Purely Visible-Light-Induced Photochromism in AgâTiO2

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Purely visible-light induced photochromism in Ag-TiO2 nano-heterostructures David Maria Tobaldi, María Jésus Hortigüela, Gonzalo Otero Irurueta, Manoj Kumar Singh, Robert C. Pullar, Maria Paula Seabra, and João António Labrincha Langmuir, Just Accepted Manuscript • Publication Date (Web): 02 May 2017 Downloaded from http://pubs.acs.org on May 4, 2017

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Purely visible-light induced photochromism in Ag-TiO2 nano-heterostructures

D.M. Tobaldi,a* M.J. Hortigüela Gallo,b G. Otero-Irurueta,b M.K. Singh,b R.C. Pullar,a M.P. Seabra,a and J.A. Labrinchaa a

Department of Materials and Ceramic Engineering / CICECO−Aveiro Instute of Materials, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal

b

Center for Mechanical Technology and Automation – TEMA, Department of Mechanical Engineering, University of Aveiro, Campus Universitario de Santiago, 3810-193 Aveiro, Portugal

Abstract We report titania nano-heterostructures decorated with silver, exhibiting tuneable photochromic properties for the first time when stimulated only by visible white light (domestic indoor lamp), with no UV wavelengths. Photochromic materials show reversible colour changes under light exposure. However, all inorganic photochromic nanoparticles (NPs) require UV light to operate. Conventionally, multicolour photochromism in Ag-TiO2 films involves a change in colour to brownish-grey during UV-light irradiation (i.e. reduction of Ag+ to Ag0), and a (re)bleaching (i.e. (re)oxidation of Ag0 to colourless Ag+) upon visible-light exposure. In this work, on the contrary, we demonstrate visible-light induced photochromism (ranging from yellow to violet) of 1-10 mol% Ag-modified titania NPs with both spectroscopic and colourimetric CIEL*a*b* analyses. This is not a bleaching of the UV-induced colour, but a change in colour itself under exposure to visible light, and it is shown to be a completely different mechanism – driven by the interfacial charge transfer (IFCT) of an electron from the valence band of TiO2 to that of the AgxO clusters that surround the titania – to the usual UV-triggered photochromism reported in titania-based materials. The quantity of Ag or irradiation time dictated the magnitude and degree of tuneability of the colour change, from pale yellow to dark blue, with a rapid change visible after only a few seconds, and the intensity and red-shift of surface plasmon resonance (SPR) induced under visible light also increased. This effect was reversible after annealing in the dark at 100 °C / 15 min. Photocatalytic activity (PCA) under visible light was also assessed against the abatement of nitrogen oxide pollutants, for interior use, therefore showing the co-existence of 1 ACS Paragon Plus Environment

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photochromism and photocatalysis – both triggered by the same wavelength – in the same material, making it a multifunctional material. Moreover, we also demonstrate and explain why X-ray photoelectron spectroscopy (XPS) is an unreliable technique with such materials.

KEYWORDS: multifunctional materials; optically active materials; titanium dioxide; nanoparticles; visiblelight. *

Corresponding author. Tel.: +351 234 370 041

E-mail addresses: [email protected]; [email protected]

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1. Introduction The importance of the interaction of light with nanoparticles and light-based technologies was fully recognised in 2015, which was declared the “International Year of Light” by leading foundations such as the Institute of Physics and the American Institute of Physics.1,2 Furthermore, the Photonics Industry Report 2013 shows that light-based technologies have had a major impact on the global economy, with a current market worth €300 billion and projected market value of over €600 billion in 2020. Indeed, the rate of growth in the photonics industry has more than doubled that seen in worldwide GDP (gross domestic product) between 2005 and 2011.2 This gave momentum to that expansion in research of materials at the nanoscale and devices, that we are still witnessing, in particular TiO2 nanomaterials,3,4 often coupled with noble metal nanoparticles (NPs).5 As a matter of fact, noble metal NPs are extensively studied because of their outstanding optical and photonic properties – they absorb light in the visible range because of the surface plasmon resonance (SPR) phenomenon.6 As a matter of fact, coupling (nano) silver and (nano) TiO2 is known to bring manifold benefits in the fields of materials for energy and sustainability,7 such as plasmon-enhanced catalytic/photocatalytic activity (PCA),8,9 high-density holographic storage,10 improved antibacterial activity,11 better solar energy conversion, and H2 production.12 Furthermore, this also gave rise to the search for advanced and multifunctional materials that should be able to display multiple and coexisting properties (e.g. magnetism, catalysis, electrical conduction, gas-sensing).13 One of the most common approaches for the design of multifunctional materials is the choice of “molecular building blocks”, engineered to grant a specific physical or chemical feature to the overall (nano)heterostructure,14 such as the selection of a useful nanomaterial and its combination with an ion or other NPs giving it extra degrees of functionality.15 Photochromism is described as the phenomenon in which a colour of a material changes upon exposure to electromagnetic radiation (UV/Visible/IR light).16 However, this has rarely been achieved in NPs (not thin films) under purely visible light, as UV is always needed for activation or switching. In this study, following the recent results on photochromic Cu-modified TiO2 NPs under UV light,17 and taking advantage of the photocatalytic property of TiO2, we combined titania NPs with silver. These Ag-TiO2 nano-heterojunctions showed themselves to have a tuneable, and reversible / switchable, photochromic property when exposed to a visible-light source commonly used as an indoor lighting source in offices, public places, households. They reverted to the original colour 3 ACS Paragon Plus Environment

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after simply placing the specimen in a dark oven for 15 min at a temperature as low as 100 °C. Additionally, it was shown that X-ray photoelectron spectroscopy (XPS) is an unreliable technique with such materials. This material was also demonstrated to have a superior photocatalytic activity (PCA) for the abatement of nitrogen oxides, compared to commercial TiO2 photocatalytic nano-powder Degussa P25 (P25). Thus we showed the co-existence of photochromism and photocatalysis – both triggered by the same wavelength – in the same material, making this a multifunctional material. Samples were made by way of an aqueous sol–gel nanosynthesis method developed by these authors;18 furthermore, the optical properties (e.g. the photochromic behaviour) were fully investigated through diffuse reflectance spectroscopy (DRS). It is envisioned that we could use such materials to follow, in real-time, the ongoing of photocatalytic reactions under visible light exposure, in order to provide feed-back on pollution abatement to institutions, companies, and other interested parties, via fixed electronic devices, apps and/or other wireless media.

2. Experimental 2.1 Sample preparation The synthesis of aqueous titanium(IV)hydroxide sols was carried out following a protocol that we have previously reported in detail.19 Sols were made via the carefully controlled hydrolysis and peptisation of titanium(IV)isopropoxide (Ti-i-pr, Ti(OCH(CH3)2)4) with distilled water diluted in isopropyl alcohol (IPA, propan-2-ol). In brief, one part of Ti-i-pr (Sigma Aldrich, 97%) was added to four parts of isopropyl alcohol to make a 20 vol % Ti-i-pr solution. This Ti-i-pr solution was hydrolysed by the dropwise addition of an excess of water (5:1 water:Ti-i-pr) employed as a 20 vol % solution in IPA. The acid necessary to peptise the sol (concentrated HNO3, Sigma Aldrich, 65%) was also added to this water–IPA solution, in a molar ratio of Ti4+:acid of 2.5:1. This water–IPA-acid solution was added dropwise to the Ti-i-pr solution at room temperature, while being stirred. The precipitated mixture was evaporated to a white jelly like mass on a rotary evaporator, removing the IPA. Distilled water was added to restore the mixture to the original volume, and this was then dried once more to a dried gel at 60 °C. Four silver-modified sols were prepared as well, with a Ag:TiO2 molar ratio of 1, 2, 5, and 10 mol %. Stoichiometric amounts of silver nitrate (1 M aqueous solution, Sigma Aldrich) were added to the sol, when this had a 1 M concentration. Afterward, dried 4 ACS Paragon Plus Environment

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gels were thermally treated at 450 °C under a static air flow, using an electric muffle furnace. The heating/cooling rate was 5 °C min–1, with a 2 h dwell time at the selected temperature. Samples were referred to as Ti450 (unmodified TiO2), and Ag–Ti450, 2Ag–Ti450, 5Ag–Ti450, and 10Ag–Ti450 for 1 mol %, 2 mol %, 5 mol %, and 10 mol % of Ag addition, respectively.

2.2 Sample characterisation DRS was used to investigate the photochromic behaviour of the Ag-modified specimens. Spectra were collected using a Shimadzu UV 3100 spectrometer (JP), equipped with a BaSO4 integrating sphere, in the UV-Vis spectral range (250-825 nm), 0.2 nm resolution and using a BaSO4 pellet as white reference material. Obtained DRS data were manipulated taking advantage of the KubelkaMunk function, transforming reflectance (R∞) into absorbance (A):20 =

(1)

To investigate the photochromic behaviour, 0.1 g of each sample was exposed, for different and incremental irradiation times to visible-light. The lamp used for this purpose was a Philips master PL-S 2P 9W/840, NL (Philips, NL) fluorescent lamp (the emission spectrum is shown in Fig. S1 of the electronic supplementary information, ESI). The radiant flux per unit area received by the specimens during each photochromic measurement was measured with a radiometer equipped with both UVA and visible probes (Delta OHM, HD2302.0, IT). This was assessed to be ∼25 W m–2 in the visible range (400 nm < λ < 800 nm), and nil in the UVA range (315 nm < λ < 400 nm). These irradiance values were chosen to be consistent with our previous investigation on Cu-TiO2; it has to be stressed that the use of different irradiance values would have an effect on the degree / speed of the photochromic behaviour.21 Irradiation times are intended to be consecutive and absolute: a specimen irradiated for 0.25 min and then for a further 0.25 min will be identified as 0.50 min. DRS measurement were assessed immediately after irradiation (ex-situ). The absorption bands due to the SPR of silver NPs were modelled using a Gaussian function (OriginPro, version 8.5.0), and the centroid of SPR band was then extracted. To study the reversibility of the photochromic effect, fresh un-irradiated samples were exposed to visible-light (as above) for periods ranging from 0.25 min (15 seconds) to 3 min, depending on the specimen, at room temperature, and their DRS spectra measured immediately. They were then placed in a dark oven at 100 °C for 15 min (30 min in the case of 10Ag-Ti450) to reverse the process


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and DRS was assessed again. This routine was repeated until a series of several stable optical switching cycles was achieved. Furthermore, CIEL*a*b* colour coordinates – L* (lightness, achromatic signal: 100 = white, 0 = black), a* (+red, –green) and b* (+yellow, –blue), with a* and b* chromatic channels – as well as RGB (red, green and blue) colour space coordinates, and XYZ (i.e. tristimulus values), were all extracted from the diffuse reflectance spectra via the MATLAB (version 7.12.0) software package – illuminant D65, 10° observer. x and y chromaticity coordinates were derived from the XYZ tristimulus values and used to build the CIE 1931 colour space chromaticity diagram. High resolution transmission electron microscopy (HR-TEM) was used to investigate the morphology of the samples. This was carried out on a JEOL JEM-2200FS (JP) microscope at 220 kV, which was also equipped with an energy dispersive X-ray spectroscopy (EDS) attachment (Oxford Instruments, UK), and a Gatan Ultrascan 4000 SCCD camera. Fast Fourier transform (FFT) patterns were analysed through the JEMS software suite. Samples were prepared by dispersing the NPs in IPA, and evaporating some drops of the suspension on carbon-coated copper TEM grids. Prior to the analysis, all of the TEM grids used in this work were irradiated for 240 min with the same lamp used in the photochromic tests – this was done in order to detect any likely (re)formed Ag0 NPs. X-ray photoelectron spectroscopy (XPS) was used to characterise the elemental composition of the sample and the chemical state of the titanium, silver and oxygen species. XPS spectra were acquired in an ultra-high vacuum (UHV) system with a base pressure of 2 × 10–10 mbar. The system is equipped with a hemispherical electron energy analyser (SPECS Phoibos 150), a delay-line detector and a monochromatic Al Kα (1486.74 eV) X-ray source. High resolution spectra were recorded at normal emission take-off angle and with a pass-energy of 20 eV, which provides an overall instrumental peak broadening slightly better than 0.5 eV. For XPS measurements the sample was diluted in Milli-Q water and a thin film was deposited on silicon by drop coating.


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2.2.1 Functional application: photocatalytic testing PCA of the prepared specimens was assessed for NOx abatement – i.e. the sum of nitrogen monoxide (NO) and nitrogen dioxide (NO2). Nitrous oxides are one of the major pollutants negatively influencing air quality in urban environments.22 PCA tests of NOx abatement were performed in a reactor previously described in detail by the authors.11 The initial concentration of NOx was 200 ppb; the outlet concentration of NOx gases was measured using a chemiluminescence analyser (AC-30 M, Environment SA, FR). Samples were prepared in the form of a thin layer of powder, with a constant mass (0.10 g), and accordingly an approximately constant thickness, in a 6 cm diameter Petri dish. PCA tests were assessed at room temperature – 27 ± 1 °C was the temperature inside the reactor – with a relative humidity of 31%. These parameters, controlled by means of a thermocouple placed inside the reactor chamber and a humidity sensor placed in the inlet pipe, remained stable throughout the tests. The light source employed was a LED white light, that irradiates exclusively in the visible region (Philips warm white LED bulb), placed 28 cm from the photocatalyst, and the radiant flux per unit area received by the photocatalyst during the experiments (measured with the same radiometer used for the photochromic experiments), was found to be 7 W m–2 in the visible range, and zero in the UVA (the emission spectrum of the white LED lamp is reported in Fig. S2). Once the desired concentration of NOx was obtained, the window glass was uncovered, the white LED lamp turned on, and the PCA reaction was presumed to begin. PCA tests were repeated in triplicate, using the same sample and with the same protocol as the first photocatalytic run, so as to check the repeatability, recyclability and photostability of the photocatalysts. Commercial nano-titania P25, a mixture of anatase, rutile, and amorphous phase,23 was used as a reference material in all the photocatalytic experiments.

3. Results and discussion 3.1 Photochromism 3.1.1 DRS Analysis Semi-quantitative phase analysis, structural and microstructural analyses of these specimens have already been assessed by the authors in a recent article investigating their gas sensing properties.24 Briefly, the silver-modified NPs are composed of anatase, rutile and brookite (neither metallic silver nor silver oxide(s) were detected), anatase being the dominant TiO2 polymorph in all the samples. It 7 ACS Paragon Plus Environment

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was 56.5 wt% in Ti450, increasing greatly with silver addition to values between 83.3-86.8 wt% with a slight rise with increasing Ag content. The amount of rutile and brookite greatly decreased with silver content (from 23.6 wt% in Ti450 to 8.2 wt% in 10Ag-Ti450 for brookite, and from 19.8 wt% in Ti450 to 7.4 wt% in 10Ag-Ti450 for rutile). From a microstructural investigation (using an advanced XRD method, whole powder pattern modelling, WPPM),25 silver was found not to enter the TiO2 lattice; furthermore, its presence delayed the crystal growth of TiO2, i.e. anatase average domain diameter was 10.4 nm in Ti450 but only 5.2 nm in 10Ag-Ti450 (cf Table S1). This was confirmed by HR-TEM images of 2Ag-Ti450 (Fig. 1a-c), in which very small silver nanocrystals are shown to be clustered around the larger titania NPs. DRS spectra prior to exposing the specimens to visible-light are depicted in Fig. 2. All the spectra (i.e. unmodified and Ag-modified TiO2) exhibited the same characteristic absorption edge at around 400 nm, assigned to the metal-ligand charge transfer (MLCT) Ti4+−O2− in TiO2.26 Besides this feature, Ag-modified samples also exhibited an absorption tail in the visible region, at around 425 nm in AgTi450. The width of this absorption tail extends with an increasing amount of silver, up to the entire visible region in 10Ag-Ti450. It can be ascribed to electron transfer from the valence band of TiO2 to that of the AgxO clusters that surround the titania (inter-facial charge transfer, IFCT),27 similar to that observed in Cu-modified TiO2.17,28 Although the Ag-TiO2 specimens were yellowish in colour (see Table 1, and top part of Fig. 2), no signature from the metallic Ag0 SPR band appears in the DRS spectra. This signifies that the only detectable signature of Ag, at this un-irradiated stage, is that from the TiO2-AgxO IFCT – in other words, silver is likely present as oxide and, as its molar content increases in the nano-heterostructures, the IFCT absorption extends itself, spanning the whole visible region (cf Fig. 2). Furthermore, as AgO is very unstable at room temperature conditions,29 we can reasonably assume that the silver is present as Ag(I) oxide (Ag2O). It must be also highlighted that the SPR band of pure metallic Ag0 NPs depends strongly on their shape and size,30 within a dielectric environment.31 For 10 nm diameter silver NPs in water, the SPR is located at around 385 nm.32 DRS was likewise used to monitor and study the evolving change in colour with visible-light exposure time, i.e. photochromism, of the Ag-modified TiO2 samples. As a Ag-TiO2 sample is exposed to visible-light for an increasing time, changes are seen to occur in the spectrum. This is graphically shown by the UV-Vis spectra, Figs 3a-d. Furthermore, this is also visually displayed in selected renderings of the RGB colour space (top parts of Fig. 3a-d), and also in the chromaticity diagrams discussed below. The RGB renderings of every measurement are shown in figures S4a-d.


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The photochromic trend of Ag-Ti450 is shown in Fig. 3a. There was little change in the spectrum with up to 1 min of visible-light irradiation time (see inset Fig. 3a). With between 3 and 10 min of visible-light irradiation time, the absorption in the visible range increased its intensity, and after 15 min, a weak SPR band appeared in the visible region, centred around ~500 nm, and accompanied by a stronger change in colour, as shown in the top part of Fig. 3a. Such a band is assigned to the SPR of metallic Ag NPs (re)forming on the surface of the specimen with extended visible-light irradiation.30 Increasing the visible-light irradiation time further led to an increase in the absorption on-set of this SPR band, as well as an increase in its breadth and intensity, and a red-shift of its centroid to higher wavelengths (= lower energy). A similar, but faster, behaviour was observed for 2Ag-Ti450, where the SPR band already had a well-defined shape after only 5 min of visible-light irradiation time (Fig. 3b). In specimens with greater silver content, i.e. 5Ag-Ti450 and 10Ag-Ti450, an even greater effect was seen (Fig. 3c,d), as the SPR band appeared in 1 mol%), reaction (3) overrules reactions (9), (10) and (11), thus “starving” the photocatalysis of electrons. This also confirms the belief that, above an optimum amount, silver might have detrimental effects on the PCA of TiO2 based photocatalysts,47 behaving as a recombination centre for the photo-generated holes and electrons.60


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4. Conclusions TiO2 was modified with silver (0, 1, 2, 5, and 10 mol% Ag) by means of an environmentally friendly, green and low-cost sol-gel method, and the co-existence of photochromism and photocatalysis – both triggered by the same wavelength – was shown to be in the same material, thus making it a multifunctional material. Although in conventional multicolour photochromism a white light is used to reverse the colour induced by a UV-light, these Ag-modified specimens showed themselves to be photochromic upon purely white-light irradiation, turning from yellow to dark blue/violet. This photochromic behaviour was thoroughly monitored for the first time, assessing with both optical spectroscopy and colourimetric analyses. It was ascertained that, upon increasing visible-light irradiation time, silver was reduced to its metallic state, creating (re)formed Ag0 NPs decorating the larger TiO2 NPs, and that the size of those (re)formed Ag0 NPs increased with further visible-light exposure. Indirect evidence of this was the progressive red-shifting of the Ag0 NPs SPR centre, which followed an exponential increase, the order of which depended on the silver amount in the specimens – i.e. a shifting from a first order exponential growth function for the addition of 1 mol% Ag, to a 3rd order exponential growth function for 10Ag-Ti450. This red-shift could also be used to qualitatively monitor the physical growth of the Ag0 NPs on the titania surface. This growth in size of the (re)formed Ag0 NPs determined their photochromic properties, and hence their change in colour. Moreover, the induced photochromism can be tuned by simply modifying the exposure time, or the Ag content in the titania – the higher the Ag mol% in TiO2, the faster the (re)formation of the Ag0 NPs. XPS experiments demonstrate that the properties of the Ag-TiO2 drastically changed during irradiation with X-rays, and this made it impossible to reliably detect the silver oxidation state at the un-irradiated stage. Furthermore, Ag-Ti450 and 2Ag-Ti450 PCA – using a white LED lamp, thus simulating an indoor situation – were assessed as photocatalysts against NOx abatement, and results were compared with those of the commercial standard P25. Our experiments showed that both Ag-Ti450 and 2AgTi450 displayed a higher, plasmon enhanced, PCA than that of P25. Moreover, Ag-Ti450 was more photocatalytically active than 2Ag-Ti450, thus confirming that an excessive content of silver is detrimental for the PCA. Finally, as the photochromic behaviour is known to be influenced by the irradiance,21 future investigations will follow the change in photochromic properties of such materials with the variations in power or time of the irradiance received by the surface of the specimens using monochromatic visible lasers. 18 ACS Paragon Plus Environment

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Acknowledgements D.M. Tobaldi is grateful to the ECO-SEE project (funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement no 609234). R.C. Pullar acknowledges the support of FCT grant SFRH/BPD/97115/2013. G. Otero-Irurueta would like to thank FCT from Portugal for his Post-Doctoral research grant (SFRH/BPD/90562/2012). This work was developed in the scope of the project CICECO–Aveiro Institute of Materials (ref. FCT UID/CTM/50011/2013), financed by national funds through the FCT/MEC and when applicable co-financed by FEDER under the PT2020 Partnership Agreement. Professors L.D. Carlos and R.A.S. Ferreira (Physics Department and CICECO−Aveiro InsƟtute of Materials, University of Aveiro, Portugal) are kindly acknowledged for the constructive and fruitful discussions. M. Ferro and RNME – University of Aveiro, FCT Project REDE/1509/RME/2005 – are also acknowledged for HR-TEM analysis


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Figures and Captions

Figure 1 – HR-TEM images of 2Ag-Ti450: a) smaller silver NPs are clustered around larger anatase and rutile NPs, with their respective crystallographic planes being shown; in the inset the EDS analysis is shown. b) A Ag0 NP is highlighted by the yellow dashed circle, and the inset displays its FFT pattern, described in the zone axis (ZA) [111]; also, the d(101) crystallographic planes of an anatase NP are shown. c) Silver NPs clustered around larger anatase NPs, with their d(111) and d(101) crystallographic planes shown; a halo of amorphous phase is also visible, marked by the yellow dashed line.

Figure 2 – DRS spectra of the samples prior to visible-light exposure. The top part of the figure shows a visual rendering (according to the RGB colour space measurements) of the colour of the samples, prior to visible-light irradiation.

Figure 3 – DRS spectra of Ag-modified samples, before and after visible-light irradiation. a) Ag-Ti450; b) 2Ag-Ti450; c) 5Ag-Ti450; d) 10Ag-Ti450. The top part of the figure shows a visual rendering (according to the RGB colour space measurements) of the evolution in colour change of the samples with visible-light irradiation time.

Figure 4 – Evolution of Ag0 NPs SPR centroid with increasing visible-light irradiation exposure time. a) AgTi450: the coefficient of determination, R2, of the single exponential growth function adopted for the fitting was 0.995; b) 2Ag-Ti450: R2 of the double exponential growth function adopted for the fitting was 0.988; c)


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5Ag-Ti450: R2 for a double exponential growth function was 0.992; d) 10Ag-Ti450: R2 for the 3rd order exponential growth function adopted for the fitting was 0.999. The estimated standard deviation was < 0.5 nm for every measurement.

Figure 5 – Evolution of the absorbance ratio A700/A450 with increasing visible-light irradiation exposure time. a) Ag-Ti450: the coefficient of determination, R2, of the single exponential growth function adopted for the fitting was 0.999; b) 2Ag-Ti450: R2 of the 3rd order exponential growth function adopted for the fitting was 0.999; c) 5Ag-Ti450: R2 for a 3rd order exponential growth function was 0.999; d) 10Ag-Ti450: R2 for the 3rd order exponential growth function adopted for the fitting was 0.999.

Figure 6 – Photochromic recovery with repeated visible-light (yellow spheres) / dark @ 100 °C (grey spheres) cycles, the y-axis representing the change in the CIE b* chromatic channel. The bars underneath show the RGB renders of the switching colours. a) Specimen Ag-Ti450; b) 2Ag-Ti450; c) 5Ag-Ti450; d) 10AgTi450.

Figure 7 – a) and b) XPS maps of Ti 2p and Ag 3d scans as a function of the BE, respectively. c) and d) show the initial (black), an intermediate (green) and final scans (red) of Ti 2p and Ag 3d core levels, respectively.

Figure 8 – Photocatalytic NOx abatement tests, in triplicate, of Ti450, Ag-Ti450 and 2Ag-Ti450, with commercial P25 as a comparison, using the white LED lamp.


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Figure 9 – Evolution of the apparent optical Eg with the visible-light irradiation time. The dark grey and red dashed lines represent the fitting obtained by using a 3rd order exponential decay function, for specimens Ag-Ti450 and 2Ag-Ti450, respectively. R2 = 0.991, and 0.994, respectively.

Figure 10 – Proposed mechanism for the prolonged lifetime of the photo-generated pair (e–/h+) in an anatase-rutile mixed phase TiO2 decorated with Ag NPs. The IFCT and visible-light irradiation promote Ag2O photo-reduction into Ag0 NPs. CB electrons are stabilised by interphase transfer that extends the lifetime of h+. Electrochemical potentials of the band edges of TiO2 and Ag2O, with respect to the absolute vacuum scale (AVS), are from the literature (ECB of rutile = –3.9 eV; ECB of anatase = –4.3 eV).61–63


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Fig. 1a


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Fig. 1b


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Fig. 1c


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Fig. 2

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Fig. 3a

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Fig. 3b

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Fig. 3c

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Fig. 3d

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Fig. 4a

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Fig. 4b

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Fig, 4c

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Fig. 4d

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Fig. 5b

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Fig. 5c

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Fig. 5d

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Fig. 6b

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Fig. 6d

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Fig. 7


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Fig. 8

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Fig. 10


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TABLES Table 1 – CIEL*a*b* coordinates, RGB colour space coordinates, and visual rendering of the samples (according to the RGB colour space).

Sample

CIEL*a*b* coordinates

L*

a*

b*

Ti450

99.5

−1.3

3.1

Ag-Ti450

84.2

−2.2

10.3

2Ag-Ti450

82.2

–2-5

13.1

5Ag-Ti450

67.5

1.2

16.3

10Ag-Ti450

43.5

1.7

6.2


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Table 2 – Apparent optical band gap (Eg) evolution, in samples Ag-Ti450 and 2Ag-Ti450, with increasing visible-light irradiation time. Eg was calculated via the Tauc plot, assuming an indirect allowed transition. Apparent optical Eg of unmodified TiO2 was 3.00 eV (413 nm).

Visible-light irradiation time, min 0 0.25 0.5 0.75 1 3 5 7 10 15 30 45 60 90 120 180 240 300

Indirect optical Eg, eV Ag-Ti450 2Ag-Ti450 2.97 2.88 2.98 2.89 2.96 2.87 2.96 2.84 2.95 2.82 2.92 2.69 2.90 2.61 2.87 2.53 2.90 2.45 2.87 2.36 2.84 2.42 2.80 2.31 2.79 2.25 2.75 n.a. 2.72 n.a. 2.69 n.a. 2.66 n.a. 2.65 n.a.

Indirect optical Eg, nm Ag-Ti450 2Ag-Ti450 418 431 417 430 419 432 420 437 420 439 425 461 428 474 432 491 428 505 432 525 437 512 443 537 445 552 451 n.a. 456 n.a. 461 n.a. 466 n.a. 468 n.a.


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Purely Visible-Light-Induced Photochromism in AgâTiO2

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