Dependence between ionic liquid structure and mechanism of Vis

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Article

Dependence between ionic liquid structure and mechanism of Vis-induced activity of TiO obtained by the ionic liquid assisted solvothermal synthesis 2

Marta Paszkiewicz-Gawron, Marta D#ugok#cka, Wojciech Lisowski, Maria Cristina Paganini, Elio Giamello, Tomasz Klimczuk, Monika Paszkiewicz, Ewelina Grabowska, Adriana Zaleska-Medynska, and Justyna #uczak ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04291 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on January 4, 2018

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ACS Sustainable Chemistry & Engineering

Dependence between ionic liquid structure and mechanism of Vis-induced activity of TiO2 obtained by the ionic liquid assisted solvothermal synthesis Marta Paszkiewicz-Gawrona, Marta Długokęckab, Wojciech Lisowskic, Maria Cristina Paganinid, Elio Giamellod, Tomasz Klimczuke, Monika Paszkiewiczf, Ewelina Grabowskaa, Adriana Zaleska-Medynskaa, Justyna Łuczakb a

Department of Environmental Technology, Faculty of Chemistry, University of Gdansk, Wita

Stwosza 63, 80-308 Gdansk, Poland b

Department of Chemical Technology, Faculty of Chemistry, Gdansk University of Technology,

G. Narutowicza 11/12, 80-233 Gdansk, Poland c

Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw,

Poland d

Department of Chemistry, University of Turin, Via P. Giuria 7, I-10125 Torino. Italy

e

Department of Solid State Physics, Faculty of Applied Physics and Mathematics, Gdansk

University of Technology, 80-233 Gdansk, Poland f

Department of Environmental Analysis, Faculty of Chemistry, University of

Gdansk, ul. Wita Stwosza 63, 80-308 Gdansk, Poland

ABSTRACT: Due to tremendous structural diversity of ionic liquids (ILs) simple transfer of observations performed for one IL used for IL-TiO2 preparation on different samples is not

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possible. Therefore, four ionic liquids, all containing distinct nitrogen-bearing organic cations (pyridinium, pyrrolidinium, ammonium, imidazolium), were used for the first time for the ILTiO2 composites preparation. The role of individual IL’s cation in the synthesis of TiO2 microspheres as well as the effect of the IL structure on the mechanism of the Vis light-induced photoactivity of IL-TiO2 was presented and discussed in regard to structure, morphology, absorption properties, elemental composition and reactive species involved in the photocatalytic reaction of phenol degradation. The successful modification of the TiO2 with organic IL species including possible interactions between IL and TiO2 surface as well as the TiO2 matrix (doping with N) were confirmed. The sample that exhibited the highest photoactivity under Vis irradiation (58%) was TiO2 prepared in a presence of 1-butylpyridinium chloride with 1:3 (IL: precursor) molar ratio. For this sample, the highest partial decomposition of cationic species of IL was observed resulting in interaction of nitrogen atoms with deeper sites of TiO2 (Ti-Nx) as well as the highest surface defects in a form of Ti3+. The superoxide radical species O2•- were found to be main active species responsible for high efficiency of degradation under Vis irradiation.

KEYWORDS: ionic liquid, photocatalysis, titanium dioxide, visible light activity, surface interactions, N-doping

INTRODUCTION

The TiO2 photocatalysts are the most commonly-used semiconductor materials for environmental applications such as air/water purification, wastewater treatment, bacteria inactivation and water splitting.1 However, the practical application of TiO2 has been impeded by its low efficiency for utilizing solar energy and its low quantum yield resulting from its wide


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band gap and high recombination rate of photogenerated charge carriers.2 It is known that the performance and thus the applications of TiO2 are strongly dependent on its own crystalline structure, morphology (exposed crystal faces, shape and pore volume and structure) and size of the particles. Therefore, to enhance the activity and widespread the application of TiO2 photocatalysis studies are focused on the adjusting and control of these factors.3 Additionally, the microstructure and electronic structure, surface properties, and the photoactivity of TiO2 are largely dependent on the synthesis conditions and procedures. For instance, the hydroand solvothermal methods have been widely used due to their simplicity and short reaction time. Very recently, ionic liquids (ILs) have been used in preparation of TiO2 particles serving as cosolvents4-6, templating agents7-10, capping/morphology-controlling agents5, 11, 12 and microwave absorbers.13-16. The main advantage of using ILs as solvents is their high polarity (higher than most of the molecular solvents) determining solubility of the reagents.17 Additionally, relatively higher viscosity of the ILs (in comparison to water or classical organic solvents) can be also beneficial since decreases the particles aggregation rate by viscous stabilization.18 ILs have been also used as structure formatting agents (templating and capping agents) due to their amphiphilicity19, thus structural organization through self-assembly, and tunable properties, therefore ability for controlling the nucleation and growth of the nano- and microstructures.9 Latterly, it was observed that ILs introduced during synthesis can interact with TiO2 surface resulting in enhancement of UV or visible light response of semiconductor.12,

16, 20

Surface

modified TiO2 nanocomposites by 1-butyl-3-methylimidazolium hydroxide [BMIM][OH] ionic liquid, synthesized by hydrothermal method, shows a red-shifted optical response.21 Zhong et al revealed that the addition of 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6] during the synthesis of TiO2 has a strong effect on the transport properties of the photogenerated


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charges under UV radiation as well as it increases the hydroxyl content on the surface of the photocatalysts.22

Jing

sulfonyl)imidazolium

et

al.

hydrogen

synthesized sulfate

TiO2

modified

[ABsIM][HSO4],

by

which

1-allyl-3-(butyl-4showed

prominent

photoelectrochemical water oxidation performance promoted by UV light, exhibiting a ten-fold photocurrent compared to the unmodified TiO2 under the same conditions.23 Qi et al. found that the addition of 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4] can enhance or suppress the photocatalytic degradation of methyl orange (MO) or rhodamine B (RhB) again under UV irradiation, respectively, depending on the trapping and transfer of the photogenerated electrons and holes.24 Improvement of photocatalytic activity of IL-assisted TiO2 was also ascribed to doping of titania by IL-building elements. For instance, N, C and F co-doped rutile titania nanorods were prepared in the presence of [BMIM][BF4] and showed a remarkable improvement in photocatalytic degradation of Congo red under visible light irradiation.7 Also Yu et al. used [BMIM][BF4] as a dopant source of F, B and C for the synthesis of fluorinated B, C co-doped anatase TiO2 via the hydrothermal method.25 Based on the literature data, it could be stated that ILs introduced during the synthesis route could improve visible light response of wide band gap semiconductors (e.g. TiO2) due to: (i) doping of TiO2 by non-metal elements constituting ILs (such as C, F, P, B)7, 25, 26; (ii) direct sensitization of TiO210; (iii) surface complex charge transfer27; (iv) favoring oxygen vacancies and Ti3+ species formation during synthesis16 and (v) affecting transport of photogenerated charges.24 Notwithstanding, it needs to be mentioned that in these very little research on the improvement of Vis-induced photocatalytic activity of TiO2 by ILs, one compound that is [BMIM][BF4], was used, whereas various mechanisms proposed. Moreover, due to structural diversity of ILs simple transfer of observations performed for one IL on different samples is not


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possible. It is not clear at this moment how to correlate the structure of ILs and preparation conditions with their properties. In addition, in most cases photocatalytic activity of TiO2 obtained via ILs-assisted preparation route are estimated in UV mediated reaction or using dyes as model compounds. This fact makes mechanism interpretation more difficult due to sensitization ability of dyes.2 What is more, active species involved in the photodegradation reactions were rarely investigated. In our previous work, we investigated the influence of the alkyl chain length in the imidazolium cation of IL on photoactivity of IL-TiO2 photocatalysts, however under UV-Vis irradiation.12 Later on, we found out that the selected ILs allowed to receive inorganic materials capable to use visible photons, besides UV ones and that type of the anion highly influence the surface properties and the visible response of TiO2 particles prepared under IL assistance.27 As a natural continuation, in this work, we present novel TiO2 materials that were successfully prepared using IL-assisted solvothermal method and show the effect of the IL’s cation type and IL content on the morphology, surface properties and photoactivity of TiO2 microparticles. Four ILs, all based on the chloride anions and on a series of distinct, nitrogen containing, cations (namely: 1-benzyl-3-methylimidazolium [BenMIM], 1-butylpyridinium [BPy], 1-butyl1-methylpyrrolidinium [BMPyr] and tetrabutylammonium [TBA]) were used for the first time for investigation of the role of ILs in the enhancement of TiO2 photoactivity (Figure 1). To confirm the IL-TiO2 composites formation, that is effective interactions between inorganic TiO2 and organic IL and to verify doping of IL’s elements in the titania bulk structure the elemental analysis of the photocatalysts and analysis of the reaction medium before and after solvothermal synthesis were performed. To provide insight into the oxidative species participating in the degradation mechanism, electron paramagnetic resonance spectroscopy was employed.


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Moreover, the role of the active species in the process of the degradation was evaluated by using different types of active species scavengers.

Figure 1. The structure of ionic liquids used during the synthesis: 1-benzyl-3methylimidazolium chloride [BenMIM][Cl], 1-butylpyridinium chloride [BPy][Cl], 1-butyl-1metylpyrrolidinium chloride [BMPyr][Cl] and tetrabutylammonium chloride [TBA][Cl] EXPERIMENTAL Titanium n-butoxide (TBOT), hydrochloric acid and ethanol were provided by Sigma Aldrich, and

used

as

received.

The

ionic

liquids

1-benzyl-3-methylimidazolium

chloride,

1-butylpyridinium chloride, 1-butyl-1-methylpyrrolidinium chloride and tetrabutylammonium chloride with purity of ≥ 99%, were obtained from Merck. Preparation of IL-assisted TiO2 particles Anatase IL-TiO2 were synthetized according to the procedure developed in our previous work using TBOT as precursor dissolved in an absolute ethanol, with addition of HCl, distilled water and selected IL. Various molar ratios of IL to TBOT (IL:TBOT) were used in the experiments (Table 1). The homogeneous solution was transferred to a 200 ml Teflon-lined stainless steel autoclave and solvothermal reaction was performed at 180°C for 24 h.12 After cooling to the


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room temperature, the product was washed with deionized water and ethanol with the volume ratio of 10:1, respectively. In the typical synthesis, threefold washing followed by product decantation was applied. In the additional experiments, the sample was washed five times and separation was performed by centrifugation (sample marked with* in Table 1) or samples were not washed at all (marked with** Table1). After washing step, the samples were dried at 60°C for 12 h, and finally calcined for 2 h at 200°C. Characterization of IL-assisted TiO2 particles Identification of the TiO2 phases, powder X-ray diffraction (PXRD) measurements were conducted using Philips/PANalytical X'Pert Pro MPD diffractometer (PANalytical, Almelo, the Netherlands) with Cu-Kα radiation. The morphology and size distribution of the IL-TiO2 samples were measured by scanning electron microscope (SEM), Hitachi Microscope TM-1000, under high vacuum with an accelerating voltage 15 kV. BET surface area and pore size of the photocatalysts (physical adsorption and desorption of nitrogen at 77 K) was measured by Micromeritics Gemini V200 Shimadzu instrument equipped in the VacPrep 061 Degasser. A Nicolet Evolution 220 UV-Vis spectrophotometer (Thermo) was used to obtain the diffuse reflectance UV-Vis absorption spectra of the samples, for which the baseline was performed using BaSO4. A Thermo Scientific Flash 2000 CHNS analyzer was used to determine the elemental composition of the synthesized materials. X-ray photoelectron spectroscopy (XPS) experiments were performed on a PHI 5000 VersaProbeTM (ULVAC-PHI) spectrometer with monochromatic Al Kα radiation (hν = 1486.6 eV). The X-ray beam was focused to a diameter of 100 µm, and the measured area was defined as a 250 x 250 µm. The highresolution (HR) XPS spectra were collected by the hemispherical analyzer at a pass energy of 23.5 eV, an energy step size of 0.1 eV and a photoelectron take off angle of 45° with respect to


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the surface plane. CasaXPS software was used to evaluate the XPS data. The HR XPS spectra were deconvoluted using a Shirley background and a Gaussian peak shape with 30% Lorentzian character. The binding energy (BE) scale of all detected spectra was referenced to the Ti 2p3/2 core level (BE=458.6 eV). Measurement of photocatalytic activity Photoactivity under Vis irradiation was determined using phenol as model compound. The photocatalyst (0.125 g) was dispersed in 25 ml of phenol solution (0.21 mmol/dm3) and poured to a cylindrical reactor with quartz window. The suspension was aerated (5 dm3/h) and degradation experiments were performed at temperature of 10 ± 0.5°C kept by of water bath. As an irradiation source, 1000 W Xenon lamp, 6271H Oriel with an optical filter >420 nm, GG 420was used. The phenol concentration was determined by the colorimetric method (λmax = 480 nm) using UV-Vis spectrophotometer (Evolution 220, Thermo-Scientific). The photocatalytic degradation runs were preceded by a blind test in the absence of photocatalysts or illumination. Controlled photoactivity experiments under Vis irradiation were carried out using different scavengers (ammonium oxalate as a scavenger of photogenerated holes, AgNO3 for electrons, benzoquinone for superoxide radical species, and tert-butyl alcohol for hydroxyl radical species). The scavenger concentration was equal to the phenol content, and experiments were performed analogously to the photocatalytic degradation of phenol described above, except that the scavengers were added to the reaction system. Additional characteristic was performed by Electron paramagnetic resonance (EPR) measurements. For the spectroscopic characterization of all the samples a quartz tubular cell allowing treatments under vacuum and in situ EPR measurements was employed. X-band CW-


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EPR spectra were detected at 77 K on a Bruker EMX spectrometer (microwave frequency 9.46 GHz) equipped with a cylindrical cavity. A microwave power of 1 mW, a modulation amplitude of 0.2 mT and a modulation frequency of 100 KHz were used. The decomposition level of [BPy] and [BenMIM] cations was analyzed using HPLC (Shimadzu) system equipped with an diode array detector SPD-M20A, pump LC-20AD, autosampler SIL 20AHT, column oven CTO-10ASvp and degasser DGU-20A5R. HPLC-grade acetonitrile with addition of 0.025% trifluoroacetic acid (v/v) and deionized water containing 0.025% TFA (v/v) were used as a mobile phase A and B, respectively. The separation was carried out with Hypersil Gold aQ column 150 x 4.6; 5 µm (Thermo Scientific, Waltham) and an isocratic program: 5 % of mobile phase A and 95% of phase B at room temperature. The flow rate was 1 mL min-1, and the elution profiles were monitored at 205 nm for [BenMIM][Cl] and 258 nm for [BPy][Cl]. Each sample (before and after solvothermal reaction) was measured in triplicate. The decomposition level of [BMPyr] and [TBA] cations were analyzed by using Agilent 1200 Series LC system (Agilent Technologies, Inc., Santa Clara, USA) coupled to HCT ultra ion trap mass spectrometer (Bruker Daltonics, Bremen, Germany) with an electrospray ionization (ESI) source. The mass spectrometer was operated in the positive ion mode using a capillary voltage of 4.0 kV. Ions with m/z 50–400 were monitored in a full scan mode. Nitrogen was employed as the nebulizer gas (30 psi) and the dry gas (10 L min−1). Analyses were performed using a Thermo Hypersil Gold aQ 150 x 4.6; 5 µm column (150 mm x 4.6 mm, 5 µm). The mixture of acetonitrile (10 %, v/v) with 5 mM ammonium acetate buffer in deionized water was used as mobile phase. The flow rate of mobile phase was 0.4 mL min -1. Each sample (before and after solvothermal reaction) was measured in triplicate.


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The decomposition level was calculated as: ߟ‫ܮܫ‬ሺ%ሻ = 100 ∙ ‫ܥ‬଴ −

஼ ஼బ

(1)

where: C0 – is the initial concentrations of cations of ILs; C – is the concentrations of cations of ILs after the solvothermal reaction. RESULTS AND DISCUSSION To investigate the effect of the selected ILs on the improvement of visible light induced photoactivity, mechanism of phenol degradation as well as active species involved in this process twenty four IL-TiO2 photocatalysts were prepared using four ILs with various molar ratios of IL to Ti precursor (IL:TBOT): 1:10; 1:8; 1:5; 1:3; 1:2 and 1:1 (Table 1). This mode of action let us to select proper IL:TBOT molar ratio that enable to obtain the photocatalyst with the highest photoactivity under visible irradiation for specific IL. The selected photocatalyst were subsequently characterized taking into account surface and optical properties; elemental composition, thereby interactions between IL and TiO2. Finally, photocatalytic performance accompanied by active species responsible for visible light activity were determined. This thorough characteristics let us to conclude about mechanism and the role of IL of photocatalytic improvement under visible irradiation. Photocatalytic activity of IL-TiO2 under Vis irradiation The photocatalytic efficiency of the IL-assisted samples was estimated using aqueous solution of phenol as a model contaminate. Phenol was chosen due to high toxicity and no absorption in the visible spectrum in contrast to organic dyes (Rhodamine B, methylene blue) often used as model contaminations in the photocatalytic activity experiments.28 The photocatalytic activity of IL-TiO2 semiconductors under visible irradiation (optical filter >420 nm) is shown in Table 1. It was found that amount of IL (molar ratio of IL:TBOT used for synthesis) needed to obtain IL-TiO2 samples with the highest photoactivity is not constant and differed for various IL


10

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structures. Moreover, the type of IL also affected their photoactivity. The highest degradation efficiency of phenol was revealed for TiO2 samples obtained in a presence of [BPy][Cl] followed by [BMPyr][Cl]. For the series of [BPy][Cl]-TiO2 semiconductors photodegradation efficiency increased with increasing quantity of IL up to IL:TBOT molar ratio of 1:3. In addition, the ability of pollutant degradation of TiO2_BPy(1:3) sample (58%) was significantly higher than value determined for reference TiO2 (13%). Increase of photoactivity was accompanied by rise of specific surface area (up to 215 m2·g-1, Table 1). Analogous observations were made for the other ILs. Hence, the terminal IL:TBOT ratios which determine the highest improvement of the photocatalytic activity were 1:2 for [BenMIM][Cl]; 1:5 for [BMPyr][Cl]; 1:2 [TBA][Cl], respectively. Again, higher photocatalytic activity of these samples is accompanied by their larger specific surface area (Table 1). This, in turn, may provide more active sites and shorten the bulk diffusion length of charge carriers, thus suppressing bulk recombination.16 Moreover, the type of washing method and the number of washing cycles were also tested. The sample, that was washed five times showed much lower photoactivity in comparison to the sample obtained by threefold washing method or not washed at all. Samples exhibited 36%*, 58% and 60%** effectiveness of phenol degradation after 60 min. of irradiation (cut off filter λ>420 nm), respectively. Therefore, we proved that washing is a key step in the whole solvothermal/hydrothermal synthesis and affects the parameters of the obtained samples. The investigated case is undoubtedly a problem and requires further examination because solvothernal/hydrothermal method is widely used in both academic research and industry.


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Table 1. Sample label, applied ions, molar ratio of reagents (IL:TBOT), surface properties and photoactivity under visible irradiation of IL-assisted TiO2 Phenol Pore degradation volume percentage (cm3·g-1) under Vis irradiation (%)

Phenol degradation reaction rate under Vis irradiation (µ µmol dm−3 min−1)

Sample label

Ionic liquid

Molar ratio (IL:TBOT)

Specific surface area (m2·g-1)

TiO2

-

-

184

0.069

13

0.4

TiO2_BenMIM(1:10)

1:10

191

0.109

15

0.5

TiO2_BenMIM(1:8)

1:8

191

0.094

19

0.6

1:5

212

0.104

31

1.1

TiO2_BenMIM(1:3)

1:3

217

0.090

32

1.1

TiO2_BenMIM(1:2)

1:2

207

0.101

44

1.4

TiO2_BenMIM(1:1)

1:1

170

0.083

15

0.5

TiO2_BPy(1:10)

1:10

213

0.104

22

0.7

TiO2_BPy(1:8)

1:8

214

0.104

18

0.6

1:5

213

0.104

45

1.5

TiO2_BPy(1:3)

1:3

215

0.105

58; 60*; 36**

2.0; 2.3*; 1.3**

TiO2_BPy(1:2)

1:2

208

0.102

53

1.6

TiO2_BPy(1:1)

1:1

198

0.097

34

1.1

TiO2_BMPyr(1:10)

1:10

197

0.097

26

0.9

TiO2_BMPyr(1:8)

1:8

209

0.103

49

1.6

1:5

213

0.104

49

1.7

TiO2_BMPyr(1:3)

1:3

228

0.111

33

1.1

TiO2_BMPyr(1:2)

1:2

228

0.111

25

0.8

TiO2_BMPyr(1:1)

1:1

217

0.110

20

0.6

TiO2_TBA(1:10)

1:10

172

0.083

38

1.3

TiO2_TBA(1:8)

1:8

210

0.102

22

0.8

1:5

207

0.101

39

1.3

TiO2_TBA(1:3)

1:3

208

0.101

41

1.3

TiO2_TBA(1:2)

1:2

209

0.102

46

1.5

TiO2_TBA(1:1)

1:1

202

0.100

23

0.7

TiO2_BenMIM(1:5)

[BenMIM][Cl]

TiO2_BPy(1:5) [BPy][Cl]

TiO2_BMPyr(1:5) [BMPyr][Cl]

TiO2_TBA(1:5)

[TBA][Cl]

* sample obtained without a washing step, ** sample obtained by fivefold washing procedure with sample separation by centrifugation,


12

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What is interesting, further increase of the IL taken to the synthesis resulted in the opposite effect in case of both surface area and photoactivity. Decrease in photocatalytic efficiency is consistent with results of Chen 16 and Li29, who investigated effect of [BMIM][BF4] on TiO2 obtained by microwave-assisted solvothermal reaction as well as our previous results obtained for 1-alkyl-3-methylimidazolium chlorides12 and 1-butyl-3-methylimidazolium bromides, hexafluorophosphates and octylsulphates.27 Li et al. proposed that lower TiO2 photoactivity in the presence of excess of IL is related with N, B and F elements which become the recombination centers of the photo-induced electrons and holes.29 In general for all series of samples the similar effect was observed (Table 1). To correlate the structure of IL cation with ILs-TiO2 visible light induced photoactivity, only samples showing the highest photoactivity were chosen for further characterization. Structure, morphology and absorption properties of IL-TiO2 The ambient temperature powder diffraction patterns (PXRD) for four selected for further testing IL-TiO2 and reference TiO2 samples are shown in Figure 2. The experimental data are represented by points, whereas a red line is a LeBail fit to the data. Expected Bragg reflections for TiO2 – anatase model (I 41/a m d, s.g. #141), are shown by vertical bars. PXRD confirms high quality anatase (no impurities found) in our samples. The diffraction reflections are broad due to small crystallite size that was estimated from the Scherrer formula on the basis of the (101) reflection width. The IL-TiO2 samples prepared under the same conditions possessed analogous crystallinity, while shortening of the reaction time as well as lowering of temperature resulted in incomplete reaction and thus lower reaction efficiency as well as lower photoactivity. The refined lattice parameters a and b, unit cell volume and average crystallite size do not differ much and are gathered in Table 2.


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Figure 2. Room temperature powder X-ray diffraction patterns of TiO2_BenMIM(1:2), TiO2_BPy(1:3); TiO2_BMPyr(1:5); TiO2_TBA(1:2) and reference TiO2 – anatase. Bragg reflections are represented by black vertical bars, observed and calculated data are shown by black points and a red line, respectively Table 2. Lattice parameters and average crystallite size of IL-assisted TiO2 photocatalysts Sample label

a (Å)

c (Å)

V (Å3)

d (Å)

TiO2

3.7890(3)

9.497(1)

136.34(4)

63

TiO2_BenMIM(1:2)

3.7889(3)

9.505(1)

136.45(4)

54

TiO2_BPy(1:3)

3.7872(5)

9.494(2)

136.17(6)

43

TiO2_BMPyr(1:5)

3.7907(4)

9.493(2)

136.41(6)

57

2_TBA(1:2)

3.7914(3)

9.506(1)

136.65(4)

58


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The SEM analysis of four IL-TiO2 samples with the highest photocatalytic activity (Figure 3) revealed that the particles are spherical with well-developed structure. Application of [TBA][Cl] provided particles with the highest contribution of the smallest fraction, that is 40% of structures had diameters of 0.5-1 µm. The TiO2 prepared in a presence of [BMPyr][Cl] have relatively more smooth surface and formed bigger structures, 80% particles were in the range of 1-3 µm. However, the photocatalysts obtained with addition of [BenMIM][Cl] and [BPy][Cl] had the most uniform distribution and were composed mainly of particles with diameter ranging from 1 to 2 µm (76%).

Figure 3. SEM images and particle size distribution of TiO2 obtained by IL-assisted solvothermal process (a) TiO2_BenMIM(1:2), (b) TiO2_BPy(1:3); (c) TiO2_BMPyr(1:5); (d) TiO2_TBA(1:2)

The UV-Vis diffuse reflectance spectra (DRS) of the IL-assisted TiO2 samples were shown in Figure S1. The optical absorption of the samples prepared with addition of ILs was shifted to the region of 400-600 nm in comparison to reference TiO2 characterized by sharp edge


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detected at 390–400 nm. Interestingly, the highest shift, thus the highest visible light absorption improvement was not observed for the sample with the highest photoactivity, that is TiO2_BPy(1:3) but for TiO2_BMPyr(1:5) sample. The significant light absorption of TiO2_BMPyr(1:5) might be due to the higher content of Ti3+ species.30 Analogous observation, namely that optical properties do not go side by side with photocatalytic activity, was already presented in literature for diverse metal and nitrogen co-doped TiO2 photocatalysts.30-34 The proposed formation mechanism of IL-TiO2 spheres was already discussed and schematically presented in our previous studies.12, 27 Elemental composition of the IL-assisted samples The selected ILs-TiO2 samples, representing the highest photocatalytic activity in each ILseries, were analyzed using XPS. Both the elemental surface composition and chemical character of detected elements were identified35 in the high-resolution (HR) XPS spectra recorded for each sample. The results are presented in Figure 5 and Table 3. The N 1s and Cl 2p XPS spectra revealed the successful IL modification of the TiO2 photocatalysts. Nitrogen was detected on all samples in two forms at BE close to 400 eV and 397.4 eV. Fist one identify well the surface IL adspecies formation (C-NH-C, C=N-C bonds, Figure 4)12,

35

and second evidences the IL

interaction with TiO2 matrix (Ti-Nx bonds, Figure 5).35-37 The chemical character of titanium, oxygen and carbon originated from the ILs-TiO2 samples is identified in deconvoluted XPS spectra of Ti 2p, O 1s and C 1s, respectively (Figure 4) and specified in Table S2. The inspection of XPS data collected in Table S2 revealed the relative surface contribution of all detected elements. The atomic concentration (AC) ratios N/Ti, evaluated for TiO2_BPy(1:3) and TiO2_BMPyr(1:5) confirmed the smaller IL amount in these samples in comparison with the TiO2_BenMIM(1:2) and TiO2_TBA(1:2) samples. From the


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other hand, the C/Ti ratio, estimated for the first two samples were found to be much larger than for the last two samples. Since the preparation processing was similar for all samples we can address these data to the partial decomposition of cationic IL surface species in TiO2_BPy(1:3) and TiO2_BMPyr(1:5) samples and interaction of nitrogen atoms with deeper sites of TiO2. This supposition is well confirmed by relatively larger contribution of Ti-Nx species revealed in deconvoluted N1s spectra for these two samples (Figure 4, Table S2).

Figure 4. High-resolution XPS spectra of Ti 2p, O 1s, C 1s, N 1s, Cl 2p collected on the surface of ionic liquid-assisted TiO2 photocatalysts


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ACS Sustainable Chemistry & Engineering 1 2 3 Table 3. Elemental composition (in at. %) and chemical characters of titanium, oxygen and carbon and nitrogen states in 4 5 layer of [BenMIM][Cl]-, [BPy][Cl]-, [TBA][Cl]- and [BMPyr][Cl]-modified TiO2 particles, evaluated by XPS analysis 6 7 8 Ti fraction (%) O fraction (%) C fraction (%) N fraction (%) 9 -OH,C-NH-C, 10 “C” “B” Ti-Olatt Ti-Nx Ti-Osurf C=O, Sample “A” ∑ Ti ∑O ∑C ∑N ∑ Cl C=N-C, Ti(4+) Ti(3+) C-OH, C-Cl -C=O, C/N 11 397.4±0.3 529.8 530.6±0.1 Ti-O-N, C-C (at.%) (at.%) (at.%) (at.%) (at.%) 2 3 Ti-O-N 458.6 456.9± sp C-N sp C-N eV eV 531.7±0.1 eV 284.6±0.2 12 400.0 eV eV 0.1 eV 285.9±0.3 288.7±0.1 eV eV eV eV 13 97.00 3.00 67.22 80.22 16.00 3.78 2.52 57.86 17.86 24.28 0.23 87.94 12.06 0.11 11.0 14TiO2_BenMIM(1:2) 29.92 15 TiO2_BPy (1:3) 26.80 93.34 6.66 62.46 78.10 16.90 5.00 10.55 65.62 31.11 3.27 0.07 33.44 66.56 0.12 150.7 16 30.26 96.14 3,86 66.97 83.03 12.99 3.98 2.51 56.15 23.13 20.72 0.15 73.64 26.36 0.12 16.7 17 TiO2_TBA (1:2) 18 TiO _BMPyr(1:5) 27.24 92.73 7.27 62.02 72.78 20.90 6.32 10.59 62.92 31.73 5.35 0.09 59.71 40.29 0.06 117.7 19 2 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 ACS Paragon Plus Environment 45 46 47

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the surface

N/Ti

Cl/Ti

Cl/N

0.0077

0.0037

0.48

0.0026

0.0045

1.71

0.0050

0.0040

0.80

0.0033

0.0022

0.67

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Moreover the C/N values evaluated for TiO2_BPy(1:3) and TiO2_BMPyr(1:5) samples (150.7 and 117.7, respectively) are much larger than their nominal IL stoichiometry (9 for both samples). In contrary, the TiO2_BenMIM(1:2) and TiO2_TBA(1:2) samples seems more stable and exhibit the C/N values 11.0 and 16.7 and nominal C/N 5.5 and 16.0, respectively (Table S2). The large contributions of surface oxide fractions and carbon species identified in “A” and “B” fractions of C 1s spectra (Figure 4, Table S2) seems to confirm co-adsorption of partially decomposed IL on TiO2_BPy(1:3) and TiO2_BMPyr(1:5) samples. It is also interesting to note that the relative amount of the Ti3+ ions in these samples is about twice bigger than for TiO2_BenMIM(1:2) and TiO2_TBA(1:2) samples with larger IL content (Table S2). In this regard, the higher Ti3+ content accompanied by relatively larger contribution of Ti-Nx species may explain higher photocatalytic activity of TiO2_BPy(1:3) and TiO2_BMPyr(1:5). The CHNS analysis (Table S1) also revealed the presence of carbon, nitrogen and hydrogen atoms in the compositions of the IL-TiO2, thus confirmed presence of the elements characteristic for ILs used in this study. The carbon, hydrogen and nitrogen peaks may be attributed to the residual IL and decomposition products present at the surface of the TiO2 particles as well as doping of the TiO2 with nitrogen. To gain insight into the origin of the N atoms introduced to the TiO2 lattice, we also investigated the composition of the reaction environment used for titania preparation, before and after solvothermal synthesis, by using HPLC. Among the cations studied in this work, the highest decomposition during the solvothermal reaction was detected for [BPy] cation (Table 4) forming ionic liquids that provided TiO2 sample with the highest photoactivity. In comparison, degradation of [BenMIM] and [BMPyr] cations two times lower, whereas [TBA] cation was found to be the most stable entity with the decomposition efficiency below 1%. In this regard, the level of IL decomposition is dependent on the type of the cationic head group.


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These observations corroborates abovementioned XPS results as well as previous studies on thermal stability of ILs (imidazolium derivatives are generally more stable than pyridiniumbased salts38, and pyridinium cations are less stable than pyrrolidinium cations39). Table 4. Decomposition levels of the IL’s cations in the reaction conditions Ionic liquid’s cation

Degradation efficiency [%]

1-benzyl-3-methylimidazolium 1-butylpyridinium 1-butyl-1-methylpyrrolidinium tetrabutylammonium

30.8 58.0 23.6 420 nm), b) TiO2, c) TiO2_BPy 1:3 and d) TiO2_BMPy 1:2. e-g) materials irradiated under O2 with UV and visible light (full lamp): e) TiO2, f) TiO2_BPy 1:3, g) TiO2_BMPyr 1:5 Mechanism of photocatalytic activity of IL-TiO2 The sample that reveled the highest Vis-induced photocatalytic activity (58% of phenol degradation after 1h irradiation) was TiO2_BPy(1:3), that is TiO2 obtained in a presence of [BPy][Cl] ionic liquid at 1 to 3 IL:TBOT molar ratio. This sample had also the most uniform distribution (76% of particles had diameter in the range of 1 to 2 µm), the smallest lattice parameters (a = 3.7872(5) Å, c = 9.494(2) Å), unit cell volume (136.17(6) Å3) and average crystallite size (43 nm), hence the largest surface area (215 m2 g-1). This properties resulted in more active sites that shorten the bulk diffusion length of the charge carriers and suppress their


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recombination, thus resulted in the higher photoactivity.42 The TiO2_BPy(1:3) sample consisted of 26.80 at.% of Ti (6.66 % of which were Ti3+), 62.46 at.% of O, 10.55 at.% of C, 0.07 at.% of N and 0.12 at.% of Cl. As has been shown previously (based on the HPLC and XPS measurements) the partial decomposition of cations of ILs takes place. Decomposition of [BPy][Cl] takes part with the relatively higher extend in comparison to other ILs resulting in the co-adsorption of the partially decomposed IL at the TiO2 surface and larger contribution of interaction of nitrogen atoms with TiO2 deeper sites (formation of Ti-Nx species). For this IL contribution of Ti-Nx species (66.56% of N) in N fraction exceeds amount of IL adsorbed at the TiO2 surface represented by Ti-O-N fraction (33.44% of N). Up to now, nitrogen doping is regarded as one of the most effective approaches to extend the absorption edge of TiO2 which enhances its photocatalytic activity under Vis irradiation because of its comparable atomic size with oxygen, small ionization energy, metastable center formation, and stability. It results in the change of the banding structure of TiO2 due to the N interstitial doping, N-substitution, and the formation of surface oxygen vacancies which cause the red-shift absorption edge of TiO2.43-45 Indeed, shift of the optical absorption to the visible light region for all IL-TiO2 was observed. Analogous considerations can be performed for TiO2_BMPyr(1:5), the second most active sample. In this regard, the proposed photocatalytic mechanism for the IL-TiO2 samples under Vis irradiation, prepared in a presence of [BPy][Cl] as well as [BMPyr][Cl], was shown in Figure 7a. With the induction of N into TiO2 lattice, new impurity level including the N 2p above the O 2p valence band could be formed. Wang et al. explained that the valence band position of the N-doped TiO2 does not shift upward due to interstitial N-doping in spite of formation of the N 2p surface state 46. Electrons on this N 2p surface state would transit to the conduction band of


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the TiO2 upon absorption of light in longer wavelength, resulting in enhanced absorption of TiO2 on visible light.46 As a consequence, electrons can be transferred from the N 2p surface state to the conduction band leading to the formation of electron and hole pairs. Finally electrons accumulated at the surface of photocatalysts could be scavenged by oxygen molecules resulting in formation of the highly oxidative species.47 In comparison for photocatalysts prepared in the presence of ammonium [TBA][Cl] and imidazolium [BenMIM][Cl] ionic liquids much lower contribution of Ti-Nx species was observed (Fig. 4), thus mainly surface modification in a form of Ti-O-N species take place. Therefore, we can assume that photoactivity of these samples under visible light is due to formation of charge transfer (CT) complex between TiO2 and ionic liquid (surface adsorbate). What is important neither of the substrates have ability to absorb Vis irradiation. According to this mechanism (Creutz-Brunchwig-Sutin model) photoexcited electron is transferred from ionic liquid in the ground state (the excited state of the adsorbate is not involved) to the conduction band of TiO2. In this regard, we assumed that between TiO2, being the semiconductor of n-type, and chloride anion of IL (where HOMO orbital is located) new energy levels are formed (Figure 7b). This is in agreement with our previous study27 performed for 1-butyl-3-methylimidazolium bromide ionic liquid, where charge transfer between the bromide anion and molecular oxygen interacting with the vacancy of TiO2 was a main source of photoactivity induced by visible photons. It is also worth to mention that the relative amount of the Ti3+ ions in the TiO2_BPy(1:3) sample is about twice higher than in the TiO2_BenMIM(1:2) and TiO2_TBA(1:2) samples, although the molar ratio of IL to TBOT used for their synthesis was higher (see details in Table S2). In this regard, the presence of Ti3+ can introduce impurity states below the conduction


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band.48 Taking into consideration the bandgap modification both by the presence of Ti3+ and Ndoping, it is supposed that the interaction between Ti3+ and N is responsible for a high phenol photo-oxidation performance of TiO2 under visible irradiation. Electrons from conduction band or Ti3+ state could react with oxygen molecules providing oxidative species such as superoxide radical anions, being the main active species responsible for high efficiency of photodegradation. In the meantime, the holes that are generated in the valence band or N-dopant levels could react with hydroxyl anions or water providing hydroxyl radicals. In view of this, the Ti3+ content accompanied by contribution of Ti-Nx species may reduce the photon excitation energy from the VB to the CB under visible-light irradiation, thus explain enhanced photocatalytic activity of TiO2_BPy(1:3) and TiO2_BMPyr(1:5) samples in these conditions.

Figure 7. Proposed mechanisms of IL-TiO2 photocatalysts excitation by visible light: (a) Ndoped TiO2 formed during solvothermal reaction between TBOT and decomposed [BPy][Cl] and


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[BMPyr][Cl]; and (b) surface complex formed by adsorption of [BenMIM][Cl] and [TBA][Cl] at the surface of growing TiO2 microspheres CONCLUSIONS Highly active under visible irradiation IL-TiO2 composites were obtained through simple ionic liquid assisted-solvothermal synthesis method using ILs composed of distinct, nitrogen containing, cations and chloride anions. We revealed that preparation of IL-TiO2 have to be preceded by careful selection not only the IL’s structure but also its content since it strongly determines the morphological feature as well as photoactive properties. Unfortunately, in most publications this parameter is omitted when the IL-TiO2 properties are characterized. Mechanism of IL-TiO2 photoactivity strictly depends on the ionic liquid structure. Ionic liquids that were prone to undergo thermal decomposition in the synthesis conditions (pyridinium and pyrrolidinium) were characterized by larger contribution of Ti-Nx and Ti3+ species. This resulted in the change of the banding structure of TiO2 due to the N interstitial doping, Ti3+ states as well as formation of surface oxygen vacancies which together cause the red-shift absorption edge of TiO2. On the contrary, for more stable ionic liquids (imidazolium and ammonium) formation electronic coupling, that is the interaction between IL and the surface of semiconductor, in a form of surface complex was concluded. New energy level formed due to arrangement of molecular orbitals take part in the electron transfer from ionic liquid adsorbate to the conduction band of TiO2. Moreover, the photocatalytic activity for all samples under Vis irradiation was found to be attributable mainly to O2•-, however other forms of reactive oxygen species, such as •OH, H2O2 and HO2• radicals were also involved in the phenol degradation process.


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These comparative studies give us better phenomenological insight into the performance of the IL-TiO2 photocatalysts and better prospects for optimizing reaction components of choice. They are a part of a very broad, but almost untouched issue in the field of IL-TiO2 composites, namely (i) which structural descriptors of ILs are crucial for preparation of the photocatalyst with desired morphology and properties and (ii) how to predict the properties of the IL-TiO2 materials on the basis of the structure and properties of ionic liquids. The Authors hope that results presented here will stimulate further studies designed to increase the photoactivity of TiO2 as well as better understanding of IL’s role in mechanism of heterogenous photocatalysis, therefore influence the potential for commercial application. Supporting Information. CHNS analysis and UV-Vis absorption spectra of the IL-TiO2 photocatalysts AUTHOR INFORMATION Corresponding Author * E-mail addresses: [email protected] (J. Łuczak), [email protected] (A. Zaleska-Medynska). ACKNOWLEDGMENT The author J.Ł. acknowledges funding from the National Science Center within program SONATA 8 (grant entitled: “Influence of the ionic liquid structure on interactions with TiO2 particles in ionic liquid assisted hydrothermal synthesis”), contract No. 2014/15/D/ST5/0274. REFERENCES 1. Fujishima, A.; Rao, T. N.; Tryk, D. A., Titanium dioxide photocatalysis. J. Photochem. Photobiol., C 2000, 1 (1), 1-21, DOI 10.1016/S1389-5567(00)00002-2.


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For Table of Contents Use Only

Selected nitrogen-bearing ionic liquids allow to obtain visible-light responded TiO2 photocatalysts for organic pollutants degradation.


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Dependence between ionic liquid structure and mechanism of Vis

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