# Photoelectrochemical Behavior of Silver Nanoparticles inside Mesoporous Titania: Plasmon-Induced Charge Separation Effect

Nelly Couzon, Mathieu Maillard, Fernand Chassagneux, Arnaud Brioude, Laurence Bois

► To cite this version:

Nelly Couzon, Mathieu Maillard, Fernand Chassagneux, Arnaud Brioude, Laurence Bois. Photoelectrochemical Behavior of Silver Nanoparticles inside Mesoporous Titania: Plasmon-Induced Charge Separation Effect. Langmuir, 2018, 35 (7), pp.2517-2526. 10.1021/acs.langmuir.8b03617. hal-02087646

HAL Id: hal-02087646

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Photo-electrochemical Behavior of Silver

Nanoparticles inside Mesoporous Titania:

Plasmon induced charge separation effect

Nelly Couzon<sup>a</sup>, Mathieu Maillard<sup>a</sup>, Fernand Chassagneux<sup>a</sup>, Arnaud Brioude<sup>a</sup>, Laurence Bois<sup>a</sup>

Laboratoire Multimatériaux et Interfaces, UMR CNRS 5615, Université Claude Bernard Lyon 1,
France.

KEYWORDS. Silver nanoparticles, mesoporous titania, cyclic voltammetry, photochromism, plasmon.

ABSTRACT

The self-assembly block copolymer method was used to synthesize mesoporous titania films and
silver nanoparticles (NPs) were grown inside the films. Such silver NPs-titania films are known
for their multicolor photochromic properties, due to a photo-oxidation reaction of silver in the
presence of titania under light excitation which is attributed to a plasmon induced charge
separation (PICS). Here, the photo-electrochemical properties of these composite films have
been investigated at different light wavelengths and chemical environment in order to
characterize the light-induced redox reactivity modifications. Cyclic voltammetry (CV) study
shows that the Ag⁺ electro-reduction peak potential varies depending on the light irradiation, which determines the state of the silver nanoparticles complexed or not by titania.

Introduction

Solar energy harvesting is among the main challenge in material design and metallic
nanoparticles have often been associated with semiconductors to improve their properties
for applications such as solar fuel generation¹, photocatalysis², photovoltaics³, chemical
sensing⁴ and information storage.⁵ The close contact between metallic nanoparticles and
semiconductors allows charge transfer between them⁶ and is a key parameter for charge
separation required for most solar energy conversion applications. Accordingly, using
mesoporous semiconductors, such as TiO₂, as a host for metallic nanoparticles NPs is a
common strategy to increase the surface contact and enhance the properties of the
composite. The photo-electrochemical properties of silver NPs embedded in mesoporous
titania films at various light wavelengths are presented in this work and have been
compared to other systems to elucidate the reaction mechanism. Silver NPs are known to
present Surface Plasmon Resonance (SPR), which is due to the conduction band electrons
oscillation induced by light interaction and leads to exceptional optical properties. Under
UV illumination, electronic transfer from TiO₂ to silver NPs has been proved,⁷ via a blue
shift of the silver NPs SPR. Under visible light, a charge-transfer mechanism from silver
NPs to TiO₂ was evidenced by Tian *et al*⁸ and called Plasmon Induced Charge Separation
(PICS). In this process, silver NPs surface plasmon resonance generate hot electrons with
enough energy to cross the Schottky barrier.⁸,⁹ Electrons are injected into the TiO₂
conduction band, inducing charge separation, which is at the origin of the enhanced
photocatalytic properties.¹⁰ This effect leads to Ag oxidation, as well as Ag⁺ reduction on the surface of the titania support and more specifically on silver NPs.¹¹,¹² A negative shift of the standard redox Ag⁺/Ag potential under illumination was also evidenced¹³,¹⁴ as well as an increase in the semiconductor conductivity.¹⁵,¹⁶ The photocatalytic properties of TiO₂ are thus generally improved in the presence of silver NPs, if the particles are protected from oxidation with a silica shell for example.¹⁷ Another important application of TiO₂-silver NPs concerns data storage using the photochromism effect.¹⁸⁻²² Indeed, light irradiation induces some silver particles photo-oxidation. The disappearance and/or change in morphology of the particles provokes a material coloration change, allowing data storage.²³⁻²⁵ Such a photochromic properties can also be observed by applying a potential on a silver NPs-titania electrode.²⁶,²⁷ Potentiometric sensing has been developed¹⁶ in which the electron injection from silver NPs to TiO₂ induces a negative potential shift.¹³ The role played by water has been highlighted and the photo-anodic dissolution of Ag NPs to Ag⁺ and silver cathodic deposition proved.²⁸

The synthesis of mesoporous titania films is based on the sol-gel process, using amphiphilic block copolymers undergoing a self-assembly process.²⁹ Formation of silver NPs inside such mesoporous templates has been developed.³⁰ An efficient metallic NP confinement is reached by immersion of the mesoporous TiO₂ film in a silver salt solution, and then in a formaldehyde solution to reduce the Ag⁺ species and form metallic NPs.³¹,³² We initially studied the photo-electrochemical properties of a TiO₂-Ag composite, showing that the Ag⁺ electro-reduction potential can be modulated by light irradiation and that silver repartition within the mesoporous matrix is modified.³³ In this work, we synthesized silver NPs embedded in mesoporous TiO₂ films and characterized them through scanning electron microscopy (SEM), UV-visible
absorption and photo-electrochemistry at different wavelengths. The present study describes the specific effect of wavelength on the silver ions reduction potential and probes the chemical bonding between the silver NPs and titania matrix. A comparison is done with the photo-electrochemical behavior of silver species with another environment: silica instead of titania, silver/gold alloy instead of pure silver, ionic Ag⁺ instead of AgNPs in order to probe the complexation of silver NPs by titania.³⁴⁻³⁶

# Experimental

## Materials

Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer, Pluronic P-123 ((EO)₂₀-(PO)₇₀-(EO)₂₀), Pluronic F-127 ((EO)₁₀₆-(PO)₇₀-(EO)₁₀₆), tetrabutylorthotitanate (Ti(OBu)₄), tetraethoxysilane (Si(OEt)₄), silver nitrate (AgNO₃), HAuCl₄, HCl and formaldehyde (CH₂O) were all purchased from Sigma-Aldrich and used as received. The FTO (fluorine-doped tin oxide) substrates (3 × 3 cm, 600 nm of FTO-deposited by CVD) were purchased from Solems.

## Mesoporous electrodes preparation

The titania sol was prepared according to previously published results³¹ by mixing Ti(OBu)₄ (3.4 g), anhydrous ethanol (12 g), hydrochloric acid (HCl, 37%) (3 mL) and a triblock copolymer P-123 (1 g). The polymer is first dissolved in ethanol. Hydrochloric acid is added dropwise on Ti(OBu)₄ under stirring. Then, the two solutions are mixed together. After stirring for 30 minutes, films were then deposited by a dip-coating method on FTO substrates with a withdrawal speed of 2 mm.s⁻¹. The sol can be used within 7
days. The films were aged under ambient conditions (T = 23 °C, RH = 30-40%) for 24 h
and then calcined at 350 °C for 2 h, with a heating ramp of 1 °C min⁻¹.

In order to form silver NPs inside the porous oxide films, the titania films were immersed in an ammoniacal silver nitrate solution (50 mM) for 15 min. Then the silver ions were chemically reduced using formaldehyde, by immersing films for 15 min at 80 °C in a solution of 12 mM formaldehyde in ethanol. The resulting films, with a red /dark brown color, were stored in a dark place under ambient conditions.

Three others reference samples have been prepared wherein we modified only one
parameter of the original sample in order to elucidate the mechanism: we replace titanium
dioxide by silica, we introduced silver as ions in solution instead of nanoparticles in the
film and we replaced silver nanoparticles by bimetallic silver-gold nanoparticles. The first
reference experiment was done with silver NPs in a mesoporous silica film. The silica sol
was prepared by mixing Si(OEt)₄ (4 g), ethanol (12 g), H₂O acifidied to pH 2 with
chlorohydric acid (HCl, 37%) (1.76 g) and F-127 triblock copolymer (1 g). F-127
polymer is dissolved in ethanol. Acidified water is added to Si(OEt)₄. Then, the two
solutions are mixed together. The silica films are deposited, aged and calcined by the
same method than the titania film. The preparation method for silver NPs is also the same.
The resulting film is yellow colored.

To understand the role of silver NPs, another reference experiment was done in which
Ag⁺ was not introduced in the titania pores. An empty mesoporous titania film was
directly used in a cyclic voltammetry (CV) experiment. The electrolyte is composed of
Na₂SO₄ (1 M) with silver nitrate (6. 10⁻⁴ M).
A sample in which silver was partially replaced with gold was prepared in order to study the influence of a modified interface between silver and titania. To change from silver NPs to gold ones, a galvanic replacement was performed on the Ag NPs - mesoporous titania films. The film was immersed in a HAuCl₄ solution (40 mM) for 15 min. The color slowly changed from red-brown to purple, indicating the formation of metallic gold NPs within the matrix. The resulting films were stored, after rinsing, in a dark place under ambient conditions.³⁸

For porosity measurements, TiO₂ powder was prepared using the same preparation method described above. The TiO₂ sol was heated at 50°C for 2 days then the solid calcined at 350°C for 2 h, with a heating ramp of 1°C.min⁻¹.

## Characterization

SEM images were obtained using a Zeiss Merlin Compact SEM with an in-lens detector at a low acceleration voltage of 5 kV. Energy dispersive X-ray analysis (EDX) was used to determine the amount of silver within the oxide film. Transmission electron microscopy (TEM) was performed on a TOPCON instrument operating at 200 keV. In a typical procedure, TiO₂-Ag film fragments of non-controlled thickness were stripped off by scratching the samples with a razor blade and were then deposited on a carbon-coated copper grid. The UV-visible absorption spectra of samples were obtained using a Safas UVmc spectrometer measuring absorbance from films. Scans were measured between 300 and 1000 nm with a 2 nm resolution. Textural characterization was realized using nitrogen adsorption/desorption isotherms on a BelsorpMini (Bel Instruments, Japan). Prior to analysis, the sample was outgassed under vacuum (at 100°C for 4 h). Pore size distribution (max radius) and mesoporous volume were calculated from the adsorption
branch of the isotherms using the Barrett-Joyner-Halenda (BJH) method. The total porous
volume was measured at P/P₀=0.98. The surface area was determined by the Brunauer-
Emmett-Teller (BET) approach.

All electrochemical measurements were performed using a Volta lab PST006
Potentiostat with a three-electrode cell designed by Zahner Gmbh. A spiral Pt wire was
used as the counter electrode and the reference electrode was a Re-5B Ag/AgCl electrode
from BASi with a reference potential of 0.21 V/NHE. The working electrode was the
titania-Ag NPs film deposited on a 3x3 cm FTO-covered glass substrate. The
electrochemically active area of the film was 3.6 cm². LED's at 455, 530 and 660 nm
were purchased from LED ENGINE Co. and calibrated with a flame spectrometer from
Ocean Optic.³³ They have a nominal flux of 329, 51 and 260 mW/cm² respectively at a
working distance between the source and the sample of d = 20 mm. Measurements were
carried out in a neutral electrolyte, 1 M Na₂SO₄, at room temperature. Solutions were not
necessarily degassed since we had previously verified that degassing had no significant
effect on the present experiments. During illumination experiments, samples were
irradiated from the back. A typical CV experiment was performed with a potential sweep
rate of 100 mV/s between -0.5 and +0.5 V. An OCP (Open Circuit Potential) was
measured during one minute before each CV experiment (each consisting of 3 cycles).
Three consecutive OCP-CV experiments were done alternating on and off light exposure.
# Results & Discussion

## Characterisation before photo-electrochemical measurements

SEM pictures of the composite show that silver NPs of 30 nm on average cover the worm-like mesoporous titania surface (Fig. 1a). In order to analyze the nature and the thickness of the film, the sample was scraped and a fractional film was observed under SEM (Fig. 1b). Smaller silver particles (8 nm) are present in the whole depth of the titania film, showing channels of 6-8 nm diameter (Fig. 1b). The two sides of the film (air and FTO interface) are easily distinguished due to the roughness of the FTO surface: titania film at the interface with FTO is corrugated whereas the upper surface is extremely flat. Pore size was measured using the nitrogen adsorption technique on TiO₂ powder (Figure S1) and was found to be 6 nm. The porosity is estimated at 50% from the mesoporous volume calculated with the BJH method. From EDX analysis, the Ag/Ti atomic ratio is around 0.3. Considering a titania film thickness of 200 nm and a porosity of 50%, the SEM image analysis of Fig. 1a (with Image J) allows us to estimate that the atomic ratio of silver on the surface relative to total titania is equal to 0.022. Compared to the 0.3 ratio value obtained by EDX, the silver located on the surface represents only 7% of the total silver content, whereas the remaining 93% are within the pores of the 200 nm thick film. Details of this analysis are presented in ESI 2.
<figure><img src="image_2.png" /><figcaption>Figure 1: SEM images of a mesoporous titania-Ag NPs electrode before CV, a: top; b: profile after scratching the film.</figcaption></figure>

The high density of silver NPs, with particles size comprised between 5 nm and 11 nm can also be seen on the TEM picture (Fig. 2a). On the high resolution TEM image (Fig. 2b), a *single* silver nanoparticle with diameter of 10 nm is *noted* *visible*.
<figure><img src="image_3.png" /><figcaption>Figure 2: TEM (a) and HRTEM (b) images of a mesoporous titania-Ag NPs electrode before CV.</figcaption></figure>

The UV-visible spectrum of mesoporous titania-Ag NPs shows an absorption band at 490 nm (blue curve, Fig. 3), while mesoporous titania has no absorption in the visible part (gray curve, Fig. 3). The band related to the SPR of silver NPs is very broad and red-shifted compared to what is usually found in silver NPs embedded in solid matrices, centred between 400 and 440 nm with a 100 nm full width at half maximum, as shown for instance in Fig. 3 (black curve) with the spectrum of mesoporous silica -Ag NPs.<sup>19,38</sup> The shift and broadening observed in solid matrix is usually due to resonance condition modification according to the surrounding medium refractive index change. Hence a silver particle totally surrounded by the high refractive index of titania
would exhibit a plasmon resonance comprised between 460 and 480 nm. Taking into account
that titania is not perfectly crystallized and silver nanoparticles are not fully surrounded by
titania, the refractive index (1.8-2.0) effect only partially explains the observed shift.³²,³⁹,⁴⁰ The
other parameters that would explain this unusual width may be due to the shape and size
distribution of silver NPs but it does not fully correlate with the actual data as mostly small and
spheroidal particles are observed from TEM observations. Another factor is related to the
possible surface oxidation of silver NPs in contact with titania.⁴¹ Recently, an interfacial charge
transfer between silver and titania was also invoked.³⁴⁻³⁶,⁴² The titania-silver interaction is of
polar-covalent nature, with the titania attracting electronic charge from the silver cluster.⁴² This
complexed state between titania and silver NPs partly explains the unusual broad absorption
spectrum obtained for the silver NPs-titania composite.

<figure><img src="image_4.png" /><figcaption>Figure 3 : UV visible absorption spectra of mesoporous titania electrode (grey), mesoporous titania-Ag NPs electrode before CV (blue) and mesoporous silica-Ag NPs electrode before CV (black).</figcaption></figure>

Photo-Electrochemical experiments
## Influence of light wavelength.

OCP and CV acquisitions between -500 and 500 mV with the light on are performed three times, followed by the same acquisitions with the light off. This sequence is repeated twice (Scheme 1). Each CV consists of 3 consecutive scans starting from -500 to 500 mV. The purpose of this sequence is to probe both the instantaneous and afterglow effect of light exposure. For a better clarity, the second cycle is shown in Figures 4 & 5, the first cycle is shown in supplementary information (Figures S2 & S3).

<figure><img src="image_5.png" /><figcaption>Scheme 1: Sequences of OCP and CV used with light on or off.</figcaption></figure>

The CV results for Ag NP-TiO₂ films are shown in Figure 4 for an acquisition without light followed by two acquisitions with light. The former acquisition is presented in black (curve I) and the latter are the colored curves (curves II & III). The second cycle done without light (curve I, Fig. 4a) shows an oxidation current related to the oxidation processes Ag⁰/Ag⁺ as a single peak at 290 mV. The oxidation is only partially reversible since a broad cathodic peak, related to the Ag⁺/Ag⁰ reduction process, is observed at -90 and +10 mV (noted as a black star on the picture), corresponding to a voltage difference of ΔE = 280 mV.

Under irradiation at 455 nm, the oxidation peak is not impacted whereas the cathodic peak is significantly modified (curves II & III, Fig. 4a). The component at -90/10 mV has decreased in intensity and a new cathodic peak appears at 160 mV and its intensity increases during the acquisitions (indicated by the black arrow in Fig. 4a), when the light
is on. Ag⁺ electro-reduction is globally shifted from 150 mV towards positive values (Table 1 and Fig. 4a).

In the experiment performed with 530 nm irradiation (curve I, Fig. 4b), the first acquisition done without light shows an anodic peak at 240 mV and two cathodic peaks at 106 and -110 mV. In the following acquisitions performed with the light on, the anodic peak is preserved, but the cathodic region is changed (curves II & III, Fig. 4b). The peak at 100 mV increases (shown by the black arrow in Fig. 4b), with another one at -30 mV, while the peak at -115 mV has decreased. Therefore, under light exposure at λ=530 nm, the influence is very clear on the Ag⁺ reduction peak potential: globally shifting from -110 to 100 mV, with a second component at -30 mV.

In the last experiment at 660 nm (curve I, Fig. 4c), an initial broad cathodic peak is observed at 10 mV without light. A second broad cathodic peak is also observed at -300 mV. However, with the light on, the main peak shifts to 70 mV and increases in intensity. Under 660 nm, a limited shift of the Ag⁺ reduction potential is observed, from 10 to 70 mV (Table 1 and curves II & III, Fig. 4c), as this irradiation wavelength does not have a great influence on the Ag⁺ electro-reduction process.

An empty mesoporous titania electrode has also been used within the same electrochemical conditions, between -0.5 V and 0.5 V and under visible light illumination. This experiment shows that there is no titania electro-activity under these conditions, which would not be the case for instance if we had used UV illumination and larger potentials.⁴³
<table><caption>Table 1: Summary of Ag⁺ reduction potential of mesoporous titania-Ag NPs electrode for CV with light off followed with 3 acquisitions under light irradiation.</caption><thead><tr><th>Light wavelength (nm)</th><th>Ag⁺ reduction potential light off (mV)</th><th>Ag⁺ reduction potential light on (mV)</th><th>Delta (mV)</th></tr></thead><tbody><tr><td>455</td><td>-90 and +10</td><td>160</td><td>150</td></tr><tr><td>530</td><td>-115 and +106</td><td>-30 and +100</td><td>210</td></tr><tr><td>660</td><td>10</td><td>70</td><td>60</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>Figure 4: CV experiments of mesoporous titania-Ag NPs (second scan); black curves (I) correspond to the CV without irradiation preceding two CVs with irradiation; Pale colored curves (II) are the first following CV with light on; dark colored curves (III) are the third CV with light on. a: under light irradiation at 455 nm (blue), b: 530 nm (green) and c: 660 nm (red). Stars denote the cathodic peak positions.</figcaption></figure>
Figure 5 shows the voltammogram evolution after the light has been switched off. For example, when the 455 nm irradiation is turned off (Fig. 5a and Table 2), the cathodic peak initially observed at 155 mV (curve I) disappears while a new cathodic peak appears at -55 mV (curves II & III), whereas after the 530 nm irradiation has been switched off, the cathodic peak at 100 mV (curve I) is damped and a new peak appears at -110 mV (curves II & III, Fig. 5b and Table 2). At 660 nm, the first acquisition with the light on gives a cathodic peak at 70 mV (curve I, Fig. 5c and Table 2). In the following acquisitions with the light off (curves II & III), the peak shifts to 50 mV and its intensity decreases.

In Figure 5, the Ag⁺ reduction intensity decreases progressively as the light is off, while in Figure 4, it increases as the light is on.

All these observations show that the reduction potential of the Ag⁺ species is related to the electrode illumination and then probably to the silver NPs excitation state. Without irradiation, a complexation between silver NPs and titania induces an accumulation of positive charges within the silver NPs³⁴⁻³⁶,⁴¹ which is a limitation for further Ag⁺ electro-reduction on the metallic surface.⁴²

<table><caption>**Table 2:** Summary of Ag⁺ reduction potential of mesoporous titania-Ag NPs electrode with the acquisition with light on followed with 3 acquisitions with light off.</caption><thead><tr><th>Light wavelength (nm)</th><th>Ag<sup>+</sup> reduction potential light on (mV)</th><th>Ag<sup>+</sup> reduction potential light off (mV)</th><th>Delta (mV)</th></tr></thead><tbody><tr><td>455</td><td>155</td><td>-55</td><td>210</td></tr><tr><td>530</td><td>100</td><td>-110</td><td>210</td></tr><tr><td>660</td><td>70</td><td>50</td><td>20</td></tr></tbody></table>
<figure><img src="image_7.png" /><figcaption>Figure 5: CV experiments of mesoporous titania-Ag NPs (second scan). Colored curves (I) correspond to the CV under irradiation preceding two CV without irradiation. The grey curves (II) correspond to the first CV done without irradiation and the black curves (III) correspond to the second CV done without irradiation. a: under light irradiation at 455 nm (blue), b: 530 nm (green) and c: 660 nm (red). Stars denote the cathodic peak positions.</figcaption></figure>
When irradiated within the SPR wavelength range, electron transfer from excited silver NPs to titania is made possible. PICS process occurs and silver nanoparticles are partially oxidized, forming Ag⁺ species and electrons (scheme 2). PICS modifies the state of charge transfer occurring between the silver particles and titania and we assume most of silver NPs have lost their complexation with titania (average irradiance corresponds to about 10⁴ photons per nanoparticles per second). Once nanoparticles are no longer complexed by titania, silver ions reduction is facilitated on the pristine metallic nanoparticles. These silver particles can act more efficiently as seeds for further Ag⁺ electro-reduction as no partial positive charges are present on the surface. It explains that, a new electro-reduction peak Ag⁺ shifted towards positive potentials is observed, meaning that Ag⁺ electro-reduction occurs more easily under light excitation.

<figure><img src="image_8.png" /><figcaption>Scheme 2: Picture representing complexation of surface silver atoms from TiO₂. PICS induces a cathodic polarization of nanoparticles and the release of silver ions, the two factors facilitate the Ag⁺ electro-reduction on Ag particles.</figcaption></figure>

After the light irradiation is stopped (Fig. 5), the system returns to the initial state because the interfacial charge transfer between silver particles and titania occurs. When light irradiation starts (Fig. 4), a rapid electro-reduction potential switch is noted due to the silver NPs availability. A slow evolution can also be observed as the intensity increase of the Ag⁺ electro-reduction peak: as new Ag⁺ species are progressively formed during the photo-oxidation process; and eventually participate in the electro-reduction process, inducing a current intensity increase (Fig. 4).
Characterisation after electrochemistry at various light wavelengths

Comparison of Ag NPs–mesoporous titania films before (Fig. 1) and after (Fig. 6) the photoelectrochemical experiment, as observed by SEM, clearly shows a strong effect of irradiation wavelength on the Ag NPs morphology. Under 455 nm, silver aggregates of 200 nm have grown, while small particles are still present. Under 530 nm, the silver particle growth is less important: few silver particles from 50 to 100 nm have appeared. This growth of silver aggregates occurs mainly at the upper surface, as already observed by a lot of works about photochromic films TiO₂-Ag.¹² Under 660 nm, the initial silver particle morphology is preserved, as light has no effect on the film redox activity at this wavelength. SEM pictures show that the growth of silver aggregates is enhanced after an electrochemical experiment under 455 nm irradiation, much more than under 530 nm and 660 nm. These observations translate the well-described photochromic properties of these composites, which are here assisted by the voltammetry cycling. Indeed, it is well depicted that the Ag⁺ photo-generated, and in our experiment Ag⁺ electro-generated, diffuse and recombine with the electrons transferred via titania on a non-resonant silver particle, which explains the growth of these non-resonant particles.¹²
<figure><img src="image_9.png" /><figcaption>Figure 6: SEM images of a mesoporous titania-Ag NPs electrode after CV under, a: 455 nm; b: 530 nm; c: 660 nm irradiation.</figcaption></figure>

UV visible absorption spectra (fig. 7) also show that the silver particles morphology is strongly impacted by CV under 455 nm, since absorption has strongly decreased, which is correlated to the silver aggregate growth. SEM pictures and UV visible absorption both show that the growth enhancement of silver aggregates is more pronounced after an electrochemical experiment under 450 nm irradiation than under 530 nm and 660 nm. These observations translate the well-known photochromic properties of these composites.²³
<figure><img src="image_10.png" /><figcaption>Fig. 7 UV visible absorption spectra of mesoporous titania-Ag NPs before (black) and after photo-electrochemical at 455 nm (blue), 530 nm (green) and 660 nm (red).</figcaption></figure>

Experiment with silver-mesoporous silica electrode

To confirm the importance of the strong interaction of silver NPs with the titania matrix
on the observed photo-electrochemical behavior, an experiment was performed with
silver NPs in a silica matrix (Fig. 8). In this case, no complexation between silver NPs
and silica is expected, since such systems do not present any photochromic behaviour and
surface plasmon resonance of silver nanoparticles in SiO2 only undergoes a limited shift.

<figure><img src="image_11.png" /><figcaption>Figure 8: CV experiment of mesoporous SiO2-Ag NPs sample without light.</figcaption></figure>
Indeed, the main silver electro-reduction occurs at 150 mV (Fig. 8), which is approximately the Ag⁺ electro-reduction potential in mesoporous TiO₂, obtained under irradiation at 455 nm. Here, irradiation in the SPR does not induce any change in the Ag⁺ electro-reduction potential. This absence of variation of the Ag⁺ electro-reduction peak potential could be related to the absence of any complexation between silver NPs and silica. This experiment confirms the role of the complexation state between silver NPs and titania surface, modulated by light excitation.

Experiment with Ag⁺ species outside the mesoporous titania electrode

To confirm the key role of finely-dispersed silver NPs on the observed photo-
electrochemical behavior, an experiment was performed without nanoparticles. A silver
salt (6.10⁻⁴ M) was first dissolved in the Na₂SO₄ electrolyte (1 M) and a pristine
mesoporous titania film was used as the working electrode. In that case, the formation of
silver NPs inside the mesoporous film was not favored. During CV (Fig. 9), we observe a
strong Ag⁺ reduction (red dotted square) at the beginning of the experiment, associated
with the formation of silver aggregates outside the electrode (Fig. 10).
<figure><img src="image_12.png" /><figcaption>Figure 9: CV experiments of the mesoporous titania electrode with AgNO₃ introduced in the electrolyte, under light irradiation at 455 nm (blue) and after irradiation (black).</figcaption></figure>

The sample has a metallic aspect (gray color) and large aggregates are formed on the top side of the electrode, at the interface with the electrolyte. The SEM picture (Fig. 10) of the titania film at the end of the CV experiment confirms the presence of a high density of large silver aggregates. From EDX analysis, the Ag/Ti atomic ratio is found to be 0.45. From SEM image analysis of Figure 10, we estimate that the atomic ratio of silver on the surface relative to the titania film is equal to 0.36, i.e. 80% of the total amount of silver. This confirms that in this experiment, silver is mainly located on the titania electrode surface while in the case of the mesoporous TiO₂-Ag NPs, the silver located on the surface represents only 7% of the total silver. In addition, a broad and intense silver oxidation peak (350 mV) is related to the partial oxidation of these large aggregates on the titania surface.
<figure><img src="image_13.png" /><figcaption>Figure 10: SEM images of mesoporous titania electrode after CV under 455 nm with AgNO₃ introduced in the electrolyte.</figcaption></figure>

The Ag⁺ reduction potential (150 mV) is very similar to the potential obtained in mesoporous titania under 455 nm excitation (Fig. 4a) and in mesoporous silica (Fig. 8). In the absence of Ag NPs in the titania porosity, interface area between metallic silver and titania is reduced and the interfacial charge transfer is less efficient. Illumination at 455 nm consequently does not change the Ag⁺ electro-reduction, as silver NPs are not present in this experiment and no longer exhibit plasmon resonance absorption.

## Experiment with silver/gold-mesoporous titania electrode

To further confirm the importance of the close vicinity of silver NPs and TiO₂ in the Ag⁺ electro-reduction process, the silver NPs were partially replaced by gold ones by galvanic replacement using a HAuCl₄ solution. The assumption is the introduction of gold on the nanoparticle surface, would limit the charge transfer interface between silver NPs and titania. EDX analysis confirmed the partial replacement of silver by gold since the Ag/Ti atomic ratio changed from 0.3 to 0.17, while the Au/Ti atomic ratio is about 0.32. The same CV experiment was performed with this mesoporous TiO₂-AgAu NPs electrode (Fig. 11).
<figure><img src="image_14.png" /><figcaption>Figure 11: CV experiments of the mesoporous TiO₂-Ag NPs sample (black) and TiO₂-AgAu NPs (green) without light.</figcaption></figure>

Silver electro-oxidation occurred at 300 mV while electro-reduction is around 220 mV. This small potential difference of 80 mV demonstrates that redox reaction is more reversible on silver-gold than on pure silver NPs embedded in mesoporous titania, which corroborates the hypothesis of a complex forming between metallic silver and titania. Finally, lower intensities of the silver redox peaks can be explained by the lower silver concentration in this gold-modified electrode. Furthermore, silver electro-reduction potential is no more light sensitive, proving the importance of the interface between silver NPs and TiO₂.

## Mechanism

Based on the above experiments, we have sufficient information to describe a mechanism for the observed silver ion reduction potential modulation inside titania under light excitation.³³ In the absence of light, the electro-reduction of Ag⁺ on metallic silver NPs is hindered. A possible explanation of this hindrance effect is that silver NPs in close vicinity to TiO₂ undergo an
interfacial charge transfer to the TiO₂ matrix, similar to a complexation.³⁴⁻³⁶ Light exposure leads to generation of electrons and Ag⁺ species formed by the photo-oxidation of excited silver nanoparticles. The consequence is a decomplexation of the silver NPs from TiO₂. The Ag⁺ electro-reduction is no longer hindered. This phenomenon has been quantified as a switch in the silver ion reduction peak potential, clearly visible at the surface plasmon maximum absorption wavelength, i.e. 455 nm. In this case, Ag⁺ electro-reduction occurs at the expected potential, on silver NPs acting as preferential growth sites. Experiment with silver NPs in silica shows that Ag⁺ electro-reduction is not hindered in that case as no complexation effect is expected between silver NPs and silica. The results obtained when an external source of silver ions is added in the electrolyte is in agreement with this hypothesis, as the electro-reduction of Ag⁺ is no longer related to the light exposure in the absence of silver NPs. Experiments performed on a gold modified electrode also confirm the same hypothesis as Ag⁺ electro-reduction is not hindered. The introduction of gold reduces the interfacial charge transfer occurring between titania and pure silver NPs. This observation can be compared to the photocatalytic loss activity in bimetallic AgAu-titania photocatalyst⁴⁵, which corroborates the charge transfer mechanism occurring from silver towards titania under visible light excitation.¹³ We also observed potential-assisted photochromic properties from these materials: during CV experiments under 455 nm exposure, silver aggregates growth is strongly enhanced, via a PICS effect occurring within the titania-silver system, while under 530 nm the growth is reduced and vanishes under 660 nm exposure. At the end of the CV experiments, samples irradiated at 530 and 660 nm seem to keep their initial purple colour, while sample irradiated at 455 nm appeared discoloured, as a direct consequence of the aggregate growth mechanism. All these photo-electrochemical observations give an insight of the influence of the SPR light excitation on the chemical bound between silver
NPs and titania and can be related to the photochromic properties of silver NPs-mesoporous titania composites.

## Conclusion

The present study describes the specific effect of wavelength on the silver ions reduction potential and probes the chemical bonding between silver NPs and the mesoporous titania matrix. We investigated the photo-electrochemical properties of silver NPs embedded in mesoporous titania films at various light wavelengths and interpreted them using a complexation effect. In the absence of light, the Ag⁺ electro-reduction inside mesoporous titania is hindered due to the complexation of silver NPs by titania. Under light excitation in the surface plasmon of silver NPs, the interfacial charge transfer with titania is interrupted and the Ag⁺ electro-reduction occurs with a higher reversibility on silver NPs. Such behavior, observed in this photo-electrochemical study, is closely related to the photochromic properties of mesoporous titania-Ag NPs films and emphasizes the importance of light-induced charge separation on chemical interactions.

## ASSOCIATED CONTENT

**Supporting Information.** Figure S1: Nitrogen adsorption/desorption isotherm and BJH curve (adsorption branch) of mesoporous TiO₂ sample (V<sub>meso</sub>= 0.26 cm³.g⁻¹; pore radius = 3 nm); ESI 2: Surface silver estimation. Figure S2: CV experiments of mesoporous titania-Ag NPs (first scan); black curves (I) correspond to the CV without irradiation preceding two CVs with irradiation; Pale colored curves (II) are the first following CV with light on; dark colored curves (III) are the third CV with light on. a: under light irradiation at 455 nm
(blue), b: 530 nm (green) and c: 660 nm (red). Stars denote the cathodic peak positions.

Figure S3: CV experiments of mesoporous titania-Ag NPs (first scan). Colored curves (I) correspond to the CV under irradiation preceding two CV without irradiation. The grey curves (II) correspond to the first CV done without irradiation and the black curves (III) correspond to the second CV done without irradiation. a: under light irradiation at 455 nm (blue), b: 530 nm (green) and c: 660 nm (red). Stars denote the cathodic peak positions.

## AUTHOR INFORMATION

### Corresponding Author

* Phone: +33 4 72 44 81 66, e-mail: laurence.bois@univ-lyon1.fr

### Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

### ACKNOWLEDGEMENT

This work was supported by the LABEX iMUST (ANR-10-LABX-0064) of the Université de Lyon, within the program "Investing in the Future" (ANR-11-IDEX-0007) by the French National Research Agency (ANR). The authors gratefully acknowledge the CTμ platform of electronic microscopy (University Lyon 1) and the CLYM (consortium Lyon Saint-Etienne de microscopie).
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