RESEARCH ARTICLE

 

Novel silver-doped NiTiO₃: auto-combustion synthesis, characterization and photovoltaic measurements

 

 

S. Mostafa Hosseinpour-Mashkani; Mahnaz Maddahfar; Ali Sobhani-Nasab^*

Young Researchers and Elites Club, Arak Branch, Islamic Azad University, Arak, Iran

 

 

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ABSTRACT

Novel silver-doped nickel titanate nanoparticles (Ag-NiTiO₃) were successfully prepared via a sol-gel method in the presence of stearyl alcohol as the capping agent and solvent. The formation of pure crystallized nickel titanate and silver-doped nickel titanate was occurred when the precursor was heat-treated at 700 °C in air for 150 and 60 min, respectively. The structural, morphological, and optical properties of obtained products were characterized by techniques such as X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, Energy dispersive X-ray microanalysis (EDX), ultraviolet-visible (UV-vis), and scanning electron microscopy (SEM). The magnetic property of the prepared Ag-NiTiO₃ nanoparticles was also investigated with vibrating sample magnetometer (VSM). To fabricate a FTO/TiO₂/Ag-NiTiO₃/Pt-FTO solar cell, Ag-NiTiO₃ film was directly deposited on top of the TiO₂ prepared by electrophoresis deposition method. Furthermore, solar cell result indicates that an inexpensive solar cell could be developed by the synthesized Ag-NiTiO₃ nanoparticles.

Keywords: Ag-NiTiO₃, sol-gel method, semiconductor, photovoltaic, doping.

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1. Introduction

Nanomaterials have gained much attention among materials and their properties do not only depend on their composition but also on their shape and size distribution.^1-3 Functional inorganic metal titanate of the general formula MTiO₃ (M%Pb, Ba, Sr, Ni, Fe, Zn, Cu, and Cr) has always been a topic of burgeoning interest largely because they display magnificent dielectric,⁴ ferroelectric,⁵ pyroelectrical,⁶ piezoelectric,⁷ magneto restrictive,⁸ and electro-optical⁹ characteristics. Such superior combination of functional properties ranked them as 'intelligent materials' for the development of sensors,^10,11 microelectronic devices,¹² and photocatalytic application.^13-14 Nickel titanate (NiTiO₃), a member of this well-known family, has a wide range of the above-mentioned properties, and has been extensively described in literature in crystal thin film and powder formats. NiTiO₃ is stable up to high temperature (1000 °C) even in air. Furthermore, the dielectric constant and magnetic susceptibility of NiTiO₃ is 13 K and 0.0096 X_m.¹⁵ There are several methods for synthesizing pure nickel titanate material such as sol-gel,^16-17 polymeric precursor method,¹⁸ electro spinning technique,¹⁹ modified Pechini method,²⁰ etc. The sol-gel technique has many advantages over other fabrication techniques such as homogeneity, stoichiometry control, purity, ease of processing, controlling the composition, and ability to coat large and complex area substrates.¹⁶ In this work, we report the synthesis of the pure Ag-NiTiO₃ nanoparticles via the sol-gel method using stearyl alcohol as the capping agent. This synthetic technique can easily be controlled and is more convenient for synthesis of pure mixed metal oxides nanoparticles. Furthermore, the possibility of developing a solar cell with ITO/Ag-NiTiO₃/TiO₂/Pt-ITO structure was also investigated.

 

2. Experimental

2.1. Characterization

X-ray diffraction (XRD) pattern was recorded by a Philips-X'PertPro (St Laurent, Quebec, Canada), X-ray diffractometer using Ni-filtered Cu Ka radiation at scan range of 10 < 20 <80 were used. The energy dispersive spectrometry (EDS) analysis was studied by XL30, Philips microscope (Phillips Company, Netherlands). Scanning electron microscopy (SEM) image was obtained on LEO-1455VP (Phillips, Amsterdam, Netherlands) equipped with an energy dispersive X-ray spectroscopy. Fourier transform infrared (FT-IR) spectra were recorded on Magna-IR, spectrometer 550 Nicolet with 0.125 cm^-1 resolution in KBr pellets in the range of 400-1000 cm^-1. Photocurrent density-voltage (J-V) curve was measured by using computerized digital multimeters (Ivium-n-Stat Multichannel potentiostat) and a variable load. A 300 W metal xenon lamp (Luzchem) served as a simulated sunlight source, and its light intensity (or radiant power) was adjusted to simulated AM 1.5 radiation at 100 mW cm^-1 with a filter for this purpose. The magnetic measurement of samples were carried out in a vibrating sample magnetometer (VSM) (Meghnatis Daghigh Kavir Co., Kashan Kavir, Iran) at room temperature in an applied magnetic field sweeping at ±10 000 Oe.

2.2. Synthesis of NiTiO₃ Nanoparticles

Nickel nitrate hexahydrate Ni(NO₃)₂6H₂O, tetra-n-butyl titanate (TNBT), and stearyl alcohol used in experiments were all of analytical grade and used without any further purification. At first, 5 g of stearyl alcohol was melted in a beaker at 60 °C and then 0.34 g of nickel nitrate was added to the melted stearyl alcohol and stirring to form a transparent solution. Afterwards, 5 mL of TNBT was added to the above solution, stirring to form a homogeneous sol, naturally cooling down to room temperature, and drying in an oven for 12 h to obtain dried gel. Finally, the gel was calcined at 700 °C for 150 min in air to obtain NiTiO₃ nanoparticles.

2.3. Synthesis of Ag-NiTiO₃ Nanoparticles

In a typical synthesis procedure, the stoichiometric ratios of NiTiO₃ (1 mmol) and AgNO₃ (0.5 mmol) were dissolved in 30 mL of distilled water under magnetic stirring to form a homogeneous solution. Afterwards, the obtained precipitate was collected by filtration and washed with absolute ethanol and distilled water several times. Finally, the precipitate was calcined at 700 °C for 60 min in air to obtain Ag-NiTiO₃ nanoparticles.

2.4. Cell Fabrication

Electrophoresis deposition (EPD) was utilized to the prepare TiO₂ films. During EPD, the cleaned FTO (Fluorine doped Tin Oxide) glass remained at a positive potential (anode) while a pure steel mesh was used as the counter (cathode) electrode. The linear distance between the two electrodes was about 3 cm. Power was supplied by a Megatek Pro-grammable DC Power Supply (MP-3005D). The applied voltage was 10 V. The deposition cycle was repeated 4 times, each time 5 s, and the temperature of the electrolyte solution was kept constant at 25 °C. The coated substrates were air-dried. The apparent area of the film waslxlcm. The resulting layer was annealed under air flow at 500 °C for 30 min. Electrolyte solution consisted of 120 mg L^-1 of I₂,48mLL^-1 of acetone, and 20 mL L^-1 of water. For deposition of Ag-NiTiO₃ powder on the FTO glass substrate, a paste of Ag-NiTiO₃ was initially prepared. The slurry was produced by mixing and grinding 1.0 g of the nanometer sized Ag-NiTiO₃ with ethanol and water in several steps. Afterwards, the ground slurry was sonicated with ultrasonic horn (Sonicator 3000; Bandeline, MS 72, Germany) and then mixed with terpineol and ethyl cellulose as binders. After removing the ethanol and water with a rotary-evaporator, the final paste was prepared. The prepared Ag-NiTiO₃ paste was coated on TiO₂ film by a doctor blade technique. After that the electrode was gradually heated under an air flow at 450 °C for 30 min. Counter-electrode was made from deposition of a Pt solution on FTO glass. Afterwards, this electrode was placed over TiO₂/Ag-NiTiO₃electrode. Sealing was accomplished by pressing the two electrodes together on a double hot-plate at a temperature of about 110 °C. The redox electrolyte consisting of 0.05 M of LiI, 0.05 M of I₂, and 0.5 M of 4-tert-butylpyridine in acetonitrile as a solvent was introduced into the cell through one of the two small holes drilled in the counter electrode. Finally, these two holes were sealed by a small square of sealing sheet and characterized by I-V test.

 

3. Results and Discussion

Crystalline structure and phase purity of as-prepared Ag-NiTiO₃ nanoparticles has been determined using XRD. The XRD pattern of Ag-NiTiO₃ nanoparticles is shown in Fig. 1. Figure 1 shows the formation of rhombohedral phase of NiTiO₃ (space group R-3, JCPDS No. 83-0198) along with cubic phase of Ag (JCPDS No. 87-0717). From XRD data, the crystallite diameter (Dc) of Ag-NiTiO₃ nanoparticle was calculated to be 18 nm using the Scherrer equation:²¹

where β is the breadth of the observed diffraction line at its half intensity maximum, K is the so-called shape factor, which usually takes a value of about 0.9, and λ is the wavelength of X-ray source used in XRD.

The morphology of the Ag-NiTiO₃ nanoparticles has been examined by SEM analysis (Fig. 2). According to the Fig. 2, it seems that the products mainly composed of small, spherical nanoparticles with average size of about 45 nm.

 

 

EDS analysis was used in order to evaluate the chemical composition and purity of final products, as shown in Fig. 3. The EDS spectrum of Ag-NiTiO3 nanoparticles shows the presence of Ni, Ti, Ag, and O elements in these nanoparticles. Furthermore, neither N nor C signals are detected in the EDS spectrum, which means the product is pure and free of any impurity.

 

 

The FT-IR spectrum of Ag-NiTiO₃ nanoparticles in the range 400-4000 cm^-1 is shown in Fig. 4. The absorption band at 3424 cm^-1 is attributable to the v(OH) stretching mode and the weak absorption band observed at 1741 cm^-1 is due to the bending vibration of absorbed water²² which indicates the presence of physisorbed water molecules linked to Ag-NiTiO₃ nanoparticles. Besides, the absorption band at 911 cm^-1 belongs to the characteristic vibrations of the inorganic Ti-O-Ti network.²³ Furthermore, the absorption peaks at 536 and 685 cm^-1 correspond to the Ti-O bending mode of TiO₂.²⁴ Moreover, the band at 446 cm^-1 can be assigned to the absorption band of Ni-O, which confirms the formation of NiTiO₃.¹⁴

The diffused reflectance spectrum of the as-prepared Ag-NiTiO₃ nanoparticles is shown in Fig. 5. The fundamental absorption edge in most semiconductors follows the exponential law. Using the absorption data the band gap was estimated by Tauc's relationship:

 

 

 

where a is absorption coefficient, hu is the photon energy, a0 and h are the constants, Eg is the optical band gap of the material, and n depends on the type of electronic transition and can be any value between 0.5 and 3. The energy gap of the Ag-NiTiO3 nanoparticles is determined by extrapolating the linear portion of the plots of (ahn)² against hn to the energy axis, as shown in Fig. 5. The Eg value is calculated as 2.1 eV for the Ag-NiTiO3 nanoparticles which is lower than pure NiTiO3 nanoparticles.¹⁴ Moreover, the band gap of pure NiTiO3 nanoparticles is slightly red shifted with add Ag on the surface of NiTiO3. There is lowering of the band gap energy of the NiTiO3 with add AgNO3. This fall in band gap energy with add Ag might be due to the metallic clusters that introduce localized energy levels in the NiTiO3 band gap. The electrons can be excited with lower energy from the valence band to these levels rather than to the conduction band of the semiconductor. Similar observations were also reported in literatures for Ag deposited semiconductor.^25,26

I-V characterization of a typical solar cell fabricated using in situ approach is shown in Fig. 6. The measurement of the current density voltage (I-V) curve for Ag-NiTiO₃ was carried out under the illumination of AM1.5 (100 mW cm^-2). Device characteristics are as follows: V_oc = 0.3 V, J_sc = 0.167 mA cm^-2, FF = 3.2, and h = 0.16. In comparison with J-V characterization of NiTiO₃ nanoparticles,¹⁴ dispersion of Ag over the surface of NiTiO₃ nanoparticles causes the surface plasmon resonance (SPR) and enhances the broad absorption in the entire visible region of the solar spectrum. Therefore, dispersion of Ag over NiTiO₃ nano-particles could be the better alternative to enhance the absorption of visible light by NiTiO₃ nanoparticles for effective solar cell.

The hysteresis loop of Ag-NiTiO₃ nanoparticles was studied to examine its magnetic properties (Fig. 7). According to the Fig. 7, at 300 K the remanent magnetization (Mr) is 0.002 emu g¹, the coercive field (Hc) is 80 Oe, and the magnetization at saturation (Ms) is 0.06 emu g^-1 (the saturation magnetization Ms was determined from the extrapolation of curve of HM versus H).

 

 

4. Conclusions

In this work, Ag-NiTiO₃ nanoparticles were successfully synthesized via the sol-gel method. Stearyl alcohol played role as the capping agent and solvent. VSM result indicates a ferromagnetic behaviour for synthesized Ag-NiTiO₃ nanoparticles. A preliminary study on the possibility of developing a solar cell having FTO/TiO₂/Ag-NiTiO₃/Pt-FTO structure was also performed. Based on the VSM and photovoltaic measurement, magnetization at saturation (Ms) and conversion efficiency are 0.06 emu g^-1 and η = 0.16, respectively. UV-Vis result confirmed the existence of strong plasmonic band induced by Ag nano-particles via surface plasmon resonance. photovoltaic result was found to be significantly enhanced for the Ag doped NiTiO₃ caused due to the plasmonic resonance induced by AgNO₃.

 

Acknowledgement

Authors are grateful to council of University of Arak for providing financial support to undertake this work.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

 

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Received 24 March 2016
Revised 26 July 2016
Accepted 9 January 2017

 

 

* To whom correspondence should be addressed. E-mail: ali.sobhaninasab@gmail.com