Synthesis and characterization of barium zinc ferrite nanoparticles: Working electrode for dye sensitized solar cell applications

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ScienceDirect Solar Energy xxx (2014) xxx–xxx www.elsevier.com/locate/solener

Synthesis and characterization of barium zinc ferrite nanoparticles: Working electrode for dye sensitized solar cell applications R. Tholkappiyan, K. Vishista ⇑ Department of Physics, Anna University, Chennai 25, India

Communicated by: Associate Editor Smagul Zh. Karazhanov

Abstract Spinel type barium zinc ferrite (Zn1xBaxFe2O4) nanoparticles with compositions of barium (x = 0.01–0.15) were prepared by an auto combustion method using glycine as fuel and nitrates as precursors. The formation mechanisms of these ferrite nanoparticles are briefly discussed. The prepared samples were characterized by powder X-ray Diffraction analysis (XRD) and confirm the formation of pure phase zinc ferrite with cubic structure. The average crystallite size was found to vary from 39.5 nm to 47.6 nm. X-ray Photoelectron Spectroscopy (XPS) was used to analyze the elemental composition and oxidation states of the elements in the ferrite samples. Detailed photoelectron peaks of Zn 2p, Fe 2p, O 1s and Ba 3d with corresponding binding energy are presented in the XPS spectrum. The optical band gap values increased from 2.42 eV to 2.50 eV with increase in barium concentration as determined from UV–Diffuse Reflectance Spectroscopy (DRS) using Tauc relation. The current–voltage (J–V) curve for DSSC based on barium zinc ferrite nanoparticles sensitized with Eosin yellowish dye was characterized by J–V measurements. It exhibited a maximum optimal energy conversion efficiency of around 0.0027% for barium doped zinc ferrite nanoparticles whereas the cell based on the pure zinc ferrite nanoparticles gave efficiency of approximately 0.0014% and enhanced open circuit voltage and current are obtained. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Zinc ferrite; Combustion method; Fuel; Nanoparticle; Oxidation state; Dye sensitized solar cells

1. Introduction In the field of solar cell technology, currently much attention is directed to the development of dye-sensitized solar cell (DSSC). This is because the solar spectrum consists of 5% UV (k = 200–400 nm), 52% infrared (k > 400 nm) and 43% visible (k = 400–800 nm). Since visible light constitutes a large fraction of solar energy, one of the greatest challenges is to focus on ferrite materials that exhibit high efficiency when illuminated by visible light photons from solar spectrum. Ferrite materials like zinc ferrite have spinel structure of the type A2þ B3þ 2 O4 where A and B refer to the metal ions at tetrahedral and octahe⇑ Corresponding author. Tel.: +91 44 22358717.

E-mail address: [email protected] (K. Vishista).

dral sites respectively in the oxygen lattice (Abedini Khorramia et al., 2011). Zinc ferrite (ZnFe2O4) belongs to the normal spinel ferrite system in bulk form whereas cubic spinel system in nanoscale forms. In addition to this, Zinc ferrite is an n-type semiconductor material that can be used in visible light photocatalytic applications due to its smaller band gap value (1.9 eV). It has the ability to absorb visible light from the solar spectrum and thus is a potentially useful solar energy conversion material (Suk Jang, 2009). Researchers are interested in ferrite materials for the development of water splitting for hydrogen energy production (Inoue et al., 2004), purification of water and air applications (Hoffman et al., 1995). In the past, many methods like mechano chemical reaction (Yang et al., 2004), co-precipitation method (Ping, 2009), low temperature method (Li et al., 1996), sol–gel

http://dx.doi.org/10.1016/j.solener.2014.02.003 0038-092X/Ó 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Tholkappiyan, R., Vishista, K. Synthesis and characterization of barium zinc ferrite nanoparticles: Working electrode for dye sensitized solar cell applications. Sol. Energy (2014), http://dx.doi.org/10.1016/j.solener.2014.02.003

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R. Tholkappiyan, K. Vishista / Solar Energy xxx (2014) xxx–xxx

method (Gatelyte et al., 2011), wet-milling process (Ozcan et al., 2005), co-precipitation–air oxidation method (Li et al., 2010), hydrothermal synthesis (Shu-Hong et al., 2003), microwave-assisted solvothermal methods (Blanco-Gutierrez et al., 2011) and reviews have been used to produce nano ZnFe2O4 materials and have been reported in literature. Among them combustion method has great opportunities for large scale production of nanoparticles in a short span of time (20 min). It is cost effective and can produce pure phase nanoparticles with the desired shape. This method is mainly based on mixing the reactants that oxidize easily such as zinc nitrate, iron nitrate and an organic fuel which acts as a reducing agent. In order to initiate the ignition of the mixture, an external heat source is needed. This ensures the self-propagating character of an exothermic redox reaction. The fuel to metal nitrate ratio has to be carefully considered as they play a vital role in total combustion process. Barium with its interesting optical behaviour has been found to increase the optical property of various ferrites. However, no reports have been published on producing barium doped zinc ferrite by auto combustion method. In this investigation, pure and barium doped zinc ferrite with chemical compositions of Zn1xBaxFe2O4 (x = 0.01, 0.05, 0.10, 0.15) were synthesized by the auto combustion method. The crystalline phases of the samples were identified by X-ray diffraction. The surface phenomenon and oxidation state of these samples were characterized by X-ray Photoelectron Spectroscopy (XPS). Detailed surface analysis of core level spectra of Zn 2p, Fe 2p, Ba 3d and O 1s peak were recorded. The optical property of these samples was measured by using UV–Diffuse Reflectance Spectroscopy (DRS). DSSC was fabricated using barium zinc ferrite nanoparticles as a working electrode. The current–voltage measurements were performed for the DSSC prepared from barium zinc ferrite nanoparticles sensitized with Eosin yellowish dye.

For further heat treatment, the gel obtained was kept on a hot plate at 200 °C for 15 min. The process of auto-ignition started and produced a dry brown resin with the emission of a large amount of gaseous fumes. Brownish zinc ferrite ash was obtained after the completion of the combustion process. The time taken between the initial ignition and the end of the reaction forming a zinc ferrite powder was less than 20 s. Finally, as-prepared nanoscale zinc ferrites samples were kept out of the hot plate and ground using an agate mortar and pestle to form fine powders. The entire reaction process is illustrated in Fig. 1. The fine powders were stored for characterization. The obtained samples were indicated as ZF, ZBF1, ZBF5, ZBF10 and ZBF15. 2.2. Preparation of working electrode and treatment with dye The doctor blade technique was used to prepare the thin layer of nanostructured films (Katsaros et al., 2002). In typical process, the colloidal paste was prepared using the following steps; (i) First, 0.1 g of ZnFe2O4 nanoparticles was ground with the help of appropriate amounts of distilled water and acetylacetone for 10 min to form a viscous paste. (ii) The viscous paste was slowly added with 0.3 ml of distilled water until desirable viscosity was attained. (iii) Finally, 20 ll of surfactant (Triton X-100) was slowly added with grinding for 10 min. The resulting ZnFe2O4 paste is uniformly dispersed on the conducting substrate using the doctor blade technique. After coating, the films were dried in air atmosphere at room temperature. Then, the films were heat treated at 450 °C for 20 min. For fabricating the DSSC, these calcined films were then soaked into the solution containing ethanol and Eosin yellowish dye for 24 h at room temperature. The dye-sensitized electrodes were then rinsed with absolute ethanol to remove the physisorbed dye molecules on the surface.

2. Experimental Ba (NO3)2/20ml H2O

2.1. Synthesis of barium zinc ferrite nanoparticles Pure and barium doped zinc ferrite nanoparticles with chemical composition Zn1xBaxFe2O4 (x = 0.01, 0.05, 0.10, 0.15) were prepared by the auto combustion method. All analytical reagents were used without further purification. Initially stoichiometric amounts of Zn(NO3)26H2O (Merck), Fe(NO3)39H2O (Himedia, 98–100% purity), Ba(NO3)2 (Himedia, 99%purity) were dissolved in 20 ml of distilled water, then 10 ml of nitric acid (HNO3) was mixed with the aqueous solution which resulted in a clear solution. Appropriate amounts of fuel (reducing agent) were slowly added to the above solution and the mixture was stirred continuously until it was completely dissolved. To evaporate the water content in the solution, the mixture was heated in a magnetic stirrer with a hot plate at 100 °C for 2 h and it became a dark brown viscous gel.

Zn(NO3)2.6H2O/ 20ml H2O

Fe(NO3)3.9H2O/ 20 ml H2O

Nitric Acid Fuel Mixed Complexes Stirring with heat at 100oC Colloidal gel Auto Combustion Powder precursors Fig. 1. Synthesis scheme of the Zn1xBaxFe2O4 (x = 0.01, 0.05, 0.10, 0.15) nanoparticles by auto combustion method.

Please cite this article in press as: Tholkappiyan, R., Vishista, K. Synthesis and characterization of barium zinc ferrite nanoparticles: Working electrode for dye sensitized solar cell applications. Sol. Energy (2014), http://dx.doi.org/10.1016/j.solener.2014.02.003

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2.3. Preparation of counter electrode The counter electrode was prepared by uniformly spreading an ITO substrate with a thin layer of 5 mM chloroplatinic acid hexahydrate (H2PtCl6) in isopropyl alcohol. The prepared platinum-coated counter electrode was then calcined at 450 °C for 20 min under ambient air atmosphere. 2.4. Preparation of electrolyte For preparation of electrolyte, initially 0.04 g of TiO2 (5 nm) nanoparticles act as inorganic filler was uniformly dispersed into 50 ml of acetonitrile. Then, appropriate amounts of iodide (I2, 0.05 M) and lithium iodide (LiI, 0.1 M) were added as redox couple. The resulting redox couple was slowly introduced into the poly-ethylene oxide and stirred well for 24 h to get good polymer dispersed electrolyte. 2.5. Cell assembly To assemble the DSSC, an electrolyte solution was sandwiched between the platinum-coated counter electrode and working electrode and then pressed gently. A parafilm was used as a spacer between two electrodes. In order to avoid the further sealing of the cell, a binder clip was fixed externally to maintain the mechanical grip of the cell. Finally, the DSSC was assembled. The schematic representation of assembled DSSC is shown in Fig. 13. 2.6. Characterizations The crystal structure of the synthesized powder was identified by a Bruker D2 X-ray diffractometer using Cu Ka radiation (k = 0.15405 nm) in the range of 10–80° with step mode of 0.2/min. Information about the oxidation states of these samples was obtained from X-ray Photoelectron Spectroscopy (XPS) using Kratos Analytical Axis Ultra DLD with Al Ka1 source. The energy of an X-ray photon of 1.486 keV with pass energy of 160 eV was used for the survey spectrum and 40 eV for narrow scans. The spectra were collected using the combination of electrostatic and magnetic lens (hybrid mode) for an analyzed area of (700  300 lm). The angle between the normal to the sample surface and the direction of photoelectron collection are perpendicular to each other. Surface charging effects were minimized using a charge balance operating at 3.6 V and 1.8 V maintained as filament bias. The optical properties of the powder samples were performed by a Shimadzu UV2450 UV–vis diffuse reflection spectrophotometer. A photocurrent–voltage characteristic was measured using M-91900 Oriel Class-A Simulator (Newport). The devices under analysis were illuminated with a xenon lamp as a light source having an intensity of 100 mW/cm2. The light intensity of the illumination source was checked by using a calibrated light intensity meter (OPHIR laser measurement group, NOVA Oriel). A current–voltage

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characteristic was measured by using computer controlled Auto lab PGSTAT302N electrochemical workstation. 3. Results and discussion 3.1. Effect of fuel on the auto combustion synthesis of Zn1xBaxFe2O4 (x = 0.01, 0.05, 0.10, 0.15) The combustion process mainly depends upon the amount and various types of fuels used due to the high temperature involved. According to the concepts of propellant chemistry, the oxidizing valence from the metal nitrates is equal to reducing valence by the fuel. In this process, Zn1xBaxFe2O4 nanoparticles were synthesized with glycine as fuel. The oxidizing and reducing valences of elements as: Zinc = +2, Barium = +2, iron = +3, Nitrogen = 0, carbon = +4, oxygen = 2, nitrogen = 0 and hydrogen = 1. In the case of glycine–nitrate combustion process, the chemical redox reaction can be expressed as given below, ð1  xÞZnðNO3 Þ2  6H2 O þ xBaðNO3 Þ2 þ 2FeðNO3 Þ3  9H2 O þ 4:44NH2 CH2 COOH ! Zn1x Bax Fe2 O4 þ 5:72N2 " þ8:88CO2 " þ11:1H2 O "

ð1Þ

In the above reaction, total reducing valence of glycine (NH2CH2COOH) is +9 and the total valance of nitrates is 40. The stoichiometric composition of the glycine– nitrate mixture becomes 2  (15) + 1  (10) + n (+9) = 0, n = 4.44 mol in the reaction. Here, fuel (NH2CH2COOH) – nitrates composition indicates glycine=NO ¼ 1:48 (glycine-to-nitrate ion ratio). In this 3 present work, we consider the stoichiometric amount of fuel to nitrate composition as 1.48. This results in the formation of pure and nanosize Zn1xBaxFe2O4 (x = 0. 01, 0.05, 0.10, 0.15) particles. 3.2. XRD studies The crystal structure of pure and barium doped zinc ferrite are identified by X-ray diffraction. Fig. 2 shows the powder X-ray Diffraction (XRD) pattern of the prepared samples. All the peaks in this pattern are well matched and examined as a cubic ZnFe2O4 with spinel structure (JCPDS file No. 82-1049). No other impurity peaks are observed in the XRD pattern. The sharp diffraction peak at (311) represents the high degree of crystalline nature of the sample. Using XRD data, the values of average crystallite size, X-ray density and lattice constant are calculated and listed in Table 1. The crystallite size (d) of this ferrite is determined using Scherrer formula (Kouam et al., 2008), d ¼ 0:9k=ðb cos hÞ

ð2Þ

where d is the average crystallite size, k (0.154 nm) is the wavelength of X-ray, b is the full-width half maximum (FWHM) of the maximum intensity peak. The X-ray density of all samples was calculated from the equation,

Please cite this article in press as: Tholkappiyan, R., Vishista, K. Synthesis and characterization of barium zinc ferrite nanoparticles: Working electrode for dye sensitized solar cell applications. Sol. Energy (2014), http://dx.doi.org/10.1016/j.solener.2014.02.003

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Fig. 2. X-ray diffraction pattern of the prepared (a) ZF, (b) ZBF1, (c) ZBF5, (d) ZBF10 and (e) ZBF15 nanoparticles by auto combustion method using glycine as fuel.

Table 1 ˚ ) and Average Crystallite size (nm), X-ray density (dx), Lattice Constant (A Band Gap (eV) for ZF, ZBF1, ZBF5, ZBF10 and ZBF15 nanoparticles. Sample

Average crystallite size(nm)

X-ray density (dx) g/cm3

Lattice ˚) constant (A

Band gap (eV)

ZF ZBF1 ZBF5 ZBF10 ZBF15

47.6 45.4 41.8 40.7 39.5

5.5643 5.5497 5.5105 5.4874 5.4586

8.4409 8.4420 8.4508 8.4591 8.4672

2.42 2.42 2.43 2.44 2.50

d x ¼ Z  M=VN A

Fig. 3. FTIR spectra of prepared (a) ZF, (b) ZBF1, (c) ZBF5, (d) ZBF10 and (e) ZBF15 nanoparticles.

˚ ), the lattice constant increases (Ahmed Fe2+ ion (0.74 A et al., 1997). The X-ray density (dx) for all the samples decreases with increases of barium content due to increase of the lattice constant (Shahane et al., 2010). 3.3. FTIR analysis

ð3Þ

where Z is the number of molecules/formula unit (Z = 8 for spinel system), M is the molecular mass of the sample, V is the unit cell volume, NA is the Avogadro’s number = 6.023  1023. The average crystallite size for pure zinc ferrite was 47.6 nm. This is a higher value than 14 nm which is prepared via polyethylene glycol (PEG)-assisted route by Koseoglu et al. (2008). While introducing barium content in zinc ferrite the average crystallite size decreased from 45.4 nm to 39.5 nm. These values are listed in Table 1. The observed decrease in crystallite size is due to cation stoichiometry (Ladgaonkar and Vaingainkar, 1998). As the zinc concentration of the sample is replaced by barium, it is seen that Zn2+ does not have strong preference for occupying only the tetrahedral site and can also occupy the octahedral site. This accounts for the decrease in particle size as the concentration of barium is increased. In the present study, the lattice constant increases with the increase in barium content up to x = 0.15 in zinc ferrites. This may be explained on the basis of small fraction (<20% as mentioned in XPS studies) of Fe2+ ion present in tetrahedral site and Fe3+ ion in octahedral site. Since ˚ ) is lower than that of the ionic radius of Fe3+ ion (0.64 A

The Fourier transform infrared spectroscopy (FT-IR) is a useful tool to get the information about the vibrational modes of the materials. As shown in Fig. 3 the FT-IR spectra of prepared nanoparticles were recorded at room temperature in the wavenumber region of 400–4000 cm1. Appearance of pair of peaks in the lower wavenumber region in the FTIR spectra of the prepared compounds further confirms the formation of spinel type barium zinc ferrites. The lower frequency bands (m1) observed in the range of 400–500 cm1 corresponds to metal–oxygen stretching vibrations in octahedral site. The higher frequency (m2) in the range of 500–750 cm1 attribute to Fe3+–O2 stretching vibrations in tetrahedral group (Gabal, 2009). This difference in the band frequencies of m1 and m2 is due to the difference in the metal–oxygen distance for the tetrahedral and octahedral metal ions (Labde et al., 2003). The vibrational frequencies m1, m2 corresponding to the position of tetrahedral and octahedral metal complexes are reported in Table 2. It is to be noted that the metal–oxygen bond length changes with barium doping. This shift in peaks also confirms the dopant interaction with the host system (Priyadharsini et al., 2009; Prakash and Baijal, 1984). 3.4. XPS studies In order to obtain information about the surface chemical composition and oxidation states of the prepared samples, X-ray Photoelectron Spectroscopy (XPS) was used.

Please cite this article in press as: Tholkappiyan, R., Vishista, K. Synthesis and characterization of barium zinc ferrite nanoparticles: Working electrode for dye sensitized solar cell applications. Sol. Energy (2014), http://dx.doi.org/10.1016/j.solener.2014.02.003

R. Tholkappiyan, K. Vishista / Solar Energy xxx (2014) xxx–xxx Table 2 Band positions (m1 and m2) of prepared nanoparticles obtained from FTIR analysis. Samples

Octahedral position m1 (cm1)

Tetrahedral position m2 (cm1)

ZF ZBF1 ZBF5 ZBF10 ZBF15

403.2 419.4 448.3 465.9 471.3

570.4 561.8 550.7 533.1 529.3

Fig. 4(a) shows survey XPS spectrum of ZF and ZBF5 nanoparticles in which elements such as Zn, Fe, Ba, C and O are detected. No nitrogen was detected, which implies the complete decomposition of metal nitrate precursors corresponding to fuel stoichiometry used in this combustion process. The data of atomic ratios for ZF and ZBF5 nanoparticles are listed in Table 3. From this table the ratio of Zn/Fe/O in the sample is very close to 1/2/4 agreeing with the expected chemical formula of ZnFe2O4 nanoparticles. The Zn 2p, Fe 2p, Ba 3d core shell XPS Spectra of ZF and ZBF5 nanoparticles are shown in Fig. 4(b–d). For detailed analysis of these Spectra CASA XPS, version 2.2.14 software package was used. Types of shirley background were used as a baseline for all peaks, and the data were curve-resolved using an 80% Gaussian/20% Lorentzian sum. Fig. 5a show the deconvolution spectra of Zn 2p peak for ZF nanoparticles. Due to spin orbital coupling, the two major components such as Zn 2p3/2 and Zn 2p1/2 were exhibited. The values of spin orbital splitting for a pure Zn 2p core level is 23.2 eV. Upon deconvolution, Zn 2p3/2 peak can be fitted into two contributions with the BE

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values of about 1018.68 eV and 1020.86 eV and confirmed the presence of Zn2+ ion in the spinel type zinc ferrite (Druska et al., 1999). The photoelectron peaks fitted at 1040.9 eV and 1043.25 eV corresponding to the binding energies of Zn 2p1/2. After doping of barium (x = 0.05) in zinc ferrite, Zn2+ ion remains in the same oxidation state as shown in Fig. 5b. Deconvolution spectra for ZF nanoparticles of Fe 2p peak are shown in Fig. 6a. Based on spin orbital splitting, the Fe 2p peak is splitted into Fe 2p3/2 and Fe 2p1/2 fine peaks. The Fe 2p3/2 peak exhibits two major separate peaks at 708.22 eV and 710.55 eV corresponding to the binding energies of Fe2+ and Fe3+ ion are identified in the ZF. The deconvolution spectra of Fe 2p1/2 peak with corresponding binding energy of 723.11 eV was observed. In additional, the peak at 718.01 eV exists in-between the Fe 2p1/2 and Fe 2p3/2 which indicates that it is satellite peak. This Fe 2p3/2 peak indicates the presence of Fe3+ ion on the surface of the ZF nanoparticles. Furthermore, a difference of 8 eV from the satellite peak to Fe 2p3/2 peak is identified as a Fe3+ state in the ZF (Jiagang et al., 2011). However, in case of ZBF5 there is no change in the oxidation states of Fe3+ ion as shown in Fig. 6b. The XPS deconvolution spectra of Ba 3d peak for ZBF5 are shown in Fig. 7. It consists of Ba 3d5/2 and Ba 3d3/2 components. The Ba 3d5/2 peak can be fitted into three peaks with binding energies of 780.91 eV, 777.75 eV and 767.23 eV which is denoted as Ba2+, Ba1+ and Ba0 ions and shows that an oxygen atom is bound with Ba atom to form an Ba–O bond (Gao et al., 2007). In this study, the observed binding energy values of Ba 3d peak indicate the presence of Ba2+ ion which is in good agreement with similar result reported by Negrila Catalinet et al. where

Fig. 4. (a) XPS survey spectrum, (b) XPS fine spectra of Zn 2p, (c) XPS fine spectra of Fe 2p, (d) XPS spectra of Ba 3d of ZF and ZBF5.

Please cite this article in press as: Tholkappiyan, R., Vishista, K. Synthesis and characterization of barium zinc ferrite nanoparticles: Working electrode for dye sensitized solar cell applications. Sol. Energy (2014), http://dx.doi.org/10.1016/j.solener.2014.02.003

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Table 3 Data of Atomic ratios obtained from XPS for pure and barium (x = 0. 05) doped zinc ferrite. Atomic ratio (%)

Zn

Fe

O

C

Ba

ZF ZBF5

6.07 4.17

12.46 13.97

25.31 29.82

56.16 49.66

– 2.38

Fig. 7. Deconvolution spectra of Ba 3d peak.

Fig. 5. Deconvolution spectra of Zn 2p peak for ZF (a) and ZBF5 (b).

Fig. 8. XPS Spectra for O1s peak (upper) for ZF, O1s peak (lower) for ZBF5.

posed into three peaks which is labeled as A, B and C, with binding energies at 528.14 eV, 529.85 eV and 531.30 eV respectively. The peak at 528.14 eV is assigned to the oxygen in the lattice O2. The peak at 529.85 eV is related to OH absorbed on the surface of the ZF. The peak at 531.30 eV represents the –H–O–H content absorbed at the sample ZF surface. Upon doping of Ba (x = 0.05), the intensity of peak labeled at A increases with binding energy of 528.26 eV it is due to the increase the l7attice O2. Furthermore, decreased intensity of peaks labeled at B and C with binding energies of 529.72 eV, and 531.51 eV depicts the decrease in the OH- and -H–O–H as shown in Fig. 8(b). Fig. 6. Deconvolution spectra of Fe 2p peak for ZF (a) and ZBF5 (b).

binding energy value of Ba 3d5/2 is 779.96 eV in case of BaTi4O9. In the present case, the deconvolution spectra of Ba 3d3/2 peak can be fitted into two peaks with a binding energy of 792.63 eV and 789.02 eV. Fig. 8(a and b) show the deconvolution of O1s spectra for ZF and ZBF5 nanoparticles. Fig. 8(a) can be decom-

3.5. SEM analysis To elucidate the morphology of the prepared nanoparticles, Scanning Electron Microscopy (SEM) were used. Fig. 9 shows the SEM micrographs of the synthesized pure and various concentrations of barium doped zinc ferrite ferrites nanoparticles. From these micrographs, the prepared nanoparticles are agglomerated with spongy network

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structure. Voids or pores are also observed in the synthesized nanoparticles. This is due to mixture of oxidizing and reducing species during the combustion process. Once, it reaches the critical temperature; large amount of gases such as oxygen, nitrogen and carbon are released. Hence the pores or voids are formed in the synthesized nanoparticles. 3.6. UV–DRS studies The effect of barium concentration on the optical properties of zinc ferrite nanoparticles was studied by UV–Vis diffuse reflectance measurement. Fig. 10 shows the UV– Reflectance spectra of the composition Zn1xBaxFe2O4 (x = 0.01, 0.05, 0.10, 0.15). It is obvious that the obtained sample shows the brown colour. From the figure, the zinc ferrite nanoparticles show excellent visible light absorption in the range of 200–500 nm. The absorption band of Bazinc ferrite in the visible light region can be ascribed to an intrinsic band transition but not to the surface states (Xu et al., 2007). It is known that ZnFe2O4 is one of the normal spinel-type compounds with cubic structure in

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which Zn2+, Ba2+ occupy tetrahedral and Fe3+ occupies octahedral sites (Abedini Khorramia et al., 2011). The energy band structures of the Ba-zinc ferrite is generally defined by using the O-2p orbital as the valence band and the Fe-3d orbital as the conduction band (Xu et al., 2009). Therefore, the absorption of Ba-zinc ferrite in the visible region can be attributed to the photo electron transition from O 2p level into Fe 3d level. From the optical reflectance data, the optical absorption coefficient can be calculated by the Kubelka–Munk function (Villa et al., 2012), 2

a ¼ ð1  RÞ =2R

ð4Þ

where a is the absorption coefficient, and R is the diffuse reflectance. The optical band gap energy Eg is obtained using the Tauc equation (Dakhel, 2010), ðahmÞ ¼ Aðhm  Eg Þ

n

ð5Þ

where A is a constant, hm is the energy of a photon and n is the index that can be characterized in the absorption process. Using the optical absorption coefficient (a), the optical

Fig. 9. SEM micrographs of (a) ZF, (b) ZBF1, (c) ZBF5, (d) ZBF10 and (e) ZBF15 nanoparticles.

Please cite this article in press as: Tholkappiyan, R., Vishista, K. Synthesis and characterization of barium zinc ferrite nanoparticles: Working electrode for dye sensitized solar cell applications. Sol. Energy (2014), http://dx.doi.org/10.1016/j.solener.2014.02.003

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Fig. 10. UV–vis diffuse reflectance spectra for the prepared (a) ZF, (b) ZBF1, (c) ZBF5, (d) ZBF10 and (e) ZBF15 nanoparticles.

band gap values of pure and barium doped zinc ferrites can be calculated. In this work, the band gap between the O-2p level and the Fe-3d level of the ZF is 2.42 eV which is in good agreement with previous result reported by Gao et al. (2010) with band gap value of 2.23 eV for the zinc ferrite nanowire sample. The Tauc plots of (ahm)2 against hm (eV) for the composition Zn1xBaxFe2O4 (x = 0.01, 0.05, 0.10, 0.15) are shown in Fig. 11. Increasing the barium dopant concentration the band gap value increases as listed

in Table 1. This indicates that no intermediate energy levels are formed between the valence band and conduction band. Hence, the energy required to excite the photo electron from the O-2p level into the Fe-3d level goes on increasing as band gap increases. Also, percentage of reflectance increases with increase of Ba concentration as shown in Fig. 10. So Ba–Zn ferrite shows a blue shift of the reflectance band region. The material prepared displays excellent absorption band in the visible region of the spectrum which justifies its photocatalyst applications. Additionally, the magnetic nature of the zinc ferrite sample was confirmed by a liquid method as shown in Fig. 12a. When the zinc ferrite nanoparticles were dispersed in water a homogeneous dark brown dispersion occurs. If a bar magnet is placed beside the glass bottle containing these nanoparticles, it shows a separable property i.e. nanoparticles were collected near the magnet within 20 min and a clear solution was obtained (Fig. 12b). After removing the magnetic field, the nanoparticles were homogeneously dispersed again (Fig. 12c). Thus the zinc ferrite nanoparticles show excellent magnetic response and can be easily separated from the dispersed solution by an external magnetic field. This proves the effectiveness and reversible responses of the nanoparticles when subjected to magnetic field. 3.7. Current–voltage measurements The current–voltage (J–V) characteristics for DSSC constructed using barium zinc ferrite nanoparticles sensitized with Eosin yellowish dye under illumination light are shown in Fig. 14. The detailed parameters such as short-

Fig. 11. Tauc plots of (ahm)2 against hm (eV) of (a) ZF, (b) ZBF1, (c) ZBF5, (d) ZBF10 and (e) ZBF15 nanoparticles.

Please cite this article in press as: Tholkappiyan, R., Vishista, K. Synthesis and characterization of barium zinc ferrite nanoparticles: Working electrode for dye sensitized solar cell applications. Sol. Energy (2014), http://dx.doi.org/10.1016/j.solener.2014.02.003

R. Tholkappiyan, K. Vishista / Solar Energy xxx (2014) xxx–xxx

Fig. 12. (a) Zinc ferrite nanoparticles in water without the magnetic field, (b) Magnetic separation of nanoparticles from suspension under imposed magnetic field, (c) sample bottle with removal of magnetic field.

circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF) and efficiency (g) of the measured and calculated values obtained from typical J–V curve for barium zinc ferrite nanoparticles as working electrodes are summarized in Table 4. It is seen from the Table 4 that the cell constructed using barium zinc ferrites films gives a clear improvement in Jsc and g (%) when compared to the undoped ZF film based DSSC. The generation of Jsc of the fabricated DSSC cell mainly depends on the amount of dye adsorbed on the surface of material, structure, light harvesting efficiency and electron injection ability of the dye (Shanmugam et al., 2013; Jose et al., 2009). The fill factor (FF) of the fabricated DSSC is calculated using the equation, FF ¼ P max =J sc  V oc

ð6Þ

9

Fig. 14. Current–voltage (I–V) characteristics of dye sensitized solar cell (DSSC) using zinc barium ferrites nanoparticles as working electrodes.

where Pmax is the maximum power that can be extracted from the solar cell, Jsc is the short circuit current in A/ m2, Voc is the open circuit voltage in V. The FF of ZF nanoparticles was 0.308. While introducing barium content in zinc ferrite nanoparticles, the FF increased from 0.282 for Ba (x = 0.05) to 0.362 for Ba (x = 0.15). This indicates the reduction in recombination between the photoexcited carriers in the electrodes and tri-iodide ions in electrolyte (De Angelis et al., 2007). In this work, the Voc increased from 342 to 536 (mV) for pure and barium doped zinc ferrite nanoparticles. This increase may be due to the lowering of recombination rate. The power conversion efficiency (g) of the fabricated ZnFe2O4-based cell is given as, g ¼ J sc  V oc  FF =P light  100%

ð7Þ

where Plight is the power density of the incident solar radiation in W/m2. Recently in 2013, Mohammad Hossein Habibi et al reported the maximum efficiency of 0.25% for zinc ferrite sol (ZnFe2O4) nanocomposite with titania thin film layer sensitized with D35 dye by employing gel electrolyte. Comparing with Mohammad Hossein Habibi et al, in this work the cell based on the barium doped zinc ferrite nanoparticles working electrode exhibited the maximum efficiency of around 0.0027% whereas the cell based on the pure nanoparticles working electrode gave efficiency of approximately 0.0014%.

Table 4 Photovoltaic parameters of the fabricated DSSC.

Fig. 13. Schematic representation of DSSC using Eosin yellowish dye.

Samples

Jsc (lA/cm2)

Voc (mV)

FF

Efficiency (g) (%)

ZF ZBF5 ZBF15

653 764 824

342 426 536

0.308 0.282 0.362

0.0014 0.0020 0.0027

Please cite this article in press as: Tholkappiyan, R., Vishista, K. Synthesis and characterization of barium zinc ferrite nanoparticles: Working electrode for dye sensitized solar cell applications. Sol. Energy (2014), http://dx.doi.org/10.1016/j.solener.2014.02.003

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R. Tholkappiyan, K. Vishista / Solar Energy xxx (2014) xxx–xxx

4. Conclusion By auto combustion method, pure and various concentrations of barium doped zinc ferrite were prepared using glycine as fuel. The formation of the cubic ZnFe2O4 spinel structure was confirmed by X-ray Diffraction (XRD) and no other impurity phase was detected. It is found that as the barium concentration of the sample increases, the particle size and X-ray density decreases. Lattice constant, also shows an increasing trend with increasing barium concentration. As observed by XPS, zinc and barium was found to be in the 2+ oxidation state and iron was found to be in the 3+ oxidation state on the surface in pure and barium (x = 0.05) doped zinc ferrite sample respectively. The optical band gap value obtained from the UV–DRS studies were observed in the range from 2.42 eV to 2.50 eV as barium concentration increases with corresponding decrease in the average crystallite size. From the J–V characterization results, it was inferred that increasing the barium content in zinc ferrite increased Voc from 342 to 536 mV which leads to lowering of recombination rate. The DSSC cell based on the barium doped zinc ferrite nanoparticles working electrode exhibited the maximum efficiency of around 0.0027% whereas the cell based on the pure nanoparticles working electrode gave efficiency of approximately 0.0014% and enhanced open circuit voltage and current are obtained. Acknowledgements The authors thank Prof. K. Ramachandran and Mr. Raja manikam for permitting us to make use of the I–V characterization facilities from the school of physics, Madurai kamaraj University, India. References Abedini Khorramia, S., Mahmoudzadeha, G., Madania, S.S., Gharib, F., 2011. Effect of calcination temperature on the particle sizes of zinc ferrite prepared by a combination of sol-gel auto combustion and ultrasonic irradiation techniques. J. Ceram. Process. Res. 12 (5), 504– 508. Jang, Jum Suk, Borse, Pramod H., Lee, Jae Sung, Jung, Ok-Sang, Cho, Chae-Ryong, Jeong, Euh Duck, Ha, Myoung Gyu, Won, Mi Sook, Kim, Hyun Gyu, 2009. Synthesis of nanocrystalline ZnFe2O4 by polymerized complex method for its visible light photocatalytic application: an efficient photo-oxidant. Bull. Korean Chem. Soc. 30 (8). Inoue, M., Hasegawa, N., Uehara, R., Gokon, N., Kaneko, H., Tamaura, Y., 2004. Solar hydrogen generation with H2O/ZnO/MnFe2O4 system. Sol. Energy 76, 309–315. Hoffman, M.R., Martin, S.T., Choi, W., Bahnemann, D.W., 1995. Environmental applications of semiconductor photocatalysis. Chem. Rev. 95, 69. Yang, Huaming, Zhang, Xiangchao, Huang, Chenghuan, Yang, Wuguo, Qiu, Guanzhou, 2004. Synthesis of ZnFe2O4 nanocrystallites by mechanochemical reaction. J. Phys. Chem. Solids 65, 1329–1332. Ping, Ren, Junxi, Zhang, Huiyong, Deng, 2009. Preparation and microstructure of spinel zinc ferrite ZnFe2O4 by Co-precipitation method. J. Wuhan Univ. Technol. – Mater. Sci. Ed., 927–930.

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Please cite this article in press as: Tholkappiyan, R., Vishista, K. Synthesis and characterization of barium zinc ferrite nanoparticles: Working electrode for dye sensitized solar cell applications. Sol. Energy (2014), http://dx.doi.org/10.1016/j.solener.2014.02.003