Hydrogenated black-TiOx: A facile and scalable synthesis for environmental water purification

Accepted Manuscript Title: Hydrogenated black-TiOx: a Facile and Scalable Synthesis for Environmental Water Purification Authors: M. Zimbone, G. Cacciato, M. Boutinguiza, A. Gulino, M. Cantarella, V. Privitera, M.G. Grimaldi PII: DOI: Reference:

S0920-5861(18)30290-6 https://doi.org/10.1016/j.cattod.2018.03.040 CATTOD 11323

To appear in:

Catalysis Today

Received date: Revised date: Accepted date:

19-6-2017 27-1-2018 22-3-2018

Please cite this article as: Zimbone M, Cacciato G, Boutinguiza M, Gulino A, Cantarella M, Privitera V, Grimaldi MG, Hydrogenated black-TiOx: a Facile and Scalable Synthesis for Environmental Water Purification, Catalysis Today (2010), https://doi.org/10.1016/j.cattod.2018.03.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Hydrogenated black-TiOx: a Facile and Scalable Synthesis for Environmental Water Purification

Massimo Zimbone1, Giuseppe Cacciato2* [email protected] , Mohamed Boutinguiza3, Antonino Gulino4, Maria Cantarella5, Vitorio Privitera6, Maria Grazia Grimaldi7

CNR-IMM, via S. Sofia 64, 95123 Catania, Italy

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CNR-IMM, via S. Sofia 64, 95123 Catania, Italy

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Departamento de Física Aplicada, E.T.S. Ingenieros Industriales de Vigo, Rúa Maxwell, s/n, Campus

Universitario 36310 Vigo, Spain

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Dipartimento di Scienze Chimiche Viale Andrea Doria, 6 Catania

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CNR-IMM, via S. Sofia 64, 95123 Catania, Italy

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CNR-IMM, via S. Sofia 64, 95123 Catania, Italy

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CNR-IMM, via S. Sofia 64, 95123 Catania, Italy

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Dipartimento di Fisica e Astronomia, Università di Catania, via S. Sofia 64, 95123 Catania, Italy

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Corresponding author: Giuseppe Cacciato, CNR-IMM, via S. Sofia 64, 95123 Catania, Italy

Hydrogenated black-TiOx: a Facile and Scalable Synthesis for Environmental photocatalytic Water

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Purification

M. Zimbonea, G. Cacciatoa,(*), M. Boutinguizab, A. Gulinoc, M. Cantarellaa, V. Priviteraa, M. G. Grimaldia,d

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CNR-IMM, via S. Sofia 64, 95123 Catania, Italy

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Grupo de Aplicaciones Industriales de los Láseres, Departamento de Física Aplicada, E.T.S. Ingenieros Industriales de

Vigo, Rúa Maxwell, s/n, Campus Universitario 36310 Vigo, Spain

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Dipartimento di Scienze Chimiche - Università di Catania and I.N.S.T.M. UdR of Catania Viale Andrea Doria 6,

95125 Catania, Italy d

Dipartimento di Fisica e Astronomia, Università di Catania, via S. Sofia 64, 95123 Catania, Italy

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corresponding author: Giuseppe Cacciato, [email protected]

Highlights

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Graphical abstract

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 Novel, scalable laser synthesis of a photoelectrochemical diode has been proposed  The black-TiOx shows high light absorbance and high hydrogen content  Amorphous structure and H inclusion allow high photocatalytic performance  The material shows long term stability

Abstract In the last few years, intense efforts have been devoted to the development of new photoactive materials for solar-driven water purification. In particular, the black-TiOx has attracted growing interest demonstrating the required capabilities. We report on the synthesis, characterization and application of hydrogenated-black 2

titanium oxide films for photocatalytic water treatment. Moreover, we compared different experimental conditions in order to maximise the photocatalytic activity of the material while maintaining an industrially compatible and green synthesis approach. The scalability and robustness of the procedure is further demonstrated by using different lasers: a 1064 nm laser, a 532 nm laser at low repetition rate and a 532 nm high repetition rate laser at different scan speeds. The morphology of the irradiated surfaces depends on the

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laser wavelength, while light adsorption in both UV and visible range is higher than 85% in every case. Amorphous and highly hydrogenated phases were found to be the most abundant although sub-

stoichiometric crystalline oxides (TiO and Ti2O3) are also found depending on the laser used. Owing to the

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presence of a metallic Ti support, a monolithic photochemical diode is realised by depositing a film of Pt

nanoparticles on the back-side of the samples. Photocatalytic activity tests reported high degradation rates for

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methylene blue dye with a maximum quantum efficiency of 0.054% observed on the sample irradiated with the 532 nm laser. Short and long-term photo-stability was investigated resulting in a good reliability of the

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manufactured diode.

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Keywords: Pulsed Laser Irradiation in Liquid, TiOx, porous, titanium, photocatalysis

1. Introduction

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R. Smalley (Nobel laureate in chemistry in 1996) listed water among humanities top ten problems together with energy, food and pollution. His considerations are still topical: the demand for fresh clean

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water sources is still a huge issue especially in developing countries, where the access to pure water is often difficult and the lack of infrastructures does not allow efficient and extensive water purification and disinfection. Moreover, the conventional water treatments methodologies suffer large limitations, from the

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environmental point of view, they require the realization and maintenance of large industrial plants and the intensive use of chemicals [1]. In this sense, photocatalysis applied to water and wastewater purification can give an effective contribution addressing the urgent water environmental problem. Novel nanotechnologies can improve the efficiency of photocatalysis (because of their large ratio surface/volume) and permit the realization of new devices and methodologies that run at low operational cost, even in places unequipped with infrastructures[2,3]. 3

TiO2-based photocatalysis has attracted great attention due to its scale economy, stability, abundance, non-toxicity, photo-activity and ability to purify air and water from pollution and contaminants[4]. TiO2 is one of the most studied and well-known material after silicon and silicon carbide due to its remarkable surface peculiarities [5]. It is commonly used in large amounts in several applications such as coating material in construction [6], paints, colour additive in foods, in the pharmaceutical industry,

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as an anode in PEC (or in DSSC) [7]or as an antifogg coating [6], self-cleaning surfaces [4]. Moreover, due to its biocompatibility, it is used in surgery and dentistry. It has also been employed in microelectronics

[8,9], for sensor [10] and electro-optical applications [11]. Uses of TiO2 for water purification have been

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addressed only in recently [12-15]

The choice of TiO2 for visible light activated water purification has been delayed because of its wide

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band gap (~3.2 eV) making it inefficient for solar-driven applications. Until 2011, black sub-stoichiometric crystalline or amorphous TiO2 phases have been considered to exhibit negligible photo-catalytic activity

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under both visible and UV illumination. A remarkable step further was been presented by Chen and co-

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workers [16] in 2011. Indeed, they stated that the photo-activity of crystalline TiO2 can be enhanced by an

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amorphous (hydrogenated) layer of black titanium oxides on crystalline core nanoparticles. After the

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publication of their paper, TiO2 amorphous phase gained renewed interest. Numerous studies came out in order to improve the activity of the TiO2 under visible radiation and in order to understand the different

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properties of black-TiO2. It was well known that TiO2 turns blue or black when Ti+4 is reduced. Ti+3 states formed in the reduced titania are considered responsible for the darkening of the surface and the decreasing

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of the photo-activity rate acting as recombination centres. Annealing of TiO2 in hydrogen (or in various gases) [17], electrochemical insertion of H [18] at atmospheric pressure, or preparation of sub-stoichiometric oxide (TiO2-x) results in black TiO2 that shows negligible photocatalytic activity. In these cases, black

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coloration partially results from the transition of Ti+3 3d1 electrons to the unoccupied Ti+3 3d1 state (related to the splitting of Ti 3d orbitals t2g and eg) or to Ti+4 3d0 states [19-22].In particular it was argued that the black coloration results from the excitation of electrons trapped at Ti(III) sites to adjacent Ti(IV) centres in a intervalence charge transfer excitation [23].This was considered for long time the only cause of the black coloration at the material surface in the sub-stoichiometric crystalline phases of titanium oxides (Magneli phases, Ti2O3 and TiO). Nevertheless, Chen at al. demonstrated that annealing in high H pressure and mild 4

temperature (~500 °C) can generate a hydrogenated amorphous black TiO2-x structure with high photocatalytic activity in the degradation of dyes under UV light illumination and good water splitting ability. Even more interesting, the appearance of Ti+3 states reduces the activity of the material to negligible values [24]. The real nature of the defect introduced by the hydrogenation has actually no consensus and is still matter of debate. Nevertheless, there are strong indications that H atoms realize a distortion of oxygen

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bonds in amorphous material. On one hand black-TiO2 possesses the above mentioned remarkable properties, on the other hand, its synthesis is disadvantageous, requiring long annealing treatments (up to 5 days) in high pressure of hydrogen

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(up to 20 bar). Another important drawback is related to the use of the nanoparticles for water purification purposes:,a further step is required in order to remove or recover nanoparticles after the photocatalytic

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treatment when dispersed in solution. Since the impact of nanoparticles on the environment is still unclear, avoiding the dispersion of nanomaterial may represent an advantage and, thus, the realization of a black

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photocatalytic nanostructured film immobilized onto a fixed substrate is highly recommended.

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Recently, our group developed an alternative, industrially compatible, environmentally friendly

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technique suitable for the synthesis of black TiOx films (on a solid substrate) with significant photocatalytic

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activity[25-26]. By irradiating the surface of a titanium foil immersed in water with a pulsed high power laser, we observed the formation of a nanostructured TiOx film [27]. The film is black (showing a high

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absorption in the visible and UV range) and is composed by amorphous sub-stoichiometric titanium oxides. This material has been demonstrated to have a high amount of bound -OH groups and showed photocatalytic

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activity in both UV and visible range [25]. Its photocatalytic activity was further improved with metal Pt nanoparticles grafting, realizing a stacked layered structure (TiOx/Ti/PtNps), able to enhance oxygenmediated electron scavenging [28-32].

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In this work, we analyse and discuss the synthesis, properties and scalability of black TiOx films

formed on the surface of titanium foils undergoing different laser irradiations. We used three different lasers: two (1064 nm and 532 nm at low repetition rate) are typically used for research purposes and the other one (532 nm at high repetition rate) is commonly used in industrial metallurgy. We observed the morphology, light absorption, composition, crystallinity, and photocatalytic activity as a function of the laser parameters. The long term stability of black hydrogenated TiOx film was also investigated. In particular, we shield light 5

on the connections between structural, optical and photo-catalytical properties of the hydrogenated TiOx material obtained by pulsed laser irradiation in liquid. We demonstrate flexibility, scalability, robustness of the synthesis procedure. Although morphology, and internal structure depends on the laser wavelength and other experimental parameters, the optical and photocatalytical properties are basically the same in all

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experimental conditions explored.

2. Experimental Details

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2.1 Sample Preparation

The synthesis of the TiOx film was performed by irradiating a titanium metal foil (Goodfellow,

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purity 99.9%, as rolled) by pulsed laser in water. Samples were irradiated with three different lasers: 1064

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nm wavelength laser at 10 Hz, 532 nm wavelength laser at low repetition rate and 532 nm wavelength laser

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at high repetition rate. Samples are named “1064 nm”, “532 nm LR” and “532 nm HR”, respectively.

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For “1064 nm” samples, irradiation was performed with a Nd:YAG Giant G790-30 at 1064 nm and 10 Hz repetition rate and 600 mJ energy per pulse. The laser was focused using a lens (focal length of 20

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cm), on the bottom of a Teflon vessel filled with 5 ml of deionized Milli-Q water (resistivity 18 MΩ·cm). Fluence was varied between 4 and 18 J/cm2. Accumulated fluence FA is evaluated as the actual fluence

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multiplied by the number of pulses. Samples chosen for optical and structural analyses were irradiated at a fluence of 10 J/cm2 and the surface was scanned (manually) until a 1 cm2 area of uniform black TiOx was

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obtained.

For “532 nm LR” samples, the laser was a Nd:YAG Giant G790-30 at 532 nm, 10 Hz repetition rate

and 80 mJ energy per pulse. The laser beam crosses horizontally a quartz cuvette filled with some ml of

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deionized Milli-Q water (resistivity 18 MΩ·cm). The path length of the laser was adjusted to be 30 m in order to smooth the hot spot of the beam by realizing a diffraction pattern with a coherence area of about 4 mm in diameter. A Gaussian intensity profile with a full width at half maximum of 1 mm was obtained. Samples chosen for optical and structural analyses were irradiated at a fluence of 2 J/cm2 and the surface was scanned (manually) until a 1 cm2 area of black TiOx was synthesized.

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The “532 nm high HR” samples were synthesized irradiating the titanium metal foil by a diodepumped Nd:YVO4 laser operating at wavelength of 532 nm, a repetition rate (RR) of 20 kHz, and a pulse duration of 15 ns. A nominal energy per pulse of 0.3 J was used. Fluence (F0) was evaluated to be 2 J/cm2 considering a diameter of the spot to be 133 µm. The laser was focused on the bottom of a vessel. Target lies beneath 9 mm of deionized Milli-Q water. Surface was scanned 5 times with a scanning speed (Ssp) between

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5 and 100 mm/s. Samples have a macroscopic surface area of 1 cm2. Three samples were obtained at different scanning speed (5, 50 and 100 mm/s). Accumulated fluence, defined as the energy deposited for

units of sample area, was calculated as the product of the number of pulsed times the fluence for single pulse.

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An accumulated fluence of 1040 J/cm2, 104 J/cm2 and 52 J/cm2 can be extracted by the above mentioned experimental parameter. Samples chosen for structural and photo-catalytic analyses were irradiated at a

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scanning speed of 100 mm/s.

The synthesis of platinum nanoparticles (PtNps) was performed by pulsed laser ablation in liquid

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methodology with the same experimental apparatus described above for the 1064 nm irradiation. Platinum

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metal plate (purity 99%) was purchased by Sigma Aldrich. Nanoparticles, dispersed in water, are stable for

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some months. Pt nanoparticles are 20 nm in diameter (as measured by dynamic light scattering) and have a

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high surface/mass ratio of the order of 20 m2/g [33].

A monolithic chemical diode was thus manufactured in order to tests the photocatalytic activity of

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the TiOx layer. It was realized by sanding with sandpaper the back side of the titanium foil in order to have a rough surface, then depositing about 3 ml (drop by drop) of a PtNps solution at 90°C and waiting until

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complete water evaporation. A continuous layer of platinum nanoparticles onto the substrate is formed and TiOx/Ti/PtNps structure was realized. SEM micrographs of isolated Pt nanoparticles and PtNps layer deposited on the rear side of the sample are shown in Figure 1a S.I and Figure 1b S.I. respectively.

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In Figure 1 it is shown the manufacture procedure for the monolithic chemical diode. Irradiation of

the Ti target and a representative photo of the irradiated sample are depicted in the upper part of Figure 1. The schematic synthesis procedure of PtNps and a photo of the colloidal nanoparticles solution are reported in the lower part of the same figure while in right side the monolithic chemical diode, composed by a TiOx/Ti/PtNps stacked layer structure is showed . Single Rutile TiO2 crystal was purchased by sigma Aldrich and used as reference. 7

2.2 Methods The crystalline structure of the TiOx/Ti samples was determined by glazing angle (0.5°) X-Ray Diffraction by using a Bruker D-9000 instrument (Cu Kα) and Bruker diffraction suite software for the diffraction analysis.

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Raman analyses were performed with an XploRA Jobin-Yvon spectrometer equipped with a 1800/mm grating and ×100 objective. Excitation wavelength was 633 nm.X-ray photoelectron spectroscopy (XPS) spectra were measured on the as prepared sample at 45° take-off angles relative to the surface plane

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with a PHI 5000 Versa Probe II system. Samples were excited with a monochromatized Al-Kα X-ray

radiation using a pass energy of 5.85 eV. The instrumental energy resolution was ≤ 0.3 eV. Deconvolution of

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the spectrum in the O 1s energy range was carried out by fitting the spectral profile with some symmetrical Gaussian envelopes after subtraction of the background.

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Rutherford backscattering spectrometry (RBS) and Elastic Recoil Detection Analysis (ERDA) were

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run using a 2 MeV He+ beam. RBS was performed with a scattering angle of 165° in normal incidence. The

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RUMP software was employed for the analysis of the RBS spectra. ERDA was performed at an angle of incidence of 10°. An aluminium foil was used for stopping the He beam scattered in forward.

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SEM images were acquired using a Field Emission SEM (Gemini Zeiss SUPRA™25) at working

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distance of 4 mm, using an electron beam of 5 keV and an in-lens detector for backscattered electrons. The UV–Visible reflectance spectra were collected using a Perkin-Elmer Lambda40 spectrometer in

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the wavelength range 350–900 nm with an integrating sphere (Labsphere 20). Absorbance is calculated with the formula: A=100 - R where R is the integrated reflectivity in percentage. In order to evaluate the photocatalytic activity of the TiOx nanostructured films, UV photo-

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degradation tests with Methylene Blue (MB) dye were carried out. Standard experimental procedure is described in ref [34]. Apparent Photon Efficiency in the UV range is calculated following the procedure reported in ref [25]. Quantum efficiency (QE) is calculated as the ratio between the Apparent Photon Efficiency and the measured absorbance in the wavelength range of the light source. QE represents the ratio between the number of molecules degraded for unit of time (and area) and the number of photons adsorbed by the sample per unit of time (and area). The visible light source (DULUX L BL /71, Osram) spectrum was 8

centred at 453 nm with a FWHM of about 30 nm. A long pass filter with an edge at 420 nm was inserted between light source and sample in order to avoid the presence of UV radiation. The UV light source (TL 8 W BLB 1FM, Philips) spectrum was centred at 368 nm with a FWHM lower than 10 nm and the measured irradiance was about 1.3 mW/cm2. Following the ISO10678:2010(E), photonic efficiency cannot exceed 0.1% for decolouration of MB dye. Spectra of UV and Vis lamps are shown in figure 2 S.I. together with the

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MB absorption spectrum. Black arrow indicates the edge of the filter used in the visible light irradiation. The mineralization of MB was evaluated by measuring total organic carbon (TOC) content with a TOC analyser (Shimadzu TOC-LCSH) equipped with a non-dispersive infrared detector (NDIR). The

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inorganic carbon was removed from the tested solutions by purging the acidified sample with a purified gas, the TOC content was measured after high-temperature oxidation (680°C). The samples were immersed in 2

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mL of a MB solution (starting concentration: 1.5 ∙ 10−5 M) and irradiated with a UV light at 368 nm for 240 min, the solutions were agitated every 60 min. The system was covered with a quartz glass, to avoid the

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evaporation of the solution during the experiment. TOC content of each solution in contact with samples was

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3.1 XRD and Raman analysis

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3. Results and discussion

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measured after 240 min of UV irradiation.

A peculiar characteristic of laser irradiation is that it allows the preparation of compounds out of

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thermodynamic equilibrium due to the high pressure and temperature achieved in the initial tens of nanoseconds and to the fast process kinetics. This fact is particular relevant for laser irradiation in liquid where the presence of liquid and cavitation bubble formation (and collapse) have a large impact on the

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kinetics. The collapse of the cavitation bubble causes the re-deposition of the ablated material on the target surface and give rise to a state of high temperature and pressure that concurs in formation of hydrogenated TiOx. In order to have structural information on the synthetized material, we performed XRD measurements. The XRD spectra of the TiOx films are shown in Figure 2. In the same figure the spectrum of the Ti substrate before laser irradiation is reported . Peaks at about 35°, 38°, 40°, and 53° relate to metallic Ti 9

substrate. These peaks are apparent as well in the irradiated samples. Peaks at 37.25° and 43.29° are ascribed to (1,1,1) and (2,0,0) reflection of titanium monoxide (TiO) respectively and peaks at 33.06° and 53.91° belong to (1,0,4) and (1,1,6) planes of Ti2O3 phase respectively. The halo clearly recognizable at low angles in the spectra of irradiated samples is a clear indication of the presence an amorphous phase. In particular for 532 nm HR sample the halo is more intense and extends up to high angles (50°). Moreover, the absence of

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peaks related to titanium oxides (only peaks related to substrate are visible) can be interpreted as a sign of the amorphous nature of the obtained material. Instread at low repetition rate (1064 nm and 532 nm LR

samples), instead, peaks related to TiO and Ti2O3 are present in the spectra, thus evidencing the presence of

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small crystallites of substoichiometric TiO2 phases.

The appearance of the broad amorphous halo, observed in the XRD spectra, in combination with the

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low intensity of the crystalline substoichiometric peaks (of TiO and Ti2O3) and the information about stoichiometry obtained with RBS analysis, induce us to state that laser irradiation synthesize a hydrogenated

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amorphous titanium oxide with eventually embedded nano-crystals of sub-stoichiometric oxides. Metastable

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crystallites are smaller in the 532 nm LR sample whereas are absent in the 532 HR.

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The preparation of crystalline metastable or amorphous phases TiO2 is associated with the high

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pressure and temperature reached during the formation of the material and the fast quenching of the temperature that allows only an incomplete rearrangement of the structure. Ti-O system has many stable

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phases very close in terms of Gibbs free energy: all these phases as TiO, Ti2O3, TinO2n-1 (see the phase diagram reported in Figure 3 S.I. ) are in the region of [O]/[Ti] ratio between 1.2 and 2. Small driving forces,

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such as strain-energy or variations in local composition are sufficient to yield metastable amorphous TiOx phases instead of the above-cited crystalline phases. Nevertheless, the main phase observed was the amorphous one. Composition of the sample obtained by RBS shows a decrease of the oxygen content

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moving from the surface to the bulk of the sample and different phases are expected in the same thickness range.

In crystalline Ti-O phases (TiO, Ti2O3, TinO2n-1, TiO2) titanium is always esa-coordinated while oxygen is 3, 4 and 6 coordinated in TiO2, Ti2O3 and TiO, respectively, correspondingly decreasing the stability from coordination number 3 to 6. The higher the O coordination the more energetic and less stable is the compound. Either oxygen coordination number or distortion of the O bonds are the main driving force for 10

the stability of the O compounds (as was shown by Adler [35]). Even in amorphous Ti-O compound, distortion of the O bond or higher coordination number create a local energy and strain increase and a consequent decrease in stability. At this point, the role of large amount of H (or H2) in the structure must be taken into account. As reported by P. Raghunath [36], Mo et al. [37] and Schmuki [17], hydrogen can be incorporated into the structure and can reduce the energy of the amorphous Ti-O compound, stabilizing it. As

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reported by Chen [16], H (or H2) insertion (due to high-pressure process) is able to destabilize the crystalline TiO2 structure and favour the formation of amorphous hydrogenated TiO2. Furthermore, hydrogen can

passivate the dangling bonds of the defects of crystalline TiO2 and in amorphous Ti-O compound and can

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relax the strain. As an example, a typical Raman spectrum of irradiated sample is reported in Figure 4 S.I..

Broad bands are apparent around 250 and 450 cm−1. We have also reported in the same figure the spectrum

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of amorphous phase of TiO2 and even in this case a broad spectra is observed. These spectra are characteristic of amorphous-like (highly disordered) phases for which the Raman signal is expected to

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mirror a smoothed version of the vibrational density of states [38]. Moreover, in the same figure we display

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the position of the main peaks of rutile, anatase and crystalline Ti2O3 phases. TiOx spectrum covers the same

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frequency range (between 180 and 500 cm−1) of the main peaks of crystalline Ti2O3 phases. It can be

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3.2 XPS analysis

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considered as a signature of sub-stoichiometric oxide phases.

XRD and Raman results suggest the presence of some lower oxidation state (Ti+3 or even Ti+2) of Ti

oxides. The presence of substoichiometric oxides relies on a local increase of electron density near the Ti

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atoms that is commonly referred to as the Ti+3 states. The presence of such oxidation states (and the relative oxygen deficient TiOx species) can be detected by XPS analysis. XPS spectrum of the 1064 nm laser irradiated TiOx sample in the Ti 2p energy region is reported in Figure 3a. The Ti 2p feature consists of the main 2p3/2, 2p1/2 spin-orbit components at 458.7 eV and 464.5 eV respectively. These values are just 0.2 eV lower with respect to those found for a TiO2 film (458.9 and 464.7 eV) prepared by thermal (700°C, 24 h) oxidation in air of a Ti foil and used here as a reference. No 11

evidences of low binding energy broadening of the doublet evident for TiO2 itself have been observed so only Ti+4 states are evident in the surface of the laser ablated film. This fact suggests the substantial absence of reduced TiO2 species on the XPS probed depth (few angstroms), being the 458.7 eV and 464.5 eV values typical of the TiO2 itself [39]. In fact, the eventual presence of Ti+3 species give rise to an XPS spectrum consisting of a pair of overlapping Ti 2p doublets because of the competition of various final state screening

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effects [40]. This finding is in accordance with the results of RBS measurements within the “smooth interface

model”. Indeed, as observed by RBS spectra, stoichiometry of the irradiated material changes continuously

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from that of TiO2 to that of pure titanium moving from the surface to about 300 nm deep. Resolution of RBS can be estimated to be about some tens of nm whereas XPS is a surface technique that probes a depth mainly corresponding to the photoelectron inelastic mean free path that for Ti 2p photoelectrons corresponds to

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some 12.5 Angstroms [41, 42]. The above consideration excludes the presence of Ti+3 states in the surface

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but is also compatible with the presence of reduced titanium down of the firsts tens of nm. This finding

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shields light on the state of the surface of the black-TiOx material.

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In order to have more insight into the O and H chemical properties the XPS of the O 1s core level is

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performed and showed in Figure 3b. Two peaks are evident but the lower intense peak shows a rather broad shape. Therefore, the overall signal was fitted using three symmetrical Gaussian components. The most

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intense peak, at 530.5 eV is consistent with the presence of TiO2, whereas, the lower intense peak revealed two components at 532.1 and 533.5 eV and is due to both water and Ti-OH hydroxide. These results were

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somewhat expected since the film was obtained by laser irradiation in water. These results is compatible with that obtained via ERDA technique for which a high concentration of H is detected. The XPS of the C1s states for the Hydrogenated black-TiOx is shown in figure 3c.

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It is apparent a signal centered at 285.0 eV consisting with the omnipresence of the so-called adventious carbon contanination. Please note that this signal is always present in all air exposed materials and its position is used for XPS calibration [42]. The atomic concentration analysis gave a carbon concentration of about 2 % percent thus excluding any possible role on the photocatalytic properties of the system.

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3.3 RBS and ERDA analysis

Information regarding the thickness and stoichiometry of the irradiated surfaces are obtained by using RBS and ERDA. In Figure 4 the RBS spectra of “1064 nm” (yellow), “532 nm LR” (pink) and “532 nm HR” (green) irradiated samples with their respective theoretical simulations (obtained via the RUMP

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software in the “Smooth interfaces” approximation) are reported. In the same figure, the RBS spectrum of a pure Ti foil before irradiation (black) and the corresponding fit are reported. A schematic profile (not in scale) of Ti and O is reported in the graph to facilitate the explanation. A monotone increase of the Ti

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concentration and a concomitant decrease of the oxygen content are observed along the three samples. Thus, RBS reveals the presence of a layer rich in sub-stoichiometric oxides. A thickness of 300 nm and an average

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value of the ratio [O]/[Ti] of about 3/2, can be roughly extracted by simulating a TiOx layer with a very sharp interface on Ti substrate. In addition to RBS measurements, we performed ERDA analysis in order to

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quantify the amount of hydrogen into the TiOx film. This technique does not allow us to discriminate the

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chemical surrounding of hydrogen in the film (i.e. we do not distinguish if hydrogen is present in O-H, H2, or

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Ti-H chemical states), but gives us an estimation of its quantity in the sample. For comparison purpose, Ti

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foil has been used as reference for the initial H content in the irradiated samples. It was found that the Ti foil and the black TiOx have a dose of 1.2·1016 atoms/cm2 and 4.5·1016 atoms/cm2 respectively. Notably, the

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amount of hydrogen in black TiOx is much higher (3.75 times) than in the Ti foil. The content in the Ti foil is ascribable to the hydrogen adsorption on the free surface of the metal. Indeed, this signal is seen as a peak in

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the ERDA spectra related to the surface; on the contrary, in the TiOx film, the hydrogen signal spreads into the bulk of the sample and is not localized at the surface. Unfortunately, because of the huge energy straggling of forward scattered H in the ERDA

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experiment set-up, spectra cannot be easily simulated and is not trivial to extract a precise profile. Nevertheless, the total hydrogen dose based on a calibration sample can be extracted. By using the approximate value of the thickness obtained by RBS analysis (i.e. 300 nm) we obtain the value of 1.5·1021 atoms/cm3 (1.6 %) of hydrogen. This value is 5 times higher of that was found by Chen (0.3%) by hydrogenating of a TiO2 nano-powder at 20 atm for 5 days [43] and can plenty justify the black coloration of the sample [4]. 13

A more accurate simulation of the RBS spectra can be obtained by considering a smooth oxygen and titanium profile. “Smooth interfaces” simulation is showed in Figures 5 S.I., 6 S.I. and 7 S.I., for 1064 nm, 532 nm LR and 532 nm HR lasers respectively. Within the “Smooth interfaces” model, the content of oxygen decreases from the surface to the bulk sample. Simulation of the layers can be obtained by considering a ratio [O]/[Ti] that decreases from 2 (that is the TiO2 stoichiometry) to zero (the value expected

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for pure titanium) moving from the surface to about 300 nm deep. Hydrogen content decreases in

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concentration moving from the surface to the bulk.

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3.4 SEM analysis

We now consider the morphology of the three samples realized with different wavelength and

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repetition rate.

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The morphology of the 1064 nm wavelength laser irradiated film is constituted by caves with a

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complex branched structure as evidenced by SEM inspection in Figure 5a. These features were deepl

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investigated in our previous works [27] and typically appear at the surface of the laser irradiated region once an accumulated fluence of 50 J/cm2 is reached. In Figure 5b and 5c, SEM images of the 532 nm wavelength

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laser irradiated sample are shown for low (LR) and high repetition rate (HR) laser respectively. These structures are very similar to each other but some differences are evident in comparison to 1064 nm laser

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irradiated surface. In the latter two cases (532 nm laser), the structures are composed mainly by cracks and pits that are apparent along (or very close to) cracks. Pits are smooth and do not show the complex branched structure characteristic of the 1064 nm wavelength laser irradiated samples. A close inspection of the surface

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of 532 nm laser irradiated sample shown the presence of subsurface rounded structures resembling underskin bubbles appearing very close to cracks. Difference in morphology between 1064 and 532 nm are not trivial and must be related to the complex dynamic during the formation of the black film. Indeed, laser irradiation induces a temperature rise on the irradiated spot and the presence of water environment enhances the diffusion of oxygen and hydrogen into the surface. The presence of cracks is clearly due to oxidation of the titanium and to the consequent thermal fatigue [44-46]. Moreover, the presence of cracks induces light 14

trapping [35] that is able to modify and increase locally the laser energy absorption. The morphological evolution of surface topology strongly depends on this localized effect, leading to a nano- and microstructuring of the surface close to the cracks edges [44,47]. Laser radiation, absorbed by the sample, induces a localized increase of the temperature and a hot and dense plasma forms on the laser spot (especially close to the cracks edges) during the first nanoseconds of irradiation. The expansion of atoms ejected from the spot

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is confined by the surrounding liquid environment. So, high temperatures (even up to 4000° K) and pressure (~100 atm) can actually be reached [48,49]. The temperature rise enhances the oxygen and hydrogen

diffusion through the metal surface and initiate redox reactions in the bulk. In the timescale of microseconds,

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a cavitation bubble appears in the liquid, over the irradiated spot [48]. The bubble collapse is believed

responsible for the violent back deposition of black TiOx on the target surface [49], the creation of cracks,

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bubble, and pits on the surface. We claim that this process generates the reported complex morphology. The differences in the 1064 nm and 532 nm laser irradiation arise by the different amount of re-deposited

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material. At 1064 nm and at a fluence of 10 J/cm2 a larger amount of re-deposited material is expected. This

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material, constituted mainly by oxidized titanium, water and hydrogen gas, covers the substructures realized

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previously and the complex branched structure of Figure 5a. At the 532 nm laser irradiation the fluence is

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lower (2 J/cm2) and the expected re-deposited material amount should be reduced, so making clear pits, under-skin bubbles and cracks. Any rigorous kinetic analysis of the formation of the morphology of black

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TiOx is behind of the scope of present discussion and will be reported in a separate paper.

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3.5 SEM analysis at various scanning speed

In the present study, we want to focus our attention on the possibility of synthesize black TiOx films

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using visible and commercial lasers. In order to demonstrate the scalability of the process, we performed the TiOx synthesis by using a commercial, high repetition rate (20 kHz @ 532 nm wavelength) laser used for industrial metallurgical processes. We irradiated the surface of the Ti foil by using three different scanning speeds (5, 50 and 100 mm/s). In Figure 6 the low and high magnification SEM images of the surface of the samples are reported. The low magnification morphology is different in the studied cases and, in particular, at low scanning rate the surface is composed by large depressions in the scale length of several tens of 15

microns but as soon as the scanning speed increases, the surface appears to be smoother. These features are due to the erosion of the surface caused by the ablation process and the presence of a cavitation bubble. Inter-pulse interval is 50 μs (1/20 KHz) and the cavitation bubble dynamic time constant, as reported by Sasaki et al., is about a ms [48]. A laser pulse interacts with the cavitation bubble realised by the previous pulse. Moreover, at low speed (5 mm/s), the inter-pulse distance (the distance between two neighbouring

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pulses) is 250 nm while the cavitation bubble has the dimension of several μm and the laser spot was evaluated to be 130 μm in diameter. This distance allows for a complete overlap of several pulses (more than 20) with the cavitation bubble. High energy plasma inside the bubble is formed and erosion of the substrate

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is therefore enhanced. At 50 mm/s and 100 mm/s speed, inter-pulse distance is 2.5 μm and 5 μm,

respectively, and a smaller overlap is present. Nevertheless, the low scale morphology of the three samples

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was the same as is evident in the right part of the Figure 6. In order to compare our results with the low repetition rate lasers (that show a flat surface in the large scale length) we used the sample obtained at 100

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mm/s scanning speed.

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3.6 Optical analysis

Laser irradiation in water induces a blackening of the titanium surface for all employed wavelengths

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in this work. The absorbance spectra (1 − 𝑅 %) of the black-TiOx film synthesized with the 1064 nm, 532

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nm LR and 532nm HR lasers are reported in Figure 7. The absorbance of the different samples is very similar and only minor differences can be recognised. Starting from a photon energy of 0.32 eV (3.8 µm

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wavelength) to 5.0 eV (250 nm wavelength), the measured absorbance is higher than 85% and, in particular, reaches values between 90% and 95 % in the visible region. In the graph, as a guide for eyes, horizontal dashed line at 85 % of absorbance and a vertical dashed line related to 0.32 eV are drawn. Absorbance

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decreases down to 50 %, at 0.32 eV. The reduction of the absorbance in the IR range (down to 0.32 eV) can be associated to an increase in the reflectivity caused by the metallic titanium substrate. We may estimate an upper bound of the band gap to be 0.32 eV. In the same graph, the standard solar irradiation spectrum for 1.5 AM is reported. Despite the wavelength of the laser used in the synthesis, the obtained black TiOx has high absorbance in the full range of solar radiation spectrum. According to literature studies, blackening of the titanium is obtained in three different ways: 16

1) Classic reduction of TiO2 in vacuum at high temperature with the formation of Ti+3 states [17] 2) hydrogen insertion (as interstitial) by an electrochemical cathodic reduction as reported by Fuijshima [4] 3) more recently hydrogenation at mild temperature and high pressure of hydrogen as proposed by Chen at al. [16].

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Chen et al., differing from other two methods, attributed the black colouration to a shift of the valence band inasmuch he observed extra band gap states near the valence band in UPS spectrum. Ti3+ related states were found to not contribute to these extra band gap states of the hydrogenated TiO2 nano-crystals and

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moreover as Ti+3 states show up, after intense UV irradiation, concomitantly disappear the near valence band states [24]. These states have to be different in nature respect to the Ti+3 and must be connected to the

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experimental procedure used by Chen to synthesize the black-TiO2. In particular, they are due to the incorporation of H caused by the high temperature and high H pressure of the synthesis process. Indeed, at

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temperature (200°C) and high H pressure (20 bar) for 5 days, the hydrogen diffuses into the lattice.

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Hydrogen destabilizes the ordered structure of the crystalline TiO2 and stabilizes the lattice disorder by

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passivating the dangling bonds. Raghunath et al. in 2013 elucidated the working mechanism for the

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incorporation of H into the TiO2 structure and the consequent amorphization [43]. According to this mechanism, H atoms migrate into the TiO2 forming 2HO-species exothermically, which further transform

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into H2O and result in the formation of O-vacancies and surface disorder. [43]. Hydrogen realizes a distortion of the oxygen environment that creates the intra-gap states close to the valence band. The conduction band

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minimum (CBM) is weakly influenced upon the O bound distortion because it is formed meanly by Ti 3d states and because of the larger energy for the distortion of the Ti bound. Whereas the valence band maximum (VBM), realized mainly by O 2p states, is strongly affected by a distortion of oxygen environment

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realizing mid gap states near the VBM [50]. Some researchers support the idea that H is introduced into crystalline TiO2 in different configurations. For example, Mo et al. assert that [51] H can be incorporated into the lattice occupying the oxygen vacancy site (Ho), or occupying an interstitial position (IH), forming a weak O-H bond. The Ho is believed to be realized at high temperature and high H pressure conditions [51]. The formation of black TiOx with laser ablation in water is realized under high pressure of hydrogen and high temperature and the formation of Ho may be a favourable process. When pulsed laser pulse 17

impinges on a Ti metal surface an increase of the temperature occurs in the first nanoseconds. Temperature of 4000 K was recently estimated (for irradiation of 10 J/cm2 pulse and 10 ns pulse duration) after the laser pulse and it decreases to about 1000 K, 1 us after the laser pulse [43]. In this condition a plasma plume is formed. The expansion of the plume is slowed down by the confined effect of the liquid environment and high pressure (of the order of ~100 atm) is reached. Moreover a cavitation bubble appears in the liquid.

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Formation, expansion, shrinking and collapse of the cavitation bubble is observed in hundreds microsecond timescale [43]. Plume expands inside the bubble and cools down. Ti, O and H specimen (contained in the

plume and in the cavitation bubble) react realising the titanium oxide nanoparticles in liquid. The collapse of

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the cavitation bubble causes the re-deposition of the ablated material on the target surface and gives rise to a new state of high temperature and pressure that concurs in the formation of hydrogenated TiOx. The

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presence of hydrogen together with the high pressure (observed during the collapse of the cavitation bubble) realises a reductive environment that favours the formation of sub-stoichiometric titanium oxide material. It

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is worth noting that the surface of the titanium oxide is in contact with water and suffers a complete

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oxidation (as probed by XPS measurements) while the material beneath the surface (as an example 50 nm

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under the surface and probed by RBS XRD or Raman) is reduced. Thus, the oxidation of the titanium foil in

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PLIL occurs under high hydrogen pressure i.e. in reductive environment so that an under-stoichiometric Hdoped titanium oxide is formed. From this point of view, the process results similar to the process proposed

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by Chen et al., although the latter process has to persist several days (from 5 to 20 days) while tens of microseconds are instead necessary in the laser procedure. It is worth noting that temperature and pressure,

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in PLIL, are several orders of magnitude higher than the standard Chen process (as reported previously, 102 atm and 103 °K compared to 20 atm and 200-500°C). The higher H pressure and higher temperature drive a faster formation of the black TiOx. Another interesting difference between black-TiOx and black-TiO2 arises

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from the smaller value of the band gap observed in the latter (lower than 0.32eV). Probably sub-gap states realize a strong distortion of the O bound and very high energy levels inside the band-gap appear. This is different from what was observed by Chen et al., that estimated a gap of about 1 eV.

3.7 Proposed Band structure

18

At this point we may, heuristically, suggest a possible band structure. On the surface, an amorphous titanium oxide is realised with Ti+4 oxidation state. Distortion and extra bounds, caused by H introduction, creates a dense band of mid gap states near the surface. The band gap results narrowed to a value lower than 0.32 eV due to the presence of these localised trap states. Titanium atom and conduction band, on the other side, is only weakly influenced by this distortion and only a small lowering of the CB is expected. Following

localized state to the lowered conduction band as reported in the scheme of Figure 8a.

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this framework, trapped holes and conduction electrons are generated by promoting an electron from a

Moving from the surface into the bulk of TiOx, a gradient of oxygen and hydrogen contents is

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expected in accordance with RBS and ERDA measurements. The TiOx layer exhibits multiple stoichiometry related to the oxygen profile. We may suppose that, similarly to TiO2, an upward bend bending of the

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conduction band may be possible near the surface caused by the presence in the surface of -OH groups. Photogenerated electron-holes couples, once created, can be separated efficiently in this layer. Electrons may

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diffuse easily to the back side of the sample, while holes can diffuse through the TiOx layer to the surface. A

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schematic of the process is reported in the Figure 8b. The intra-bandgap trap states create a “leaky” dielectric

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without loss of oxidation power.

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that allows holes to be transported to the surface (in a similar was to what was discussed by Lewis et al [7])

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3.8 Photocatalytic activity analysis

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The high light absorbance, the presence of Ti+4 oxidation states on the surface, the high amount of H in the layer, the presence of oxygen (and hydrogen) gradient and the presence of Ti-OH on the surface, have a great influence on the photodegradation of contaminants. In the present paragraph, we present the

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photocatalytic performances of the black TiOx synthesized by using different wavelengths and repetition rates by measuring the discoloration of a methylene blue (MB) dye solution under visible and UV illumination. The MB concentration decreases as function of illumination time for all samples following a pseudo first-order kinetic law. The curves are fitted in the range between 2 and 8 hours with a decreasing exponential following the formula: 𝐶/𝐶0 = 𝑒 −𝑘𝑡 , where k is the discoloration rate constant. 19

The value of the discoloration rate obtained by using visible light is reported in Figure 9 for “1064 nm” (@1064, red), “532 nm LR” (@532LR, green) and “532 nm HR” (@532HR,blue) sample. In the same graph are shown the degradation rate of MB without any catalyst and with a TiO2 reference. A discoloration rate of 0.051, 0.045, 0.027, 0.013 and 0.015 h-1 (± 0.01) is measured for “1064 nm”, “532 nm LR”, “532 nm HR”, TiO2 and MB solution respectively. Samples TiO2 and MB are inactive inside the experimental errors

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while the black TiOx shown activity under visible illumination. In figure 10a, it is shown the discoloration rate of the MB under UV illumination. Laser synthesized samples are shown as bars while dotted line represents the value obtained without the catalyst (MB). A

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discoloration rate of 0.18 ± 0.01 h-1cm-2, 0.24 ± 0.01 h-1cm-2, 0.15 ± 0.01 h-1cm-2 and 0.02 ± 0.01 h-1cm-2 is

measured for “1064 nm”, “532 nm LR”, “532 nm HR” and MB reference solution, respectively. By using the results of the fit, a quantum efficiency of 0.040, 0.054, 0.033 % are calculated for “1064 nm”, “532 nm LR”

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and “532 nm HR” respectively. These results are noticeable taking into account that commercial

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photocatalytic glasses possess a quantum efficiency of about 0.025 % [52]. In a control experiment the

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quantum efficiency for a sample irradiated at 1064 nm without PtNps film is evaluated to be 0.011%: the

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increase of the quantum efficiency adding the PtNps film is about 380% [25].

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The laser synthetized chemical diode is able to decolour a MB solution efficiently by oxidizing it. Nevertheless, the discolouration is only the first step in the degradation pathway of MB and a complete

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mineralisation of carbon, nitrogen and sulphur heteroatoms into CO2, NH4+, NO3- SO4-2 may be possible [53]. In order to verify the ability in removing organic contaminants from the solution, we measured the residual

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total of organic carbon (TOC) amount of a MB solution after UV illumination. In Figure 10b the percentage of removed organic substances (in the MB solutions) after 240 min of UV light illumination is shown for the three samples. The “532 nm LR” laser irradiated sample is the most active in mineralising almost 30 % of

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the organic compounds while “1064 nm” and “532 nm HR” are less active with a removal of about 10 %. The measured TOC data resemble the trend of photodegradation presented in Figure 10a though it is worth noting that the removal rate is less than that observed for the decolouration due to the presence of reaction side products of MB.

3.9 Photocatalytic activity mechanism 20

In order to explain the high photocatalytic activity of our samples, we have to consider the overall photocatalytic process. Photocatalysis is a complex phenomenon that involves several steps in which the adsorption of the radiation and generation of electrons and holes couple are only the first. In order to have high photoactivity, i) migration of the electrons and holes to the surface, ii) trapping of e and h on surface defects, and iii) interaction of trapped electrons and holes with water or contaminants, must be considered

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[54-63]. In order to give a clear picture of the photocatalytic process we describe the holes path, in the

following paragraph, and the electron path, in the next one. These pathways are related each other since the

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overall photocatalytic process needs to discharge the same amount of electrons and holes in solution in order to maintain charge neutrality.

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A high transfer rate of holes to the solution is necessary in order to achieve a high value for the photoactivity. In particular, holes, once created, should be separated from the electrons (in order to avoid

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recombination) and then transferred to the surface, trapped at the surface defects and eventually should

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interact with dye. The presence of trapped states, due to hydrogenated amorphous material close to the

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surface, increases both the absorption of light and the trapping of holes near the surface defects. XPS and

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absorption spectrum show the presence of Ti+4 states and a black amorphous phase. Gopal et al.[64] found defects (lying on the surface of TiO2) that involve Ti+4 states and are able to trap holes. These states are

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related to a distortion of oxygen bounds or a high value of oxygen coordination. In black TiOx, distortion and extra bounds, caused by H introduction, create a dense band of deep localized acceptor states near the surface

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as earlier discussed. However, holes or trapped holes, in order to oxidize the (MB) dye, must have adequate oxidation power and must lie lower (in scheme 2) the redox level of oxidation of MB (i.e. 1.08 eV@SHE). The mechanism for achieving high photoactivity in TiO2 is driven by an efficient electron scavenging

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process. Indeed electrons must be transferred to the liquid medium in order to maintain charge neutrality of the solid materials. In fact, an accumulation of electrons induces a high electron-hole recombination and thus reduces or even stops the transfer of oxidizing holes to the liquid, stopping de facto the photo-catalytical activity. Fuijshima in 2008[4] reported a complex mechanism of “self-organization” (involving incorporation of H) at the surface able to eliminate the excess of negative charge. In TiO2 mechanism of photocatalysis, hydrogen insertion occurs due to the presence of photoelectrons. As illumination proceeds, hydrogen 21

insertion create H rich regions. The interstitial hydrogen acts as a donor and for high concentration of H, these regions act as metals. These regions are called “wire” and are able to transfer electrons into the liquid avoiding the barrier for electrons realised at the interface TiO2 and liquid [4]. In order to maximize the efficiency of the transfer process we developed a monolithic photochemical diode by using the TiOx/Ti/PtNps structure. This structure is able to improve both migration and scavenging

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of electrons. As noted above, electrons must be scavenged efficiently in order to avoid recombination: an accumulation of charges into the semiconductor causes a decrease in the efficiency of the transfer and,

therefore, a decrease in the photoactivity.. Here in order to reduce the accumulation of electrons and enhance

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the activity of the material, we deposited a thin porous film (of some µm) of Pt nanoparticles on the back-

side of the Ti target. We realized a TiOx/Ti/PtNps sandwich recognized as a “monolithic chemical diode”. Platinum nanoparticles favour the scavenging of electrons because of the affinity with oxygen [65] and due

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to the high surface/mass ratio (about 20 m2/g). Indeed, the most important and generally accepted reaction

A

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 for eliminating electrons in titanium oxides is the formation the superoxide radical O2 (ads)  eCB   O2 

[66-70]. This reaction is often considered the rate limiting step (bottleneck) of the photocatalytic process

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Moreover, positioning of the PtNps on the back-side, in contact with Ti substrate, avoids the formation of a

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Schottky barrier between titanium oxide and Pt that should reduce the efficiency of electrons scavenging [71,72]. Electrons photogenerated near the surface of the TiOx film, may diffuse freely into the conduction

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band and are transferred to metal Ti and, finally, to the PtNps layer. There, electrons are scavenged by O2 adsorbed on the Pt following the above-mentioned reaction. Moreover, oxygen gradient allows electrons to

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have a driving force for the diffusion towards Ti. This situation is schematically reported in the Figure 8b.

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3.10 Photo-stability analysis

The photostability of the hydrogenated TiOx material is a key parameter looking forward to a real

application in the environmental remediation field. In this paragraph, we want to describe preliminary photostability tests under UV illumination. Although TiO2 is known to be stable under UV radiation and photocatalysis, hydrogenated TiOx may undergo degradation. So, we performed measurements of the photocatalytic activity after repeated discoloration cycles, at both short and long periods of illumination. In 22

Figure 11a, we report the photodegradation rate k for “1064 nm”, “532 nm LR”, “532 nm HR” and for MB after repeated measurements within the same month. Photocatalytic activity for all the samples decays after 5 days (120 h) of continuous illumination. The decrease of the activity follows an exponential decay in all cases with a decay time of about 1 day. After 5 days of continuous illumination, the activity drop to a negligible value ( k = ~ 0.04 h-1). Such behaviour is substantially similar for all samples: it is independent on

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the laser irradiation wavelength (1064 or 532 nm). It is worth noting that samples are irradiated without interruption for 5 days long. Each day the solution was substituted with a fresh one and the experiment was run. The same measurements were repeated after 19 days by using the same samples. In such time, the

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samples were stored in water. Surprisingly, the photocatalytic activityhas restored. Sample irradiated at 532 nm LR has the main k increase (from 0.04 h-1 to 0.2 h-1) while the samples irradiated at 1064 nm and 532 nm

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HR increase both the degradation value k to about 0.12 h-1. We illuminated the samples continuously for 5 days (from day 19 to day 23) and we observed a decrease of the activity similar to the one observed

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previously. A third series of measurements was performed after 38 days by using the same samples and same

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procedure. Similar results have been found. It is worth noting that values of photocatalytic activity obtained

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in the three series of experiments (day 1, day 19 and day 38) are similar as reported in the Figure 11 b. At

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day 19 and at day 38, the results come after storage in water lasting 14 days. This behaviour is substantially similar for all the samples and it is independent on the laser wavelength used for the synthesis.

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Some possible mechanisms for the inactivation of the TiOx or Pt film may be taken into account: structural alteration of the surface, corrosion or poisoning with the by-product of the degradation adsorbed

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on the surface. Corrosion may be excluded because of the SEM analysis of sample after the illumination shows no signs of decomposition after 120 h of UV illumination. We may exclude structural alteration, otherwise we would not have observed the recovery of the photocatalytic activity. Moreover, this behaviour

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is substantially similar for all the samples and is independent on the laser wavelength used for the synthesis. Thus, we attribute the degradation of the activity (after one week of illumination) to a sort of poisoning effect of the surface while long term activity is insensible of ageing effects. Poisoning of TiO2, realisation of unstable surface defects or even synthesis of H2 bubble on the Pt side of the samples may be the cause of the degradation drop observed experimentally. In order to confirm, this hypotheses and attribute give a more accurate explanation of the results very long term stability tests are under the way. Further analyses are 23

necessary to improve the understanding of the inactivation of photocatalytic activity after prolonged illumination as well as for the recovering of the activity after a period of inactivation. In particular, in view of real application of the proposed technology, the role played by surface defects both in TiO2 and in the PtNps layer should be investigated in the long term time scale.

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4. Conclusion

An extensive characterization of an industrially, compatible and “green” synthesis of hydrogenated TiOx

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was reported. Scalability was demonstrated by using three different lasers at 1064 nm and at 532 nm

wavlengths, at low repetition rate and at high repetition rate. Black TiOx was obtained by irradiating a pure

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Ti foil in all cases and a thin black film of some hundreds of nm was synthesized. Samples (irrespective of the lasers and fluence employed) have a high absorbance (> 85%) in visible and UV range that allows their

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use for sun-driven applications. Irradiated surface is composed by cracks or complex holes depending on the

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wavelength of the laser used for the irradiation. Bulk TiOx is composed mainly by substoichiometric

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amorphous titanium oxide phases with a high concentration of hydrogen (1.6 %) for all laser irradiation.

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Embedded crystalline phases (TiO and Ti2O3) are found in low repetition lasers although amorphous phase is the most abundant. The synthetized material presents Ti+4 states on the surface. These are considered to be

PT

responsible for holes trapping and for the enhancement of photocatalysis. Owing to the presence of the metallic Ti support, a monolithic photochemical diode is realised by depositing a film of Pt nanoparticles

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(synthetized by pulsed laser ablation in water) on the back-side of the samples. Photocatalytic activity tests results in high degradation rate with a quantum efficiency of 0.054 % observed by sample irradiated with 532 nm laser at low repetition rate. Short and long term stability was investigated. The activity decays after

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120 hours of illumination to negligible value for all samples but it recovers after 14 days of storing in water. Implications of this behaviour were proposed.

5. Acknowledgments

24

Authors are grateful to Prof. S. Mirabella for very useful discussions and to Dr. Eric Barbagiovanni for reading and correcting the manuscript. This research has been supported by the FP7 European Project WATER (Grant Agreement n. 316082). The Government of Spain is acknowledged for the Mobility Grant of

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PT

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Senior Professors and Researchers (MAT2015-71459-C2-P, (Grant PRX15/00088)).

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Bibliography [1] G. Cacciato, M. Zimbone, F. Ruffino, M. G. Grimaldi, "TiO2 Nanostructures and Nanocomposites for Sustainable Photocatalytic Water Purification". In Green Nanotechnology, M. Larramendy, S. Soloneski, Eds.; InTech (2016) ISBN 978-953-51-2410-8, Print ISBN 978-953-51-2409-2, Published: June 1, 2016. DOI: 10.5772/62620 [2]S. Malato, M.I. Maldonado, P. Fernández-Ibáñez, I. Oller, I. Polo, R. Sánchez-Moreno. Decontamination and disinfection of water by solar photocatalysis: The pilot plants of the Plataforma Solar de Almeria. Materials Science in Semiconductor Processing 42 (2016) 15-23.

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[3]S. Giannakis, M.I. Polo-López, D. Spuhler, J.A. Sánchez-Pérez, P. Fernández-Ibáñez, C. Pulgarin. Solar disinfection is an augmentable, in situ-generated photo-Fenton reaction - Part 2: A review of the applications for drinking water and wastewater disinfection. Applied Cataysis B: Environmental, 199 (2016) 199-223.

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[4]A. Fujishima, X. Zhangb, D. A. Tryk. TiO2 photocatalysis and related surface phenomena. Surface Science Reports 63 (2008) 515_582

[5] M. A. Henderson A surface science perspective on TiO2 photocatalysis Surface Science Reports 66 (2011) 185–297

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[6] K. Hashimoto, H. Irie, A. Fujishima. TiO2 Photocatalysis: A Historical Overview and Future Prospects Jpn. J. Appl. Phys., Vol. 44, No. 12 (2005)

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[7] S. Hu, M. R. Shaner, J. A. Beardslee, M. Lichterman, B. S. Brunschwig, N. S. Lewis. Amorphous TiO2 coatings stabilize Si, GaAs, and GaP photoanodes for efficient water oxidation. SCIENCE 30 MAY 2014, VOL 344 ISSUE 6187

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[8] J J. Yang, F. Miao, M. D. Pickett, D. A. A. Ohlberg, D. R Stewart, C. N. Lau, R. S. Williams, The mechanism of electroforming of metal oxide memristive switches. Nanotechnology, Volume 20, Number 21 [9] L. Qingjiang, A. Khiat, I. Salaoru, C. Papavassiliou, X. Hui, T. Prodromakisa, Memory Impedance in TiO2 based Metal-Insulator-Metal Devices.Sci Rep. 2014; 4: 4522

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[10] T.M. Apple, P. Gajardo, C. Dybowski. NMR, ESR, and TPD study of hydrogen adsorption on rhodium/titanium dioxide catalyst. Journal of Catalysis (1981), 68 (1), 103-8 CODEN: JCTLA5; ISSN:00219517

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[11] X. Chen, L. Liu, Z. Liu; M. A. Marcus, W.C. Wang, N.A. Oyler, M.E. Grass, B. Mao, P.A. Glans, P. Yu, J. Guo, S.S. Mao. Properties of disorder-engineered black titanium dioxide nanoparticles through hydrogenation. Scientific Reports (2013), 3 (), 1510, 7 pp. CODEN: SRCEC3; ISSN:2045-2322.

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[12]M. Zimbone, M.A. Buccheri, G. Cacciato, R. Sanz,G. Rappazzo, S. Boninelli,R. Reitano, L. Romano, V. Privitera ,M.G. Grimaldi. Photocatalytical and antibacterial activity of TiO2 nanoparticles obtained by laser ablation in water; Applied Catalysis B: Environmental, Vol. 165, pp. 487-494, 2015

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[13]V. Scuderi, G. Impellizzeri, M. Zimbone, R. Sanz, A. Di Mauro, M. A. Buccheria, M. M., A. Terrasi,G. Rappazzo, G. Nicotra, V Privitera. Rapid synthesis of photoactive hydrogenated TiO2nanoplumes. Applied Catalysis B: Environmental 183 (2016) 328–334 [14]M. Zimbone, G.Cacciato, M.A.Buccheri, R.Sanz, N.Piluso, R.Reitano, F.LaVia, M.G. Grimaldi, V.Privitera. Photocatalytical activity of amorphous hydrogenated TiO2 obtained by pulsed laser ablation in liquid. Materials Science in Semiconductor Processing Volume: 42 Special Issue: SI Pages: 28-31 [15]V. Scuderi, G. Impellizzeri, L. Romano, M. Scuderi, M. V. Brundo, K. Bergum, M. Zimbone, R. Sanz, M. A. Buccheri, F. Simone, G. Nicotra, B.G. Svensson, M. G. Grimaldi. An enhanced photocatalytic response of nanometric TiO2 wrapping of Au nanoparticles for eco-friendly water applications. Nanoscale, 2014, 6, 11189 26

[16] X. Chen, L. Liu, P. Y. Yu, S. S. Mao. Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. SCIENCE VOL 331 11 FEBRUARY 2011 DOI: 10.1126/science.1200448 [17] N. Liu, C. Schneider, D. Freitag, M. Hartmann, U. Venkatesan, J. Müller, E. Spiecker, P. Schmuki. Black TiO2 Nanotubes: Cocatalyst-Free Open-Circuit Hydrogen Generation. Nano Lett., 2014, 14 (6), pp 3309–3313. DOI: 10.1021/nl500710j [18] A. Ghicov, H. Tsuchiya, R. Hahn, J. M. Macak, A. G. Munoz, P. Schmuki. TiO2 nanotubes: H+ insertion and strong electrochromic effects. Electrochemistry Communications 8 (2006) 528–532

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[19] Z. Wang, C. Yang, T. Lin, H. Yin, P. Chen, D. Wan, F. Xu, F. Huang, J. Lin, X. Xie, M. Jiang, Visible-light photocatalytic, solar thermal and photoelectrochemical properties of aluminium-reduced black titania. Energy Environ. Sci. 2013, 6, 3007.

SC R

[20] Q. Zhu, Y. Peng, L. Lin, C.-M. Fan, G.-Q. Gao, R.-X. Wang, A.-W. Xu. Stable blue TiO2−x nanoparticles for efficient visible light photocatalysts. J. Mater. Chem. A 2014, 2, 4429.

[21] C. Di Valentin, G. Pacchioni, A. Selloni, Reduced and n-Type Doped TiO2: Nature of Ti3+ Species. J. Phys. Chem. C 2009, 113, 20543.

U

[22] X. Liu, G. Zhu, X. Wang, X. Yuan, T. Lin, F. Huang. Progress in Black Titania: A New Material for Advanced Photocatalysis. Adv. Energy Mater. 2016, 1600452

N

[23] S Eriksen, R. G. Egdell, Electronic excitations at oxygen deficient TiO2(110) surfaces: A study by EELS. Surf. Sci. 1987, 180, 263

M

A

[24] X. Chen1, L. Liu, Z. Liu, M. A. Marcus, W.C. Wang, N. A. Oyler, M. E. Grass, B. Mao, P.A. Glans, P. Y. Yu, J. Guo, S. S. Mao. Properties of Disorder-Engineered Black Titanium Dioxide Nanoparticles through Hydrogenation. Scientific Reports | 3 : 1510 | DOI: 10.1038/srep01510

ED

[25]M.Zimbone, G.Cacciato, R.Sanz, R.Carles, A.Gulino, V.Privitera, M.G. Grimaldi; Black TiOx photocatalyst obtained by laser irradiation in water. Catalysis Communications 84, 11-15 (2016).

PT

[26]M. Zimbone, G. Cacciato, M. Boutinguiza, V. Privitera, M. G. Grimaldi. Laser irradiation in water for the novel, scalable synthesis of black TiOx photocatalyst for environmental remediation. Beilstein J. Nanotechnol. 2017, 8, 196–202. doi:10.3762/bjnano.8.21

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[27]G. Cacciato, M. Zimbone, F. Ruffino, V. Privitera, and M. G. Grimaldi. Morphology study of nanoporous black TiOx synthesized by laser-based method. Phys. Status Solidi B, 1600835 (2017) / DOI 10.1002/pssb.201600835 [28]M. Liu, R. Inde, M. Nishikawa, X. Qiu, D. Atarashi, E. Sakai, Y. Nosaka, K. Hashimoto,M. Miyauchi, Enhanced Photoactivity with Nanocluster-Grafted Titanium Dioxide Photocatalysts. ACS Nano 8 (7) (2014) 7229–7238.

A

[29] Huogen Yu, Hiroshi Irie, Kazuhito Hashimoto. Conduction Band Energy Level Control of Titanium Dioxide: Toward an Efficient Visible-Light-Sensitive Photocatalyst. J. Am. Chem. Soc., 2010, 132 (20), pp 6898–6899 [30] G. Cacciato,M. Bayle, A. Pugliara, C. Bonafos, M. Zimbone, V. Privitera,M.G. Grimaldi, R. Carles. Enhancing carrier generation in TiO2 by a synergistic effect between plasmon resonance in Ag nanoparticles and optical interference. Nanoscale 7 (2015) 13468. [31] M. Liu, X. Qiu, M. Miyauchi, K. Hashimoto, Cu(II) Oxide Amorphous Nanoclusters Grafted Ti3+ SelfDoped TiO2: An Efficient Visible Light Photocatalyst. Chem. Mater. 23 (2011) 5282–5286.

27

[32] X. Qiu, M. Miyauchi, K. Sunada, M. Minoshima, M. Liu, Y. Lu, D. Li, Y. Shimodaira, Y. Hosogi, Y. Kuroda, K. Hashimoto. Hybrid CuxO/TiO2 Nanocomposites As Risk-Reduction Materials in Indoor Environments. ACS Nano, 2012, 6 (2), pp 1609–1618. DOI: 10.1021/nn2045888 [33] A. Di Mauro, M. Zimbone, M. Scuderi, G. Nicotra, M.E. Fragalà, G. Impellizzeri, Effect of Pt Nanoparticles on the Photocatalytic Activity of ZnO Nanofibers. Nanoscale Res. Lett. 10 (2015) 484. [34] ISO:10678 Determination of photocatalytical activity of surfaces in an aqueous medium by degradation methylene blue. [35] D.Adler. Amorphous-Semiconductor Devices. Scientific American 236, 36 - 48 (1977)

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doi:10.1038/scientificamerican0577-36

SC R

[36] P. Raghunath, W. F. Huang, and M. C. Lin Quantum chemical elucidation of the mechanism for hydrogenation of TiO2anatase crystals The Journal of Chemical Phy sics 138, 154705 (2013); doi: http://dx.doi.org/10.1063/1.4799800

[37] L.B. Mo, Y. Wang, Y. Bai, Q.Y. Xiang, Q. Li, W.-Q. Yao, J.-O. Wang, K. Ibrahim, H.-H. Wang, C.-H. Wan, J.-Li Cao. Hydrogen Impurity Defects in Rutile TiO2. Scientific Reports 5, Article number: 17634 (2015) doi:10.1038/srep17634

U

[38]R. Shuker, R.W. Gammon, Raman-Scattering Selection-Rule Breaking and the Density of States in Amorphous Materials Phys. Rev. Lett. 25 (1970) 222–225.

A

N

[39] A.E. Taverner, A. Gulino, R.G. Egdell, T.J. Tate. A Photoemission Study of electron States in Sb-ion Implanted TiO2 (110). Appl. Surf. Sci., 1995, 90, 383-387

M

[40]A. Gulino, A.E. Taverner, S. Warren, P. Harris, R.G. Egdell, A Photoemission Study of Sb-Doped TiO2. Surf. Sci., 1994, 315, 351-361] [41] D. Briggs, J. T. Grant. Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, IMP, Chichester, 2003

ED

[42] A. Gulino, Structural and Electronic Characterization of Self-Assembled Molecular Nanoarchitectures by X-ray Photoelectron Spectroscopy. Anal. Bioanal. Chem. 2013, 405, 1479 –1495.

PT

[43] P. Raghunath, W. F. Huang, and M. C. Lin Quantum chemical elucidation of the mechanism for hydrogenation of TiO2anatase crystals The Journal of Chemical Phy sics 138, 154705 (2013); doi: http://dx.doi.org/10.1063/1.4799800

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[44]A.F. Pan, W.J. Wang∗, X.S. Mei, B.X. Zheng, Z.X. Yan. Cracks growth behaviors of commercial pure titanium undernanosecond laser irradiation for formation of nanostructure-coveredmicrostructures (with sub5 m). Applied Surface Science 387 (2016) 1046–1053. [45] B.K. Nayak, M.C. Gupta, Ultrafast laser-induced self-organized conicalmicro/nano surface structures and their origin, Opt. Laser Eng. 48 (2010)966–973.

A

[46] Y. Li, Z. Cui, W. Wang, C. Lin, H.L. Tsai, Formation of linkednanostructure-textured mound-shaped microstructures on stainless steelsurface via femtosecond laser ablation, Appl. Surf. Sci. 324 (2015) 775– 783. [47] ] V.I. Emel’yanov,Mechanism of laser-induced self organization of Nano and Microstructures of surface relief in air and in liquid environment: G. Yang (Ed.), Laser Ablation in Liquids: Principles and Applications in the Preparation of Nanomaterials, CRC Press 2009, pp.1-102. [48] K. Sasaki, Dynamics of liquid-phase laser ablation, in: G. Yang (Ed.), Laser Ablation in Liquids: Principles and Applications in the Preparation of Nanomaterials, CRC Press 2009, pp. 269–295. 28

[49] S. Ibrahimkutty , P. Wagener, T. dos Santos Rolo, D. Karpov, A. Menzel, T. Baumbach, S. Barcikowski, A.Plech. A hierarchical view on material formation during pulsed-laser synthesis of nanoparticles in liquid. Scientific Reports | 5:16313 | DOI: 10.1038/srep16313 [50] L. Liu, P. P. Yu, X. Chen, S. S. Mao and D. Z. Shen, Hydrogenation and Disorder in Engineered Black TiO2 Phys. Rev. Lett., 2013, 111, 065505 [51] L.B. Mo, Y. Wang, Y. Bai, Q.Y. Xiang, Q. Li, W.-Q. Yao, J.-O. Wang, K. Ibrahim, H.-H. Wang, C.-H. Wan, J.-Li Cao. Hydrogen Impurity Defects in Rutile TiO2. Scientific Reports 5, Article number: 17634 (2015) doi:10.1038/srep17634

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[52] R. Fateh, R. Dillert, D. Bahnemann, Self-Cleaning Properties, Mechanical Stability, and Adhesion Strength of Transparent Photocatalytic TiO2–ZnO Coatings on Polycarbonate. ACS Appl. Mater. Interfaces 6 (2014) 2270–2278.

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[53]A. Houas, H. Lachheb, Mohamed Ksibi, Elimame Elaloui, Chantal Guillard, Jean-Marie Herrmann. Photocatalytic degradation pathway of methylene blue in water. Applied Catalysis B: Environmental Volume 31, Issue 2, 4 May 2001, Pages 145–157 [54] Y. Ma, J. Yao, Comparison of photodegradative rate of rhodamine B assisted by two kinds of TI02 films. Chemosphere 38 (1999) 2407.

U

[55] M. Cho, H. Chung, W. Choi, J. Yoona, Linear correlation between inactivation of E. coli and OH radical concentration in TiO2 photocatalytic disinfection. Water Res. 38 (2004) 1069–1077.

A

N

[56] R. Comparelli, E. Fanizza, M.L. Curri, P.D. Cozzoli, G. Mascolo, R. Passino, A.Agostiano, Photocatalytic degradation of azo dyes by organic-capped anatase TiO2 nanocrystals immobilized onto substrates Appl. Catal. B 55 (2005) 81.

M

[57] H. Haick, Y. Paz, Long-Range Effects of Noble Metals on the Photocatalytic Properties of Titanium Dioxide J. Phys. Chem. B 107 (2003) 2319.

PT

ED

[58]M.-S. Wong, W-C Chu, D-S Sun, H-S Huang, J-H Chen, P-J Tsai, N-T Lin, M-S Yu, S-F Hsu, S-L Wang, H-H Chang, Visible-Light-Induced Bactericidal Activity of a Nitrogen-Doped Titanium Photocatalyst against Human Pathogens Appl. Environ. Microbiol. 72 (2006) 6111.

CC E

[59] Z.X. Lu, L. Zhou, Z.L. Zhang, W.L. Shi, Z.X. Xie, H.Y. Xie, D.W. Pang, P. Shen, Cell Damage Induced by Photocatalysis of TiO2 Thin Films. Langmuir 19 (2003) 8765.

[60] G. Fu, P.S. Vary, C.T. Lin, Anatase TiO2 nanocomposites for antimicrobial coatings, J. Phys. Chem. B 109 (2005) 8889.

A

[61] L.K. Adams, D.Y. Lyon, P.J. Alvarez, Comparative eco-toxicity of nanoscale TiO 2, SiO 2, and ZnO water suspensions. J. Water Res. 40 (2006) 3527. [62] M.V.Brundo, R.Pecoraro, F.Marino, A.Salvaggio, D.Tibullo, S.Saccone, V. Bramanti, M.A. Buccheri, G.Impellizzeri, V.Scuderi, M.Zimbone,V.Privitera; Toxicity evaluation of new engineered nanomaterials in zebrafish. Frontiers in Physiology, section Aquatic Physiology Front Physiol. 2016; 7: 130 (2016 ).

[63] M.V. Berridge, P.M. Herst, A.S. Tan, Tetrazolium dyes as tools in cell biology: New insights into their cellular reduction. Biotechnol. Annu. Rev. 11 (2005)127–152. 29

[64] N.O. Gopal, H.H. Lo, S.C. Sheu, S.-C. Ke. A Potential Site for Trapping Photogenerated Holes on Rutile TiO2 Surface as Revealed by EPR Spectroscopy: An Avenue for Enhancing Photocatalytic Activity. J. Am. Chem. Soc., 2010, 132 (32), pp 10982–10983 [65] C.L. Munich, Y. Zhou, A.M. Holder, A.W.Weimer, C.B.Musgrave, Effect of Surface Deposited Pt on the Photoactivity of TiO2. J. Phys. Chem. C 116 (2012) 10138–10149. [66] B. Lee, W. Liaw, J. Lou, Photocatalytic Decolorization of Methylene Blue in Aqueous TiO2 Suspension. Environ. Eng. Sci. 16 (165) (1999).

IP T

[67] S. Lakshmi, R. Renganathan, S. Fujita, Photobleaching of methylene blue sensitised by TiO2: an ambiguous system?. J. Photochem. Photobiol. A: Chem. 127(1999) 123. [68] X. Li, G. Liu, J. Zhao, Two competitive primary processes in the photodegradation of cationic triarylmethane dyes under visible irradiation in TiO2 dispersions. New J. Chem. 23 (1999) 1193.

SC R

[69] R. Sanz, L.Romano, M. Zimbone, M. A. Buccheri, V. Scuderi, G. Impellizzeri,M. Scuderi, G. Nicotra, J. Jensen, V. Privitera. UV-black rutile TiO2: An antireflective photocatalytic nanostructure. Journal of Applied Physics 117, 074903 (2015); [70] T. Tatsuma, S. Tachibana, T. Miwa, D.A. Trky, A. Fujishima, Remote Bleaching of Methylene Blue by UV-Irradiated TiO2 in the Gas Phase. J. Phys. Chem. B 103(1999) 8033.

U

[71] G.A. Somorjai, K.M. Bratlie, M.O. Montano, J.Y. Park, Dynamics of Surface Catalyzed Reactions; the Roles of Surface Defects, Surface Diffusion, and Hot Electrons. J. Phys. Chem. B 110 (2006) 20014–20022.

A

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PT

ED

M

A

N

[72] J. Young Park, J.R. Renzas, A.M. Contreras, G.A. Somorjai, The genesis and importance of oxide– metal interface controlled heterogeneous catalysis; the catalytic nanodiode. Top. Catal. 46 (2007) 217–222.

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Figure caption

Figure 1: Schematic representation of the sample preparation steps: TOP: laser irradiation of the Ti foil in water. Schematic and photo of the sample after irradiation (black TiOx); DOWN: synthesis of Pt nanoparticles via laser ablation in water and photo of the Pt Nps colloidal solution; RIGHT: schematic

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representation of the sample used for photocatalytic activitytest: black TiOx/Ti/PtNps sample. Pt nanoparticles are deposited in the back side of the irradiated Ti foil.

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Figure 2: XRD spectra of the TiOx/Ti film and the Ti foil (before irradiation). The main

characteristic peaks at 35°, 38°, 40°, and 53° are related to metallic Ti substrate. The main feature of the

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black TiOx sample relies on the pronounced amorphous signal observed at low scattering angles. Peaks at 37.25° and 43.29° are ascribed to (1,1,1) and (2,0,0) reflection of TiO and peaks at 33.06° and 53.91° belong

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to (1,0,4) and (1,1,6) planes of Ti2O3 phase.

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Figure 3: Al Kα excited XPS of the black-TiOx sample. a) refers to the Ti 2p and b) refers to O 1s

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and c) refers to C 1s binding energy region. The black line refers to the experimental TiOx profile and red line refers to TiO2 sheet taken as a reference. In Figure b the green, dark cyan and magenta line refers to the

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530.5, 532.1 and 533.5 eV component, blue line refers to the background and red line superimposed to the

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experimental profile refers to the sum of all Gaussian components.

Figure 4: RBS spectra of the TiOx/Ti film. Green , violet and yellow curves are obtained by 1064

nm, 532 nm LR and 532 nm HR samples respectively. Simulations of the spectra are also reported. Right

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upper side: a schematic (not in scale) representation of the irradiated sample is reported.

Figure 5: SEM image of the black surface after laser irradiation by using the a) 1064 nm, b) 532 nm LR (low repetition) rate and c) 532 nm (HR) high repetition rate laser. Film obtained irradiating at 1064 nm is constituted by caves with a complex branched structure while in both the 532 nm lasers irradiated samples

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the surface is constituted by plates separated by cracks. Pits and rounded structures resembling “under-skin” bubbles are also apparent into the cracks and in the plates respectively.

Figure 6: SEM image of the black surface after laser irradiation by using 532 nm high repetition rate laser. Scan was performed at three different speeds (5, 50 and 100 mm/s). In the left and right side, low and

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high magnification SEM images are shown. Low magnification image obtained at low scan speed (5 mm/s) shows depressions of 10 μm in diameter and 12.5 μm spaced; these features disappear using 50 and 100

mm/s scanning speed. The smoother surface is attributable to the 100 mm/s sample. High magnification

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images (right side) showed very similar morphology for all the scanning speed employed in these

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experiments.

Figure 7: Absorbance spectra of the TiOx/Ti film in the IR, Vis and UV range for 1064 nm 532 nm

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high and low repetition rate laser. Horizontal and vertical dashed lines are drawn as guides for eyes. The

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horizontal line represents the 85% in absorbance and the vertical line is related to 0.35 eV as photon energy.

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In the same graph it is reported (right scale) the solar irradiation spectrum.

Figure 8: Schematic representation of the band structure of the TiOx/Ti/PtNps. a) schematic of the levels of the TiOx. Solid lines represent the conduction band (formed mainly by Ti+4 states) and valence band

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(mainly formed by O-2 states). Conduction band lowering and valence band rising due to the amorphous

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structure of TiOx are evidenced by dashed line. b) Schematic of the photocatalysis process. An upward bend bending of the conduction band near the surface of the TiOx layer may cause a diffusion of the photoelectrons through the Ti bulk to the PtNps layer. Photo-electrons may reduce the oxygen to superoxide in the

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PtNps side while the photo-holes oxidize the MB dye. Fermi level and redox potential of MB+/MB and O2/O2-*are drawn in the same graph.

Figure 9: Discoloration rate under visible light illumination for 1064 nm (@1064, red), 532 nm LR (@532LR, green) and “532 nm HR” (@532HR,blue) sample. The discoloration of MB solution with reference TiO2 (black) and without photo catalyst (gray) is also reported. 32

Figure 10: a) Discoloration rate under UV illumination in presence of photocatalyst: 1064 nm (@1064, red), 532 nm LR (@532LR, green) and “532 nm HR” (@532HR,blue). The discoloration of a MB solution without photo catalyst (black) is also reported for comparison purpose as a horizontal dash line. b) Percentage of removed organic substances obtained by TOC analysis in the MB solutions after 240 min

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under UV light in the presence of the three samples.

Figure 11: Long term stability of the MB discolouration rate. The discolouration rate obtained with

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sample irradiated at 1064 nm (red), 532 nm low repetition rate (green), 532 nm high repetition rate(blue) and the discoloration rate of a MB solution without photocatalyst (black) is reported as a function of the

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irradiation time. Figure shown a break in the irradiation of 14 days in which sample is stored in water

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without UV irradiation. A subsequent irradiation for successive 5 day performed.

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