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PVA assisted low temperature anatase to rutile phase transformation (ART) and properties of titania nanoparticles
Accepted Manuscript PVA Assisted Low Temperature Anatase to Rutile Phase Transformation (ART) and Properties of Titania Nanoparticles Shrabani Mondal, Rashmi Madhuri, Prashant K. Sharma PII:
S0925-8388(15)30203-6
DOI:
10.1016/j.jallcom.2015.06.087
Reference:
JALCOM 34429
To appear in:
Journal of Alloys and Compounds
Received Date: 4 February 2015 Revised Date:
8 June 2015
Accepted Date: 10 June 2015
Please cite this article as: S. Mondal, R. Madhuri, P.K. Sharma, PVA Assisted Low Temperature Anatase to Rutile Phase Transformation (ART) and Properties of Titania Nanoparticles, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.06.087. 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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Graphical Abstract
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Present work reports anatase to rutile phase transformation (ART) of titania nanoparticles at very low temperature (180° C) just by introducing polyvinyl alcohol (PVA) during co-precipitation followed by hydrothermal synthesis.
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PVA Assisted Low Temperature Anatase to Rutile Phase Transformation (ART) and Properties of Titania Nanoparticles Shrabani Mondal1, Rashmi Madhuri2, Prashant K. Sharma1,† 1
ABSTRACT
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Functional Nanomaterials Research Laboratory, Department of Applied Physics, Indian School of Mines (ISM), Dhanbad 826004, India 2 Department of Applied Chemistry, Indian School of Mines (ISM), Dhanbad 826004, India † Corresponding Author Email: [email protected]
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Anatase to rutile phase transformation (ART) of titania nanoparticles is observed at very low temperature (180° C) just by introducing polyvinyl alcohol (PVA) during co-precipitation
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followed by hydrothermal synthesis. The detailed investigations pertaining to the structural, optical and electrochemical properties of the nanosized titania and titania/PVA nanohybrid has been carried out. The crystallite size and crystal structure is confirmed using X-ray diffraction (XRD). Transmission electron microscopic (TEM) image reveals formation of spherical NPs in both the cases. Identification of functional groups is done using Fourier
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transform infrared spectroscopy (FTIR). The photoluminescence studies showed that emission slightly shifts towards higher wavelength side with remarkable decrease in intensity for TiO2/PVA nanocomposite (rutile samples). The remarkable decrease in PL intensity in
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TiO2/PVA nanocomposite (rutile samples) is explained considering the surface passivation during growth process. Ion transportation is monitored via Cyclic voltammetric (CV) and
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Electrochemical Impedance Spectroscopy (EIS) measurements. A significant enhancement of peak cathodic current in case of nanocomposite modified electrode is observed. It is assumed that TiO2/PVA (rutile) nanoparticles provided the conducting path for the electrons and hence enhanced the electrochemical reaction.
Keywords: Nanostructured materials: chemical synthesis: X-ray diffraction: phase transitions: transmission electron microscopy: electrochemical impedance spectroscopy.
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ACCEPTED MANUSCRIPT 1. Introduction Inorganic–organic hybrid materials are of profound interest to chemists, physicists and material scientists in recent years. These new and versatile kind of materials are superior to ordinary inorganic or organic one. Though they possess the individual properties
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of its components, still, some enhanced or new functions are exhibited in their hybrid. Size and shape dependent unique properties of nanomaterials permit more flexibility in physical, optical, chemical and electrical properties of these hybrids [1].
Poly vinyl alcohol (PVA) is a well known member of organic family. It is non toxic,
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biocompatible, water soluble synthetic polymer [2]. It has superior adhesive, film forming and emulsifying properties [3]. Highly active –OH group present in its polymer chain allows better
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reactivity with other materials. PVA has been chosen as organic component in most of the previous studies related to of TiO2–polymer hybrid material. In 1960’s, Dr. Fujishima of Japan first discovered Titanium as a photo catalyst [4-6]. Since then numerous efforts has been devoted in studying the new properties by restructuring TiO2 in nano scale. Till now,
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TiO2–polymer hybrid material is one of the most studied multi functional nanomaterial. It has vast applications extending from environmental monitoring to microelectronic devices. To name a few, photo-electrochemical activity [7], UV detector, ultrasonic sensor, gas sensor
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[8], solar energy conversion [9], self cleaning, anode material for Li ion batteries, coating material for optical elements [10-12] etc. Environmental compatibility, non toxicity, phase
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stability at high temperature, low price and easy synthesis protocol are other inherent properties of TiO2 which makes it useful in all fundamental studies and practical applications [13]. Some of the stated applications are dependent on the properties achieved after modification of TiO2 with some foreign compounds (inorganic/organic). In the above scenario, concerning the wide field of application of these two materials, TiO2/PVA hybrid is more attractive to researchers. So far a variety of works has been attempted to expand the diversity of TiO2 nanomaterials by adding organic compound PVA. A special concern has devoted to improve the catalytic property of TiO2 [14-16]. Yang et al. has established a novel visible light active photo catalyst by modifying TiO2 NPs with conjugate polymer PVA [17].
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ACCEPTED MANUSCRIPT Abbasizadeh et al. has successively applied TiO2 /PVA nanocomposite in detecting relatively small dose of heavy metals, Ni (13.0 mg/g), U (36.1 mg/g), Cd (49.0 mg/g) in wastewater [18]. Transparency in visible region along with strong UV absorption capability of TiO2/PVA nanohybrid is highly demanded in cosmetic, optical, food packaging industry [19].
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Sugumaran et al. has achieved excellent optical behavior such as very high transmittance, wide band gap, low absorption coefficient, low extinction coefficient with high refractive index in the hybrid system than pure PVA. This group has also demonstrated considerably enhanced dielectric properties compared to the virgin samples in the same work [3]. Most
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frequent synthesis procedure found in literature is the hydrolysis of Ti (IV) precursor in acidic medium [20-22].
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It is well known that TiO2 exists in three polymorphic phases, (a) anatase, space group I41/amd (b) rutile, space group P42/mnm and (c) brookite, space group Pcab [23]. Besides these three, two multiplicity of rutile phase at high pressure forms TiO2 (II) with a PbO2 structure [24] and TiO2 (H) with a hollandite structure [25]. Due to lesser density of
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anatase phase (3.894g/cm3) it undergoes transition into rutile (4.25g/cm3) in order to attain better stability. Transition temperature ranges from 450°C to 1200°C [26, 27]. TiO2 is n-type indirect band gap semiconductor in its pure form. Anatase phase posses band gap energy
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3.2 eV and for rutile it is 3.02 eV [28].
In the present work we have adopted a two step bottom up approach for sample
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preparation. Majority of the works focused on polymorphic Titania, formation of mixed phase TiO2 (anatase and rutile) has reported [29, 30] below the temperature 450°C. Depending upon the additives and preparation conditions the transformation rate has also been sighted [31]. According to best of our knowledge it is the first time where we are reporting the fully transformed rutile structures from anatase at quite lower temperature just by adding PVA (organic ligand) with Ti precursor. Subsequent changes in structural and optical properties are monitored. Role of PVA in electron transfer mechanism is also explored. 2. Materials and Methods
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ACCEPTED MANUSCRIPT 2.1. Chemicals Used For synthesis purpose, Titanium Chloride (TiCl3) with 99.99 % purity (Sigma Aldrich, Germany), PVA (E. Merck Limited, India), MW=145,000, Sodium hydroxide (NaOH) and Ethanol (E. Merck Limited, India) were used as starting materials. All chemicals were of
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analytical reagent grade and were directly used without any further purification. 2.2. Synthesis of Pure TiO2
Titanium chloride (TiCl3) was used as raw material to prepare pure TiO2 nanoparticles (NPs) by two step chemical route. Co-precipitation followed by hydrothermal method. 1 mL
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of TiCl3 solution was added into 40 mL of NaOH (0.4 mol/L) aqueous solution under continuous stirring for few hours. Occurrence of blue precipitate confirms the formation of
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Titanium hydroxide [Ti(OH)3]. In the next step the precipitate was centrifuged and washed with de-ionized water and ethanol for several times to remove NaCl and excessive NaOH. Washing process was continued until the pH value of the filtrate was about 7. The obtained filtrate was dispersed into 70 mL of de-ionized water with magnetic stirring for 10 min to form
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blue slurry. The mixture was placed in an autoclave maintained at 180 °C for 6 h and allowed to cool to room temperature gradually. Stable anatase TiO2 dispersion was formed and no post-synthetic purification process was required. TiO2 nanostructures were separated
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via centrifugation. Finally, the obtained product was dried at 40˚C in a hot air oven. Possible chemical reactions can be described as follows.
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(1) (2)
2.3. Synthesis of TiO2/PVA Nanocomposites At first a viscous solution of PVA matrix was formed by adding 4 wt% of PVA into 100 ml double distilled water under more than 3 hours of vigorous stirring at 80˚C until a colorless transparent solution was obtained. Then the solution was allowed to cool at room temperature gradually. In another beaker 1mL TiCl3 was added into 40 mL of 0.4 molar
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ACCEPTED MANUSCRIPT aqueous NaOH solution under vigorous stirring. At that time colorless PVA solution was introduced into the aqueous NaOH solution. After few times of stirring the precipitate was obtained. Finally the precipitate was separated out and washed with de-ionized water and ethanol until pH value reached 7. The obtained filtrate was dispersed into 70 mL of de-
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ionized water under stirring and then transferred into a stainless steel autoclave maintained at 180 °C for 6 h. After cooling down to normal tem perature the solution was centrifuged and collected. Finally, the obtained product was dried at 40˚C in a hot air oven.
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3. Instrumentation Used
Bruker D8 Focus powder diffractometer with CuKα radiation (λ=1.5406 Å) was used
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for recording X-ray diffraction (XRD) patterns in the range 2θ= 20° -70°. All the measurements were carried out at room temperature with operating condition 30 kV, 25 mA. Surface morphology of the NPs were observed using field emission scanning electron microscope (FESEM), model Supra-55, ZEISS, Germany. Transmission electron microscopy
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(TEM), high resolution transmission electron microscopy (HRTEM) and energy dispersive Xray spectroscopy (EDX) were performed using Tecnai 30 G2 S-Twin microscope operated at 300 kV. The fourier transformed infrared (FT-IR) spectra was recorded in the range 400-
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4000 cm-1 on a Perkin-Elmer Spectrum Bx (Perkin-Elmer, U.S.A.). KBr pellets mixed with very small amount of samples were used for IR analysis. Photoluminescence spectroscopy
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was employed to examine emission properties of the samples. LS 55 Perkin Elmer spectrophotometer with a xenon discharge lamp (340nm excitation wavelength) was used for this purpose. For the exploration of electrochemical performance CH instrument of model no. 660 C, USA is used.
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ACCEPTED MANUSCRIPT 4. Result and Discussion Fig. 1 represents XRD spectra of prepared TiO2 and TiO2/PVA nanocomposites. Appearance of the peaks at the 2θ positions 25.6°, 38.11°, 48.49°, 54.22 ° and their relative intensities are consistent with the anatase TiO2 according to the JCPDS file number 86-
2θ=27.43°, 36.07°, 38.86°, 41.35°, 53.89°, 56.41° are
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1157. Whereas, diffraction pattern of TiO2/PVA nanocomposite having peak positions at in excellent agreement with standard
JCPDS file number 89-0555 for rutile TiO2. The average crystallite sizes of the samples are
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calculated using Scherrer equation [32] for the major prominent peaks and taking their
(1)
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average.
Here, λ = wavelength of X-Ray used, θ is the Bragg’s angle, β = full width at half maxima on 2θ scale. After fitting the curves properly (χ2=0.99) full width half maxima was estimated. Value of lattice parameters for tetragonal TiO2 (a=b≠c) for most intense peaks has been
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calculated using the following formula:
(2)
Where, dhkl is the inter planer separation corresponding to miller indices h, k, l and a, b, c are
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the lattice constants. In pure TiO2 samples, the lattice constant were calculated to be a=b= 3.69 nm and c= 9.36 nm. For TiO2/PVA nanocomposites, the lattice constant was calculated
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to be a=b= 4.49 nm, c=2.88nm. These values are nearly equal to the values found in JCPDS files for anatase and rutile structures. Role of introducing PVA is extensively observable from XRD spectra. Formation of comparatively smaller sized particles as well as anatase to rutile phase transformation (ART) at relatively lower temperature (180° C). The average calculated crystallite size changes from 17 nm to 9 nm just by capping PVA on TiO2 nanoparticles (NPs). Perhaps the nucleation (anatase) process during reaction is suppressed by PVA which consequences smaller NPs. To justify the observed phase transformation, it is important to consider the phase stability criteria. Literature reveals this kind of transformation
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ACCEPTED MANUSCRIPT among polymorphic phases of TiO2 as a function of several parameters such as initial phase, initial particle size, concentration of foreign impurities, annealing temperature and reaction atmosphere as well [33-36]. According to Mahshid et al. activation energy is the key factor which controls the phase transformation at low temperature [37]. In NPs, due to effectively
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larger surface area, activation energy will be less. A sharp decrease in particle size after introducing PVA thus acts as a motive behind this anatase to rutile phase transformation. In chemical synthesis procedure NPs growth mechanism is governed by surface precipitation of solvated atoms or clustering of few atoms or aggregation of atoms along some preferred
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direction [38].
Oswald-ripening (O-R) based precipitation mechanism during first step and oriented
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attachment (OA) mechanism during hydrothermal process has responded positively in NPs growth process. Smaller particles possess larger chemical potentials associated with their surfaces [39]. This excess energy promotes the relative solubility of smaller particles into bigger one. Kinetics of O-R follows the power law [40]:
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(3)
Where, D0 is the initial particle size, D (t) is the size at time t, k is the rate constant for the limiting step and exponent n is dependent on nature of rate limiting step. Penn et al. has first
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discovered another new kind of growth mechanism (OA) during hydrothermal treatment of TiO2 NPs [40-42]. In this procedure NPs itself perform as building block in crystal growth.
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Crystals grown via OA pathway posses several defects such as edge dislocation, stacking fault, twins etc. in the grain boundary regions [43]. Consequently some kinds of strain due to deformation of host lattice will generate. Investigation of induced strain has been carried out with the help of Williamson-Hall plots (Fig. 2). In the present case we observe a negative slope of magnitude 0.006 in pure sample whereas a positive slope (0.0142) is noticed in the TiO2/PVA nanocomposites sample. This is as a consequence of decrease in crystallite size with the introduction of PVA. If the NPs are tightly drawn together by some means inside the solution then in some cases thermal energy cannot afford the exploration in all orientations.
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ACCEPTED MANUSCRIPT Hence a vast number of contact points align themselves in a particular direction to avoid these interfaces [31]. At room temperature rutile is the most stable phase of bulk TiO2 whereas in nano region it is anatase. Average surface energies for anatase and rutile phase predicted by static molecular dynamics (MD) calculations are 1.3 and 1.9 J/m2 respectively
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[44, 45]. It is already known that contribution of average surface energy to the total free energy is a function of the surface area. In this scenario among all polymorphic phases of TiO2 surface contribution of anatase is least, which helps it to show better stability in nanoscale. Finnegan et al has found 465°C as the tr ansformation temperature during the
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heating process of anatase TiO2 having particle size 10-15 nm [46]. More interestingly it is
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observed that for a particular size (Dc≈ 14 nm) a crossover region among the polymorphic phases exists. Anatase and rutile both possess equal free energies for this critical size [35, 47, 48]. This may explain why rutile formed only after anatase NPs has coarsened. In the assistance of PVA this critical size range (10-15 nm) has been achieved which initiated the transformation to rutile phase.
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Both anatase and rutile TiO2 samples were prepared followed by same synthesis protocols as well as reaction conditions, only difference is addition/introduction of PVA in case of synthesis of rutile samples. Our chemical reaction has produced a pure anatase
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phase samples [confirmed using XRD results; see Fig. 1 (a)] when reaction is carried out without PVA, whereas, introduction of PVA resulted in pure rutile phase samples [confirmed
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using XRD results; see Fig. 1 (b)] by following the same synthesis process. In this case TiO2, the size of nanoparticle is very important and critical factor to obtain anatase to rutile phase transformation. As already reported by the other workers, TiO2 nanoparticles within the critical size range (Dc=10-15 nm) can undergo such phase transformation. Zhang et. al. [44] and Oliver et. al. [45] has already performed the empirical and theoretical studies focused on the size dependence of relative phase stability among the polymorphs of titania nanoparticles. Below a certain particle size (for TiO2 ≈14 nm) the free energy curve shows a crossover between these two phases. Consequently bulk rutile is stable relative to anatase
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ACCEPTED MANUSCRIPT by an amount of free energy (∆G) ≈ 67 kJ mol-1. Here it is worth to mention that PVA acts as a surfactant and passivates the surface of TiO2 nanoparticles and restricts the growth of nanoparticles within the size range of 15 nm, as confirmed by XRD and TEM results. Hence we can conclude that in our work anatase to rutile phase transformation is extremely a size
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dependent effect and PVA has controlled the growth process and subsequently the particle size of TiO2 nanoparticles, and thus assisted in Anatase to Rutile phase transformation process by restricting the growth of TiO2 nanopaticles with in 15 nm.
Fig. 3 shows representative FESEM, TEM and HRTEM images of TiO2 and
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TiO2/PVA hybrid NPs. Formation of smaller sized NPs in case of composite is clear from
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FESEM and TEM micrographs. We also observe a wide distribution of nearly spherical NPs throughout the surface. The imaged lattice spacing 0.34 nm corresponds to (101) lattice planes of anatase TiO2 (inset of figure 3c). Lattice Spacing of 0.30 nm (inset of figure 3d) designates (110) planes which is also observed in the XRD spectra for rutile TiO2. This observation again confirms the phase transformation after addition of PVA. Lesser
clear in TEM images.
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agglomeration with distinguish spherical particles in the composite system is even more
To determine the structural properties of the inorganic-organic nano hybrid FTIR
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spectroscopy has been employed in the wavelength region 4000-500 cm-1. Functional groups present in the samples have been determined from the dips of the transmittance vs
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wavenumber spectra (Fig. 4). Fig. 4 shows FTIR spectra of (a) pure TiO2 and (b) pure PVA and (c) TiO2/PVA hybrid NPs. Inset of Fig. 4 represent zoomed view of the IR spectra shown in (c) in the region 2400cm-1 -4000 cm-1. Band near to 3440 cm-1 confirms the presence of Ti-OH stretching vibration. Stretching band of O-H is known to be the characteristics IR band of alcohol or phenols [49]. For pure PVA this mode occurs at 3470 cm-1 [50]. In present case this band has shifted slightly with broadening width. May be broadening is due to the specific interaction of TiO2 NPs with the PVA matrix. Bands in the region 2800 to 3100 cm-1 corresponds to the C-H vibration which forms the basic organic structure of PVA. A broad bands around 3444 cm-1 to 3800 cm-1 which are the characteristics of tetrahedral Page 9 of 25
ACCEPTED MANUSCRIPT coordinated vacancies, Ti-OH has appeared in the spectra indicates again the existence of TiO2 NPs into the composite [51]. Absence of any band at 2130 cm-1 indicates that Ti3+ ions are not present in the sample. Absorption in the range 1010-1200 cm-1 is found due to O-O stretching vibration. 1400 cm-1 band is caused by lattice vibration of TiO2 NPs. Two more
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intense bands are found in the composite system at 1636 and 1332 cm-1 which originates from the water absorbed and O containing carbon interacting with TiO2 NPs. Vibration due to –CH2 groups of PVA at 2921 cm-1 has also seen [52]. Other bands at 820 cm-1 and 520cm-1 belong to Ti-O-Ti asymmetric stretching [50]. Observed bands in pure sample is not clearly
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visible in the composite as organic structure has suppressed those bands.
Fig. 5 illustrates the photoluminescence features of prepared samples recorded at
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room temperature using a xenon discharge lamp as source. We have recorded the excitation spectra (Fig. 5a) for pure sample. The emission profiles (Fig. 5b) for both the cases are captured particularly by the excitation of 340 nm found from the excitation spectra previously. For clear visualization of the exact emission peaks we have de-convoluted the
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spectrums (Fig. 5c, d). In TiO2 samples the emission peaks originates due to three physical reasons: oxygen vacancies [53, 54], surface states/defects [55], self trapped excitations [53, 56]. Oxygen vacancies as well as Ti4+ cations associated with the oxygen vacancies
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contribute a major part in the surface states [57, 58]. A weak emission at 420 nm (Fig. 5d, peak 3) is visible in case of composite. Y. Lei et al. has found an intrinsic level transition
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located at 425 nm in TiO2 NPs [59]. So we can assign this 420 nm band as in earlier case rather than a surface state assisted transition. Bluish green emission (Fig. 5c, peak 3 & 5d, peak 6) at 485 nm (2.55 eV) is originated from intrinsic levels rather than surface defects. This peak in both spectra is allocated as a self trapped excitation band localized on the octahedral site. Charge transfer from Ti3+ ion to oxygen anion species in TiO68-complex [53] is its origin. A broad emission peak observed from 335 nm - 415 nm and centered at 380 nm (Fig. 5d, peak 1) can be attributed as the near band edge emission of rutile TiO2 [60]. Tripathi et al. have reported that the existence of compressive strain increases PL intensity rather than tensile strain [61]. Oxygen vacancies can create deep level defect states in the
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ACCEPTED MANUSCRIPT oxide samples. In our case same phenomena is observed from the comparative PL spectra represented. Deep traps at 0.59 eV below the conduction band gives rise emission at 460 nm in TiO2 [53, 54]. We also observe two such kinds of emissions around 458 nm in both cases (Fig. 5c, peak 2 and 5d, peak 5) which may be denoted as emission mainly from
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oxygen vacant states. Band at 438 nm (Fig. 5c, d) is observed in rutile sample. Emission bands located at 521 nm (Fig. 5c, peak 4 and 5d, peak 7) is generated by the radiative recombination of self trapped excitons and the transitions originated from the surface states of the NPs [62]. As PVA is a surface passivating agent so it has quenched a significant
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number of surface traps which has affected the intensity to a greater extent. PL band in rutile
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TiO2 at long wavelength side ≅ 835 nm is attributed to the oxygen vacancies [62]. Although, due to measurement limitation (maximum PL spectral range 200 nm -700 nm) of our instrument, we were unable to record the PL spectra in near infrared region (≅ 835 nm; attributed to the oxygen vacancies). Emissions from TiO2 in the visible region can be attributed to different Ti vacancies, oxygen vacancies and interstitial fields [61].
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Cyclic voltammetric (CV) and electrochemical impedance spectroscopy (EIS) are executed by conventional 3-electrode electrochemical cell consisted of one Pt wire as counter electrode, Ag/AgCl reference electrode and modified pencil graphite electrode as
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working electrode 10.0 ml electrochemical cell consist of 1M KCL solution as a supporting electrolyte. Prior to electrode modification with synthesized nanocomposite, the graphite
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pencil rods were dipped in 0.1M HNO3 for 2min. After that, the rods were washed with distilled water and rubbed with cotton. The treated pencil rods were housed in a micropipette tip and wrapped with copper wire at one end, while other end was left free for further modification. The as-prepared pencil graphite electrode was further modified with a suspension of NPs (0.01 mg) dispersed in 1.0 mL DMSO by simple dip-coating method. The graphite electrode is kept fixed into the solution for few minute and then dried at room temperature naturally before executing further study. CV and EIS measurements are performed at room temperature. For CV measurement, a blank run was taken in the
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ACCEPTED MANUSCRIPT potential window of -1.0 to +1.0V, in the absence of analyte. After that, 1mM ferricynide solution was added to the electrochemical cell and runs were recorded in the same ponetial range. Voltammetry is an analytical technique to keep the record of transfer of electron as a function of applied potential. Current potential response is a precious mean to find out the
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capacitive behaviour of any material. Fig. 6a, shows the CV profile of NPs modified electrodes in the Potential range - 0.1V to 0.1V for 1mM K3Fe (CN)6. Typical C-V graph clearly shows the appearance of redox peaks for K3Fe (CN)6 at the same positions as reported earlier. Profile demonstrates significant amount of enhancement in peak cathodic
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current in case of nanocomposite modified electrode. Major reason behind this output is the smaller size of hybrid NPs. Available larger surface area of TiO2/PVA NPs offer better
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contact area which helps a lot to faster the rate of redox reaction. It can be concluded that PVA leads larger exposure of TiO2 NPs with ions of the electrolyte. In other word, NPs have served the role of conducting path for the smooth progress of ion transportation within the cell.
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Besides the fact that TiO2/PVA hybrid nanostructure has smaller size and hence large available surface area, the CV and EIS are greatly affected by the crystal structure also. Anatase TiO2 has normal tetragonal crystal structure. Where as Rutile has a body-
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centred tetragonal unit cell. This means, anatase unit cell is more elongated as compared to rutile phase. In the rutile form, the Ti-atoms occupy the least space. This makes the rutile
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form the most stable as well as the density of Ti-atom will be more in case of rutile TiO2/PVA hybrid nanostructure. This provides better conducting path which results in the reduction of recombination of photogenerated electrons and holes and hence better CV and EIS response.
Under the positive scan when potential is sufficiently high to oxidize Fe(CN)6-4, gradual increase in anodic current is obtained. But some of the ions get depleted on the electrode surface and anodic current decays after few times. In reverse scan process cathodic current is generated due to the reduction process. CV profile is a result of rapid
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ACCEPTED MANUSCRIPT generation of different oxidation states of the molecule via electrochemical reaction. The plausible reactions are mentioned below: Fe(CN)6-4 Fe(CN)- + e ………… Anodic Current
(6)
Fe(CN)6-3 + e Fe(CN)-…….Cathodic Current
(7)
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Evaluation of the impedance performance is conducted using electrochemical impedance spectroscopy in presence of 1mM K3Fe (CN)6 solution as probe in support of 0.1M KCL solution. All the measurements are done over the frequency range 100 KHz to 100 MHz under the biasing potential -0.45 V with 5mv alternating voltage. Plotting the real
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part of impedance along X-axis (Z’) and imaginary part along Y-axis (Z’’) desired impedance profile is collected (Fig. 6b). In EIS, the semicircle diameter denotes the electron-transfer
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resistance. Decrease in semicircle diameter point out that composite NPs modified electrode is highly efficient in ion transportation than pure one. Hence EIS results are consistent with the CV results. Rapid transfer of ions among electrode and electrolyte within the electrolytic cell derive this kind of behavior. All in all application of PVA has brought drastic change in
5. Conclusions
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the TiO2 NPs modified electrode which is beneficial in electrochemical reaction.
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Anatase to rutile phase transformation (ART) of titania nanoparticles is achieved at very low temperature (180° C) just by introducing p olyvinyl alcohol (PVA) during co-
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precipitation followed by hydrothermal synthesis. The photoluminescence studies showed that emission slightly shifts towards higher wavelength side with remarkable decrease in intensity for PVA/TiO2nanocomposite (rutile samples). The remarkable decrease in PL intensity in PVA/TiO2nanocomposite (rutile samples) is explained considering the surface passivation during growth process. Ion transportation is monitored via CV and EIS spectroscopy. In both cases PVA/TiO2nanocomposite (rutile samples) modified electrode showed better response in ion transportation within the electrolytic cell than pure NPs decorated electrode. The reason may be ascribed to the larger exposure area of composite with the electrolyte. The prepared PVA/TiO2 nanocomposite seems to be one of the
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ACCEPTED MANUSCRIPT promising candidates for modern age lighting, electrochemical sensing and display applications.
Acknowledgement
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Authors are thankful to Department of Science and Technology, Government of India for sanction of Fast Track Research Project for Young Scientists to Dr. Rashmi Madhuri (Ref. No.: SB/FT/CS-155/2012) and Dr. Prashant K. Sharma (Ref. No.: SR/FTP/PS157/2011). Dr. Sharma (FRS/34/2012-2013/APH) and Dr. Madhuri (FRS/43/2013-2014/AC)
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are also thankful to Indian School of Mines, Dhanbad for grant of Major Research Project under Faculty Research Scheme. We are also thankful to Board of Research in Nuclear
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Sciences (BRNS), Department of Atomic Energy, Government of India for major research project (Ref. No. 34/14/21/2014-BRNS). Shrabani is also thankful to Indian School of Mines,
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Dhanbad for Junior Research Fellowship.
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Figure Captions
Fig. 1: X-ray diffraction spectra of (a) pure TiO2 NPs and (b) TiO2/PVA nanocomposite.
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Fig. 2: Williamson-Hall plots of (a) pure TiO2 NPs and (b) TiO2/PVA nanocomposite.
Fig. 3: Field emission scanning electron microscopic images of (a) TiO2 (b) TiO2/PVA NPs
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and transmission electron microscopic images of (c) TiO2 and (d) TiO2/PVA NPs. Inset
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represents ultra high resolution images.
Fig. 4: Fourier transform infra-red spectra of (a) pure TiO2 and (b) pure PVA and (c) TiO2/PVA hybrid NPs. Inset represent inlarged view of the IR spectra shown in (c) in the region 2400cm-1 -4000 cm-1.
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Fig. 5: Room temperature photoluminescence (a) excitation spectra of pure TiO2(b) emission spectra of TiO2 and TiO2/PVA NPs using excitation wavelength 340 nm. (c) and (d)
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represent emission spectra of pure and composite after de-convolution.
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Fig. 6: (a) Comparative CV profiles (b) EIS curves of NPs modified electrode.
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Research Highlights: •
Low temperature phase transformation of TiO2 nanoparticles from anatase to rutile Role of PVA in phase transformation
•
Synthesis of spherical shaped uniformly distributed PVA capped TiO2 NPs
•
Explained the charge transfer process among anatase to rutile phase
Enhanced electrochemical performance of TiO2/PVA nanohybrid modified
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electrode than pure TiO2 NPs modified electrode
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•
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transformation via luminescence studies.
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•
S0925-8388(15)30203-6
DOI:
10.1016/j.jallcom.2015.06.087
Reference:
JALCOM 34429
To appear in:
Journal of Alloys and Compounds
Received Date: 4 February 2015 Revised Date:
8 June 2015
Accepted Date: 10 June 2015
Please cite this article as: S. Mondal, R. Madhuri, P.K. Sharma, PVA Assisted Low Temperature Anatase to Rutile Phase Transformation (ART) and Properties of Titania Nanoparticles, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.06.087. 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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Graphical Abstract
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Present work reports anatase to rutile phase transformation (ART) of titania nanoparticles at very low temperature (180° C) just by introducing polyvinyl alcohol (PVA) during co-precipitation followed by hydrothermal synthesis.
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PVA Assisted Low Temperature Anatase to Rutile Phase Transformation (ART) and Properties of Titania Nanoparticles Shrabani Mondal1, Rashmi Madhuri2, Prashant K. Sharma1,† 1
ABSTRACT
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Functional Nanomaterials Research Laboratory, Department of Applied Physics, Indian School of Mines (ISM), Dhanbad 826004, India 2 Department of Applied Chemistry, Indian School of Mines (ISM), Dhanbad 826004, India † Corresponding Author Email: [email protected]
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Anatase to rutile phase transformation (ART) of titania nanoparticles is observed at very low temperature (180° C) just by introducing polyvinyl alcohol (PVA) during co-precipitation
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followed by hydrothermal synthesis. The detailed investigations pertaining to the structural, optical and electrochemical properties of the nanosized titania and titania/PVA nanohybrid has been carried out. The crystallite size and crystal structure is confirmed using X-ray diffraction (XRD). Transmission electron microscopic (TEM) image reveals formation of spherical NPs in both the cases. Identification of functional groups is done using Fourier
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transform infrared spectroscopy (FTIR). The photoluminescence studies showed that emission slightly shifts towards higher wavelength side with remarkable decrease in intensity for TiO2/PVA nanocomposite (rutile samples). The remarkable decrease in PL intensity in
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TiO2/PVA nanocomposite (rutile samples) is explained considering the surface passivation during growth process. Ion transportation is monitored via Cyclic voltammetric (CV) and
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Electrochemical Impedance Spectroscopy (EIS) measurements. A significant enhancement of peak cathodic current in case of nanocomposite modified electrode is observed. It is assumed that TiO2/PVA (rutile) nanoparticles provided the conducting path for the electrons and hence enhanced the electrochemical reaction.
Keywords: Nanostructured materials: chemical synthesis: X-ray diffraction: phase transitions: transmission electron microscopy: electrochemical impedance spectroscopy.
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ACCEPTED MANUSCRIPT 1. Introduction Inorganic–organic hybrid materials are of profound interest to chemists, physicists and material scientists in recent years. These new and versatile kind of materials are superior to ordinary inorganic or organic one. Though they possess the individual properties
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of its components, still, some enhanced or new functions are exhibited in their hybrid. Size and shape dependent unique properties of nanomaterials permit more flexibility in physical, optical, chemical and electrical properties of these hybrids [1].
Poly vinyl alcohol (PVA) is a well known member of organic family. It is non toxic,
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biocompatible, water soluble synthetic polymer [2]. It has superior adhesive, film forming and emulsifying properties [3]. Highly active –OH group present in its polymer chain allows better
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reactivity with other materials. PVA has been chosen as organic component in most of the previous studies related to of TiO2–polymer hybrid material. In 1960’s, Dr. Fujishima of Japan first discovered Titanium as a photo catalyst [4-6]. Since then numerous efforts has been devoted in studying the new properties by restructuring TiO2 in nano scale. Till now,
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TiO2–polymer hybrid material is one of the most studied multi functional nanomaterial. It has vast applications extending from environmental monitoring to microelectronic devices. To name a few, photo-electrochemical activity [7], UV detector, ultrasonic sensor, gas sensor
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[8], solar energy conversion [9], self cleaning, anode material for Li ion batteries, coating material for optical elements [10-12] etc. Environmental compatibility, non toxicity, phase
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stability at high temperature, low price and easy synthesis protocol are other inherent properties of TiO2 which makes it useful in all fundamental studies and practical applications [13]. Some of the stated applications are dependent on the properties achieved after modification of TiO2 with some foreign compounds (inorganic/organic). In the above scenario, concerning the wide field of application of these two materials, TiO2/PVA hybrid is more attractive to researchers. So far a variety of works has been attempted to expand the diversity of TiO2 nanomaterials by adding organic compound PVA. A special concern has devoted to improve the catalytic property of TiO2 [14-16]. Yang et al. has established a novel visible light active photo catalyst by modifying TiO2 NPs with conjugate polymer PVA [17].
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ACCEPTED MANUSCRIPT Abbasizadeh et al. has successively applied TiO2 /PVA nanocomposite in detecting relatively small dose of heavy metals, Ni (13.0 mg/g), U (36.1 mg/g), Cd (49.0 mg/g) in wastewater [18]. Transparency in visible region along with strong UV absorption capability of TiO2/PVA nanohybrid is highly demanded in cosmetic, optical, food packaging industry [19].
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Sugumaran et al. has achieved excellent optical behavior such as very high transmittance, wide band gap, low absorption coefficient, low extinction coefficient with high refractive index in the hybrid system than pure PVA. This group has also demonstrated considerably enhanced dielectric properties compared to the virgin samples in the same work [3]. Most
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frequent synthesis procedure found in literature is the hydrolysis of Ti (IV) precursor in acidic medium [20-22].
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It is well known that TiO2 exists in three polymorphic phases, (a) anatase, space group I41/amd (b) rutile, space group P42/mnm and (c) brookite, space group Pcab [23]. Besides these three, two multiplicity of rutile phase at high pressure forms TiO2 (II) with a PbO2 structure [24] and TiO2 (H) with a hollandite structure [25]. Due to lesser density of
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anatase phase (3.894g/cm3) it undergoes transition into rutile (4.25g/cm3) in order to attain better stability. Transition temperature ranges from 450°C to 1200°C [26, 27]. TiO2 is n-type indirect band gap semiconductor in its pure form. Anatase phase posses band gap energy
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3.2 eV and for rutile it is 3.02 eV [28].
In the present work we have adopted a two step bottom up approach for sample
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preparation. Majority of the works focused on polymorphic Titania, formation of mixed phase TiO2 (anatase and rutile) has reported [29, 30] below the temperature 450°C. Depending upon the additives and preparation conditions the transformation rate has also been sighted [31]. According to best of our knowledge it is the first time where we are reporting the fully transformed rutile structures from anatase at quite lower temperature just by adding PVA (organic ligand) with Ti precursor. Subsequent changes in structural and optical properties are monitored. Role of PVA in electron transfer mechanism is also explored. 2. Materials and Methods
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ACCEPTED MANUSCRIPT 2.1. Chemicals Used For synthesis purpose, Titanium Chloride (TiCl3) with 99.99 % purity (Sigma Aldrich, Germany), PVA (E. Merck Limited, India), MW=145,000, Sodium hydroxide (NaOH) and Ethanol (E. Merck Limited, India) were used as starting materials. All chemicals were of
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analytical reagent grade and were directly used without any further purification. 2.2. Synthesis of Pure TiO2
Titanium chloride (TiCl3) was used as raw material to prepare pure TiO2 nanoparticles (NPs) by two step chemical route. Co-precipitation followed by hydrothermal method. 1 mL
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of TiCl3 solution was added into 40 mL of NaOH (0.4 mol/L) aqueous solution under continuous stirring for few hours. Occurrence of blue precipitate confirms the formation of
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Titanium hydroxide [Ti(OH)3]. In the next step the precipitate was centrifuged and washed with de-ionized water and ethanol for several times to remove NaCl and excessive NaOH. Washing process was continued until the pH value of the filtrate was about 7. The obtained filtrate was dispersed into 70 mL of de-ionized water with magnetic stirring for 10 min to form
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blue slurry. The mixture was placed in an autoclave maintained at 180 °C for 6 h and allowed to cool to room temperature gradually. Stable anatase TiO2 dispersion was formed and no post-synthetic purification process was required. TiO2 nanostructures were separated
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via centrifugation. Finally, the obtained product was dried at 40˚C in a hot air oven. Possible chemical reactions can be described as follows.
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(1) (2)
2.3. Synthesis of TiO2/PVA Nanocomposites At first a viscous solution of PVA matrix was formed by adding 4 wt% of PVA into 100 ml double distilled water under more than 3 hours of vigorous stirring at 80˚C until a colorless transparent solution was obtained. Then the solution was allowed to cool at room temperature gradually. In another beaker 1mL TiCl3 was added into 40 mL of 0.4 molar
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ACCEPTED MANUSCRIPT aqueous NaOH solution under vigorous stirring. At that time colorless PVA solution was introduced into the aqueous NaOH solution. After few times of stirring the precipitate was obtained. Finally the precipitate was separated out and washed with de-ionized water and ethanol until pH value reached 7. The obtained filtrate was dispersed into 70 mL of de-
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ionized water under stirring and then transferred into a stainless steel autoclave maintained at 180 °C for 6 h. After cooling down to normal tem perature the solution was centrifuged and collected. Finally, the obtained product was dried at 40˚C in a hot air oven.
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3. Instrumentation Used
Bruker D8 Focus powder diffractometer with CuKα radiation (λ=1.5406 Å) was used
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for recording X-ray diffraction (XRD) patterns in the range 2θ= 20° -70°. All the measurements were carried out at room temperature with operating condition 30 kV, 25 mA. Surface morphology of the NPs were observed using field emission scanning electron microscope (FESEM), model Supra-55, ZEISS, Germany. Transmission electron microscopy
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(TEM), high resolution transmission electron microscopy (HRTEM) and energy dispersive Xray spectroscopy (EDX) were performed using Tecnai 30 G2 S-Twin microscope operated at 300 kV. The fourier transformed infrared (FT-IR) spectra was recorded in the range 400-
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4000 cm-1 on a Perkin-Elmer Spectrum Bx (Perkin-Elmer, U.S.A.). KBr pellets mixed with very small amount of samples were used for IR analysis. Photoluminescence spectroscopy
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was employed to examine emission properties of the samples. LS 55 Perkin Elmer spectrophotometer with a xenon discharge lamp (340nm excitation wavelength) was used for this purpose. For the exploration of electrochemical performance CH instrument of model no. 660 C, USA is used.
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ACCEPTED MANUSCRIPT 4. Result and Discussion Fig. 1 represents XRD spectra of prepared TiO2 and TiO2/PVA nanocomposites. Appearance of the peaks at the 2θ positions 25.6°, 38.11°, 48.49°, 54.22 ° and their relative intensities are consistent with the anatase TiO2 according to the JCPDS file number 86-
2θ=27.43°, 36.07°, 38.86°, 41.35°, 53.89°, 56.41° are
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1157. Whereas, diffraction pattern of TiO2/PVA nanocomposite having peak positions at in excellent agreement with standard
JCPDS file number 89-0555 for rutile TiO2. The average crystallite sizes of the samples are
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calculated using Scherrer equation [32] for the major prominent peaks and taking their
(1)
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average.
Here, λ = wavelength of X-Ray used, θ is the Bragg’s angle, β = full width at half maxima on 2θ scale. After fitting the curves properly (χ2=0.99) full width half maxima was estimated. Value of lattice parameters for tetragonal TiO2 (a=b≠c) for most intense peaks has been
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calculated using the following formula:
(2)
Where, dhkl is the inter planer separation corresponding to miller indices h, k, l and a, b, c are
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the lattice constants. In pure TiO2 samples, the lattice constant were calculated to be a=b= 3.69 nm and c= 9.36 nm. For TiO2/PVA nanocomposites, the lattice constant was calculated
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to be a=b= 4.49 nm, c=2.88nm. These values are nearly equal to the values found in JCPDS files for anatase and rutile structures. Role of introducing PVA is extensively observable from XRD spectra. Formation of comparatively smaller sized particles as well as anatase to rutile phase transformation (ART) at relatively lower temperature (180° C). The average calculated crystallite size changes from 17 nm to 9 nm just by capping PVA on TiO2 nanoparticles (NPs). Perhaps the nucleation (anatase) process during reaction is suppressed by PVA which consequences smaller NPs. To justify the observed phase transformation, it is important to consider the phase stability criteria. Literature reveals this kind of transformation
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ACCEPTED MANUSCRIPT among polymorphic phases of TiO2 as a function of several parameters such as initial phase, initial particle size, concentration of foreign impurities, annealing temperature and reaction atmosphere as well [33-36]. According to Mahshid et al. activation energy is the key factor which controls the phase transformation at low temperature [37]. In NPs, due to effectively
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larger surface area, activation energy will be less. A sharp decrease in particle size after introducing PVA thus acts as a motive behind this anatase to rutile phase transformation. In chemical synthesis procedure NPs growth mechanism is governed by surface precipitation of solvated atoms or clustering of few atoms or aggregation of atoms along some preferred
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direction [38].
Oswald-ripening (O-R) based precipitation mechanism during first step and oriented
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attachment (OA) mechanism during hydrothermal process has responded positively in NPs growth process. Smaller particles possess larger chemical potentials associated with their surfaces [39]. This excess energy promotes the relative solubility of smaller particles into bigger one. Kinetics of O-R follows the power law [40]:
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(3)
Where, D0 is the initial particle size, D (t) is the size at time t, k is the rate constant for the limiting step and exponent n is dependent on nature of rate limiting step. Penn et al. has first
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discovered another new kind of growth mechanism (OA) during hydrothermal treatment of TiO2 NPs [40-42]. In this procedure NPs itself perform as building block in crystal growth.
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Crystals grown via OA pathway posses several defects such as edge dislocation, stacking fault, twins etc. in the grain boundary regions [43]. Consequently some kinds of strain due to deformation of host lattice will generate. Investigation of induced strain has been carried out with the help of Williamson-Hall plots (Fig. 2). In the present case we observe a negative slope of magnitude 0.006 in pure sample whereas a positive slope (0.0142) is noticed in the TiO2/PVA nanocomposites sample. This is as a consequence of decrease in crystallite size with the introduction of PVA. If the NPs are tightly drawn together by some means inside the solution then in some cases thermal energy cannot afford the exploration in all orientations.
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ACCEPTED MANUSCRIPT Hence a vast number of contact points align themselves in a particular direction to avoid these interfaces [31]. At room temperature rutile is the most stable phase of bulk TiO2 whereas in nano region it is anatase. Average surface energies for anatase and rutile phase predicted by static molecular dynamics (MD) calculations are 1.3 and 1.9 J/m2 respectively
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[44, 45]. It is already known that contribution of average surface energy to the total free energy is a function of the surface area. In this scenario among all polymorphic phases of TiO2 surface contribution of anatase is least, which helps it to show better stability in nanoscale. Finnegan et al has found 465°C as the tr ansformation temperature during the
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heating process of anatase TiO2 having particle size 10-15 nm [46]. More interestingly it is
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observed that for a particular size (Dc≈ 14 nm) a crossover region among the polymorphic phases exists. Anatase and rutile both possess equal free energies for this critical size [35, 47, 48]. This may explain why rutile formed only after anatase NPs has coarsened. In the assistance of PVA this critical size range (10-15 nm) has been achieved which initiated the transformation to rutile phase.
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Both anatase and rutile TiO2 samples were prepared followed by same synthesis protocols as well as reaction conditions, only difference is addition/introduction of PVA in case of synthesis of rutile samples. Our chemical reaction has produced a pure anatase
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phase samples [confirmed using XRD results; see Fig. 1 (a)] when reaction is carried out without PVA, whereas, introduction of PVA resulted in pure rutile phase samples [confirmed
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using XRD results; see Fig. 1 (b)] by following the same synthesis process. In this case TiO2, the size of nanoparticle is very important and critical factor to obtain anatase to rutile phase transformation. As already reported by the other workers, TiO2 nanoparticles within the critical size range (Dc=10-15 nm) can undergo such phase transformation. Zhang et. al. [44] and Oliver et. al. [45] has already performed the empirical and theoretical studies focused on the size dependence of relative phase stability among the polymorphs of titania nanoparticles. Below a certain particle size (for TiO2 ≈14 nm) the free energy curve shows a crossover between these two phases. Consequently bulk rutile is stable relative to anatase
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ACCEPTED MANUSCRIPT by an amount of free energy (∆G) ≈ 67 kJ mol-1. Here it is worth to mention that PVA acts as a surfactant and passivates the surface of TiO2 nanoparticles and restricts the growth of nanoparticles within the size range of 15 nm, as confirmed by XRD and TEM results. Hence we can conclude that in our work anatase to rutile phase transformation is extremely a size
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dependent effect and PVA has controlled the growth process and subsequently the particle size of TiO2 nanoparticles, and thus assisted in Anatase to Rutile phase transformation process by restricting the growth of TiO2 nanopaticles with in 15 nm.
Fig. 3 shows representative FESEM, TEM and HRTEM images of TiO2 and
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TiO2/PVA hybrid NPs. Formation of smaller sized NPs in case of composite is clear from
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FESEM and TEM micrographs. We also observe a wide distribution of nearly spherical NPs throughout the surface. The imaged lattice spacing 0.34 nm corresponds to (101) lattice planes of anatase TiO2 (inset of figure 3c). Lattice Spacing of 0.30 nm (inset of figure 3d) designates (110) planes which is also observed in the XRD spectra for rutile TiO2. This observation again confirms the phase transformation after addition of PVA. Lesser
clear in TEM images.
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agglomeration with distinguish spherical particles in the composite system is even more
To determine the structural properties of the inorganic-organic nano hybrid FTIR
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spectroscopy has been employed in the wavelength region 4000-500 cm-1. Functional groups present in the samples have been determined from the dips of the transmittance vs
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wavenumber spectra (Fig. 4). Fig. 4 shows FTIR spectra of (a) pure TiO2 and (b) pure PVA and (c) TiO2/PVA hybrid NPs. Inset of Fig. 4 represent zoomed view of the IR spectra shown in (c) in the region 2400cm-1 -4000 cm-1. Band near to 3440 cm-1 confirms the presence of Ti-OH stretching vibration. Stretching band of O-H is known to be the characteristics IR band of alcohol or phenols [49]. For pure PVA this mode occurs at 3470 cm-1 [50]. In present case this band has shifted slightly with broadening width. May be broadening is due to the specific interaction of TiO2 NPs with the PVA matrix. Bands in the region 2800 to 3100 cm-1 corresponds to the C-H vibration which forms the basic organic structure of PVA. A broad bands around 3444 cm-1 to 3800 cm-1 which are the characteristics of tetrahedral Page 9 of 25
ACCEPTED MANUSCRIPT coordinated vacancies, Ti-OH has appeared in the spectra indicates again the existence of TiO2 NPs into the composite [51]. Absence of any band at 2130 cm-1 indicates that Ti3+ ions are not present in the sample. Absorption in the range 1010-1200 cm-1 is found due to O-O stretching vibration. 1400 cm-1 band is caused by lattice vibration of TiO2 NPs. Two more
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intense bands are found in the composite system at 1636 and 1332 cm-1 which originates from the water absorbed and O containing carbon interacting with TiO2 NPs. Vibration due to –CH2 groups of PVA at 2921 cm-1 has also seen [52]. Other bands at 820 cm-1 and 520cm-1 belong to Ti-O-Ti asymmetric stretching [50]. Observed bands in pure sample is not clearly
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visible in the composite as organic structure has suppressed those bands.
Fig. 5 illustrates the photoluminescence features of prepared samples recorded at
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room temperature using a xenon discharge lamp as source. We have recorded the excitation spectra (Fig. 5a) for pure sample. The emission profiles (Fig. 5b) for both the cases are captured particularly by the excitation of 340 nm found from the excitation spectra previously. For clear visualization of the exact emission peaks we have de-convoluted the
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spectrums (Fig. 5c, d). In TiO2 samples the emission peaks originates due to three physical reasons: oxygen vacancies [53, 54], surface states/defects [55], self trapped excitations [53, 56]. Oxygen vacancies as well as Ti4+ cations associated with the oxygen vacancies
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contribute a major part in the surface states [57, 58]. A weak emission at 420 nm (Fig. 5d, peak 3) is visible in case of composite. Y. Lei et al. has found an intrinsic level transition
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located at 425 nm in TiO2 NPs [59]. So we can assign this 420 nm band as in earlier case rather than a surface state assisted transition. Bluish green emission (Fig. 5c, peak 3 & 5d, peak 6) at 485 nm (2.55 eV) is originated from intrinsic levels rather than surface defects. This peak in both spectra is allocated as a self trapped excitation band localized on the octahedral site. Charge transfer from Ti3+ ion to oxygen anion species in TiO68-complex [53] is its origin. A broad emission peak observed from 335 nm - 415 nm and centered at 380 nm (Fig. 5d, peak 1) can be attributed as the near band edge emission of rutile TiO2 [60]. Tripathi et al. have reported that the existence of compressive strain increases PL intensity rather than tensile strain [61]. Oxygen vacancies can create deep level defect states in the
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ACCEPTED MANUSCRIPT oxide samples. In our case same phenomena is observed from the comparative PL spectra represented. Deep traps at 0.59 eV below the conduction band gives rise emission at 460 nm in TiO2 [53, 54]. We also observe two such kinds of emissions around 458 nm in both cases (Fig. 5c, peak 2 and 5d, peak 5) which may be denoted as emission mainly from
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oxygen vacant states. Band at 438 nm (Fig. 5c, d) is observed in rutile sample. Emission bands located at 521 nm (Fig. 5c, peak 4 and 5d, peak 7) is generated by the radiative recombination of self trapped excitons and the transitions originated from the surface states of the NPs [62]. As PVA is a surface passivating agent so it has quenched a significant
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number of surface traps which has affected the intensity to a greater extent. PL band in rutile
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TiO2 at long wavelength side ≅ 835 nm is attributed to the oxygen vacancies [62]. Although, due to measurement limitation (maximum PL spectral range 200 nm -700 nm) of our instrument, we were unable to record the PL spectra in near infrared region (≅ 835 nm; attributed to the oxygen vacancies). Emissions from TiO2 in the visible region can be attributed to different Ti vacancies, oxygen vacancies and interstitial fields [61].
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Cyclic voltammetric (CV) and electrochemical impedance spectroscopy (EIS) are executed by conventional 3-electrode electrochemical cell consisted of one Pt wire as counter electrode, Ag/AgCl reference electrode and modified pencil graphite electrode as
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working electrode 10.0 ml electrochemical cell consist of 1M KCL solution as a supporting electrolyte. Prior to electrode modification with synthesized nanocomposite, the graphite
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pencil rods were dipped in 0.1M HNO3 for 2min. After that, the rods were washed with distilled water and rubbed with cotton. The treated pencil rods were housed in a micropipette tip and wrapped with copper wire at one end, while other end was left free for further modification. The as-prepared pencil graphite electrode was further modified with a suspension of NPs (0.01 mg) dispersed in 1.0 mL DMSO by simple dip-coating method. The graphite electrode is kept fixed into the solution for few minute and then dried at room temperature naturally before executing further study. CV and EIS measurements are performed at room temperature. For CV measurement, a blank run was taken in the
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ACCEPTED MANUSCRIPT potential window of -1.0 to +1.0V, in the absence of analyte. After that, 1mM ferricynide solution was added to the electrochemical cell and runs were recorded in the same ponetial range. Voltammetry is an analytical technique to keep the record of transfer of electron as a function of applied potential. Current potential response is a precious mean to find out the
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capacitive behaviour of any material. Fig. 6a, shows the CV profile of NPs modified electrodes in the Potential range - 0.1V to 0.1V for 1mM K3Fe (CN)6. Typical C-V graph clearly shows the appearance of redox peaks for K3Fe (CN)6 at the same positions as reported earlier. Profile demonstrates significant amount of enhancement in peak cathodic
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current in case of nanocomposite modified electrode. Major reason behind this output is the smaller size of hybrid NPs. Available larger surface area of TiO2/PVA NPs offer better
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contact area which helps a lot to faster the rate of redox reaction. It can be concluded that PVA leads larger exposure of TiO2 NPs with ions of the electrolyte. In other word, NPs have served the role of conducting path for the smooth progress of ion transportation within the cell.
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Besides the fact that TiO2/PVA hybrid nanostructure has smaller size and hence large available surface area, the CV and EIS are greatly affected by the crystal structure also. Anatase TiO2 has normal tetragonal crystal structure. Where as Rutile has a body-
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centred tetragonal unit cell. This means, anatase unit cell is more elongated as compared to rutile phase. In the rutile form, the Ti-atoms occupy the least space. This makes the rutile
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form the most stable as well as the density of Ti-atom will be more in case of rutile TiO2/PVA hybrid nanostructure. This provides better conducting path which results in the reduction of recombination of photogenerated electrons and holes and hence better CV and EIS response.
Under the positive scan when potential is sufficiently high to oxidize Fe(CN)6-4, gradual increase in anodic current is obtained. But some of the ions get depleted on the electrode surface and anodic current decays after few times. In reverse scan process cathodic current is generated due to the reduction process. CV profile is a result of rapid
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ACCEPTED MANUSCRIPT generation of different oxidation states of the molecule via electrochemical reaction. The plausible reactions are mentioned below: Fe(CN)6-4 Fe(CN)- + e ………… Anodic Current
(6)
Fe(CN)6-3 + e Fe(CN)-…….Cathodic Current
(7)
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Evaluation of the impedance performance is conducted using electrochemical impedance spectroscopy in presence of 1mM K3Fe (CN)6 solution as probe in support of 0.1M KCL solution. All the measurements are done over the frequency range 100 KHz to 100 MHz under the biasing potential -0.45 V with 5mv alternating voltage. Plotting the real
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part of impedance along X-axis (Z’) and imaginary part along Y-axis (Z’’) desired impedance profile is collected (Fig. 6b). In EIS, the semicircle diameter denotes the electron-transfer
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resistance. Decrease in semicircle diameter point out that composite NPs modified electrode is highly efficient in ion transportation than pure one. Hence EIS results are consistent with the CV results. Rapid transfer of ions among electrode and electrolyte within the electrolytic cell derive this kind of behavior. All in all application of PVA has brought drastic change in
5. Conclusions
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the TiO2 NPs modified electrode which is beneficial in electrochemical reaction.
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Anatase to rutile phase transformation (ART) of titania nanoparticles is achieved at very low temperature (180° C) just by introducing p olyvinyl alcohol (PVA) during co-
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precipitation followed by hydrothermal synthesis. The photoluminescence studies showed that emission slightly shifts towards higher wavelength side with remarkable decrease in intensity for PVA/TiO2nanocomposite (rutile samples). The remarkable decrease in PL intensity in PVA/TiO2nanocomposite (rutile samples) is explained considering the surface passivation during growth process. Ion transportation is monitored via CV and EIS spectroscopy. In both cases PVA/TiO2nanocomposite (rutile samples) modified electrode showed better response in ion transportation within the electrolytic cell than pure NPs decorated electrode. The reason may be ascribed to the larger exposure area of composite with the electrolyte. The prepared PVA/TiO2 nanocomposite seems to be one of the
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ACCEPTED MANUSCRIPT promising candidates for modern age lighting, electrochemical sensing and display applications.
Acknowledgement
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Authors are thankful to Department of Science and Technology, Government of India for sanction of Fast Track Research Project for Young Scientists to Dr. Rashmi Madhuri (Ref. No.: SB/FT/CS-155/2012) and Dr. Prashant K. Sharma (Ref. No.: SR/FTP/PS157/2011). Dr. Sharma (FRS/34/2012-2013/APH) and Dr. Madhuri (FRS/43/2013-2014/AC)
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are also thankful to Indian School of Mines, Dhanbad for grant of Major Research Project under Faculty Research Scheme. We are also thankful to Board of Research in Nuclear
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Sciences (BRNS), Department of Atomic Energy, Government of India for major research project (Ref. No. 34/14/21/2014-BRNS). Shrabani is also thankful to Indian School of Mines,
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Dhanbad for Junior Research Fellowship.
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Figure Captions
Fig. 1: X-ray diffraction spectra of (a) pure TiO2 NPs and (b) TiO2/PVA nanocomposite.
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Fig. 2: Williamson-Hall plots of (a) pure TiO2 NPs and (b) TiO2/PVA nanocomposite.
Fig. 3: Field emission scanning electron microscopic images of (a) TiO2 (b) TiO2/PVA NPs
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and transmission electron microscopic images of (c) TiO2 and (d) TiO2/PVA NPs. Inset
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represents ultra high resolution images.
Fig. 4: Fourier transform infra-red spectra of (a) pure TiO2 and (b) pure PVA and (c) TiO2/PVA hybrid NPs. Inset represent inlarged view of the IR spectra shown in (c) in the region 2400cm-1 -4000 cm-1.
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Fig. 5: Room temperature photoluminescence (a) excitation spectra of pure TiO2(b) emission spectra of TiO2 and TiO2/PVA NPs using excitation wavelength 340 nm. (c) and (d)
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represent emission spectra of pure and composite after de-convolution.
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Fig. 6: (a) Comparative CV profiles (b) EIS curves of NPs modified electrode.
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Research Highlights: •
Low temperature phase transformation of TiO2 nanoparticles from anatase to rutile Role of PVA in phase transformation
•
Synthesis of spherical shaped uniformly distributed PVA capped TiO2 NPs
•
Explained the charge transfer process among anatase to rutile phase
Enhanced electrochemical performance of TiO2/PVA nanohybrid modified
EP
TE D
M AN U
electrode than pure TiO2 NPs modified electrode
AC C
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SC
transformation via luminescence studies.
RI PT
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