Fully Reversible Electrically Induced Photochromic-Like Behaviour of Ag:TiO 2 Thin Films

: ATiO 2 thinfilm, preparedonfluorine-dopedindiumtinoxide(FTO)-coatedglasssubstrate, from commercial off-the-shelf terpinol-based paste, was used to directly adsorb Ag plasmonic nanoparticles cappedwithpolyvinylpyrollidone(PVP)coating. TheTiO 2 filmwassinteredbeforethesurfaceentrapment of Ag nanoparticles. The composite was evaluated in terms of spectroelectrochemical measurements, cyclic voltammetry as well as structural methods such as scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). It was found that the Ag nanoparticles are effectively adsorbed on the TiO 2 film, while application of controlled voltages leads to a fully reversible shift of the plasmon peak from 413 nm at oxidation inducing voltages to 440 nm at reducing voltages. This phenomenon allows for the fabrication of a simple photonic switch at either or both wavelengths. The phenomenon of the plasmon shift is due to a combination of plasmon shift related to the form and dielectric environment of the nanoparticles.

The TiO 2 films are highly photosensitive and exhibit optically induced properties. They are also applicable in non-linear optics and optical devices [27,28]. However, one of their main disadvantages is that their large energy band gap (E g = 3-3.4 eV), that lies in the ultraviolet (UV) region, limits

Preparation of Silver Nanoparticles (AgNPs) and TiO 2 -AgNPs Films
We dissolved 0.01 g/mL of PVP at room temperature in triply distilled H 2 O (10 mL), to which 100 µL of 0.5 M AgNO 3 solution were slowly added under stirring for one hour. The solution was kept for 24 hours after which its colour darkened. Similar reactions were achieved by an extra addition of equal volumes of 0.5 M Na 2 S aqueous solution to the AgNO 3 :PVP mixture. TiO 2 films were immersed in AgNO 3 :PVP solution and kept for 24 hours. This procedure enables the reduction of Ag + [35], its agglomeration to nanoparticles as well as the prevention of further agglomeration to very large nanoparticles [36].

Film Characterization
The adsorption onto the TiO 2 films was monitored by recording the UV-Vis absorption spectra of the immobilized films at room temperature using a Shimadzu UV-1800 spectrophotometer. Contributions to the spectra from scatter and absorption by the TiO 2 film alone were subtracted by the use of AgNPs-free reference films. Prior to all spectroscopic measurements, the films were removed from the immobilization solution and rinsed in a buffer (methanol) solution several times to remove non-immobilized nanoparticles or excessive AgNO 3 solution. The photocatalytic process can be associated with the Ag-TiO 2 films, where ultra bandgap irradiation of titania generates an electron-hole pair, with possibility to reduce silver ions at the surface of the titania to silver metal. The conditions under experiments were free from UV irradiation. Typically, the Ag-TiO 2 prepared films appeared dark brown black in transmitted light. Chemical and thermal reduction have also been used to prepare Ag-TiO 2 films which, however, lacked the switching mechanism reported here.
XRD analyses of the TiO 2 films on FTO glass with or without AgNPs were performed using a Bruker D8 advance X-Ray Diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) using a Cu Kα-radiation source set with an anode current of 40 mA and accelerating voltage of 40 kV with a scanning speed 0.015 degrees/second. The diffraction patterns were indexed by comparison with the Joint Committee on Powder Diffraction Standards (JCPDS) files number 21-1276 and 21-1272 for rutile and anatase respectively. The morphology and thickness of the TiO 2 film was analysed by a ZEISS EVO MA 10 SEM equipped with an energy-dispersive spectrometer (EDS, Oxford Instruments, 129 eV resolution). The thin films were in some cases sputtered with gold, of 5 nm thickness, in order increase the conductivity of the samples prior the SEM imaging. TEM studies of the TiO 2 nanoparticles, crystallites and Ag nanocrystals were carried out with a Philips CM20 electron microscope equipped with a Gatan GIF200 energy filter.

Electrochemical Measurements
Electrochemical and spectroelectrochemical experiments were performed using an Autolab PGStat 101 potentiostat. The spectroelectrochemical cell was a 6 mL, three-electrode teflon cell with quartz windows, employing a platinum mesh flag as the counter electrode, an Ag/AgCl/KCl sat reference electrode, and the Ag-TiO 2 film on FTO conducting glass as the working electrode. The electrochemical cell had an inlet and an outlet for passing gas into it. The electrolyte, an aqueous solution of 10 mM sodium phosphate (pH 7), was thoroughly de-aerated by bubbling with Argon prior to any electrochemical measurements and an Argon atmosphere was maintained throughout the measurements. For spectroelectrochemistry, the above cell was incorporated in the sample compartment of the Shimadzu UV-1800 spectrophotometer, and the absorption changes were monitored as a function of the applied potential. All potentials are reported against Ag/AgCl and all experiments were carried out at room temperature.

Results and Discussion
The surface morphology, structure and thickness of the TiO 2 films prior and after deposition of AgNPs were analysed by SEM. The SEM images (Figure 1a) of TiO 2 film showed disordered porosity and comprise a rigid, porous network of TiO 2 nanoparticles of average size 30-40 nm. The film exhibits great homogeneity and even size distribution, while all nanoparticles are bonded together through the sintering process, creating a rich mesoporous surface. The thickness of the coated titania films (Figure 1b) was found to be around 6 µm by analysing the cross-section SEM images. These results confirm that the mesoporous film structure of TiO 2 could provide an excellent surface for the AgNPs to diffuse throughout the porous structure. In addition, the porous film could provide many active sites for electrocatalytic reactions. Mesoporous layers are most suitable for immobilizing electroactive compounds, as the surface area available for sensitization and hence electrochemistry can be increased by over two orders of magnitude with respect to a flat electrode, while ensuring satisfactory access to the pores [37]. Agglomerates (showed with a red arrow in Figure 1c) are formed when AgNPs are deposited on top of the TiO 2 films. The AgNPs agglomerates are shown as white dots on the SEM images at the sample surface and exhibit a large size distribution from 200 to 800 nm, with an average size of 600 nm. Back-scattered electrons (BSE) images have also provided evidence towards the different chemical content of the agglomerates with respect to that of the surface. The EDS spectrum of Ag-TiO 2 (Figure 1d) shows the main peaks of Ti, O, Ag confirming the large amount of silver present. The extra addition of equal volumes of 0.5 M Na 2 S in the AgNO 3 :PVP mixture, which most probably would have passivated the surface of the AgNPs with a thin layer of Ag 2 S, was not detected in the EDS spectra, yet it is considered that a thin layer of Ag 2 S may have formed on the AgNPs surface. Finally, EDS elemental analysis led to the conclusion that the TiO 2 :Ag molar ratio at the surface was 8, while in the Supplementary Information (SI Figure S1) a surface EDS mapping can be observed that indicates the successful Ag coverage.   The color change of the Ag-TiO2 films is significant upon bias application. Figure 3 presents the thin films of TiO2 as prepared from Dyesol ( Figure 3a) and from Degussa ( Figure 3d). After immersing the TiO2 films in the AgNPs solution the film color changes (Figure 3b), since AgNPs were adsorbed on the TiO2 film. In order to perform the electrochemical measurements, negative bias was applied in the film and the color of the film became darker (Figure 3c,e). The color change of the Ag-TiO 2 films is significant upon bias application. Figure 3 presents the thin films of TiO 2 as prepared from Dyesol ( Figure 3a) and from Degussa ( Figure 3d). After immersing the TiO 2 films in the AgNPs solution the film color changes (Figure 3b), since AgNPs were adsorbed on the TiO 2 film. In order to perform the electrochemical measurements, negative bias was applied in the film and the color of the film became darker (Figure 3c,e).  In order to investigate any changes in the crystal structure of the TiO2 films affected by the AgNPs, the XRD patterns ( Figure 4) of FTO-conducting substrates, TiO2 and Ag-TiO2 film electrodes were measured. The XRD of TiO2 and AgNPs-TiO2 electrodes revealed similar characteristic peaks at 2θ: 25.28°, 37.8° and 48.05° which correspond to the indices of anatase TiO2 (101), (004) and (200) hkl planes and are consistent with the reported values of the JCPDS file (21-1272). As expected, no characteristic peaks that correspond to the rutile TiO2 indices were observed since the Dyesol TiO2 paste is 100% anatase. The only difference was observed on the relative intensities of the peaks, which may be due to the fact that doping alters the crystallinity but not the crystal structure of the Ag-TiO2 films. In addition, two slightly intense peaks at 26.54° and 38° which correspond to the plane indices 110 and 200 of the FTO glass could be observed. The XRD pattern of the AgNPs-TiO2 film showed a new peak at 44.4° which can be assigned to the (200) plane of Ag. Furthermore, a closer look of the peak at 38° of the Ag-TiO2, which corresponds to the (200) FTO index, also showed a small shoulder at 38.1° which can be assigned to the Ag (111) plane. These two peaks agree with the JCPDs card of Ag as presented in Figure 4. No rutile phase or any other modification is observed for the Ag-TiO2 film depending on AgNPs incorporation. In order to investigate any changes in the crystal structure of the TiO 2 films affected by the AgNPs, the XRD patterns ( Figure 4) of FTO-conducting substrates, TiO 2 and Ag-TiO 2 film electrodes were measured. The XRD of TiO 2 and AgNPs-TiO 2 electrodes revealed similar characteristic peaks at 2θ: 25.28 • , 37.8 • and 48.05 • which correspond to the indices of anatase TiO 2 (101), (004) and (200) hkl planes and are consistent with the reported values of the JCPDS file (21-1272). As expected, no characteristic peaks that correspond to the rutile TiO 2 indices were observed since the Dyesol TiO 2 paste is 100% anatase. The only difference was observed on the relative intensities of the peaks, which may be due to the fact that doping alters the crystallinity but not the crystal structure of the Ag-TiO 2 films. In addition, two slightly intense peaks at 26.54 • and 38 • which correspond to the plane indices 110 and 200 of the FTO glass could be observed. The XRD pattern of the AgNPs-TiO 2 film showed a new peak at 44.4 • which can be assigned to the (200) plane of Ag. Furthermore, a closer look of the peak at 38 • of the Ag-TiO 2 , which corresponds to the (200) FTO index, also showed a small shoulder at 38.1 • which can be assigned to the Ag (111) plane. These two peaks agree with the JCPDs card of Ag as presented in Figure 4. No rutile phase or any other modification is observed for the Ag-TiO 2 film depending on AgNPs incorporation.  The TiO2 films deposited on conducting FTO glass slides had been soaked in solutions of AgNPs and rinsed with NaH2PO4 buffer to remove any loosely bound nanoparticles. The mp TiO2 films combine transparency in the visible region of the electromagnetic spectrum with a high surface area accessible to molecules from a surrounding solution (e.g., AgNPs). In many cases in the past, this allowed the adsorbed molecules (dyes, proteins, electrochromic species and surfactants) to achieve the densities required for informative electronic absorption spectroscopy, whether that was for the development of solar cells, electrochemical biosensors, electrochromic devices or catalytic applications [3,4,9,27,38,39]. Therefore, the transparency of the electrodes can be useful as it could allow the AgNPs adsorption process and their electrochromic properties to be monitored by UV-Vis absorption spectroscopy, which is a technique also used by other authors for the structural characterization of the AgNPs in a dielectric matrix [27,[38][39][40][41]. Figure 5 shows the optical absorption spectra of the AgNPs solution used as dopants for the surface of the TiO2 films. A relatively broad absorption band is located at 452 nm properly corresponding to the size and form of the AgNPs; this peak is due to the SPR of the AgNPs and is within the spectrum range (400-450 nm) reported for them depending on their shape and size [27,30,38,39,41]. Adsorption of AgNPs on TiO2 films results in light coloration (see Figure 3) of the films, indicating that a large amount of AgNPs has been immobilized into the mesoporous TiO2 film. Also shown is the absorption spectrum of a bare TiO2 film which is transparent and colourless in the visible region, showing a characteristic absorption increase below 400 nm due to the onset of TiO2 band gap excitation. Therefore, the optical transparency of the TiO2 allows the adsorption process of the AgNPs to be monitored by UV-Vis absorption [30,42]. The electronic absorption spectra ( Figure  5) for the nanocomposite Ag-TiO2 film showed features typical of AgNPs superimposed on a background arising from scattering by the TiO2 layer. The resulting spectrum of AgNPs on the TiO2 film showed a characteristic absorption band at 413 nm (a clear blue shift in comparison with the absorption spectra of the AgNPs in solution). The Ag-TiO2 film exhibits a much narrower, blue shift and more defined optical spectrum which means that the active metallic cores of the deposited AgNPs are smaller than those in the solution and probably spherical [27,38,41,43]  The TiO 2 films deposited on conducting FTO glass slides had been soaked in solutions of AgNPs and rinsed with NaH 2¬ PO 4 buffer to remove any loosely bound nanoparticles. The mp TiO 2 films combine transparency in the visible region of the electromagnetic spectrum with a high surface area accessible to molecules from a surrounding solution (e.g., AgNPs). In many cases in the past, this allowed the adsorbed molecules (dyes, proteins, electrochromic species and surfactants) to achieve the densities required for informative electronic absorption spectroscopy, whether that was for the development of solar cells, electrochemical biosensors, electrochromic devices or catalytic applications [3,4,9,27,38,39]. Therefore, the transparency of the electrodes can be useful as it could allow the AgNPs adsorption process and their electrochromic properties to be monitored by UV-Vis absorption spectroscopy, which is a technique also used by other authors for the structural characterization of the AgNPs in a dielectric matrix [27,[38][39][40][41]. Figure 5 shows the optical absorption spectra of the AgNPs solution used as dopants for the surface of the TiO 2 films. A relatively broad absorption band is located at 452 nm properly corresponding to the size and form of the AgNPs; this peak is due to the SPR of the AgNPs and is within the spectrum range (400-450 nm) reported for them depending on their shape and size [27,30,38,39,41]. Adsorption of AgNPs on TiO 2 films results in light coloration (see Figure 3) of the films, indicating that a large amount of AgNPs has been immobilized into the mesoporous TiO 2 film. Also shown is the absorption spectrum of a bare TiO 2 film which is transparent and colourless in the visible region, showing a characteristic absorption increase below 400 nm due to the onset of TiO 2 band gap excitation. Therefore, the optical transparency of the TiO 2 allows the adsorption process of the AgNPs to be monitored by UV-Vis absorption [30,42]. The electronic absorption spectra ( Figure 5) for the nanocomposite Ag-TiO 2 film showed features typical of AgNPs superimposed on a background arising from scattering by the TiO 2 layer. The resulting spectrum of AgNPs on the TiO 2 film showed a characteristic absorption band at 413 nm (a clear blue shift in comparison with the absorption spectra of the AgNPs in solution). The Ag-TiO 2 film exhibits a much narrower, blue shift and more defined optical spectrum which means that the active metallic cores of the deposited AgNPs are smaller than those in the solution and probably spherical [27,38,41,43]. The shift is related to the size of the AgNPs and to the interaction between TiO 2 and Ag, as well as to the fact that the majority of the AgNPs that have filled the pores are smaller than the pores themselves. No band is observed of organic residuals, which were used to bind the TiO2 nanoparticles, remaining in the film due to the titania film sintering at 450 • C. If the observed plasmonic bands were broad between 510 and 590 nm, that would have implied that the nanoparticles are large and/or of non spherical nature [44]. Increasing the AgNO 3 solution concentration, where the AgNPs were created, had as a result the increase of the absorbance of the Ag-TiO 2 film, as was also reported by other researchers in the past [30]. There is also the possibility that some of the non-reduced silver ions (Ag + ) could be adsorbed on the hydroxylated TiO 2 surface according to the following reaction, as suggested by other authors also in the past: Ag + + TiO 2 − → Ti-O-Ag + H + [45].

AgNP 12 nm
The plasmon resonance in the Ag-TiO 2 thin films strongly depends on the crystalline phase and dielectric constant of the TiO 2 matrix [46]. In theory, the SPR peak wavelength increases with increasing dielectric constant of the matrix and depends on the refractive index.
Coatings 2020, 10, x FOR PEER REVIEW 9 of 16 to the size of the AgNPs and to the interaction between TiO2 and Ag, as well as to the fact that the majority of the AgNPs that have filled the pores are smaller than the pores themselves. No band is observed of organic residuals, which were used to bind the TiO2 nanoparticles, remaining in the film due to the titania film sintering at 450 °C. If the observed plasmonic bands were broad between 510 and 590 nm, that would have implied that the nanoparticles are large and/or of non spherical nature [44]. Increasing the AgNO3 solution concentration, where the AgNPs were created, had as a result the increase of the absorbance of the Ag-TiO2 film, as was also reported by other researchers in the past [30]. There is also the possibility that some of the non-reduced silver ions (Ag + ) could be adsorbed on the hydroxylated TiO2 surface according to the following reaction, as suggested by other authors also in the past: Ag + + TiO2 − → Ti-O-Ag + H + [45]. The plasmon resonance in the Ag-TiO2 thin films strongly depends on the crystalline phase and dielectric constant of the TiO2 matrix [46]. In theory, the SPR peak wavelength increases with increasing dielectric constant of the matrix and depends on the refractive index. It should be mentioned that the form of the absorption profile does not only signify that the 415 nm absorption peak of the Ag-TiO2 films is characteristic of spherical AgNPs. It also carries information due to the band absence between 520 and 540 nm which is usually related to bigger AgNPs or AgNPs dimmers. Similarly, no band is observed around 620 nm which is usually due to non-spherical Ag nanoparticles or to larger particles or to manifestation of higher-order plasmon modes called quadrupolar modes. No band is also observed around 670 nm which suggests a longitudinal plasmon band of Ag nanorods. Any broadness of the observed absorption bands could refer to different morphologies of the deposited nanoparticles, to broad particle size distribution, and to agglomeration processes, while all the above discussion is in agreement with the TEM analysis.
The optical band gap of the Ag-TiO2 films is expected to be smaller to that of pure (blank) TiO2 films due to the effect of the AgNPs, as decrease of Eg with silver addition has also been reported by other authors [27,38,39,41]. Also, the Ag + ions probably exist on the surface of the anatase TiO2 films by forming Ag-O-Ti bonds [47], which may introduce trap states affecting the energy band gap. Finally, it is possible that the potential applied can oxidize or reduce the Ag nanoparticles to silver oxide and reverse this as suggested by Kuzma et al [48]. Figure 6 presents the spectroelectrochemical spectrum for a Ag-TiO2 film electrode after remaining for 2 min at each negative applied potential (−0. It should be mentioned that the form of the absorption profile does not only signify that the 415 nm absorption peak of the Ag-TiO 2 films is characteristic of spherical AgNPs. It also carries information due to the band absence between 520 and 540 nm which is usually related to bigger AgNPs or AgNPs dimmers. Similarly, no band is observed around 620 nm which is usually due to non-spherical Ag nanoparticles or to larger particles or to manifestation of higher-order plasmon modes called quadrupolar modes. No band is also observed around 670 nm which suggests a longitudinal plasmon band of Ag nanorods. Any broadness of the observed absorption bands could refer to different morphologies of the deposited nanoparticles, to broad particle size distribution, and to agglomeration processes, while all the above discussion is in agreement with the TEM analysis. The optical band gap of the Ag-TiO 2 films is expected to be smaller to that of pure (blank) TiO 2 films due to the effect of the AgNPs, as decrease of E g with silver addition has also been reported by other authors [27,38,39,41]. Also, the Ag + ions probably exist on the surface of the anatase TiO 2 films by forming Ag-O-Ti bonds [47], which may introduce trap states affecting the energy band gap. Finally, it is possible that the potential applied can oxidize or reduce the Ag nanoparticles to silver oxide and reverse this as suggested by Kuzma et al [48]. Figure 6 presents the spectroelectrochemical spectrum for a Ag-TiO 2 film electrode after remaining for 2 min at each negative applied potential (−0.1 to −1.1 V) applied. The plasmon (or Soret) band at 413 nm up until the application of −0.4 V remains constant, but upon the application of −0.5 V, it starts to increase in intensity without though changing shape. The application of −0.6 V to the film causes a big increase of the peak which at the same time becomes thinner and sharper. Afterwards, and by gradually applying more negative biases up to −1.1 V, the peak at 413 nm continues to rise and at the same time becomes sharper. However, several minutes after the end of the spectroelectrochemical measurements and the application of no voltage, the absorbance of the film shows that it has not returned to its initial state, but rather a shift of the main peak from 413 nm to 440 nm has been observed.
band at 413 nm up until the application of −0.4 V remains constant, but upon the application of −0.5 V, it starts to increase in intensity without though changing shape. The application of −0.6 V to the film causes a big increase of the peak which at the same time becomes thinner and sharper. Afterwards, and by gradually applying more negative biases up to −1.1 V, the peak at 413 nm continues to rise and at the same time becomes sharper. However, several minutes after the end of the spectroelectrochemical measurements and the application of no voltage, the absorbance of the film shows that it has not returned to its initial state, but rather a shift of the main peak from 413 nm to 440 nm has been observed.
Following this route, increasing negative biases (−0.2 to −0.9) were applied again to the doped film and its absorption spectra were recorded. Figure 7 shows that upon the application of up to −0.4 V, a small increase in the size of the peak at 440 nm was monitored and a slight blue shift. However, upon the application of higher negative biases (−0.8 or −0.9 V) the peak shifts back to 413 nm. Figure  7 also shows that upon the application of a positive bias (0 to 0.6 V) the peak shifts again to 440 nm. The switch from 413 nm to 440 nm, depending on the bias that was applied to the film, was repeated several times and always with success.    Following this route, increasing negative biases (−0.2 to −0.9) were applied again to the doped film and its absorption spectra were recorded. Figure 7 shows that upon the application of up to −0.4 V, a small increase in the size of the peak at 440 nm was monitored and a slight blue shift. However, upon the application of higher negative biases (−0.8 or −0.9 V) the peak shifts back to 413 nm. Figure 7 also shows that upon the application of a positive bias (0 to 0.6 V) the peak shifts again to 440 nm. The switch from 413 nm to 440 nm, depending on the bias that was applied to the film, was repeated several times and always with success.
Coatings 2020, 10, x FOR PEER REVIEW 10 of 16 band at 413 nm up until the application of −0.4 V remains constant, but upon the application of −0.5 V, it starts to increase in intensity without though changing shape. The application of −0.6 V to the film causes a big increase of the peak which at the same time becomes thinner and sharper. Afterwards, and by gradually applying more negative biases up to −1.1 V, the peak at 413 nm continues to rise and at the same time becomes sharper. However, several minutes after the end of the spectroelectrochemical measurements and the application of no voltage, the absorbance of the film shows that it has not returned to its initial state, but rather a shift of the main peak from 413 nm to 440 nm has been observed. Following this route, increasing negative biases (−0.2 to −0.9) were applied again to the doped film and its absorption spectra were recorded. Figure 7 shows that upon the application of up to −0.4 V, a small increase in the size of the peak at 440 nm was monitored and a slight blue shift. However, upon the application of higher negative biases (−0.8 or −0.9 V) the peak shifts back to 413 nm. Figure  7 also shows that upon the application of a positive bias (0 to 0.6 V) the peak shifts again to 440 nm. The switch from 413 nm to 440 nm, depending on the bias that was applied to the film, was repeated several times and always with success.    In order to gain an insight into the kinetic mechanism, the optical absorbance (OA) or optical density (OD) were measured for a range of negative and positive voltages. Figure 8a,b show the curves regarding the kinetics for the absorption change at 413 (a) and 440 nm (b) respectively. The absorbance was initially monitored during the application of −0.8 V for 600 s and immediately afterwards upon the application of 0.15 V for another 600 s. Figure 8a demonstrates that the continuous application of a sufficient cathodic current (−0.8 V) causes the fast increase of the absorbance at 413 nm of the Ag-TiO 2 film in 4.5 s (τ 1/2 = 2.6 s). By stepping back the potential from −0.8 V to +0.15 V, the absorbance at 413 nm starts dropping quite fast at the first 20 s and slower afterwards until it reaches the OD value it exhibited before the application of the negative bias. Similar results were obtained in Figure 8b  In order to gain an insight into the kinetic mechanism, the optical absorbance (OA) or optical density (OD) were measured for a range of negative and positive voltages. Figure 8a,8b show the curves regarding the kinetics for the absorption change at 413 (a) and 440 nm (b) respectively. The absorbance was initially monitored during the application of −0.8 V for 600 s and immediately afterwards upon the application of 0.15 V for another 600 s. Figure 8a demonstrates that the continuous application of a sufficient cathodic current (−0.8 V) causes the fast increase of the absorbance at 413 nm of the Ag-TiO2 film in 4.5 s (τ1/2 = 2.6 s). By stepping back the potential from −0.8 V to +0.15 V, the absorbance at 413 nm starts dropping quite fast at the first 20 s and slower afterwards until it reaches the OD value it exhibited before the application of the negative bias. Similar results were obtained in Figure 8b for the kinetics for the absorption change at 440 nm. The continuous application of the −0.8 V caused the fast increase of OD at 440 nm of the Ag-TiO2 in 6 sec (τ1/2 = 3.8 s). The application of the +0.15 V caused the gradual drop of the OD at 440 nm and reached completion after 50 s. The spectroelectrochemical studies reported in Figures 6 and 7 were further supported by cyclic voltammetry (CV). The CVs of mp TiO2 films with or without the adsorption of AgNPs on their surface, in aqueous 10 mM NaH2PO4 electrolyte of pH 7, at a scan rate of 0.1 V/s, are presented in Figure 9. Upon scanning the potential of a bare semiconductive TiO2 film cathodically in a neutral pH aqueous medium (NaH2PO4, pH 7), a transition from insulating to conductive behaviour is observed. The characteristic charging/discharging currents were assigned to electron injection and storage into sub-band gap/conduction band states of the TiO2 film, until the metal oxide becomes fully degenerated once the applied potential reaches the conduction band potential [9,33,34,49]. The current shows a plateau at potentials where the film behaves as an insulator (positive biases and up to around −0.3 V). At that range the electrical response is dominated by the Helmholtz capacity of the uncovered FTO glass/electrolyte solution interface at the bottom of the TiO2 film [50,51]. At more negative potentials (−0.3 V and higher), the cathodic current displays an exponentially rising behaviour (at −0.8 V) that is considered by many authors as the reduction of superficial Ti ions [52,53]. This rising behaviour is then transformed to a peak (at −0.75 V), when the direction of voltammetry is reversed to anodic. This is considered as re-oxidation of reduced Ti ions and is a slow and irreversible process [53]. These potentials are more negative than the conduction band potential and the TiO2 film behaves as a conductive metallic electrode. As the scan rate becomes slower ( Figure 10) the height of the anodic peak progressively diminishes until, at very slow scan rates (0.01 V/s), it disappears. According to many reports [1,7,53,54] the origin of this behaviour is due to the charging/discharging of electrons in the film and a charge transfer mechanism. However, no cathodic or anodic peaks due to a redox reaction are observed. The CV integrates to approximately 0, indicating negligible Faradaic currents.
In addition, Figure 9 shows the CV of a TiO2 film electrode after the adsorption of AgNPs on its surface. The Ag-TiO2 film exhibited two pairs of redox peaks that were absent from the CV of the  The spectroelectrochemical studies reported in Figures 6 and 7 were further supported by cyclic voltammetry (CV). The CVs of mp TiO 2 films with or without the adsorption of AgNPs on their surface, in aqueous 10 mM NaH 2 PO 4 electrolyte of pH 7, at a scan rate of 0.1 V/s, are presented in Figure 9. Upon scanning the potential of a bare semiconductive TiO 2 film cathodically in a neutral pH aqueous medium (NaH 2 PO 4 , pH 7), a transition from insulating to conductive behaviour is observed. The characteristic charging/discharging currents were assigned to electron injection and storage into sub-band gap/conduction band states of the TiO 2 film, until the metal oxide becomes fully degenerated once the applied potential reaches the conduction band potential [9,33,34,49]. The current shows a plateau at potentials where the film behaves as an insulator (positive biases and up to around −0.3 V). At that range the electrical response is dominated by the Helmholtz capacity of the uncovered FTO glass/electrolyte solution interface at the bottom of the TiO 2 film [50,51]. At more negative potentials (−0.3 V and higher), the cathodic current displays an exponentially rising behaviour (at −0.8 V) that is considered by many authors as the reduction of superficial Ti ions [52,53]. This rising behaviour is then transformed to a peak (at −0.75 V), when the direction of voltammetry is reversed to anodic. This is considered as re-oxidation of reduced Ti ions and is a slow and irreversible process [53]. These potentials are more negative than the conduction band potential and the TiO 2 film behaves as a conductive metallic electrode. As the scan rate becomes slower ( Figure 10) the height of the anodic peak progressively diminishes until, at very slow scan rates (0.01 V/s), it disappears. According to many reports [1,7,53,54] the origin of this behaviour is due to the charging/discharging of electrons in the film and a charge transfer mechanism. However, no cathodic or anodic peaks due to a redox reaction are observed. The CV integrates to approximately 0, indicating negligible Faradaic currents.
In addition, Figure 9 shows the CV of a TiO 2 film electrode after the adsorption of AgNPs on its surface. The Ag-TiO 2 film exhibited two pairs of redox peaks that were absent from the CV of the bare TiO 2 film. For the first pair, a small cathodic peak is observed at +0.07 V and a small anodic peak at +0.27 V. For the second pair the cathodic peak is much bigger and appears at −0.65 V and the anodic peak which is much smaller and broader appears around −0.53 V. All these peaks are attributed to two different reduction/oxidation states of Ag on the TiO 2 films. The redox reactions are associated with Ag → Ag 2 O and Ag 2 O → Ag (oxidation and reduction peaks, respectively) [55,56]. The pair of peaks at the lower biases correspond to the reduction and oxidation of the AgNPs that have adsorbed inside the mesoporous network of the TiO 2 film and the other pair of smaller peaks most probably correspond either to small deposited AgNPs or to AgNPs adsorbed only on the outer surface of the TiO 2 film [46]. Also both re-oxidation peaks are broader and smaller to the reduction peaks due to the large band gap of the TiO 2 film. This observation most probably requires further experiments in order to draw any final conclusions. However, this redox behaviour (two sets of redox peaks, a two phase reduction) of the AgNPs on the TiO 2 films has also been monitored on immobilization studies of a heme based redox dyes, such as iron(III) 5,10,15,20-tetrakis(1-methyl-4-pyridyl) porphyrin, FeTMPyP, on the same films [33].
Coatings 2020, 10, x FOR PEER REVIEW 12 of 16 bare TiO2 film. For the first pair, a small cathodic peak is observed at +0.07 V and a small anodic peak at +0.27 V. For the second pair the cathodic peak is much bigger and appears at −0.65 V and the anodic peak which is much smaller and broader appears around −0.53 V. All these peaks are attributed to two different reduction/oxidation states of Ag on the TiO2 films. The redox reactions are associated with Ag → Ag2O and Ag2O → Ag (oxidation and reduction peaks, respectively) [55,56]. The pair of peaks at the lower biases correspond to the reduction and oxidation of the AgNPs that have adsorbed inside the mesoporous network of the TiO2 film and the other pair of smaller peaks most probably correspond either to small deposited AgNPs or to AgNPs adsorbed only on the outer surface of the TiO2 film [46]. Also both re-oxidation peaks are broader and smaller to the reduction peaks due to the large band gap of the TiO2 film. This observation most probably requires further experiments in order to draw any final conclusions. However, this redox behaviour (two sets of redox peaks, a two phase reduction) of the AgNPs on the TiO2 films has also been monitored on immobilization studies of a heme based redox dyes, such as iron(III) 5,10,15,20-tetrakis(1-methyl-4-pyridyl) porphyrin, FeTMPyP, on the same films [33]. The effect of the scan rate on the voltammetric behaviour of the Ag-TiO2 film was also investigated. Slower scan rates were applied to the Ag-TiO2 electrode in order to try to obtain a reversible peak-shaped CV. Figure 10 illustrates that even at slow scan rates no simple reversible behaviour for the Ag-TiO2 film was observed, consistent with the currents being limited by the low TiO2 conductivity at moderate potentials. However, the cathodic peak potential at −0.65 V shifted negatively with the increase of the scan rate. In addition, there is a good linear relationship between the cathodic peak current and the scan rate in the range (0.01 to 0.1 V/s), indicative of a surfaceconfined electrochemical process. This has also been observed in other systems where instead of AgNPs, surfactants or proteins were adsorbed on the TiO2 films [39]. The effect of the scan rate on the voltammetric behaviour of the Ag-TiO 2 film was also investigated. Slower scan rates were applied to the Ag-TiO 2 electrode in order to try to obtain a reversible peak-shaped CV. Figure 10 illustrates that even at slow scan rates no simple reversible behaviour for the Ag-TiO 2 film was observed, consistent with the currents being limited by the low TiO 2 conductivity at moderate potentials. However, the cathodic peak potential at −0.65 V shifted negatively with the increase of the scan rate. In addition, there is a good linear relationship between the cathodic peak current and the scan rate in the range (0.01 to 0.1 V/s), indicative of a surface-confined electrochemical process. This has also been observed in other systems where instead of AgNPs, surfactants or proteins were adsorbed on the TiO 2 films [39].
Also, it was found that the cathodic and anodic peak currents corresponding to the adsorbed AgNPs vary linearly with scan rates in the range of 10 to 100 mV·s -1 . The CVs exhibit a current increase as the scan rates are steadily increased, significantly larger when the 0.1 V/s scan rate results are compared to those of the 0.01 V/s. This is clear indication that the adsorbed AgNPs underwent a surface-controlled process. Coatings 2020, 10, x FOR PEER REVIEW 13 of 16 Figure 10. Cyclic voltammograms (CVs) in 10 mM NaH2PO4 buffer pH 7, for a Ag-TiO2 film at different scan rates.
Also, it was found that the cathodic and anodic peak currents corresponding to the adsorbed AgNPs vary linearly with scan rates in the range of 10 to 100 mV·s -1 . The CVs exhibit a current increase as the scan rates are steadily increased, significantly larger when the 0.1 V/s scan rate results are compared to those of the 0.01 V/s. This is clear indication that the adsorbed AgNPs underwent a surface-controlled process.

Conclusions
All the above experimental data clearly suggest that the electrophotochromism of the Ag-TiO2 composite is due to oxidation/reduction of the Ag nanoparticles, which forms a thin layer of Ag2O on the metallic core, forming core/shell nanoparticles; the core is metallic while the outer shell is semiconducting, where the peaks agree well with those found by Kuzma et al. [46]. In fact, it is suggested that the oxide thickness is about 1.5 nm when the composite absorbs at the 440 nm peak and at most 0.5 nm while the composite exhibits a peak at 413 nm. Furthermore, it is possible that the particles may be charged upon the application of negative potential on the Ag-TiO2 electrode, which would lead their separation due to their charging; such an effect would not be possible at positive electrode potentials since silver has a lower conduction band than that of the FTO electrode and than that of TiO2.
Author Contributions: E.T. and I.K. conceived and designed the experiments; S.K., J.P., E.T. and I.K. performed the experiments; E.T., I.K., S.K. and J.P. analyzed the data; E.T. and I.K. contributed reagents/materials/analysis tools; I.K., S.K., E.T. and J.P. wrote the paper. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding from funding agencies in the public, commercial or notfor-profit sectors.

Conclusions
All the above experimental data clearly suggest that the electrophotochromism of the Ag-TiO 2 composite is due to oxidation/reduction of the Ag nanoparticles, which forms a thin layer of Ag 2 O on the metallic core, forming core/shell nanoparticles; the core is metallic while the outer shell is semiconducting, where the peaks agree well with those found by Kuzma et al. [46]. In fact, it is suggested that the oxide thickness is about 1.5 nm when the composite absorbs at the 440 nm peak and at most 0.5 nm while the composite exhibits a peak at 413 nm. Furthermore, it is possible that the particles may be charged upon the application of negative potential on the Ag-TiO 2 electrode, which would lead their separation due to their charging; such an effect would not be possible at positive electrode potentials since silver has a lower conduction band than that of the FTO electrode and than that of TiO 2 .
Funding: This research received no external funding from funding agencies in the public, commercial or not-for-profit sectors.