Influence of new templating agent for the synthesis of coral like tio 2 nanoparticles and their photocatalytic activity

Singh, Influence of new templating agent for the synthesis of coral-like TiO2 nanoparticles and their photocatalytic activity, Journal of Science: Advanced Materials and Devices 2017, do

Influence of new templating agent for the synthesis of coral-like TiO2 nanoparticles

and their photocatalytic activity

Satwant Kaur Shahi, Navneet Kaur, Sofia Sandhu, J.S Shahi, Vasundhara Singh

DOI: 10.1016/j.jsamd.2017.07.006

Reference: JSAMD 110

To appear in: Journal of Science: Advanced Materials and Devices

Received Date: 16 May 2017

Revised Date: 12 July 2017

Accepted Date: 19 July 2017

Please cite this article as: S.K Shahi, N Kaur, S Sandhu, J Shahi, V Singh, Influence of new

templating agent for the synthesis of coral-like TiO2 nanoparticles and their photocatalytic activity,

Journal of Science: Advanced Materials and Devices (2017), doi: 10.1016/j.jsamd.2017.07.006

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

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Title Page Influence of new templating agent for the synthesis of coral-like TiO2 nanoparticles and their photocatalytic activity

Satwant Kaur Shahi a , Navneet Kaur a , Sofia Sandhu a , J S Shahi b , Vasundhara Singh a *

email:vasun7@yahoo.co.in, Tel No.: 91-172-2733263

b

Department of Physics, Panjab University, Chandigarh, 160014, INDIA

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Influence of new templating agent for the synthesis of coral-like TiO2 nanoparticles and their photocatalytic activity

Abstract

We report a low cost and environment friendly solvent system to synthesize TiO2 nanoparticles using acidic deep eutectic solvent, choline chloride / p-toluene sulphonic acid as templating and hydrolyzing agent via sol-gel method The effect of varying concentration of deep eutectic solvent has been investigated on the important physico-chemical characteristics of as-synthesized TiO2 such as phase, morphology, particle size, surface area and band gap energy A detailed characterization of the obtained nanomaterials has been performed using various techniques, including X-ray diffraction (XRD), Scanning electron microscopy (SEM), Brunauer- Emmett- Tellar (BET) surface area, Raman and Ultraviolet-visible absorption spectroscopy The spherical shaped TiO2 nanoparticles was found to

be biphasic having two phases: anatase and rutile with a crystallite size in the range of 5.6-6.8 nm These spherical shaped nanoparticles assembled together to form coral-like morphology The effect of calcination temperature on synthesized products was studied by heating at 750oC The photocatalytic activity of the prepared TiO2 materials has been evaluated by the photo-discoloration of an aqueous methyl orange dye solution (20 ppm) under UV light irradiation The results indicate that photocatalytic efficiency of anatase-rutile mixture in sample DES-3 is higher (98% within 3h) than commercially available Degussa P-25 (87% within 3h)

1 Introduction

Titanium dioxide has attracted vast attention because of its widespread technological applications in photocatalysis, photovoltaics, gas sensors, self-cleaning surfaces, white pigments, catalyst supports, lithium ion batteries, memory devices and so on [ 1-5] TiO2 can exist in three distinct phases: anatase, rutile and brookite Thermodynamically, rutile is the most stable phase in bulk material while anatase and brookite are metastable and transform to rutile exothermally and irreversibly [6-9] In general, anatase form of TiO2 has been considered to have high photocatalytic activity [10] but there are reports of higher photocatalytic activity of anatase-rutile mixture than pure anatase due to synergistic effect between two phases preventing the recombination of photogenerated electrons and holes [11]

It is well known that the performance of TiO2 in various applications and its physico-chemical properties are strongly influenced by its particle size, phase, morphology and surface area [12, 13] It is desirable to develop nanostructures via various synthetic routes that allow for controlling the phase structure, size and morphology of TiO2 in good yield on the nanoscale To date, controllable synthesis of TiO2 has gained much attention and variety

of structures have been reported [14-19] Various methods have been explored to synthesize TiO2 nanomaterials such as sol-gel, precipitation, hydrothermal, microwave assisted, chemical vapour deposition and sonochemical method

Research in deep eutectic solvents (DES), which is an upcoming media for ionothermal reactions in nanotechnology, is still in its infancy Deep eutectic solvents have now emerged as an attractive alternate to the

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conventional ionic liquids showing numerous advantages over the latter due to their ease of preparation in a pure state at low cost, nontoxic nature, more synthetically accessible, biodegradable and can easily be tailored from their inexpensive components to suit certain applications [20] Deep eutectic solvents are obtained by the complexation of

a quaternary ammonium salt with various H-bond donors, such as carboxylic acids, amides, and alcohols [21] and fulfil the requirement of highly structured solvents for the synthesis of shape controlled nanomaterials [22, 23].Few examples are reported in literature, of the use of deep eutectic solvents both as solvent and structure-directing agent

to synthesize nanomaterials In a recent study, DESs have been reviewed [24] for their use as designer solvents to produce different kinds of nanomaterials, in which they play an efficient role in determining the shape, size and morphology of nanomaterials such as self-organized TiO2 nanobamboos by using succinic acid and choline chloride [25], gold nanowires [26], spindly bismuth vanadate microtubes [27], nanoflower shaped NiO and NiCl2 nanosheets [28] ZnO nanostructures using antisolvent approach [29] and hydrangea-like micro-sized SnO by using choline chloride/urea [30]

In the present work, we have employed low cost acidic deep eutectic solvent, choline chloride (ChCl) / p-toluene sulphonic acid (PTSA) as a templating and as an acidic hydrolyzing agent to synthesize biphasic TiO2 from tetra butyl titanate precursor (TBT) via sol-gel method avoiding the use of corrosive mineral acids The complete characterization and photocatalytic activity of as-prepared samples was studied Further, TiO2 nanoparticles were calcined at 750oC to investigate the effect of heating on the structural properties of TiO2 such as phase change, particle size and based on that photocatalytic activities were determined under UV light illumination

2 Materials and methods

2.1 Reagents and Chemicals

All chemicals used in this work, tetra butyl titanate (TBT) ((Ti(OCH2CH2CH2CH3)4, 97%)), choline chloride ((CH3)3N(Cl)CH2CH2OH, 99%)), para toluene sulphonic acid (CH3C6H4SO3H, 98.5%), ethanol, Degussa P-25 (99.5%) and methyl orange were of analytical purity grade, supplied by Sigma-Aldrich and were used as received without any further purification

2.2 Preparation of acidic DES

The acidic deep eutectic solvent was prepared by using choline chloride (ChCl) and p-toluene sulphonic acid (PTSA) in 1:1 ratio The solvent was heated at 80oC under continuous magnetic stirring till a completely miscible and colourless transparent liquid was obtained

2.3 Synthesis of TiO 2

To obtain TiO2 nanoparticles via sol-gel approach, 1 ml of TBT as titania precursor was added drop wise to

3 ml of prepared DES followed by the addition of 5 ml of double distilled water The mixture was magnetically

stirred vigorously for 30 minutes at room temperature A clear and transparent homogenous solution was formed

The solution was heated and aged at 100oC for 24h in an oven The resultant products were collected, separated by centrifugation, washed thoroughly with distilled water and ethanol, dried in an oven at 70°C overnight to obtain

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TiO2 powder To investigate the effect of concentration of acidic DES on phase and morphology of TiO2, experiments were performed under similar conditions with different molar concentration of acidic DES to obtain products, which were labelled as DES-1(3 ml), DES-2 (6 ml), DES-3 (9 ml) and DES-4 (12 ml) respectively To study the effect of calcination temperature on phase structure, as-synthesized products were calcined at temperature

750oC using a multisegment programmable furnace The temperature was increased at a rate of 3.0oC/min, kept for 3h and then decreased to room temperature The calcined products were labelled as DES-1C, DES-2C, DES-3C and DES-4C

2.3 Characterisation of TiO 2 nanoparticles

XRD measurements were performed using an X-ray powder diffractometer (XPERT-PRO) operated at 45

kV and 40 mA with Cu-Kα radiation (λ=0.15406 nm) and a scan angle (2θ) of 5-80o UV-vis spectra were recorded

on spectrometer Lambda 35 (Perkin Elmer) equipped with diffuse reflectance accessory at room temperature BET surface areas were calculated by using BET (Brunauer Emmett Teller) equation thermally heated at 180oC for 2 h (Quantachrome Nova Win version 10.01) SEM images of samples were obtained using Model SU 8010 (Hitachi) Raman spectra were recorded with Raman spectrometer IHR 550, JY-HORIBA, using 488 nm wavelength of an Argon Laser with Grating-1800 grooves/mm, Detector-Palatier effect cooled ccd (NEW-PORT)

2.4 Photocatalytic experiments

The photocatalytic activity of as-synthesized TiO2 samples was investigated by measuring discoloration of methyl orange in an aqueous solution under UV irradiation and compared with commercial Degussa P-25 Discoloration was carried in an immersion well type photochemical reactor made of Pyrex glass with a water-circulating jacket maintained at room temperature, an opening for supply of oxygen and inlet through which the samples were taken from time to time during the experiment with the help of a syringe The photo-reactor was placed on a magnetic stirrer A UV lamp of 150 W was used as a light source to irradiate the solution 60 mg of TiO2 sample powder was mixed in 200 ml of 20 ppm aqueous methyl orange solution (2 g/100 ml) To attain the adsorption-desorption equilibrium, solution was stirred in the dark for 30 min 5 ml of sample was taken at regular intervals and further separated by centrifugation at 4000 rpm for 20 min The obtained upper clear solution was analysed using a UV-vis spectrometer The percentage discoloration was determined according to eqn 1:

η = 

where A o and A t are the absorbances at t=0 and time t respectively assessed by evaluating the absorbance at 463 nm

on UV-vis spectrometer

3 Results and discussions

3.1 XRD analysis

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X-ray diffraction (XRD) spectra of as-synthesized TiO2 provided the detailed crystalline phase information

as given in Fig 1 XRD patterns showed the variation between anatase (101) and rutile (110) phases with change in

concentration of acidic DES The crystalline phase, phase composition and particle size of TiO2 and calcined TiO2 was determined and compiled in Table 1 and 2 The relative TiO2 phase percentage was calculated (eqn 2) by

analyzing the diffraction peak intensities of anatase (101) and rutile (110) [31] The rutile fraction (FR) was calculated by the following equation eqn 2:

where IA and IR are the intensities of anatase peak (101) and rutile peak (110) respectively

As shown in Fig 1, Sample DES-1 and DES-2 showed the formation of pure anatase phase of TiO2 (JCPDS-21-1272) Further, increasing the concentration of acidic deep eutectic solvent lead to decrease in the content of anatase (from 100% in sample DES-1 and DES-2 to 34.7 % in DES-4) with the appearance of rutile phase (28.4% in sample DES-3 and 65.3% in DES-4) (JCPDS-21-1276) The average crystallite size of nanoparticles was

estimated by applying Debye-Scherrer equation (eqn 3) on the diffraction peaks of anatase (101) and rutile (110),

which showed small variation in size in the range of 5.6 to 6.8 nm

Where τ is the mean size of the crystalline domains, λ is the X-ray wavelength (0.154 nm), β is the FWHM of the catalyst, K = 0.89 and θ is the diffraction angle The results are presented in Table 1

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Fig 1 XRD patterns of sample DES-1 (a), DES-2 (b) DES-3 (c) and DES-4 (d)

Table 1 Physico-chemical properties of as-synthesized TiO2.

From XRD analysis of calcined samples (Fig 2), it was found that the crystal phase and composition of TiO2 nanoparticles can be entirely changed by calcination With calcination, crystallinity and size of nanoparticles increases and average size of the nanoparticles was in the range of 25 to 35 nm There was a complete

transformation of anatase to rutile phase in DES-4 as evident from Fig 2 These results are in good agreement as

reported previously in the literature that phase change from anatase to rutile is initiated at temperature higher than

600oC [32] However, complete conversion into rutile phase does not takes place for the synthesized samples

DES-Sample Volume of DES

added

TiO2 Phase

Anatase

%

Rutile

%

XRD crystallite size (nm)

SBET (m2g-1)

Band gap (Eg)

DES-1 3 ml Anatase 100 - 6.8 87.6 3.01 DES-2 6 ml Anatase 100 - 6.0 90.7 3.0 DES-3 9 ml A & R 71.6 28.4 5.9 102.4 3.0 DES-4 12 ml A & R 34.7 65.3 5.6 83.4 3.08

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1, DES-2 and DES-3, instead results into anatase–rutile mixtures with increased rutile content from 0 to 19.4

(DES-1 to DES-(DES-1C), 0 to 75.2 (DES-2 to DES-2C) and 28.4 to 93.4 % (DES-3 to DES-3C) The average crystallite sizes, phase and phase composition of various heated samples are listed in Table 2

Fig 2 XRD patterns of calcined TiO2 samples, DES-1C (a), DES-2C (b), DES-3C (c) and DES-4C (d)

3.1.1 Influence of concentration of acidic DES on the phase structure of TiO 2

It has been observed from XRD analysis that the concentration of DES has great influence on the phase structure of TiO2 By increasing the volume of acidic DES in sample DES-1 to sample DES-4, acidic concentration

of the solution increases, which results into change in phase percentage composition of TiO2 The effect of acidic character on the crystal phase of TiO2 was explained by Cheng and co-workers [33], who suggested that Ti (IV) complexes exist as octahedral coordinated complex ions in the solution He proposed that increase in acidity affect the type of bonding between [TiO6] units, which are formed during the hydrolysis of TiO2 precursor in the reaction system The decrease in acidic character of the solution increases the probability of edge shared bonding, which could favour the anatase phase formation of TiO2 as observed in sample DES-1 and DES-2 Increase in the acidity

of reaction mixture in DES-4 results into higher rutile phase composition

Table 2 Physico-chemical properties of calcined TiO2.

Sample Volume of DES

added

Phase change after calcination

Anatase

%

Rutile %

XRD crystallite size (nm)

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3.2 SEM analysis

Fig 3 represents the SEM images of the as-synthesized TiO2 nanoparticles in different reaction acidic concentrations It can be observed that these TiO2 nanoparticles in different DES concentration have variable morphologies When the low concentration of DES was used in DES-1, granular well-ordered nanospheres with grain size of ca 6 nm were observed (Fig 3 a) Further increase in the volume of DES from 3 ml to 6 ml, these nanoparticles combine (Fig 3 b) and appears to be in a process of making coral-like structure Fig 3 c and d shows the morphology of samples DES-3 and DES-4 fabricated at higher concentration of acidic DES, in which TiO2 nanospheres form nanoparticle-aggregated coral-like structure

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Fig 3 SEM images of samples DES-1 (a), DES-2 (b) DES-3 (c) and DES-4 (d)

The mechanism for the growth of TiO2 nanospheres having coral-like morphology can be described as following During crystallization process, DES is likely act as a capping agent and growth controller, preferring to selectively adsorb on the surface of nanoparticles The crystal growth takes place by the initial formation of tiny nuclei, which grow into small sized primary nanoparticles Further, these particles assemble into bigger spherical aggregates leading to the formation of assembled and thermodynamically favorable coral-like structure (Fig 3 c)

3.3 Raman spectra

Fig 4 demonstrate the Raman spectra of the most active as-synthesized TiO2 photocatalyst, sample DES-3

In bulk material, Raman lines at 144 (Eg), 197 (Eg), 639 (Eg), 399 (B1g), 513 (A1g) and 519 (B1g) cm-1 represent the presence of anatase phase [34, 35], signals at 143 (B1g), 447 (Eg), 612 (A1g) and 826 (B2g) shows the presence

of rutile [36] and signals at 128, 246, 320 and 366 cm-1 indicate the presence of Brookite phase [37] In Fig 4 of

sample DES-3, the presence of Raman signals at around 151 (Eg), 448 (Eg), 514 (A1g) and 615 (A1g) can be observed, which characterize the presence of both anatase and rutile phase, confirmed by the XRD results Due to

the small size of the nanoparticles, vibrational frequencies are broadened with respect to the bulk material

transformation of. .. [TiO< sub>6] units, which are formed during the hydrolysis of TiO< sub >2< /sub> precursor in the reaction system The decrease in acidic character of the solution increases the probability of. .. the concentration of DES has great influence on the phase structure of TiO< sub >2< /sub> By increasing the volume of acidic DES in sample DES-1 to sample DES-4, acidic concentration

of the