared to stabilize the LSPR peak wavelength. As shown in Figure 2 (b) (c), the LSPR peak wavelength was almost fixed after two, three, and four depositions. The same trend was observed at other MPS concentrations. These results suggest that Na-MPS inhibited growth of AuNPs throughout repeated layer depositions. Na-MPS addition also increased the LSPR peak intensity. The LSPR peak intensity of the sample prepared at MPS/Au = 0.34 was 2.2 times higher than that of the additive-free sample (n = 4).

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Conversely, the peak intensity of the sample prepared at MPS/Au = 1.71 was reduced by nearly 75%

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relative to the additive-free sample (measured at 0.24 times that of the additive-free sample). Thus, the effect of MPS on the LSPR peak intensity appears to depend on the amount of the added MPS.

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To clarify this effect, Fig. 3 plots the relationship between the LSPR peak intensity and MPS/Au

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ratio. Note that at some optimal Na-MPS concentration (corresponding to MPS/Au = 0.34), the peak

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intensity is higher than at the higher or lower concentrations. Because the LSPR intensity of AuNPs decreases with the particle size [37] and number of AuNPs, it seems that large quantities of Na-MPS

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decrease the AuNP size or the number of photocatalytically deposited AuNPs.

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4.2. X-ray diffraction of the (PSS/TiO2/Au)4 films

XRD patterns of the (PSS/TiO2/Au)4 composite films and TiO2 powder prepared from

TiO2 30 mass% suspension are shown in Fig.4. Although the TiO2 powder pattern was attributed to anatase, the number of TiO2 nanoparticles in the composite film was too small to yield an observable anatase peak. On the other hand, Au (111) and (200) peaks were observed in the additive-free sample and in samples fabricated at MPS/Au = 0.17, 0.34, 0.68, and 1.71. The strong peak intensity of Au in the MPS/Au = 0.17 and 0.34 samples suggests that the number of AuNPs deposited during the photocatalysis was increased at small MPS/Au ratios. On the contrary, the Au peak intensity and

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sharpness reduced as the MPS/Au ratio increased. The crystallite size was estimated from the

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FMHW value of the Au (111) peak by using the Scherrer equation and is plotted as a function of MPS ratio in Fig.5. At MPS/Au ratios of 0.68 and 1.71, the NP size was reduced to 13 nm. Such a

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small crystallite size and weak LSPR peak (see Fig. 2 (c)) indicates that excess MPS restricted the

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crystal growth of AuNPs. No Au peak was found in the MPS/Au = 3.41 sample; probably because

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the amount of photocatalytically deposited Au was too low to analyze.

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4.3. Microstructure of AuNPs in composite thin films

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Figure 6 shows bright-field STEM images of (PSS/TiO2/Au)1 and (PSS/TiO2/Au)4

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composite thin films. In n = 1 samples, the particle diameter of additive-free AuNP samples was approximately 30 nm. In the presence of Na-MPS, the particle diameter slightly increased up to MPS/Au = 0.34, but decreased to approximately 20 nm in the MPS/Au = 0.68 sample, and to 10 nm or less in MPS/Au = 1.71 and 3.41 samples. These results reveal that high concentrations of Na-MPS decrease the size of the Au particles. In previous studies, AuNPs were prepared by reduction in a homogeneous solution. The diameter of the AuNPs decreases with increasing quantity of the protective reagent because it coats the Au surface, preventing growth of its core [36]. The same tendency was observed in the photocatalytic deposition process developed in this study.

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In the additive-free n = 4 sample, the AuNP particle diameter was approximately 80 nm,

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much larger than in the n = 1 sample. However, in the presence of Na-MPS, the particle diameter did not differ between the n = 1 and n = 4 samples. As shown in Fig. 1, AuNPs prepared from

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Na-MPS-free precursor solution probably carry insufficient surface charge for a full coating of TiO2

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in the subsequent LBL process. Consequently, AuNP surfaces were partially exposed to the solution,

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encouraging growth in subsequent photocatalytic deposition processes. In the presence of Na-MPS, the MPS were linked to AuNPs and donated sufficient charge for proper coating of the TiO2 layer.

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Such bonding of the MPS to the Au surface and upper TiO2 layer prevented growth of the AuNPs

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and facilitated their nucleation on the upper TiO2 layer.

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4.4. Surface composition of the composite films

Figure 7 shows the Au/Ti molar ratios at the surfaces of additive-free and MPS/Au = 0.34

samples, estimated from XPS measurements. For these measurements, the (PSS/TiO2/Au)2 and (PSS/TiO2/Au)2+(PSS/TiO2) samples were used. In the latter, the (PSS/TiO2/Au)2 layers were coated with PSS/TiO2 layers. The PSS/TiO2 deposition decreased the Au/Ti ratio by 55% (from 0.085 to 0.037) in MPS additive-free samples and by 69% (from 0.13 to 0.040) in MPS/Au = 0.34 samples. These results reveal that Na-MPS effectively increased the TiO2 surface coverage on the AuNP

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surface. Positively charged TiO2 particles were expected to preferentially deposit on the negatively

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charged MPS-AuNPs surface. It should be noted that the Au/Ti ratio was increased in the presence of Na-MPS. In

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(PSS/TiO2/Au)2 samples prepared from the MPS/Au = 0.34 precursor solution, the Au/Ti ratio was

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1.5 times higher than in additive-free samples. This inference is supported by UV–Vis spectra (Fig.

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2) and XRD patterns (Fig. 4), which indicate higher Au content in the composite film fabricated at low concentrations of Na-MPS. Figure 8 shows the UV–Vis spectra of Au precursor solutions. The

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absorbance at 314 nm (characterizing the ligand-to-metal charge transfer of [AuCl4]− [38,39])

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decreased, and the peak wavelength blue-shifted with increasing Au/MPS ratio. These results

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suggest complex formation between the MPS and Au ion. The resulting change in the Au3+ redox

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potential and adsorption state may be responsible for the increased AuNP content.

4.5. Effect of heat treatment

Figure 9 shows the LSPR spectra of the (PSS/TiO2/Au)4 and (TiO2/Au)4 samples

fabricated at MPS/Au = 0.34. The peak wavelength was slightly shifted from 547.5 nm to 551.5 nm although the shape of the LSPR peak was relatively insensitive to the heat treatment. This suggests that the size and interparticle distance of AuNPs were maintained after the MPS and interlayered

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PSS had been removed by heat treatment.

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4.6. LSPR sensitivity of the composite films

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Figure 10 shows LSPR peak shifts in the (PSS/TiO2/Au)4 and (PSS/TiO2/Au)1 samples

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fabricated in additive-free conditions and at MPS/Au= 0.34 as functions of solvent refractive index. The additive-free (PSS/TiO2/Au)1 sample was excluded from this experiment because its LSPR peak

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was insufficiently high to observe the peak shift. In all samples, the peak wavelength increased as

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the refractive index of the solvent increased. The wavelength shift per unit of refractive index

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(nm/RIU) defines the sensitivity of the LSPR sensor. By this definition, additive-free

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(PSS/TiO2/Au)4 samples were more sensitive (102 nm/RIU, R2 = 0.911) than MPS/Au = 0.34 samples (54.3 nm/RIU, R2 = 0.938). Such differences are expected to primarily influence the AuNP particle size. As shown in Fig. 6 (a) and (b), the typical size of AuNPs fabricated in the absence and presence of Na-MPS (MPS/Au = 0.34) was 80 nm and 30 nm, respectively. According to Lee et al. [35], the LSPR sensitivity is strongly affected by the AuNP size. Larger AuNPs exhibit higher sensitivity to the refractive index of the surrounding medium [35]. The (PSS/TiO2/Au)1 sample displays a shorter LSPR peak wavelength and higher sensitivity (68.5 nm/RIU, R2 = 0.993) than the (PSS/TiO2/Au)4 sample fabricated at MPS/Au = 0.34.

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In the (PSS/TiO2/Au)1 sample, AuNPs were not completely coated with TiO2 and were partially

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exposed to the surrounding solvent. This exposure would have increased the contact area between the AuNPs and solvent, and thereby the sensitivity of the LSPR peak wavelength to the solvent

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refractive index. By contrast, in the (PSS/TiO2/Au)4 sample, most of the AuNPs were located

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between (PSS/TiO2) layers and surrounded by TiO2 particles of very high refractive index (nanatase =

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2.52 ). Therefore, the LSPR spectra peaked at longer wavelengths in the (PSS/TiO2/Au)4 samples than in the (PSS/TiO2/Au)1 sample. This result indicates that the contact area between the AuNPs

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and solvent was smaller in the (PSS/TiO2/Au)4 samples than in the (PSS/TiO2/Au)1 sample. This

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reduction, attributable to the surrounding TiO2 particles, also reduced the sensitivity of the

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(PSS/TiO2/Au)4 samples relative to the (PSS/TiO2/Au)1 sample. Malinsky et al. [40,41] referred to

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this effect as the substrate effect.

5. Conclusions

Composite thin films composed of AuNPs, PSS, and TiO2 nanoparticles were prepared by

combined LBL and photocatalytic deposition. The effects of adding Na-MPS to the HAuCl4 solution used in the photocatalytic deposition process were investigated in detail. UV–Vis spectroscopy and STEM observation revealed that Na-MPS restricted growth of the photocatalytically deposited

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AuNPs during repeated deposition cycles. When present in small quantities (low MPS/Au ratio),

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MPS slightly increased the diameter of the AuNPs deposited by photocatalysis, whereas high quantities decreased the size of the deposited AuNPs. The surface composition, evaluated by XPS,

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suggested that Na-MPS facilitated TiO2 coverage on the precipitated AuNPs. The AuNP surface

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appeared to be strongly charged by association with MPS molecules. In the four-layer sample, the

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sensitivity of the LSPR refractive index to the immersing solvents was decreased by Na-MPS

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addition; probably because the AuNP size was reduced by MPS modification.

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Figure captions

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Figure 1. Schematic of the deposition process of (PSS/TiO2/Au)n films fabricated in (a) additive-free

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conditions (b) presence of Na-MPS.

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Figure 2. UV–Vis spectra of the (PSS/TiO2/Au)n samples (n = 1–4): (a) Additive-free (b) MPS/Au =

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0.34 (c) MPS:Au = 1.71.

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Figure 3. Change in the LSPR peak intensity of (PSS/TiO2/Au)n samples with molar ratio of

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MPS/Au in the Au precursor solution: Open circles: single-layer sample; closed circles: two-layer

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sample; open triangles: three-layer sample; closed triangles: four-layer sample.

Figure 4. XRD patterns of (PSS/TiO2Au)4 samples and TiO2 powder prepared from TiO2 suspension: (a) additive-free (b) MPS/Au = 0.17 (c) MPS/Au = 0.34 (d) MPS/Au = 0.68 (e) MPS/Au = 1.71 (f) MPS/Au = 3.41 (g)TiO2 powder.

Figure 5. Relationship between crystallite size (estimated from XRD patterns of Fig. 4) and MPS/Au molar ratio.

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Figure 6. BF-STEM images of (PSS/TiO2/Au)1 and (PSS/TiO2/Au)4 samples: (a) additive-free single layer (b) MPS/Au = 0.34 single layer (c) MPS/Au = 0.68 single layer (d) MPS/Au = 1.71 single

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layer (e) additive-free four-layer (f) MPS/Au = 0.34 four-layer (g) MPS/Au = 0.68 four-layer (h)

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MPS/Au = 1.71 four-layer.

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Figure 7. Surface chemical composition of the composite films evaluated from XPS results.

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Figure 8. UV–Vis spectra of Au precursor solutions used in the photodeposition process.

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Figure 9. UV–Vis spectra of (PSS/TiO2/Au)4 films (MPS/Au = 0.34) before and after the heat treatment at 500 °C. Broken and solid lines indicate before and after the heat treatment, respectively.

Figure 10. Peak wavelengths in the LSPR spectra of composite thin films versus refractive index of the immersing solvent (a) additive-free (PSS/TiO2/Au)4 (b) MPS/Au = 0.34 (PSS/TiO2/Au)4 (c) MPS/Au = 0.34 (PSS/TiO2/Au)1.

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Page 28 of 39

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MPS modified gold nanoparticle

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+ + + + + + + + + + ++ + ++ + + + + ++ + + + + + + + + + + + + + + + ++ ++ + + + + + ++ + + + + + + + ++ + + + + + + ++ + + + ++ + + + + + + + + + + + + ++ ++ + + + + + + + + ++ +

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*Graphical Abstract (for review)

0.34

PSS/TiO2 layer

Substrate

0.68

100 nm

1.71

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MPS/Au ratio

Page 29 of 39

Figure(s)

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Figure 1

(a)

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+ + + + + + + + + + ++ + + + ++ + + + ++ + + ++ +

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Substrate

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TiO2 PSS

Photocatalytic deposition

PSS/TiO2 deposition

Au nanoparticle

(b) MPS modified Au nanoparticle

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Photocatalytic PSS/TiO2 deposition deposition + + + ++ + + + + ++++++ ++++ + ++ + + + + ++++++ ++++ + + + + + + ++ ++ + + ++ + + + + + + + + + + + + + + ++ + + + + + + + + + + ++ + + + + + + + + + + ++ + + + ++ + + + ++ + + ++ + + + + ++ + + + ++ + + ++ + + + + ++ + + + ++ + + ++ + Substrate

Substrate

Substrate

Yanagida et al. Page 30 of 39

0.4

0.4

400

600

0.0

400

600

Wavelength (nm)

0.8 0.6 0.4 0.2

800

0.0

400

600

800

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800

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0.6

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Figure 2

Yanagida et al. Page 31 of 39

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Yanagida et al. Page 32 of 39

Figure 4 (200)

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(111)

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Intensity (a.u.)

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(b)

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40 60 CuKa 2q (deg.)

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Yanagida et al. Page 33 of 39

Figure 5

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Na-MPS/HAuCl4 (mol ratio)

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40

Yanagida et al. Page 34 of 39

(c)

100 nm

100 nm

(g)

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(f)

100 nm

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100 nm (h)

100 nm

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pt

100 nm

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Yanagida et al. Page 35 of 39

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Figure 7 (PSS/TiO2/Au)2+(PSS/TiO2)

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0.08 0.06

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Au/Ti (mol ratio)

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(PSS/TiO2/Au)2

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Yanagida et al. Page 36 of 39

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2

250

300

350

400

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1

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Wavelength (nm)

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Additive-free Au/MPS = 0.17 Au/MPS = 0.34 Au/MPS = 0.68 Au/MPS = 1.71 Au/MPS = 3.41

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Figure 8

Yanagida et al. Page 37 of 39

Figure 9

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Yanagida et al. Page 38 of 39

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Figure 10

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Peak wavelength (nm)

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Refractive index

Yanagida et al. Page 39 of 39