2016-09-15T20:07:18+05:302016-09-15T20:07:10+05:302016-09-15T20:07:18+05:30Acrobat PDFMaker 11 for Worduuid:160c84b9-1dd2-44e0-a1c7-1925ca599c9buuid:1eb7fd9d-6670-461c-a74d-fdaf2611929a10xmlSynthesis of Fe-TiO2 Nanoparticles for Photoelectrochemical Generation ofHydrogen Diana Núñez De la OssaAdobe PDF Library 11.0D:20160911093048Hewlett-Packard Company
International Journal of ChemTech Research CODEN (USA): IJCRGG, ISSN: 0974-4290, ISSN(Online):2455-9555 Vol.9, No.08 pp 453-464, 2016
Synthesis of Fe-TiO2 Nanoparticles for Photoelectrochemical Generation of Hydrogen Álvaro Realpe*, Diana P. Núñez, Adriana Herrera Department of Chemical Engineering, Research Group of Modeling of Particles and Processes, University of Cartagena, Cartagena, Colombia Abstract : In this paper, nanostructured photoanodes were made from titanium dioxide (TiO2) nanoparticles doped with Fein order to modify its absorption range in the electromagnetic spectrum. The TiO2 nanoparticles were obtained by a green biological route, using an extract derived from leaves of lemon grassas a reducingagent; these nanoparticles were doped by wet impregnation mechanism and supported like thin films prepared by Dr.Blading method. The morphology, structure and optical properties were evaluated by scanning electron microscope (SEM), X – ray diffraction (XRD),UV-VIS diffuse reflectance spectroscopy (UV-VIS/DRS) and photocurrent measurements. The characterization shows nanoparticles with the photoactive anatase phase of TiO
2with an approximate size of 56 nm. Doping of Fe+3 in TiO2 resulted in a shift of absorption edge towards the visible region of solar spectrum, it was observed a decrease in the band gap energy from 3.08 to 2.66 eV with increasing the doping concentration from 0 % w/w Fe up to1.0 % Fe. The 0.5 % w/w Fe doped TiO2 photoelectrode exhibited the highest photocurrent, 225 µA at zero external bia. Keywords: Titanium Dioxide, Photoelectrode, Doped Nanoparticle, Band Gap, Photoelectrochemical Water Splitting.
1. Introduction
Renewable energy has gained importance due to efforts to reduce the environmental pollution. This type of energy resource has a relevant role in world’s future (future world stability) due to the adverse environmental impacts of fossil based generation, besides the fact that, renewable energy resources can provide a reliable and sustainable supply of energy almost indefinitely. These sources can include solar energy, hydro energy (hydropower), biomass technology, wind power and geothermal energy [1,2,3].Currently the usage of technologies based on renewable energy has spread throughout the world to provide electricity, industrial application such as heating and drying [4]. In recent years, much attention has been paid to hydrogen, as a fuel and energy carrier [5-7];it can be used for a wide variety of applications to supplement or to substitute the hydrocarbon fuels and fossil fuels [8-16] by virtue of its high specific energy[17].Actually, the most common methods to obtained energy are based on fossil fuels (e.g., steam reforming of natural gas or other light hydrocarbons, gasification of coal and other heavy hydrocarbons), but these process are associated with high levels of CO2 emissions[17, 18] other alternatives include electrolysis of water[5-7], direct and indirect thermochemical decomposition [19].However, water electrolysis process uses the electricity generated by fossil fuels, for that reason it is necessary to develop alternative methods that provide the electricity required of hydrogen generation by this mechanism. Renewable resources like solar and wind energyare sustainable methods for hydrogen production with high purity, simple and green process[20-23].Solar energy is regarded as one of the most promising renewable energies to support the sustainable development of human society [24]. According to [18] the most promising way of producing hydrogen using a renewable source is through of solar energy. Photoelectrochemical water splitting mechanism involves the splitting of water into its component gases (oxygen and hydrogen) using solar energy, in this conversion system a semiconductor photoelectrode immersed in aqueous electrolyte absorb photons and use the energy to enable a redox chemical reaction [18, 25]. Semiconducting oxides appear to be the most promising materials for photo-electrodes due to the fact that their properties may be modified over a wide range through changes in nonstoichiometry and the associated semiconducting properties[18].TiO) on the nanosecond time scale, results in low efficiency in utilizing photon[26-32]. 2 is a metal oxide semiconductor known for a wield range of applications due to its chemical stability, electronic properties, resistance to corrosion and photocorrosion in aqueous environments, non-toxicity, absorbing photochemical properties and its relatively low cost [26-28]. On account of this properties and the due to its band edges of conduction and valence matching with the redox level of the water has been subject of studies aiming at the characterization of its properties and their modification in order to use it in photoelectrochemical generation of hydrogen process [29]. Because of large band gap energy in the two main polymorphs of TiO2(3.2 eV for anatase, 3.08 for rutile), it could only excited by UV light irradiation (λ < 380 nm), which accounts for 4% of the incoming solar energy on the Earth’s surface, which leads to a very low theorical conversion efficiency [24, 28, 29]. In addition, the high recombination rate of the photogenerated electron - hole pairs (A large amount of research has been focused on improving of the visible light absorption capacity in wide band gap of semiconductors through modification of its optical absorption coefficient by sensitization with small band gap organic semiconductors and/or through narrowing of its band gap via doping[33-36]. It is also important improving the morphology and electronic structure of TiO2 for effective separation and transportation of photoexcited charge carriers[37]. Doping opens up the possibility of changing the electronic structure of TiO2 nanoparticles, altering their chemical composition and optical properties [38], by permitting more light can be utilized to initiate the photovoltaic process [39]. Doping with metal and non-metal has been studied to change optical and electronic properties of TiO2 that limit the efficiency in the photochemical processes, the photoactivity under visible light irradiation of metal-doped TiO2(including transition metals) is due to a new energy level that is introduced in the band gap by the dispersion of metal particles in the TiO2lattice, in addition, transition metal doping improvement trapping of electrons to inhibit electron– hole recombination[40]. However, the effect of the dopant depends of its abilities to trap and transfer electrons or holes [41], and its concentrations is anessential factor to determine the photoactivity of the TiO2[42]. Transition metals such as Nb, Mn, Fe, Co, Sn, Cd, and Ni have been investigated with this purpose, between these, iron has been considered an appropriate candidate due to similar size of Fe3+ and Ti4+ ion radius, 0.64 Å and 0.68 Å respectively, therefore, Fe ions might easily be incorporated with the crystal lattice of TiO2[28, 29, 39].Many different techniques has been reported for doping of TiO2including sol – gel spin coating [29, 43], wet impregnation method [44], hydrothermal [45], oxidative pyrolysis [38 46]. In the present study, Fe doped TiO2nanostructured photoanodes has been elaborated for its evaluation in a photoelectrochemical water splitting process. TiO2 nanoparticles were synthetized by “green synthesis” mechanism, and the doping was made by wet impregnation method with different concentrations of Fe. The effect of iron doping on photoelectrode behavior was studied for band gap determination of doped and undoped photoelectrodes.
2. Experimental Procedure
2.1 Materials and methods TiO2 nanoparticles were synthetized by green synthesis method, using extract derived from leaves of lemon grass, the process was divided in two stages: natural extract elaboration and synthesis of TiO2 nanoparticles. Titanium tetraisopropoxide (TTIP, 95% pure) manufactured byAlfaAesar was used as a TiO2 precursorwhile ferric chloride hexahidrate (FeCl3 6H2O) as the precursor of Fe. Ethanol (CH3CH2OH) was used to wash of nanoparticles. TiO2 thin films were supported over stainless steel sheet (5 × 5 cm2 area and 0.3 mm thick, SS 304), prior to its use, stainless steel sheet was rigorously cleaned using soapy water, acetone, isopropanol and distilled water, and then dried in ambient air. After that, it was immersed in 0.5 M acid sulfuric solution followed by rising with distilled water[47]Acronal 295 D (acronal – water 50% v/v) as adhesive to TiOsolution followed by rising with distilled water[47]Acronal 295 D (acronal – water 50% v/v) as adhesive to TiOsolution followed by rising with distilled water[47]Acronal 295 D (acronal – water 50% v/v) as adhesive to TiO2.2 Preparation of extract derived from leaves of lemon grass Lemon grass (Cimbopogon cytratus) extract was used as a reducing agent. The fresh leaves were washed with distilled water to remove the dust or any other dirt of the environment and dried at 40°C in a furnace, after that, the leaves were cut into fine pieces and grinded. Aqueous extract of lemon grass was prepared by immersion of 100 g of lemon grass in 500 ml of distilled water at 100°C and was left at rest. This extract was filtered through filter paper several times and concentrated by evaporation, heating it at 80°C until 100 ml of extract. 2.3 Synthesis of titanium dioxide nanoparticles For synthesis of TiO2 nanoparticles, 85 ml aqueous solution of 5 mM of titanium isopropoxide was stirred during 12 h, 15 ml of the aqueous extract of lemon grass was added in 5 mM solution with continuous stirring at 175 rpm and room temperature for 24 h [48]. After the reaction of lemon grass extract with TTIP, the solution was centrifuged and nanoparticles were collected and dispersed in ethanol and then in distilled water, the centrifugation process was repeated to wash out impurities. Finally, titanium dioxide nanoparticles were dried in an oven at 100°C and then calcined in a muffle furnace for 3 h, starting at 330°C and ending at 550°C, this heat treatment also allows the removal of organic matter without reacting [49]. 2.4 Metal doping semiconductor - Preparation of Fe doped TiO2 nanoparticles TiO2 nanoparticles were doped using wet impregnation method [36]. An amount of TiO2 was added in distilled water, the TiO2 slurry was stirred for 30 min (to break up the loosely attached aggregates), after, an aqueous solution of iron (III) chloride (precursor of Fe+3) with the required amount of precursor required for doping was added to TiO2 suspension. It was subject to continuous stirring at room ambient for 48 h, and subsequently driedat 80°C, the solid was then crushed and calcined in a muffle furnace at 400°C for 2 h[50]. The Fe concentration were of 0.5, 0.7 and 1.0 % w/w versus TiO2. 2.5 Elaboration of nanostructured photoelectrodes To make the photoelectrodes, TiO2 thin films were supported over stainless steel sheets. The suspension was prepared by adding TiO2 nanoparticles to acronal solution (40% w/v TiO2 – acronal), the suspension was magnetically stirred for 30 min, and an ultrasonic bath was using to improve the dispersion state of nanoparticles in solution [51], it was used acronal in the suspension to ensure the films adhesion over the substrate. The suspension (paste) was applied onto the stainless steel sheet by a technique reported by [52-55],after deposition, the film was allowed to dry in air. The deposited process was repeated two more times to obtain a multilayer film (three layers), after that, films were subject to thermal treatment of sintering at 400°C for 30 min [49, 55-57], sintering allows of evaporating the solvent, to improve the nanoparticles connectivity, mechanical strength and the electrical conductivity of the photoelectrode [57]. 2.6 Characterization 2.6.1 X – ray Diffraction analysis (XRD) X-ray diffraction analysis was performed to identify crystal phase of titanium dioxide nanoparticles using a Bruker Model D8 ADVANCE Diffractometer. 2.6.2 Scanning electron microscopy (SEM) SEM analysis was performed on a Quanta 650 FEG Scanning Electron Microscope, through this analysis was identified the particle size and morphology of nanoparticles synthesized. 2.6.3 V-VIS diffusion reflectance spectroscopy (UV-VIS/DRS) The UV-Visible diffuse Reflectancia spectra was measures in the wavelength from 200 to 800 nm using a Thermo Scientific EVOLUTION 600 spectrophotometer UV/VIS. The spectra were obtained in reflectance percentage (%R). 2.6.4 Photoelectrochemical characterization Spectral photocurrent was measured in a configuration of two electrodes[58] into a photo electrochemical cell with a TiO2 photoelectrode as a working electrode and a stainless steel counter electrode introduced in an aqueous 0.1 M sulfuric electrolyte solution. The measurement of the electrical parameter was carried out using a multimeter connected to the system through an external circuit, electrical contact was made with a copper wire. The photoanode was radiated with a lamp visible light. The distance between the working and the counter electrode was 15 cm. The separated of photogenerated charges was evaluated by photocurrent response under irradiation, undoped and Fe doped TiO2 photoelectrodes were used.
3. Results and Discussion
3.1 X – ray Diffraction analysis (XRD)
TiO2 nanoparticles formation through extract derived from leaves of lemon grass was confirmed by X-ray diffraction analysis. Figure 1 displays the XRD pattern of synthesized TiO2 nanoparticles. This figure shows well-defined peaks,indicating the crystalline nature of the phases present in the sample [59].These peaks can be denoted at 2θ value of 25.3°, 37.99°, 48.0°, 54.5°, 62.9°, which are characteristic ofthe Anatase phase of TiO2[60]. In this DRX pattern are not present the diffraction peaks belonging to the other polymorphs of TiO2, rutile and brookita [60]. An important factor towardthis result is the calcination temperature of the nanoparticles, it has been reported[61], that normally anatase-rutile phase transformation takes place in the temperature range from 600 to 700°C, in accordance with [62], the temperature range for anatase–rutile transformation shifted to higher temperature with the increase of the synthesized pH value. The transformation can be also affected by factors such as preparation conditions, precursors, impurities, oxygen vacancies and the particle size of the anatase phase obtained [61].
Figure 1. XRD pattern of TiO2 nanoparticles synthesized from aqueous leaf extract of lemon grass
The synthesis of TiO2nanoparticles has been investigated using different plant species, by using Aloe vera extract [63], Eclipta prostrata leaf[40], Jatropha curcas [64].These investigations report that biosynthesis of TiO22 nanoparticles through extract from plantsoccur due to phytochemicals as terpenes, ketones, aldehydes and flavonoids, which are the main responsible of the reactions because they may act as reducing agents of alkoxide, which gives rise to the formation of the nanoparticles of TiO.
3.2 Scanning electron microscopy (SEM)
Figure 2 shows the scanning electron microscope (SEM) of pure TiO2 nanoparticles synthesized by green synthesis method. According to the micrograph, the morphology of the TiO2 nanoparticles is approximately spherical, the nanoparticles are agglomerated together to form lot of nanoclusters.
Figure 2.SEM micrograph of TiO2 nanoparticles
The nanoparticles size distribution analysis was carried out using ImageJ program, the particles are distributed in the size 56±16 nm ranges. Figure3 shows the histogram corresponding to that distribution.
Figure 4 shows the optical reflectance diffuse properties of undoped TiO2 and doped TiO2 with Fe, and to form the thin films deposited on stainless steel sheets for studying with ultraviolet-visible (UV-vis) in the wavelength range of 200 - 800 nm. It reveals that incorporation of iron ions leads to shift of optical response toward visible part of the spectra.
Figure 4.Diffuse reflectance spectra of Fe doped TiO2 films supported
The optical energy band gap of undoped and Fe doped TiO, where α is the absorption coefficient, is constant, is the photon energy, is the optical band gap energy and denote the nature of the electronictransition interband [65-67], the absorption coefficient )was determined by applying Kubelka – Munk function [68-69], the equation for this method is represented by , where is the reflectance and is the reflectance function which is proportionally to absorption coefficient ). For anindirect optical transition in anatase TiO as a function of theFigure 5, the linear part of those graphs was extrapolated to(the energy axis), allowing to determine band gap. 2 were determined using theTauc plotmethod2[69], was plotted As indicating in Table 1, the band gap energy for undoped film is higher than band gap energy of Fe doped TiO2 films, the value of this energy gap reduces with increasing of Fe concentration, these results are in agreement with other investigations [29], [70] in which have reported that the band gap energy was reduced as the Fe concentration increases. The band gap values decreased from 3.09 eV to 2.66 eV with an increase in the Fe content from 0 to 1.0 % w/w. Some authors have found that the optical energy band gap does not change from a determined concentration of doping, Tae-hyun Lee et al reported a decrease of Eg value from 3.0 to 2.2 eV at 0 % and 2 % of Fe, but with concentration increase at 5, 10 and 20 % band gap energy of films increased to 2.8 and 2.9 eV, without shows a change for the latest (10 and 20 %) [66].
Table 1. Band gap energy and photocurrent of Fe doped TiO2 films at different doping concentration
Figure 5. Relation(αhv)½ and the photon energy of Fe – doped TiO2 nanoparticles. (a) 0 % w/w Fe, (b) 0.5 % w/w Fe, (c) 0.7 % w/w Fe, (d) 1.0 % w/w Fe.
In according to[60] by doping is achieved a modification in electronic structure of the semiconductor, through it, additional electronic states can be provide within the band gap of the TiO2. Therefore, the shift of absorption edge toward visible region is due to the electron excitation and transfer from these levels (dopant ions) to the conduction band of TiO2 requires photons with low energy than an electron excitation and transfer from the valence to conduction band [61, 71-72].In this case, the visible region is induced by a sub-band gap transition between the 3d electrons of Fe and the conduction band of TiO2 [73-74].
3.4 Photoelectrochemical characterization
Figure 6displays the photocurrent generated by the photoelectrodes under dark and illuminated conditions (visible light), without applying an external voltage for the measurements. As a result of visible light excitation in doped photoelectrodes, photocurrent trough the cell between photoanode and cathode, therefore, the addition of Fe did provide visible light absorptionin TiO2.The graphics show an increase of the photocurrent after the photonic source was turned on. The photocurrent generated when visible light was used on Fe doped TiO2 photoelectrodes allowed the generation of gases in the photoelectrochemical cell. Undoped TiO2 photoelectrode does not shows response under visible light irradiation.
Figure 6.Photocurrent as a function of time without bias voltages under illumination by visible light: (a) 0.5 % w/w Fe-TiO2 - (b) 0.7 % w/w Fe-TiO2 - (c) 1.0 % w/w Fe-TiO2
The 0.7 % w/w Fe doped TiO2 exhibited the highest photocurrent, which was 133.12 µA at zero external bias, above and below this doping photocurrent was lower. It was 87.04 µA for 0.5 % w/w and 112.62 µA for 1.0 % w/w. According to [29]this performance may be attributed to traps created within the band gap, which act as trapping center for photogenerated electron – hole pair, hence it may prolong the charges lifetime, increasing the recombination time, however, the photocurrent with 1.0 %w/w Fe decrease because at high doping concentration the recombination of this photogenerated charges is easier.[66] reported in their investigation that the photocurrent decreased when doping concentration increased due to agglomeration, because it is more difficult for photoexcited minority carriers to escape to the electrolyte. Therefore, there is an optimal dopant concentration that allowed the photoactivity reaches maximum values. This values of photocurrent can be comparable to [75] where under similar conditions (without external bias) were obtained lower photocurrent values in TiO2 photoelectrode, in [21]and [22] were obtained higher photocurrent values. From graphics can be observed that happened fluctuations in photocurrent behavior over time, one of the reasons for declines in the values of photocurrent occur is due to accumulation of bubbles (gas) on the photoelectrodes surface, which leads to a decrease in its effective area [75].
4. Conclusions
In this investigation, TiO2 nanoparticles were synthetized to elaborate photoelectrodes. The used method for synthesis allowed to obtain nanoparticles with a ban gap lower than some commercial semiconductors with a small particle size, hence it could be a viable alternative since it is also simple, low cost and environmental benignant. The incorporation of Fe into the lattice of pure TiO2 by wet impregnate reduced its bang gap from 3.09 eV to 2.66 eV, shifting the absorption edge of TiO2 toward a region of lower energy, the largest reduction was obtained with a 1.0 % w/w Fe concentration. With all Fe-TiO2 photoelectrodes photocurrent was generated through visible light radiation without bias external, the better behavior photocurrent was observed in 0.7 % w/w Fe – TiO2 photoelectrode reaching 133.12 µA. Acknowledgements The authors of this investigation are grateful to Administrative Department of Science, Technology and Innovation (COLCIENCIAS, Colombia) and to the University of Cartagena for finance and provide the space for perform this project through the program of researcher young and innovator 2015. References
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