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TiO2 Nanotubes Arrays Loaded with Ligand-free Au Nanoparticles: Enhancement in Photocatalytic Activity Marcello Marelli, Claudio Evangelisti, Maria Vittoria Diamanti, Vladimiro Dal Santo, Mariapia Pedeferri, Claudia Letizia Bianchi, Luca Schiavi, and Alberto Strini ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11436 • Publication Date (Web): 21 Oct 2016 Downloaded from http://pubs.acs.org on October 23, 2016

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ACS Applied Materials & Interfaces

TiO2 Nanotubes Arrays Loaded with Ligand-free Au Nanoparticles: Enhancement in Photocatalytic Activity

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Marcello Marelli†, Claudio Evangelisti†*, Maria Vittoria Diamanti‡, Vladimiro Dal Santo†, Maria

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Pia Pedeferri‡, Claudia L. Bianchi∫, Luca Schiavi§ and Alberto Strini§

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†

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‡ Department of Chemistry, Materials and Chemical Engineering ‘Giulio Natta’, Politecnico di Milano,Via Mancinelli

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7, 20131 Milan, Italy

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∫

Istituto di Scienze e Tecnologie Molecolari (ISTM-CNR), via Golgi, 19, 20133 Milano, Italy

Dipartimento di Chimica, Università di Milano, Via Golgi 19 — 20133 Milano, Italy

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§

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Italy

Istituto per le Tecnologie della Costruzione (ITC-CNR), via Lombardia, 49, I-20098 San Giuliano Milanese (MI),

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*Corresponding author: Dr. Claudio Evangelisti; Istituto di Scienze e Tecnologie Molecolari (ISTM-CNR), Via C.

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Golgi 19, 20133 Milano (ITALY)- phone: +390250995623

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email: [email protected]

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Keywords: titanium dioxide, nanotubes arrays, anatase, electrochemical anodization, Au

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nanoparticles, metal vapor synthesis, photocatalysis.

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Abstract

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A new protocol to synthesize size-controlled Au nanoparticles (NPs) loaded onto vertically aligned

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anatase TiO2 nanotubes arrays (TNTAs) prepared by electrochemical anodization is reported.

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Ligand-free Au NPs (< 10 nm) were deposited onto anatase TNTAs supports, finely tuning the Au

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loading by controlling the immersion time of the support into metal vapor synthesis (MVS)-derived

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Au-acetone solutions. The Au/TNTAs composites were characterized by electron microscopies

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(SEM, (S)TEM), X-ray diffraction, X-ray photoelectron spectroscopy and UV-Vis spectroscopy.

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Their photocatalytic efficiency was evaluated in toluene degradation in air at ambient conditions

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without thermal or chemical post-synthetic treatments. The role of Au loadings was pointed out,

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obtaining a three times enhancement of the pristine anatase TNTAs activity with the best sample

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containing 3.3 µg Au cm–2.

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ACS Paragon Plus Environment


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1. Introduction

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Titanium dioxide (TiO2) is the most diffused among photocatalytic material thanks to its photo-

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reactivity (band-gap of 3.2 eV and 3.0 eV for the anatase and rutile phase, respectively), high

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stability, non-toxicity, and availability.1,2 Under UV light nanostructured TiO2 is able to promote a

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wide range of reactions such as hydrogen production by water splitting,3 electricity production in

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dye sensitized solar cells,4 CO2 reduction5. Moreover, its capability to degrade organic pollutants

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such as VOCs6 (benzene, toluene, organic chlorides) and inorganic pollutants7 (NOx, SOx, NH3,

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and CO) finds interesting applications for indoor and outdoor air purification8-10. TiO2

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nanocomposites both in form of powder or coating/film have been extensively investigated as

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photoactive antimicrobial materials against microorganisms such as algae, viruses fungi and

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bacteria.11,12 However, the extremely low photocatalytic efficiency of conventional nanostructured

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TiO2 powder (quantum yields < 1 %)13 involves the requirement of a large amount of material, the

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catalyst recycling is difficult and aggregation into larger and less active particles can occur.

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Recently, in order to overcome these drawbacks, intense efforts have been focused on the

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modification of the electronic properties of nanostructured TiO2-based materials by different

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approaches, such as metal nanoparticles deposition, doping with metal and non-metal ions or

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coupling with other semiconductors.14 Among them, the loading of TiO2-based materials with Au

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nanoparticles (NPs) has been extensively investigated by several research groups.15-17 The hetero-

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junction between TiO2 surface and Au NPs leads to a rapid interfacial photo-generated electron

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transfer from TiO2 to Au NPs (Schottky barrier) increasing the separation of photogenerated e-/h+

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pairs, reducing recombination probability and increasing therefore the photocatalytic activity.18,19

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The amplitude of this effect is strongly related to the particle size, since mainly the small ones (< 10

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nm) result in higher efficiency, as well as to the Au loading.20,21 Other mechanisms as gold surface

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plasmon resonance are often invoked to explain the enhanced photoactivity of these materials under

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visible light irradiation.22-25 Different approaches have been reported for the synthesis of Au NPs

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decorated TiO2, including conventional impregnation,26 deposition-precipitation,26,27 chemical

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reduction,28 and photodeposition23,29.

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Besides, in the last years, one-dimensional nanostructured TiO2 such as nanotubes arrays (TNTAs)

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have attracted an increasing attention in (photo)catalysis due to their unique physico-chemical and

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structural properties.30-31 TNTAs show a stable large surface-to-volume ratio (> 300 m2 g–1) with no

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risk of aggregation, high sedimentation rate as well as excellent adhesion and electrical contact with

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the metallic substrate from which they are originated.32-33 Moreover, they are expected to have

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better photogenerated charge separation when compared to TiO2 NPs due to the improved electron

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transportation along the 1D channels and the decrement of inter-crystalline contacts.35


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The possibility to obtain TNTAs from the anodic oxidation of metallic titanium foils paves also the

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way to the direct realization of supported photocatalysts35-37 without the drawbacks related to the

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sintering process of photoactive titania powders38.

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In order to combine the advantages of TNTAs systems with an active Au NPs decoration,

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conventional deposition methods are generally not effective due to fast formation of large metal

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aggregates, crystalline phase-changes or the required application of complex procedures which need

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careful cleaning steps and/or post-thermal treatments. Recently, advanced methodologies enabling

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the deposition of size-controlled Au NPs homogeneously dispersed onto geometrically ordered

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TNTAs have been reported.34,35 Paramasivam et al. decorated TNTAs with Au NPs by sputtering

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technique followed by post-annealing at high temperature (450°C) to ensure the Au NPs adhesion.

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The synthesis afforded Au NPs with diameters centered at 28 nm but post-thermal treatment led to

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the formation less active rutile phase.39 Previously, Barreca et al. developed an original

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plasma/liquid phase hybrid approach based on the RF-sputtering of Au on porous titania xerogels

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obtained by the sol-gel route. The role of post-thermal treatments as well as the Au content on the

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Au particle size distribution was highlighted: post-annealing at high temperature (> 400 °C) leading

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to TiO2 anatase phase, induced the coalescence of Au agglomerates till 15 nm in diameter.12,40 Wu

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et al. adopted a pulse electrodeposition technique to deposit Au NPs with size ranging from 8 to 40

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nm onto TNTAs electrodes.35 Au NPs size was controlled by adjusting electrochemical parameters;

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however, a broadening of size distribution rather than an increase of particles number was observed

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by increasing the metal loading. Recently, Xiao et al. reported an innovative approach to the

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synthesis of Au NPs by solar light irradiation of metal cluster decorated TNTAs supports, obtaining

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binary hybrid nanocomposites with a mean Au particle size of 13 nm.37,41

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Herein we propose a simple and scalable synthetic protocol able to load Au NPs onto vertically

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aligned anatase TNTAs in mild reaction conditions (25°C). Au NPs were prepared by the metal

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vapor synthesis technique (MVS).42,43 The versatility and feasibility of this synthetic pathway in

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depositing highly dispersed metal particles onto a wide range of inorganic and polymeric catalytic

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membranes have been previously proven.44 Taking advantage of such results, the present work is

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devoted to investigate the deposition of MVS-derived ligand-free Au NPs with controlled size (< 10

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nm) onto anatase TNTAs supports leading to novel Au/TNTAs nanocomposites featuring properties

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hardly attainable by conventional synthetic routes. Au NPs in acetone solution (Au solvated metal

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atoms, SMA) were loaded onto the TNTAs surface by simple dipping the TNTAs coated samples

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directly in the Au SMA at room temperature avoiding post-annealing treatments. The Au NPs

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loading onto the TNTAs surface was easily controlled by changing the dipping time of the support

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without significant changes of Au NPs size distributions.



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The photocatalytic activity of these systems was assessed measuring the degradation of toluene in

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air at ppb level and in typical ambient condition. Toluene can be easily found both in indoor and

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outdoor polluted air and was hence selected as aromatic ambient pollutant model.45,46

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2. Experimental

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TNTAs were produced by anodizing substrates of commercial purity titanium, grade 2 following

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ASTM classification.47 Titanium sheets of approximately 10 cm x 10 cm area, 0.5 mm thickness

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were cleaned by degreasing with acetone, then immersed in a solution of 0.5 wt.% NaF/1M Na2SO4

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and connected to an activated titanium counter electrode. Anodizing was performed by applying 20

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V with a voltage ramp of 1 V s-1 and maintaining the chosen voltage for 6 h. After the treatment,

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samples were rinsed with deionized water to remove salt deposits from the electrolyte and thermally

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annealed in air at 400°C for 2 h: annealing was necessary to crystallize the obtained amorphous

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TiO2.36,48 Finally, samples were cut to the desired size of 2.5 cm x 2.5 cm. BET measurements were

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performed by Kr physisorption at 77 K (ASAP 2020, Micrometitics). X-ray diffraction (XRD)

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measurements (Philips PW 3710-Cu Kα radiation) at room temperature were used to determine the

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crystal structure of the oxide.

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Au NPs were synthesized by the MVS technique (Electronic Supplementary Information, ESI Fig.

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S1) following a previously reported procedure.43 Au vapors generated at 10-4 mBar by resistive

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heating of a alumina crucible filled with ca. 500 mg of gold pellets, was co-condensed at liquid

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nitrogen temperature (-196°C) with acetone (100 ml) in the glass reactor chamber of the MVS

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apparatus in ca. 40 min. The reactor chamber was heated to the melting point of the solid matrix and

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the resulting deep purple solution was siphoned at low temperature in a Schlenk tube and kept in a

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refrigerator at -20 °C. The Au-content in SMA solution was 5.0 mg mL–1, as determined by ICP-

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OES (Thermo Scientific ICAP6300 Duo) analysis. For this work, 25 mL of Au SMA at 0.9 mg mL–

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1

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UV−vis diffuse reflectance spectra (DRS) were collected by an Avaspec 2048-L spectrometer

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(Avantes) equipped with a Deuterium-Halogen Light Source (AvaLight DHS) and a 30 mm

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diameter integrating sphere (Avasphere 30 REFL). A Diffuse PTFE material (Avantes WS-2) was

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used as reference tile. The X-ray photoelectron spectroscopy (XPS) characterizations were carried

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out by a M-probe system (SSI - Surface Science Instruments); C1s was taken as internal reference

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for energy.

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Scanning electron microscopy (SEM) characterization were performed at 15 kV in high vacuum

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mode with a PHILIPS XL30 ESEME-FEG. SEM-EDX elemental analysis (Energy Dispersive X-

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ray spectroscopy) were carried out by a EDAX Sirion 200/400 probe. EDS data were collected on a

were obtained by dilution from the pristine SMA solution with distilled acetone.


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5 µm2 active area. Scanning transmission electron microscopy (STEM) and high resolution

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transmission electron microscopy (HRTEM) measurements were collected by using a LIBRA

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200FE ZEISS at 200 kV equipped with a high angle annular dark field detector (HAADF). Sample

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were collected scratching the surface with a sharp scalpel and collecting the fragment onto a holey

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carbon supported Cu GRID by simple adherence.49 Elemental quantitative analysis was performed

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by ICP-OES with external calibration, after complete Au dissolution in 2 mL of aqua regia solution

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at room temperature. The back side of each sample was covered by a protective polymeric acid-

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resistant layer: in this way only the gold on the front side (the active and tested side) was digested.

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The Au NPs were loaded onto the TiO2 surface by dipping the TNTAs coated samples directly in

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the Au-acetone SMA at room temperature under Ar inert atmosphere. The reactor was a

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conventional Schlenk glass tube with proper size to place upright the sample inside. In order to

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complete dip the sample slide, we used 25 mL of SMA solution (0.9 mg Au mL–1). Different Au-

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TNTAs samples were prepared by varying the dipping time (2, 10, 120, 240, 480, 1200 minutes,

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respectively). After the deposition, the samples were washed by a rinsing cycle (deionized water-

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acetone-isopropanol-deionized water in sequence) and dried in air at room temperature. A

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schematic representation of the overall deposition procedure is reported Fig 1 and S1. The

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photocatalytic activity was assessed measuring the toluene degradation with a previously described

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experimental system45 based on a continuous-flow stirred photoreactor. The system was equipped

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with six fluorescent lamps (PL-S/BLB, Philips) as UV-A source resulting in 605 ± 20 µW cm–2

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irradiance (340-400 nm range) and it was operated at constant toluene concentration (750 ± 50 nmol

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m–3) allowing a direct comparison of the obtained reaction rates. All the measurements were carried

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out at 25.0 ± 0.2 °C and 50 ± 2 R.H. (errors as 1 σ repeatability). The reaction rate per surface area

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(as mass of degraded pollutant per unit of catalytic surface and time) was calculated with the

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following equation:

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r=

Q (C − C ) A 0

(1)

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where r is the reaction rate per surface area (mol m–2 s–1), Q is the volumetric flow rate (m3 s–1), A is

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the area of the titania sample surface (m2), C and C0 are the reactor internal concentration of the

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reacting specie (mol m–3) with and without irradiation respectively. The repeatability of reaction

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rate r was calculated as error propagation in the (1) assuming ± 2 % error in the concentration C0

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and C, ± 1 % error in the supply air volumetric flow Q and ± 5 % error in the sample layer surface

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A. The measurement of system zero (i.e. the result of a measurement process without sample) gives

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the same C and C0 concentrations within the analytical system repeatability error. Further details are

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reported in the SI. All samples were catalytically tested before and after the Au NPs loading.



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3. Result and Discussion

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In order to synthesize separately the TNTAs supports and the Au NPs we used two well-established

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and supported methodologies (i.e. electrochemical-anodization and MVS respectively, Fig. S1).

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TNTAs obtained by metallic titanium foil anodization offer the benefit of growing an oxide layer

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well anchored to the metallic substrate. Scanning electron microscopy (SEM) of the bare TNTAs

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showed their nanotubular features containing self-organized nanotubes on the surface clearly

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separated from one another with inner mean diameter of 50 nm (Fig. S1d). The film thickness,

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measured from cross section SEM image, (Fig. S1e) was estimated around 1 µm. BET

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measurements highlight a growth of 95 square meter of exposed surface area every square meter of

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geometrical slide area. After annealing, only TiO2-anatase phase structure was detected by XRD

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analyses (Fig. S4). Each sample was checked for anatase purity before Au deposition in the angular

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range 20° 10 µg cm–2) a pronounced deactivation was observed.

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Fig. 6. -Toluene degradation rates: a) absolute reaction rate for each sample before and after Au NPs deposition; b) relative reaction rate for each sample after Au NPs deposition vs. Au loading. Dashed line indicates no effect (1:1 ratio). Error bars as 1 σ estimated repeatability error.

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Although several research groups studied the effect of Au NPs on the photocatalytic activity of

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TiO2-based materials, only few works deal with the use of TNTAs loaded with Au NPs in photo-

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degradation processes, particularly of airborne pollutants at typical ambient conditions. The effect

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of Au and Ag NPs on TNTAs was reported by Paramasivam et al..39 confirming that the enhancing

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effect of metal NPs in water-based oxidative processes can also be exploited in case of highly

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organized titania nanostructures. Huang et al.34 also studied TNTAs decorated with Au in the

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degradation of Rhodamine B. They found a maximum activity with a 0.68 wt. % Au loading, using

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visible-light irradiation. The degradation of benzene at high concentration in air (0.7-3.0 mol. %)

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with titania nanotubes modified by Au nanoparticles was studied by Awate et al.60 confirming the

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activity enhancing effect of Au at low loadings (~ 1 wt. %) also in case of unsupported titania

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nanotubes.

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The present study demonstrates the enhancement effect of Au NPs deposited on highly ordered

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supported nanotubes from a ligand-free Au-acetone SMA solution in the degradation of toluene at

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ambient concentration (i.e. in the ppb range). According to commonly accepted radical degradation

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mechanism (Fig. 7), the photocatalytic enhancement effect of the deposited Au NPs can be ascribed

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to the formation of a Schottky barrier at Au/TiO2 interface. This interaction slows down the ACS Paragon Plus Environment


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recombination of the photogenerated electron/hole pairs by separating the electron (that is

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preferentially transferred to the metallic nanoparticle) from the hole.16,18,19,61

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The reduction of the competitive recombination reaction results in enhancement of the redox

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reactions with the chemical species available at the catalyst surface. Finally, the positive hole

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photogenerated plays crucial role in degradation mechanism: highly reactive titanoxyl (TiO·) and

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hydroxyl (OH·) radicals are formed from the interaction with surface titanol groups and adsorbed

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water, respectively. The alternative electron injection from the Au nanoparticle to the TiO2

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conduction band is sometime invoked

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plasmon band (~ 550 nm), but in the present study the use of UV-A radiation (365 nm) rules out

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16,24,61,62

when is operating the excitation of the Au surface

this mechanism.

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Fig. 7. Proposed radical degradation mechanism on Au/TNTAs systems

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The three-time factor activity enhancement with the optimal Au loading is comparable in magnitude

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to the results reported in literature with different Au-decorated catalytic systems (e.g. P25, titania

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layers or titania unsupported nanotubes) or with very different reaction conditions (e.g. degradation

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of pollutants in water or high concentrated pollutants in air).21,63,64 The data available in literature do

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not allow a direct comparison of the absolute activity values in the different cases. It is nevertheless

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worth noting that the relative activity gain at the optimal Au loading found in the reported studies is

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comparable in magnitude despite the very different conditions used, supporting the existence of a

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common mechanism. Moreover, the enhancement factor obtained in this work with the deposition

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from ligand-free Au SMA solution is better than those reported in literature for gas-solid oxidative

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degradation processes and is equivalent to the best ones found for water-based systems.

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In the present study it was found a maximum activity enhancement for the 3.3 µg cm–2 Au loaded

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sample and a net activity degradation (< 25% of the pristine sample) for the higher loaded sample

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(11.5 µg cm–2). This is in clear agreement with previously reported studies63,64 indicating that the

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Au loading is a critical parameter for system efficiency. At relatively high loadings Au

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nanoparticles could mask the active sites of titania surface (i.e. titanols and hydroxyl groups),


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reducing the available toluene adsorption and light absorption, thus leading to an overall

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deactivation.61,64 Finally, the role of a post-deposition mild thermal treatment was investigated

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heating up sample S2 at 120°C for 30 minutes. A marked shift in the Au NPs size distribution,

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centred now around a median value of 13 nm (only the 17 % of the NPs are < 10 nm in size), was

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observed leading to a decrease in the formal particles number (Fig. S10a). On the other hand, no

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morphological changes were detected for the TNTAs support. Interestingly, the activity dropped

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down slightly above the basal value (Fig. S10b) pointing out the role of Au NPs size and particle

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surface numerical density on the photocatalytic behaviour of the Au/TNTAs composites.

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These results confirm that the loading and the size of Au NPs are critical for the optimization of the

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photocatalytic activity of TNTAs and that they must be finely tuned in order to maximize the

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photocatalytic system efficiency.

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4. Conclusions

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Size-controlled Au NPs synthesized by metal vapor synthesis technique were successfully loaded

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onto vertically aligned anatase TNTAs at room temperature, avoiding further thermal treatments

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which could induce Au NPs aggregation as well as crystal and morphological TNTAs

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modifications.

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The metal loading NPs was finely tuned by controlling the dipping time of the TNTAs support into

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the Au SMA solution. SEM, STEM, HRTEM and XPS analysis evidenced the presence of metallic

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Au NPs with a mean size less than 10 nm highly dispersed on the TNTAs support and with a strong

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particles/support interaction. The photocatalytic activity of the Au/TNTAs composites was

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evaluated in toluene degradation in air at ambient conditions highlighting the crucial role of the Au

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loading on their photocatalytic performances (three times enhancement of the pristine TNTAs

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activity was obtained with the Au/TNTAs containing 3.3 µg cm–2 of Au).

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The proposed protocol could provide a new approach for the deposition of size-controlled Au NPs

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highly dispersed onto planar supports, also on large scale, enabling the design of innovative

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composites for catalytic and photocatalytic applications. Moreover, the method here reported can be

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conveniently extended to the preparation of a wide range mono- and bimetallic nanoparticles (Cu,

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Ag, Fe, Pd, Au-Cu, Pd-Cu, etc.), pointing out the usefulness of solvated metal atoms as starting

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material to obtain nanocomposite films.

33 34 35

Supporting Information



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The Supporting Information is available free of charge on the ACS Publications website

2

at DOI: XXXXX

3

Experimental Procedure; Catalytic Tests; XRD analysis; SEM, SEM-EDX data; STEM with related

4

Au NPs size distribution; XPS survey spectra, UV−Vis diffuse reflectance spectra and Au NPs size

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distribution before and after thermal treatment with related photoactivity.

6 7 8

Acknowledgement

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MM and CE thanks the Italian Ministry of University and Scientific Research (MIUR) under the

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FIRB 2010 program (RBFR10BF5V) for the financial support. LS and AS gratefully acknowledges

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financial support from Regione Lombardia through the project "INTEGRATE, Technological

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innovations for a rational management of the built environment" Accordo Quadro Regione

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Lombardia-CNR 2013-2015.

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Reference

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Plasmon-Enhanced

Heterogeneousm

Catalysis.

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Nanophotonics

2016,

doi:


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Table of Contents Graphic

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TiO2 Nanotubes Arrays Loaded with Ligand-free Au Nanoparticles: Enhancement in Photocatalytic Activity

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Marcello Marelli, Claudio Evangelisti*, Maria Vittoria Diamanti, Vladimiro Dal Santo,

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Maria Pia Pedeferri, Claudia L. Bianchi, Luca Schiavi and Alberto Strini

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