polyhydroxyfullerene composites for formic acid photodegradation

Accepted Manuscript Title: Titanium Dioxide Nanotubes/Polyhydroxyfullerene Composites for Formic Acid Photodegradation Authors: Marwa Hamandi, Gilles Berhault, Frederic Dappozze, Chantal Guillard, Hafedh Kochkar PII: DOI: Reference:

S0169-4332(17)30960-1 http://dx.doi.org/doi:10.1016/j.apsusc.2017.03.261 APSUSC 35637

To appear in:

APSUSC

Received date: Revised date: Accepted date:

22-12-2016 15-3-2017 29-3-2017

Please cite this article as: Marwa Hamandi, Gilles Berhault, Frederic Dappozze, Chantal Guillard, Hafedh Kochkar, Titanium Dioxide Nanotubes/Polyhydroxyfullerene Composites for Formic Acid Photodegradation, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.03.261 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.

Titanium Dioxide Nanotubes / Polyhydroxyfullerene Composites for Formic Acid Photodegradation

Marwa Hamandia,, Gilles Berhault b,*, Frederic Dappozzeb, Chantal Guillardb, Hafedh Kochkara,c*

a

Université de Tunis El Manar, Faculté des Sciences de Tunis, Laboratoire de Chimie

des Matériaux et Catalyse, 2092, Tunis, Tunisia b

Institut de Recherches sur la Catalyse et l’Environnement de Lyon, IRCELYON,

CNRS, University of Lyon I, Villeurbanne 69100, France c

Laboratoire de Valorisation des Matériaux Utiles, Centre National de Recherches en

Sciences des Matériaux (CNRSM), Technopôle Borj-Cédria, 8027 Soliman, Tunisia

All correspondence should be sent to:

Dr. Hafedh Kochkar Ph:+216 22 96 04 26 Fax: +216 79 32 53 14. E-mail: [email protected] (H. Kochkar).

or to : Dr. Gilles Berhault Ph: +33 472 44 54 43 Fax: +33 472 44 53 99 E-mail: [email protected] (G. Berhault)

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Graphical Abstract

Highlights 

Polyhydroxyfullerene (PHF) decorating TiO2 nanostructured materials



PHF helps to maintain surface oxygen vacancies at the TiO2 surface



PHF improves the faradic current across the semiconductor interface



Higher photocatalytic activity is achieved for monolayer PHF onto TiO2 nanotubes.

2

Abstract The influence of polyhydroxyfullerene (PHF) on the photocatalytic properties of calcined hydrogenotitanate nanotubes (HNT) were evaluated in the present study. PHFHNT

nanocomposites

were

first

characterized

by

N2

adsorption-desorption

measurements, X-ray diffraction, X-ray photoelectron, electron paramagnetic resonance and UV-vis diffuse reflectance spectroscopies, transmission electron microscopy, photoluminescence, and photocurrent experiments. Correlation was then established with the photocatalytic properties of PHF-HNT nanocomposites during the photodegradation of formic acid. After characterization of the functionalization step of fullerene, the best efficient protocol of incorporation of PHF to HNT solids was first determined. Results showed that incorporation of PHF to HNT before calcination at 400°C is detrimental to photocatalytic activity. On the opposite, a highly photocatalytic activity was observed if PHF is incorporated to already calcined HNT followed by a second post-thermal treatment at 400°C. In a second step, the effect of the PHF loading was demonstrated showing an optimum in photocatalytic activity for 1.0 wt% PHF loading while increasing further the polyhydroxyfullerene amount becomes harmful to activity due to a shielding effect resulting from the formation of PHF multilayers.

Keywords: photocatalysis; polyhydroxyfullerene; nanocomposites; formic acid; adsorption 1. Introduction

Fullerenes have attracted an enormous interest since their discovery by Kroto and coworkers [1] due to their exceptional structural [2], magnetic [3-5], superconducting [68], electrochemical [9, 10], and excited state [11] properties. Carbon nanostructured fullerenes have also been also explored in biological applications [12-15] and photocatalysis [16, 17]. When TiO2 is irradiated with UV light, electron-hole pairs are generated. Holes can react with water and surface hydroxyl ions to produce hydroxyl radicals. At the same time, electrons can react with adsorbed molecular oxygen yielding superoxide anion

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radicals. These radicals are excellent oxidation agents for pollutant degradation. However, the high rate of recombination between photogenerated electrons and holes on TiO2 remains as a major limitation for efficiently photodegrading pollutants. As C60 exhibits high electron affinity, it is possible to enhance the photocatalytic activity of TiO2 if C60 is introduced into a TiO2 photocatalytic system. Some works on the photocatalytic activity of C60/TiO2 composites have already been reported. Apostolopoulou et al. [18] reported a simple route using successive incipient wetness impregnations followed by a heat treatment at 180 °C to form highly dispersed C60 nanoparticles on titania. Bai et al. [19] and Krishna et al. [20, 21] demonstrated that nanocomposites of polyhydroxyfullerene (PHF) and TiO2 can be spontaneously formed through self-assembly when the two components are mixed. Previous works have evaluated the photocatalytic activity of C60-TiO2 nanoparticles but only for the photodegradation of dyes such as methylene blue [22, 23], methyl orange [24] and procion red [21]. Many works combined C60 with polymers and applied these systems for photochemical solar cells to increase the photoconversion of solar energy [25, 26]. In this way, C60 was found to act as an excellent electron acceptor [27, 28]. The photochemical behavior of C60 adsorbed on TiO2 particles has also been investigated using diffuse reflectance laser flash photolysis. At submonolayer coverages, irreversible oxidation of C60 is observed on titanium dioxide particles [29]. Moreover, the lack of active sites on the fullerene cages restricts the direct use of fullerene in catalysis. There are only a few studies using fullerenes as a component in photo(electro)catalysts [30, 31]. Moreover, fullerene cages are highly hydrophobic and insoluble in water and in many other polar solvents [32] limiting their direct application especially in aqueous systems. Therefore, several approaches have been explored in order to introduce fullerenes in an aqueous or polar environment through functionalization with hydroxyl groups to form polyhydroxylated fullerene, i.e. so-called fullerenols (PHF) [12, 33-35]. PHF was first synthesized using nitronium chemistry [34] or aqueous acid [35, 36] or basic reactions [37]. PHF has a simple structure and low toxicity and is therefore convenient for practical use in photocatalytic systems. PHF is thus considered as the most promising water-soluble fullerene derivative [38-42]. Besides the hydroxyl groups, PHF can also include some other non-hydroxylic groups, such as terminal or bridged oxygen groups.

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In the present study, PHF was envisaged as a potential doping agent of TiO2 photocatalytic systems. Formic acid was chosen as a target molecule because it allows direct mineralization to CO2 and H2O without forming intermediate products and because it represents the final step of any photodegradation process of many organic compounds. A new and facile way to elaborate hybrid photocatalytic systems combining PHF and TiO2 nanotubes is therefore evaluated here. TiO2 nanotubes formed by calcination of hydrogenotitanate nanotubes have been selected for this study since previous works have evidenced the interest of using this kind of solids with high surface area and moderate anatase crystallinity for photocatalytic purposes [43-48]. Several parameters influencing photocatalytic properties were evaluated like the PHF loading and postthermal treatments. First, fullerenes were oxidized to produce PHF and were added to hydrogenotitanates before or after a calcination treatment in order to determine the most efficient method of preparation. Second, the best way of incorporating PHF was selected to investigate the effect of PHF loading and to determine the optimum PHF amount.

2. Experimental 2.1. Chemicals All the materials used were analytical reagents and used without further purification. NaOH (98%), HCl (37%), MeOH (99%) and C60 (99.9%) were supplied by Sigma Aldrich. HCOOH (FA) (80%) and H2O2 (30%) were purchased from Acros Organics. TiO2 P25 (72% anatase, 28% rutile) was purchased from Degussa-Hüls-AG Company. Ultrapure water (18 MΩ.cm-1) was used throughout the whole experiments.

2.2. Synthesis of PHF

Herein, the functionalization of C60 was adapted from [49]. Typically, a mixture of C60 (72 mg, 0.1 mmol), NaOH (240 mg, 6 mmol), and 30% H2O2 (6 mmol) were ground at room temperature in an agate mortar under air atmosphere for 15 min. During grinding, the color of the mixture turned to yellowish brown. Then 50 ml of deionized water was added to the mixture under stirring for 10 min to dissolve the as-obtained sludge. Next,

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a filtration step was performed and the filtrate was concentrated to 5 mL producing a brown precipitate. Then, MeOH was added dropwise to the filtrate to precipitate completely the as-obtained fullerenols. Finally, PHF was recovered by centrifugation at 10000 rpm for 20 min and washed thoroughly three times with methanol to ensure complete NaOH removal. The obtained material was then dried under vacuum at 80°C for 12h.

2.3. Elaboration of TiO2 Nanotubes

Titanate nanotubes were prepared following the same protocol used by Meksi et al. [48] via alkaline hydrothermal treatment. Typically, 3.0 g of TiO2 powder (P25) were treated with 90 mL of 11.25 mol.L−1 NaOH aqueous solution in a 150 mL Teflon-lined autoclave at 130°C for 20 h [50]. The resulting product was washed with distilled water and a 0.1 mol.L−1 HCl solution until the pH value of the rinsing solution reached ca. 6.5. This sample was then dried at 80°C for 24 h. For a more complete sodium removal, a second washing step was performed using a more concentrated solution of hydrochloric acid (1.0 mol.L−1). The obtained nanomaterials called HNT are formed of hydrogenotitanate nanotubes (H2Ti2O5.H2O phase, [43]). Calcination treatment was then carried out at 400°C under O2 (heating rate: 2°C min-1) to obtain TiO2 anatase nanotubes [43], denominated here as HNT400.

2.4. Preparation of C60-TiO2 Nanocomposites

PHF was added in different amounts (between 0.5 and 5.0 wt%) by incipient wetness impregnation to two different types of nanotubes namely: hydrogenotitanates (HNT) and TiO2 nanotubes (HNT400). The obtained materials were dried at 80°C for 24h to obtain HNT-xPHF or HNT400-xPHF respectively (with x, the weight percentage of PHF). The resulting solids were then heat treated at 400°C for 2h under oxygen to obtain HNT-xPHF-400 and HNT400-xPHF-400.

2.5. Characterization Techniques

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Low temperature nitrogen adsorption measurements were done using a Micromeritics ASAP 2020 instrument. The Brunauer–Emmett–Teller (BET) specific surface areas were determined by a multipoint method using the adsorption data in the relative pressure P/P0 range of 0.05–0.25. Desorption isotherms were used to determine the pore size distribution using the Barrett–Joyner–Halenda (BJH) method. The nitrogen adsorption volumes at the relative pressure (P/P0) of 0.95 were used to determine the total pore volumes. The thermal stability of the different solids was studied through thermogravimetric analysis (TGA) using a Mettler MX1 microbalance with STARe DB logiciel equipped with 70µL Al2O3 crucible. A heating rate of 5°C.min-1 was used from room temperature to 800°C under air atmosphere. The phase identification of the sample was performed by X-ray Diffraction (XRD) analyses using a Bruker D8 Advance A25 diffractometer with CuKα radiation (λ= 1.54184 Å). The “HighScore Plus” software was used for the identification of phases and the crystallite size was calculated using the Scherrer equation: L = Kλ/(βicosθ)) Where L is the crystallite size, K is taken as 1, λ is the wavelength of the X-ray radiation (CuKα = 0.15406 nm) and βi is the full width at half-maximum. Raman spectra were recorded at 20◦C using a LabRAM-HR instrument (Horiba JobinYvon) from 100 cm−1 to 1000 cm−1 with spectral resolution of 4 cm−1, a 514 nm argon–krypton RM2018 laser as incident light, and a CCD detector cooled at -75°C. The average power at the surface of the studied samples was fixed at 1 mW. The photoluminescence (PL) analysis was performed using a PerkinElmer LS55 spectrofluophotometer equipped with a Xe lamp presenting an excited wavelength at 330 nm. Electron Paramagnetic Resonance (EPR) spectra were recorded at room temperature on a Bruker ER-200D spectrometer using X-band frequencies (9.30 GHz). Quartz tubes of 5 mm o.d. were filled to a depth of 1 cm with a weighed amount of powder sample. The morphology of the different samples was studied by Transmission Electron Microscopy (TEM) and High Resolution TEM (HRTEM) using a JEOL 2010 instrument operating at 200 kV. The microscope was equipped with an ultrahighresolution polar piece (point resolution: 1.9 Å). The specimens for TEM analysis were

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prepared by dropping and drying the colloidal solution onto a holey carbon film supported on a Cu grid (300 mesh). Measurement of the band gap energies (Eg) was performed using a diffuse reflectance UV–vis AvaSpec-2048 Fiber Optic Spectrometer equipped with a symmetrical CzernyTurner design with 2048 pixel CCD detector array. Spectra were recorded from 250 nm to 800 nm. A barium sulfate (BaSO4) was used as a blank white reference. FTIR spectra were recorded under inert atmosphere on a Perkin Elmer (Spectrum BX) spectrometer in the 4000-400 cm-1 wavenumber range using a spectral resolution of 4 cm-1 and accumulating 100 scans. X-ray photoelectron spectroscopy (XPS) was performed using a KRATOS Axis Ultra spectrometer equipped with a hemi-spherical analyzer operating at a fixed pass energy of 40 eV with a 150 W Al Kα monochromatic source (1486.6 eV). The samples were pressed on an indium foil attached to the sample holder and placed into the XPS instrument. Binding energies were determined with an accuracy of ±0.2 eV. Curve fitting was performed using mixed Gaussian and Lorentzian line shape after the treatment of background by Shirley type baseline (casaXPS software, version 2.0.71). The photocurrent measurements were carried out using a Metrohm Autolab instrument. A three electrode system was employed consisting of a fullerene-TiO2 modified glassy ITO (Indium tin oxide) electrode, a saturated calomel reference electrode (SCE) and a platinum wire counter electrode. All potentials were quoted versus SCE. Sodium sulfate with a concentration of 0.5 M was used as electrolytic solution. UV irradiation was performed using a Xenon lamp (500 W) providing maximum energy at 365 nm. The distance between the bottom of the reactor and the UV source was adjusted in order to maintain a radiant flux of 5 mW.min-1 (as measured with a VLX-3W radiometer equipped with a CX-365 detector (UV-A)).

2.6. Adsorption Study of Polyhydroxyfullerene

The adsorption kinetic experiments were carried-out by mixing a suspension of the adsorbent (1 g.L-1) and a solution of PHF (100 mg.L-1). The combined adsorbent/PHF mixture was then kept under stirring in a double wall reactor at a regulated temperature of 293 K. 10 cm3 of the mixture was then sampled at regular interval times, followed by 8

centrifugation at 10000 rpm for 30 min. The supernatant was then carefully removed and the procedure was repeated twice. The concentration of free PHF in the final supernatant (Ce) was then measured by UV/vis spectroscopy using a Perkin-Elmer Instrument Lambda 45 apparatus using the 330 nm n-π* transitions of C=O bonds of PHF molecules. Adsorbed PHF (Qe) was finally calculated according to the following equation: Q =

(

)

where C0 and Ce represent the initial PHF concentration and the concentration at equilibrium (mg.L-1), respectively, V is the volume of the PHF solution (L), and m is the amount of adsorbent (g). Adsorption isotherms were studied using 30 mL of PHF solution by varying the concentrations between 20 and 200 mg.L-1. The experiments were performed for 60 minutes at a temperature of 293 K.

2.7. Photocatalytic Experiments The photocatalytic tests were performed in 30 cm3 of an aqueous solution using different concentrations of formic acid. The photoreaction was carried out in a Pyrex photoreactor (100 cm3) using an optical window with a 12.5 cm2 area. A concentration of 1 g.L−1 for each sample was used. The pH of the solutions was ca. 3.0 ± 0.5 depending on the concentration used. Stirring was set at 450 rpm. UV irradiation was performed using a high-pressure Philips HPK Hg lamp (125 W) providing maximum energy at 365 nm. An optical filter Corning 0.52 was installed to cut off wavelength below 340 nm. A circulating water cell (thickness: 2.2 cm) was used to prevent any heating of the suspension. The distance between the bottom of the reactor and the UV source was adjusted in order to maintain a radiant flux of 5 mW.min-1 (as measured with a VLX-3W radiometer equipped with a CX-365 detector (UV-A)). The suspensions were stirred in the dark for 30 min to reach adsorption equilibrium conditions prior to irradiation. The concentration of formic acid after equilibration was measured and taken as the initial concentration (C0) in order to disregard adsorption phenomena. The photocatalytic degradation of formic acid was then performed at room temperature and at natural pH (ca. 3.5). Samples were withdrawn at different interval times and the

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catalyst was separated from the liquid phase by filtration using spherical SRP filters (pore size: 0.45 µm). Samples were then analyzed using a VARIAN ProStar High Performance Liquid Chromatography (HPLC) equipped with a COREGEL-87H3 column (300 mm × 7.8 mm) and a UV–vis detector (max = 210 nm). An isocratic H2SO4 elution (5 × 10-3 mol.L-1) was used at a flow rate of 0.7 mL.min-1.

3. Results and Discussion 3.1. Functionalization of Fullerenes

After oxidation, fullerenes (C60) are transformed into a homogenous brown dispersion of highly water soluble fullerenols [51]. Creation of OH groups are evidenced through FTIR measurements (Figure 1). Indeed, FTIR spectrum of fullerene exhibits fundamental IR active modes for C60 with vibration bands at 505 and 663 cm-1 in agreement with calculated active infrared bands of the highly symmetric C60 [52, 53]. The (C=C) vibration of the C60 cage can also be observed at 1619 cm-1. In the case of PHF, the main characteristic bands corresponding to C60 are still present but with a higher intensity suggesting a breaking of the high C60 symmetry. Moreover, new bands characteristic of fullerenols are now observed [33, 54]. Bands at 1396 cm-1 (strong) and around 1080 cm-1 (weak) correspond respectively to the s(C-O-H) and (C-O) vibrations of the C-O-H structure of fullerenols. The noticeable (C=C) band tends both to shift slightly to 1637 cm-1 and to increase in intensity. According to Xing et al. [54], this shift could be related to the appearance of a small contribution at higher wavenumbers (in their case at 1710 cm-1) corresponding to the carbonyl stretching absorption band. However, the absence of a clear shoulder around 1710 cm-1 in the present study does not allow to confirm safely about the presence of a small amount of carbonyl groups using only IR measurements. Finally, the broad band due to (O-H) tends to strongly increase in intensity after functionalization. FTIR results therefore evidence the formation of a large number of hydroxyl groups at the surface of C60 during functionalization confirming the formation of polyhydroxyfullerene. Figure S1 (Supplementary data) shows the UV-vis absorption spectrum of C60 in benzene and of PHF in water. C60 exhibits two bands, at 260 nm attributed to the dipole transition from highest occupied molecular orbital (HOMO) to lowest unoccupied

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molecular orbital (LUMO) and at 310 nm corresponding to the π-π* transition of C60 [58]. The spectrum of PHF in water exhibits a maximum absorption at 260 nm attributed to π-π* transitions due to aromatic C=C groups and a shoulder at about 330 nm attributed to n-π* transitions of C=O bonds. UV-vis spectroscopy studies were also performed to determine the band gap energy values of fullerene and fullerenol using the Kubelka–Munk method. F(R)hν1/2 versus hν plots were built with F(R) = (1 − R)/2R assuming indirect band gap transition. Results show values around 1.8 eV for fullerene and 2.2 eV for PHF. This significant increase of Eg after functionalization is related to the presence of oxygenated groups at the surface of fullerene. To obtain further information about the thermal properties of PHF, TGA experiments were also carried out. Thermogravimetric profiles for PHF and C60 are shown in Figure S2. As expected, C60 exhibits a complete combustion under air around 600°C. Surprisingly, on the opposite, in the case of PHF, a progressive and slow kinetic weight loss is evidenced throughout the whole process with a loss of only 8.5% between room temperature and 800°C. The first poorly-defined loss of about 3.8% is observed between 60 and 160°C. This loss is attributed to desorption of physisorbed molecules, mainly water and methanol. A second weight loss of 3.7% is then observed between 160°C and 680°C. These results are in agreement with the study of Husebo et al. [59]. They explained this progressive steady weight loss evolution by a slow loss of water related to the high solubility of PHF. On the whole, results suggest the existence of a complexity of oxidized carbon groups with different types of oxygen functionalities that are covalently implanted on hydrophilic oxidized cluster cages.

3.2. Effect of the Calcination Treatment on the Structural and Textural Properties of PHF-TiO2 Nanocomposites 3.2.1. Effect of the Order of Incorporation of PHF on the Structural Properties of TiO2 Nanotubes

The elaboration of PHF-TiO2 nanomaterials was performed using two different approaches. In the first method, PHF (1.0 wt%) was added by impregnation to the HNT

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solid followed by a thermal treatment at 400°C under oxygen for 2 h (called HNT1%PHF-400). In the second method, HNT was first calcined at 400°C prior to PHF impregnation step while a second thermal treatment at 400 °C was carried out after PHF incorporation (called HNT400-1%PHF-400). The as-synthesized HNT nanomaterials present broad diffraction peaks mainly at 10.0°, 24.6°, 38.4°, and 48.5° which can be ascribed to the H2Ti2O5·H2O orthorhombic phase [48]. The diffractograms of samples after a first treatment at 400◦C (HNT400, HNT1%PHF-400, HNT400-1%PHF, HNT400-1%PHF-400) (Figure 2) show contributions with characteristic diffraction peaks at 25.4°, 38.1°, 48.3°, 54.0°, 55.1°, and 62.8° attributed to the (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1) and (2 0 4) reflections of the anatase phase respectively [43]. Hence, the first thermal treatment contributes to the phase transformation of the orthorhombic H2Ti2 O5·H2O phase into anatase. However, careful examination of the diffractograms (Figure 2) shows that the crystallinity of the materials strongly depends on the method of preparation. In fact, the addition of PHF onto hydrogenotitanates prior to post thermal treatment leads to a low degree of crystallization of the anatase phase after calcination at 400°C. On the opposite, adding PHF onto already calcined TiO2 ensures a higher crystallinity of the resulting material. The average anatase crystallite sizes were then calculated (Table1) according to the line width analysis of the (101) anatase reflection using the Scherrer equation. The two samples of TiO2 containing 1% PHF, HNT400-1%PHF-400 and HNT-1%PHF-400 show similar crystallite sizes of 7.4 and 7.1 nm respectively showing that the method of preparation of PHF-HNT nanocomposites does not influence significantly the TiO2 crystallite size. Raman spectra of HNT nanotubes and HNT-PHF nanocomposites are shown in Figure 3. Anatase vibrations can be observed at 144, 198, 394, 514 and 638 cm-1 and are related to the E1g, E2g, B1g, B1g/A1g and Eg active modes respectively [48]. Moreover, additional weaker bands related to the presence of the TiO2 (B) phase can be detected at 123 (sh), 238, 253, 287, 365 (sh), and 478 cm-1 [60, 61]. Previous results have shown that an intermediate TiO2(B) phase tends to be formed during the hydrogenotitanate to anatase transformation. However, compared to pure HNT400 [43], the relative intensity of vibration bands related to TiO2(B) seems here slightly stronger after calcination at 400°C. This is probably due to the presence of PHF retarding in a certain extent the

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complete transformation into anatase. Moreover, bands attributed to the TiO2(B) phase tends to decrease in intensity when going from HNT-1%PHF-400 to HNT400-1%PHF and finally HNT400-1%PHF-400. This is in agreement with XRD results showing that the incorporation of PHF before calcination at 400°C hampers more strongly the crystallization of the anatase phase. Moreover, interestingly, the E1g mode of the anatase phase presents a red shift for the HNT-1%PHF-400 with a vibration mode at 143.6 cm-1 compared to pure HNT400 (144.2 cm-1) [48], HNT400-1%PHF (144.3 cm-1), and HNT400-1%PHF-400 (144.1 cm-1). According to Huo et al. [62], this shift is related to a higher proportion of surface oxygen vacancies on HNT400 solids (onto which PHF was incorporated or not) compared to HNT-1%PHF-400. Therefore, the incorporation of PHF before calcination of the hydrogenotitanate nanotubes is detrimental to the formation of surface oxygen vacancies on the nanotubes surface. To study the morphological characteristics of PHF-HNT materials, TEM micrographs are reported in Figure 4. The HNT400 sample obtained after calcination at 400°C of hydrogenotitanate nanotubes [43] still presents a nanotubular morphology with a diameter around 10-14 nm and a wall thickness of 2-3 nm. These nanotubes form aggregated bundles forming voids and leading to the formation of intergranular porosity (Figure 4A). For 1%PHF-containing samples, whatever the order of incorporation of PHF, the nanotubular morphology is still observed showing that this elaboration method does not influence significantly the nanotubular morphology (Figures 4B-E). Moreover, TiO2 nanotubes are covered by amorphous particles with an average particle diameter of c.a. 2 nm suggesting that PHF molecules are present onto TiO2 nanotubes wall as clusters (see Figure 4B for example). High resolution images of PHF-HNT (inset in Figures 4C and 4E) confirms the presence of lattice fringes with an interlayer d spacing of 0.35 nm corresponding to the (101) plane of anatase in agreement with XRD results. In order to determine the influence of the elaboration method on the nature of the carbon species present on the HNT-PHF materials, XPS analysis of HNT-1%PHF, HNT-1%PHF-400, HNT400-1%PHF and HNT400-1%PHF-400 was performed (Supplementary data Figure S3). Results are reported in Table 2. Ti 2p core-level spectra present a unique signal at 458.8 eV assigned to Ti4+ suggesting the absence of any significant Ti3+ species or Ti-O-C entities. Calcination of HNT-1%PHF does not modify significantly the atomic percentage of C or the C/Ti atomic ratio showing an

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absence of redispersion of C after calcination if PHF is first incorporated onto hydrogenotitanate before the thermal treatment. A similar conclusion can be reached for HNT400-1%PHF materials with an absence of effect of the final calcination step on the C dispersion. However, incorporating PHF onto an already calcined HNT400 solid instead of the non-calcined hydrogenotitanate leads to an increase of both the C atomic percentage and of the C/Ti atomic ratio. This shows that a better surface dispersion of PHF can be achieved if added onto an already calcined HNT material. C 1s core-level spectra present three contributions at 284.6, 286.2, and 289.0 eV corresponding respectively to non-oxidized carbon, mono-oxygenated carbon (C-OH), and carboxylate moieties [63] (Table 2 and Figure S3). If considering the respective proportions of these three C species, a direct effect of the calcination at 400°C of HNT1%PHF materials can be noted leading to an increase of the proportion of non-oxidized carbon at the expense of mono-oxidized C-OH carbon species while the proportion of carboxylate moieties does not change. This suggests a negative effect of the calcination procedure in this case on the amount of oxidized species on fullerene cages. On the opposite, calcining a second time at 400°C HNT400-1%PHF materials leads to a slight decrease of the proportion of mono-oxidized species at the benefit of carboxylates while the percentage of non-oxidized carbon is not modified. However, when comparing HNT-1%PHF-400 and HNT400-1%PHF-400, no noticeable differences in terms of carbon species can be noticed after their final thermal treatment. HNT-1%PHF-400 and HNT400-1%PHF-400 were also compared using electron paramagnetic resonance spectroscopy. EPR spectra are reported in Figure S4 (Supplementary Information). On HNT400-1%PHF-400, only a weak signal at g = 1.99 due to the presence of a low concentration of bulk Ti3+ states [64-66] can be detected. These Ti3+ species result from the non-stoichiometric formation of Ti atoms at the grain boundary [67]. On HNT-1%PHF-400, a single line spectrum with a g value of 2.003 is observed. This signal is absent on HNT400-1%PHF-400. In the present case, the value of this g factor is too high to be attributable to Ti3+ sites. In fact, this signal is related to the presence of some paramagnetic species resulting from the formation of electron-deficient C60 moieties [68, 69]. The presence of electron-deficient C60 is not surprising since the hydrogenotitanate phase (present before calcination) is known to possess high

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adsorptive properties [70, 71]. The strong interaction between the hydrogenotitanate nanotubes and fullerenols moieties would induce some electron transfer from the stabilized fullerenols to the titanate phase. On the opposite, the much lower interaction of titania nanotubes formed after calcination of hydrogenotitanates would hamper such an electron transfer. This would also explain the higher proportion of the highly oxidized C1s contribution (BE = 289.0 eV) in XPS results for HNT-1%PHF compared to HNT400-1%PHF (Table 2). Photoluminescence (PL) properties were also determined for HNT-1%PHF-400 and HNT400-1%PHF-400 (Figure 5) using an excited wavelength at 330 nm in order to better ascertain the influence of the order of PHF incorporation on the structural properties of TiO2 nanotubes. Five main emission peaks are generally observed on PL spectra at 420, 440, 470, 485, and 530 nm [47, 48]. The peak at 420 nm is related to phonon-assisted indirect transition from edge (x) to the center () of the Brillouin zone [72]. The contribution at 440 nm is due to bulk recombination of electron-hole pairs [73]. Emission peaks at 470 nm (weak in the present case or overlapping with the 485 nm contribution) and at 530 nm are due to photogenerated electron trapping on surface oxygen defects [74] while charge transfer from Ti3+ to TiO62- octahedra occurs at 485 nm [75]. Comparing HNT-1%PHF-400 and HNT400-1%PHF-400 shows three main effects. First of all, the emission peak at 440 nm due to bulk recombination of electron-hole pairs gives rise to a much higher contribution on HNT-1%PHF-400 than on HNT4001%PHF-400. Second, a clear signal at 530 nm due to the capture of photoelectrons by surface oxygen defects is observed on HNT400-1%PHF-400 while it is absent on HNT1%PHF-400. This result is in agreement with the Raman results and confirm that incorporating PHF to HNT before calcination limits the formation of surface oxygen defects. Finally, a strong signal at 485 nm due to Ti3+ sites is now observed on HNT400-1%PHF-400 while this emission peak is quite weak on HNT-1%PHF-400. However, the nature of these Ti3+ sites may differ from the Ti3+ bulk sites observed previously by EPR. In the previous case, this signal was observed without UV irradiation of the samples and can proceed from the formation of non-stoichiometric Ti sites during the formation of the TiO2 nanotubes. On PL spectra, the Ti3+ sites are observed under UV irradiation and result from the reduction of some surface Ti4+ sites

15

by photogenerated electrons. Such a signal was also observed previously on TiO2 nanotubes [47, 48] but its relative intensity (by comparison to the emission peak due to bulk recombination) was weaker than in the present case (HNT-400: I485/I440=0.85; HNT-400-1%PHF-400: I485/I440=1.20). This easier reduction of Ti4+ sites under UV illumination to form surface Ti3+ sites may be related to the presence of the C60 clusters on the surface of TiO2 nanotubes. Through their reducing character, C60 clusters may weaken Ti-O bond strength and would favour their following reduction under UV irradiation. On the opposite, the formation of electron-deficient C60 clusters on HNT1%PHF-400 prevents from any weakening of the Ti-O bond strength induced by fullerenols. Results show that the order of incorporation of PHF into the TiO2-PHF composite influences the crystallization degree of the anatase phase and the dispersion of fullerenols. In this respect, adding PHF prior to the calcination of hydrogenotitanate into anatase leads to the strongest impact in terms of limitation of anatase crystallization and to the lowest dispersion of fullerenols onto TiO2. This mode of incorporation also limits the formation of surface oxygen vacancies as suggested by Raman analysis and favors bulk recombination of electron-hole pairs as shown by photoluminescence. Raman study also shows that the proportion of the intermediate TiO2(B) phase is the lowest for HNT400-1%PHF-400. The highest anatase purity is then achieved after incorporation of PHF onto an already calcined hydrogenotitanate followed by a subsequent second calcination step at 400°C. This way of incorporating PHF to TiO2 nanotubes also favors the formation of surface Ti3+ sites under UV illumination through probably weakening of Ti-O bond strength induced by neighboring C60 clusters. This protocol of incorporation has then been selected in the next section to further study the effect of the PHF loading.

3.2.2. Effect of the PHF Loading on Textural and Structural Properties of TiO2

In order to study the effect of the PHF loading on the textural properties of HNT-PHF nanomaterials, a set of N2 adsorption-desorption measurements were carried out as shown in Figure 6. All isotherms present type IV profiles with H3 hysteresis loops characteristic of non-rigid aggregates of particles giving rise to pores in the form of slot

16

[43]. Compared to pure HNT400, addition of PHF leads to better defined hysteresis loops suggesting that the presence of fullerenols tends to increase the mesoporosity of the HNT-PHF composites. However, one should be cautious in analysing these results since TiO2 nanotubes form aggregated bundles leading to the formation of voids. Complementary BJH pore size distributions (Figure 7) evidences a progressive shift to lower pore diameter values when increasing the PHF loading. The maximum observed for pore size distributions shift from 13.0 nm for HNT400 to 9.6 nm for HNT4005%PHF-400. This maximum is related to the inter-particles void formed between aggregated bundles of nanotubes. It shows that addition of PHF tends to decrease attractive Van der Waals forces maintaining TiO2 nanotubes into aggregated assemblies [76, 77]. Interestingly, one should note also a shoulder at 3.6 nm corresponding to the porosity resulting from the nanotubular morphology [43]. Its detection even for the sample containing 5 wt% PHF confirms that the nanotubular morphology was not suppressed by the incorporation of fullerenols. It also strongly suggests that PHF is mainly located at the outer surface of the nanotubes and not inside the tubes. This fact is confirmed by the determination of the SBET specific surface areas (Table 1). Indeed, SBET values remain relatively constant whatever the PHF loading showing that fullerenols moieties do not block the porosity resulting from the nanotubular morphology. Examination of diffractograms shows the formation of the anatase phase regardless of the amount of PHF (Figure S5). Only, a slightly lower crystallization of the anatase phase can be noticed for HNT400-x%PHF-400 compared to pure HNT400 whatever the PHF loading. Moreover, no contribution coming from PHF can be detected even with a loading of 5.0 wt% suggesting a quite well dispersion of fullerenols on the surface of TiO2 nanotubes and/or its complete amorphization. Table 1 reports the evolution of the anatase crystalline domains in function of the PHF loading. Compared to pure HNT400, the crystallite size tends to decrease from 10.4 nm for HNT400 to 7.4 nm for HNT4001%PHF-400. Increasing further the PHF loading does not tend to accentuate the decrease of the anatase crystallite sizes. UV-vis DRS spectra were also acquired for the HNT and HNT-PHF materials in order to determine their respective band gap energy values (Table 1). DRS spectra are dominated by the strong absorption of TiO2 in the UV region with identical onset values

17

around 390 nm whatever the solid considered (data not shown). Moreover, all the materials containing or not PHF show an average band gap energy value of about 3.14 eV close to the one expected for anatase (Table 1). This shows that fullerenols do not influence optical properties of the TiO2 substrate in agreement with previous results. Fullerenols are therefore mainly deposited at the outer surface of the nanotubes without creation of direct Ti-O-C links.

3.3. Effect of the Calcination Treatment on the Photocatalytic Properties of PHF-TiO2 Nanocomposites 3.3.1. Effect of the Order of Incorporation of PHF on the Photocatalytic Properties of TiO2 Nanotubes

The activity of the HNT-1%PHF-400, HNT400-1%PHF, HNT400-1%PHF-400 and their PHF-free equivalent HNT400 (used as reference) was evaluated during the photodegradation of formic acid (FA). Figure 8 reports the evolution of the amount of formic acid in solution ([FA]0 = 200 mg.L-1) in the dark and with UV illumination. Results only show a negligible decrease of the FA concentration in the dark by adsorption of formic acid. After UV irradiation, a very different comportment can be observed when comparing HNT400-1%PHF and HNT400-1%PHF-400. HNT4001%PHF presents the lowest activity with only a slight degradation rate of formic acid. Moreover, the HNT400-1%PHF catalyst is not stable during the photocatalytic test. A progressive darkening of the catalyst is observed during UV illumination and the rate of formic acid degradation decreases abruptly. This suggests that PHF tends to be reduced under UV irradiation. It seems that the interaction between PHF and TiO2 is limited to weak forces (hydrogen bonding and/or van der Waals interaction) making PHF unstable at the surface of TiO2 nanotubes under UV illumination. On the opposite, for HNT4001%PHF-400, the degradation rate strongly increases leading to complete elimination of formic acid in 60 min. This photocatalytic activity is even more important than for P25 [43] and for the pure HNT-400 solid with complete degradation after 105 min. Therefore, a second thermal treatment influences positively the photocatalytic performance of HNT-PHF nanomaterials. This can be related to a stronger anchoring of PHF moieties onto the outer surface of TiO2 nanotubes when comparing results to

18

HNT400-1%PHF. This strong anchoring would favour the generation of surface Ti3+ sites under UV illumination as shown previously by EPR and photoluminescence. Finally, the HNT-1%PHF-400 sample presents a moderate photocatalytic activity but still lower than for pure HNT400 showing that incorporation of PHF to hydrogenotitanate before calcination has a strong negative impact on photocatalytic properties of TiO2 nanotubes. In this case, the high bulk recombination rate of electronhole pairs and the limitation of the amount of surface oxygen defects able to capture photogenerated electrons combine together to strongly lower the photocatalytic activity. After 1h, only about 50% of formic acid was degraded. The determination of the initial rate (r0) of degradation for different initial FA concentrations (from 109 to 6517 µmol.L-1) allows asserting the kinetic behavior of HNT-1%PHF-400, HNT400-1%PHF-400 and their PHF-free equivalent HNT400 samples in FA photocatalytic degradation. Note that the HNT400-1%PHF sample is not considered due to its low photocatalytic activity and instability during test. Initial rates of degradation (r0) in function of FA concentration are presented in Figure 9. At low formic acid concentration, the initial rate of photodegradation increases as a function of the formic acid concentration in solution. However, beyond nearly 2000 µmol.L-1, all curves tend to level off and reach progressively a plateau. These results show that the degradation kinetics of formic acid follows the Langmuir-Hinshelwood model according to: r0 = where r0 is the initial rate of degradation, k is the rate constant of formic acid degradation and K is the adsorption constant of formic acid onto nanomaterials. Rate constant values can therefore be determined and results are summarized in Table 3. The catalytic activity of the different photocatalysts follows the increasing order: HNT1%PHF-400
19

based on the determination of the charge carrier migration time () toward the surface of the semiconductor which is size dependent and can be expressed as:

=



where R is the radius of a particle and D (5.10-3 cm2.s-1) is the diffusion coefficient of excited charge carriers [78]. A  value of 7.8 ps is obtained for HNT400 solids containing PHF whatever the method of incorporation of fullerenols (Table 3). This value is much lower than for pure HNT400 (15.9 ps) in agreement with the decrease of the anatase crystallite domains when PHF was added to HNT400 (Table 1). However, this evolution of the charge carrier migration time does not correlate with the variation of the rate constant values, kexp. Therefore, other parameters than only the TiO2 crystallite size must be involved to correctly describe the evolution of the photocatalytic activity of HNT-PHF materials. When either the hole or the electron is trapped by surface adsorbed species on the photocatalyst or by an appropriate acceptor, the non-trapped electrons or holes might react with suitable acceptor molecules in solution. The overall reaction is thus decomposed into two steps: 1) Formation of an encounter complex between the electron (or hole) acceptor and the semiconductor particle through for example formation of surface oxygen vacancies. The rate of this process will be diffusion limited. 2) Interfacial electron transfer (an electrochemical step) involving a Faradic current across the semiconductor-solution interface and characterized by a rate parameter (in cm s-1). A detailed kinetic treatment of the above reaction steps has been carried out by Marcus [79] who derived the following equation:

=

(

+ )

where R is the sum of the radii of the semiconductor particle and the electron (or hole) and D is the sum of their respective diffusion coefficients, kobs is the classical rate constant obtained experimentally and kct is the electrochemical rate parameter. If the heterogeneous charge transfer is considered much faster than diffusion (kct≫ ), in this case, the well-known Smoluchowski expression is obtained:

20

kobs= 4RD kobs values are provided in Table 3 and can be used to determine if reaction rates are diffusion limited or not. Results showed clearly a strong impact of PHF on the photocatalytic process. Comparison between kexp and kobs values shows a relatively similar evolution when considering HNT400 and HNT-1%PHF-400. These results suggest that incorporation of PHF onto hydrogenotitanate nanotubes before their calcination into TiO2 nanotubes hampers the formation of surface oxygen vacancies able to trap photogenerated electrons in agreement with Raman analysis. This restricts possibilities to lower the recombination of electron-hole pairs and decreases the intrinsic photocatalytic activity of HNT-1%PHF-400 [48]. This situation however does not apply when explaining the high photocatalytic activity of HNT400-1%PHF-400 for which there is no correlation between kexp and kobs values. Another explanation is therefore necessary to describe correctly the causes inherent to the high photocatalytic activity of HNT400-1%PHF-400. If we consider now that heterogeneous charge transfer is considered as much slower

than diffusion (an interfacial electron transfer), kct<< ; the preceding equation becomes:

= and kct can be obtained as follows : kct=

kobs

If we assume that kobs values are equal to kexp values, therefore, kct values are provided in Table 3. When comparing HNT-1%PHF-400 to HNT400-1%PHF-400, a strong increase of the kct value from 28.0 to 61.5 can be observed if PHF is incorporated onto an already calcined HNT400 solid instead of a non-calcined hydrogenotitanate HNT. This evolution of kct values is similar to the variations of kexp experimental rate constants and shows clearly an increase of the faradic current across the semiconductor interface if PHF is incorporated onto an already calcined HNT400 solid. It also emphasizes the beneficial effect resulting from a second post thermal treatment at 400°C for grafting PHF onto TiO2 nanotubes improving the interfacial electron transfer and therefore

21

increasing photocatalytic activity. On the opposite, comparison between HNT400 and HNT-1%PHF-400 shows similar kct values in both cases even if experimental rate constants are quite different. This confirms that in this latter case, the decrease of photocatalytic activity is related to a lower formation of surface oxygen vacancies if PHF is incorporated before the calcination step. Finally, in order to confirm that the incorporation of PHF onto calcined HNT400 followed by a second post thermal treatment improves the faradic current across the semiconductor interface, the lifetime evaluation of photogenerated electron-hole pairs during the photocatalytic process was performed by photocurrent measurements. Experiments were carried out at 1V, a potential value at which saturation of the photocurrent can be neglected. Photocurrent results are presented in Figure 10. Herein, reacting species are OH- ions coming from the NaOH electrolyte and adsorbed on the surface of the working electrode. The effect of PHF can be clearly identified after calcination at 400°C. In that case, the response was highly enhanced for HNT4001%PHF-400 compared to pure HNT400. This response was directly linked to the lifetime of generated electron-hole pairs. Therefore, the presence of PHF effectively decreases the bulk recombination rate enhancing the photocatalytic activity of this catalyst in agreement with photoluminescence results.

3.3.2. Effect of the PHF Loading on the Photocatalytic Properties of TiO2 Nanotubes

The effect of the PHF loading on the photocatalytic response of HNT-PHF nanomaterials was then evaluated considering the most efficient method of preparation in terms of photocatalytic activity, i.e. incorporation of PHF onto TiO2 nanotubes obtained by calcination of hydrogenotitanate at 400°C followed by a second post thermal treatment at 400°C. Results are reported in Table 4 for PHF loadings ranging between 0.5 and 5.0 wt%. Evolution of rate constant values shows a maximum at 1.0 wt% PHF loading while no clear tendency can be noticed for adsorption constant values. Increasing further the PHF amount leads first to a progressive decrease in photocatalytic activity up to 2.0 wt% while a strong loss of activity can be noticed for the photocatalyst containing 5.0 wt% PHF.

22

In order to ascertain the causes behind such an evolution of the photocatalytic activity, the adsorption isotherm of PHF onto HNT400 was determined. The amount of PHF adsorbed per gram of catalyst (Qe) is reported in Figure 11 as a function of the PHF concentration at equilibrium (Ce). The PHF adsorption isotherm can be modelized using the Langmuir model: Qads/Qmax= KadsCe/(1+KadsCe)

Where Qads is the adsorbed quantity of PHF, Qmax the maximum amount adsorbed on HNT400 (50 mg.g-1), Kads the adsorption constant of PHF, and Ce is the concentration of PHF at the adsorption equilibrium. In the first part of the curve, the Langmuir model fits with the experimental data up to a PHF concentration of 65 mg.L-1 suggesting that PHF in this concentration range forms a monolayer at the surface of the HNT400 photocatalyst. Above the threshold value of 65 mg.L-1 (corresponding to 1.5 wt% PHF), experimental adsorbed amounts of PHF at equilibrium are higher than those expected considering a Langmuir model. This is in agreement with the appearance of multilayers of PHF at the surface of TiO2 nanotubes. Moreover, from the PHF adsorption isotherm, the coverage rate (θ = Qads/Qmax) at the threshold value of 65 mg.L-1 can therefore be calculated. Result shows that only 30% of the HNT400 surface is in fact really covered. This finding suggests that the multilayer adsorption of PHF starts largely before the total coverage of the HNT400 surface. Finally, the concomitance between the maximum photocatalytic activity and the completion of a surface PHF monolayer at the surface of TiO2 nanotubes can be noticed (Figure 11). This concomitance strongly suggests that as soon as multilayers of PHF form, the photocatalytic activity decreases because the formation of PHF multilayers leads to a shielding effect blocking partly light penetration. This point was confirmed by photocurrent measurement of the HNT400-2%PHF-400 (Figure 10). Indeed, increasing the PHF loading from 1.0 to 2.0 wt% leads to a decrease of the photocurrent response. Moreover, contrary to HNT400-1%PHF-400, the photocurrent does not reach immediately a constant value as soon as UV illumination is turned on. This is in agreement with a difficulty for light penetration during UV illumination resulting from the formation of multilayers of PHF acting as a light shield.

23

4. Conclusion

The effect of polyhydroxyfullerene (PHF) on the structural, textural and morphological properties of TiO2 nanomaterials obtained by calcination of hydrogenotitanate nanotubes was evaluated in the present study. Combination of different characterization techniques allowed to determine 1) the way fullerene was functionalized into polyhydroxyfullerene (PHF), 2) the best protocol of preparation for incorporating PHF to TiO2 nanotubes, and 3) the effect of the PHF loading on the photocatalytic response for the formic acid degradation. In a first step, the formation of a large number of hydroxyl groups but also of carboxylic moieties was confirmed during functionalization showing that formation of PHF was successfully achieved. In a second step, the mode of incorporation of PHF was extensively studied. Incorporating PHF before calcination of hydrogenotitanates limits the anatase crystallization, lowers the dispersion of PHF onto TiO2, hampers the formation of surface oxygen vacancies on the TiO2 surface and therefore favors a high bulk recombination of electron-hole pairs. This leads to a lower photocatalytic response in the photodegradation of formic acid in this case. On the opposite, incorporating PHF onto already calcined TiO2 nanotube followed by a second post thermal treatment helps to maintain surface oxygen vacancies at the TiO2 surface as well as Ti3+ sites, did not modify anatase crystallinity and improves faradic current across the semiconductor interface leading to much better photocatalytic activity than pure TiO2 nanotubes. Results also show that the photocatalytic response reaches a maximum for a PHF loading of 1.0 wt%. Above this loading, the decrease of photocatalytic activity was ascribed to a shielding effect due to PHF multilayers blocking partly light penetration during photocatalysis.

Acknowledgement The authors gratefully acknowledge the financial support by the Tunisian Ministry of Higher Education and Scientific Research and of the French Ministry of Foreign Affairs in the framework of the PHC-Utique program referenced 16G 1202.

24

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29

Figure Captions Figure 1. FTIR spectra of fullerene and fullerenols.

Figure 2. XRD patterns of HNT400 and of the HNT-PHF nanomaterials: with incorporation of PHF onto hydrogenotitanate without (HNT-1%PHF) or with calcination at 400°C (HNT-1%PHF-400), with incorporation of PHF onto already calcined TiO2 without (HNT400-1%PHF) or with calcination at 400°C (HNT4001%PHF-400).

Figure 3. Raman spectra of HNT-1%PHF-400, HNT400-1%PHF, and HNT4001%PHF-400.

Figure 4. TEM micrographs of HNT after calcination at 400°C A), and of the HNT-PHF nanomaterials: with incorporation of PHF onto hydrogenotitanate without (HNT1%PHF) B) or with calcination at 400°C (HNT-1%PHF-400) C), with incorporation of PHF onto already calcined TiO2 without (HNT400-1%PHF) D) or with calcination at 400°C (HNT400-1%PHF-400) E). Arrows in Figures 4B and C indicate the presence of fullerene entities on the surface of nanotubes. Insets of Figures 4C and E shows the interplanar spacing corresponding to the (101) plane of anatase.

Figure 5: Photoluminescence spectra of HNT-1%PHF-400 (A) and of HNT-4001%PHF-400 (B).

Figure 6. N2 adsorption-desorption isotherms of HNT400 and of HNT400-x%PHF-400 with different PHF amounts (x = 1.0, 2.0 or 5.0 wt%).

Figure 7. BJH pore size distributions for the HNT400 solid and for the HNT400-x% PHF-400 materials (x = 1.0, 2.0 or 5.0 wt%).

Figure 8. Evolution with time of the concentration of formic acid in solution using HNT400, HNT-1%PHF-400, HNT400-1%PHF and HNT400-1%PHF-400, under dark

30

and UV illumination conditions. ([FA] ≈ 200 mg.L−1 ≈ 4348 µmol.L−1 ; V = 30 mL; [TiO2] = 1 g.L−1; natural pH ≈ 3).

Figure 9. Evolution of the initial rates of degradation r0 as a function of the concentration of formic acid at the adsorption equilibrium, Ce for the HNT400, HNT1%PHF-400, and HNT400-1%PHF-400 samples. Symbols refer to experiment values and curves to the Langmuir–Hinshelwood model.

Figure 10. Photocurrent responses of HNT400 and of HNT400-x%PHF-400 solids (x = 1.0 or 2.0 wt%).

Figure 11. PHF adsorption isotherm onto HNT400 showing the evolution of the adsorbed amount of PHF in function of its concentration in solution at equilibrium conditions. Comparison is made with PHF adsorbed amount values considering a Langmuir model. Comparison is also provided with the evolution of the photocatalytic response in the degradation of formic acid in function of the PHF loading.

31

505

663

3437

1396

1637

644

C60(OH)n

3422

Absorbance (a.u.)

1619

C60

4000

3600

3200

2800

2400

2000

1600

1200

800

400

-1

Wavenumbers (cm )

Figure 1 Hamandi et al.

32

Figure 2 HAMANDI et al

33

Figure 3 HAMANDI et al

34

Figure 4 HAMANDI et al

35

Figure 5 Hamandi et al.

36

400 HNT400 HNT400-1%PHF-400

Adsorbed volume (cm3.g-1)

HNT400-2%PHF-400

300

HNT400-5%PHF-400

200

100

0 0

0.2

0.4

0.6

P/P0

Figure 6 HAMANDI et al

37

0.8

1

Figure 7 Hamandi et al.

38

UV OFF

UV ON

HCOOH Concentration (mg.L-1)

200 HNT400-1%PHF

150

100

HNT-1%PHF-400

50 HNT400 HNT400-1%PHF-400

0 -30

0

30

60

90

Time (min)

Figure 8 Hamandi et al.

39

120

Figure 9 HAMANDI et al

40

8E-05 HNT400

Photocurrent (A.cm-2)

HNT400-1%PHF-400

6E-05

HNT400-2%PHF-400

4E-05

UV ON

2E-05 UV OFF

0E+00 0

20

40

60

80

100

Time (s)

Figure 10 Hamandi et al.

41

120

80

110

multilayer

monolayer 70

Qe (mg/g )

60 90

50 40

80

30

70

20 60

10 0

50 0

20

40

60

80

100

120

140

160

Ce (mg/L) Qe experimental

Qe calculated by Langmuir Model

kexp (µmolL-1min-1)

Figure 11 HAMANDI et al

42

k exp (µmolL-1min-1)

100

Table 1. Textural properties, band gap energy values and anatase crystallite sizes for PHF-TiO2 nanocomposites. Catalysts

SBET (m2 g-1)

Vmeso (cm3g-1)

Eg (eV)

Anatase crystallite size (nm)

HNT400

248

0.45

3.14

10.4

HNT400-1%PHF-400

232

0.59

3.14

7.4

HNT400-2%PHF-400

242

0.55

3.14

9.7

HNT400-5%PHF-400

257

0.62

3.14

7.6

HNT-1%PHF-400

282

0.62

3.12

7.1

Table 2. XPS results of different HNT-PHF composites: C/Ti and O/Ti atomic ratios, Ti, O, and C atomic percentages, and proportions of the different C species as obtained after decomposition of the C 1s core level spectra (non-oxidized carbon: 284.6 eV, mono-oxidized carbon C-OH: 286.2 eV, carboxylate: 289.0 eV). Catalysts

Atomic ratio

Atomic%

C/Ti

O/Ti

Ti

O

C

284.6

286.2

289.0

HNT-1%PHF

0.31

2.29

27.4

62.7

8.6

54.7

28.2

17.1

HNT-1%PHF-400

0.28

2.27

27.7

63.0

7.8

59.4

22.9

17.7

HNT-400-1%PHF

0.39

2.32

26.6

61.7

10.5

59.9

25.7

14.4

HNT-400-1%PHF-400

0.41

2.38

25.9

61.6

10.7

60.0

22.0

18.0

43

C1s core level spectra

Table 3. Rate constant values (kexp) determined using a Langmuir-Hinshelwood linearization model for the formic acid degradation using undoped and PHF-doped TiO2 nanomaterials. Comparison is also provided with rate constant kobs and kct values (see text for more information). Charge carrier migration time values are also included.

Catalysts

Pseudo-First-Order

Kinetics

Parameters k (µmol.L-1.min-1) kobsa

kexp

a.

kctb

(ps)c

HNT-400

72.6

42.3

23.0

15.9

HNT-1%PHF-400

43.5

29.5

28.0

7.8

HNT400-1%PHF-400

94.7

29.5

61.5

7.8

kobs=4DR

b

.kct Interfacial electron transfer

c

(ps)charge carrier

44

Table 4. Rate constant values (kexp) determined using a Langmuir–Hinshelwood model for the formic acid photodegradation. Results are reported for PHF-TiO2 nanocomposites with different PHF amounts and obtained by incorporation of PHF onto calcined HNT400 followed by a second post thermal treatment.

Catalysts

kexp(µmolL-1 min-1)

HNT400

72.6

HNT400-0.5%PHF-400

74.5

HNT400-1% PHF-400

94.7

HNT400-1.5%PHF-400

91.0

HNT400-2% PHF-400

84.5

HNT400-5% PHF-400

58.1

45