Preparation and characterization of Fe-doped TiO powders for solar ...

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terized by means of X-ray diffraction (XRD), Fourier-transform infrared (FT-IR), Fourier-transform Raman .... Sol- gel-derived Fe-doped TiO2 catalysts showed the pres- ence of rutile and pseudobrookite ... el, the space charge layer became thicker, and the prob- .... where CA is content ratio of anatase (%) and CR content.
Processing and Application of Ceramics 6 [1] (2012) 21–36

Preparation and characterization of Fe-doped TiO2 powders for solar light response and photocatalytic applications Ibram Ganesh*, Polkampally P. Kumar, Abhishek K. Gupta, Panakati S.C. Sekhar, Kalathur Radha, Gadhe Padmanabham, Govindan Sundararajan Laboratory for Photoelectrochemical (PEC) Cells and Advanced Ceramics, International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Balapur PO, Hyderabad-500005, A.P., India Received 22 October 2011; received in revised form 30 December 2011; accepted 7 February 2012

Abstract Different amounts of Fe-doped TiO2 (with 0.1 to 10 wt.% Fe) powders were prepared at temperatures in the range of 400 and 800  °C following a conventional co-precipitation technique and were thoroughly characterized by means of X-ray diffraction (XRD), Fourier-transform infrared (FT-IR), Fourier-transform Raman (FT-Raman), diffuse reflectance spectroscopy (DRS), BET surface area, zeta potential and flat band potential measurements. Photocatalytic ability of Fe-doped TiO2 powders was evaluated by means of methylene blue (MB) degradation experiments conducted under the irradiation of simulated solar light. Characterization results suggested that as a dopant Fe stabilized TiO2 in the form of anatase phase, reduced its band gap energy and adjusted its flat band potentials in such a way that these powders can be employed for photoelectrolysis of water into hydrogen and oxygen in photoelectrochemical (PEC) cells. The 0.1 wt.% Fe-doped TiO2 exhibited highest activity in the photocatalytic degradation of MB. The kinetic studies revealed that the MB degradation reaction follows the Langmuir-Hinshelwood first order reaction rate. Keywords: titania, doping, photocatalysis, band gap energy, semiconductor I. Introduction Recently, the photoelectrochemical (PEC) conversion of CO2 to methanol and the photo-electrolysis of water into hydrogen and oxygen have received a great deal of attention from the scientific community as these two reactions can indeed harvest solar energy in the form of chemical energy [1–5]. These reactions are popularly called as artificial photosynthesis reactions, which require thermodynamic energy inputs of 1.5 and 1.23 eV, respectively [4–9]. Greater energy inputs are required to make up for losses due to band bending (necessary in order to separate charge at the semiconductor surface), resistance losses, and overvoltage potentials. The most frequently studied material for photoelectrode is TiO2 [6,7]. Despite its high band gap energy of 3 eV, TiO2 is the most preferred photo-electrode owing to its high photo-corrosion re-

sistance in aqueous media, chemical stability, low cost and non-toxicity. Given its indirect band gap transition, the anatase in comparison to rutile and brookite phases of TiO2 is the most preferred phase for photoelectrode applications. The maximum value obtained for the photovoltage of a PEC cell equipped with a TiO2 photoanode is ~0.7–0.9 eV [10]. This implies that the PEC cell containing TiO2 based single photoelectrode requires some amount of an external bias voltage to perform water oxidation reaction [1–10]. It is known that as long as external bias voltage or energy that is generated from fossil fuels is involved the artificial photosynthesis process performed in a PEC cell cannot be declared successful [11,12]. However, for the first time, Nozik [11,12] successfully photooxidized water into hydrogen and oxygen by utilizing simultaneously n-type TiO2 as photoanode and p-GaP as photocathode without employing any external bias voltage in a PEC cell. Although Nozik’s process was not successful commercially as p-GaP undergoes photocorrosion in aqueous based PEC cells, it demonstrat-

Corresponding author: tel: +91 40 24442699, e-mail: [email protected]

*

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ed that given a stable p-type semi-conducting material with suitable band gap energy and band edges (i.e., flat band potentials), the water oxidation reaction (i.e., the energy consuming, endothermic, uphill and light reaction of natural photosynthesis) can be performed in a PEC cell without the need of any external bias voltage [11,12]. Since, TiO2 is an excellent photoelectrode material with band edges suitable for water oxidation reaction, several efforts were made to convert n-type semi-conducting behaviour of TiO2 into p-type, and also to reduce its band gap energy and the recombination of its photogenerated electron-hole pairs so that it can absorb a larger portion of sunlight that reaches the surface of the earth [4,8,13–23]. Thus, synthesis of ptype semi-conducting TiO2 is of great importance from the point of view of artificial photosynthesis. The p-type semi-conducting behaviour has been observed for TiO2 that was doped with certain metal ions such as, Fe3+, Co3+, Ni2+, Cu+, etc. [4,8,13–23]. The change in the nature of semi-conducting behaviour of TiO2 has been attributed to the hetero-unions formed between n-type TiO2 and p-type doped metal oxide [4,8,13–23]. Among the various p-type semi-conducting materials, the Fe2O3 has been identified as a promising one as it possesses a considerably low band gap energy (Ebg = 2.2 eV) and the ionic radius of Fe3+ (0.64 Å) is quite comparable to that of Ti4+ (0.68 Å). However, Fe2O3 was found to be more prone to photocorrosion and the lifetime of its photogenerated minority carriers was found to be quite short [24]. In order to bring together the beneficial properties of Fe2O3 and TiO2, the composites of these two semi-conducting oxides were prepared [16–23] and employed for photoassisted reduction of dinitrogen to ammonia, and the photoassisted degradation of waste materials in environment treatment [25,26]. The increase of Fe concentration from 0.2 to 0.5% in TiO2 has resulted in the increased photocatalytic activity of the latter material prepared at 500 °C. However, when Fe content and calcination temperatures were further increased, the photocatalytic activity was found to decrease. This decrease was attributed to the limited solubility of Fe in TiO2 (anatase or rutile), which is less than 1 wt.%. Furthermore, the hematite (α-Fe2O3) or pseudobrookite (Fe2TiO5) phases formed upon doping of >1 wt.% Fe were found to be responsible for the transfer of photogenerated charge carriers from TiO2 to Fe2O3 (or Fe2TiO5) that resulted in the decreased photocatalytic activity [16–23]. In 1995, Tsodikov et al. [21] reported the formation of solid solutions of Fex(Ti)1−0.75xO2-δ; 0.01200 mL/min) for >2  h and continued the same during voltage measurements. Initially, the pH of the solution was adjusted to pH=1–2 prior to recording the voltage readings using a solution of HNO3 (0.1 M). The light source was a Xenon arc lamp of 500 W (Solar Simulator, Oriel 91160) having AM 1.5G filter and a monochromator (Newport74125 model) with a bandwidth of 5 nm. Stable photovoltages were recorded after about 30 min of changing the pH value using a multi-meter (Agilent, Singapore) [32,33]. All the potentials presented here are against NHE reference. 2.4 Characterization A Gemini Micromeritics analyzer (Micromeritics, Norcross, USA) was used for Brunauer-Emmett-Teller (BET) surface area measurements. The BET surface area was measured by nitrogen physisorption at liquid nitrogen temperature (–196 °C) by considering 0.162 nm2 as the area of cross section of N2 molecule. Prior to measurements, the samples were evacuated (up to 1×10−3 Torr) at 180  °C for 2  h. Phase analysis, crystallite size and lattice parameter values of the powders were determined by X-ray diffraction (Bruker’s D8 advance system, Bruker’s AXS, GmbH, Germany) using Cu Kα radiation source. To obtain quantitative information of phases, the most intense peak of the individual phase was taken into consideration. The peak heights of all the phases were summed up and the percentage concentration of a particular phase was estimated from the ratio of the strongest peak of that phase to the sum of various phases present in a sys-

tem [34,35]. The crystallite sizes of the powders were estimated with the help of Debye-Scherrer equation (hkl = K·λ/βhkl cosθ; where K is a constant taken as 1 and β is the integral breadth that depends on the width of the particular hkl plane; λ = 1.5406 Å, the wavelength of the Cu Kα source; and θ is the Bragg’s angle) using the XRD data of the strongest reflection of the major phase [34,35]. The lattice parameter values were determined using equation 1/d(hkl)2 = (h2+k2)/a2 + l2/c2 (where the value of d(hkl), for an XRD peak, was determined from Bragg’s law, 2d(hkl)·sinθ = n·λ). Here, hkl is the crystal plane indices, d(hkl) is the distance between crystal planes of (hkl), and a, c are the lattice parameters (for tetragonal anatase and rutile phases of TiO2: a  =  b  ≠  c). The planes, (101) and (200) for anatase (ICDD File No.: 03-065-5714), and the (110) and (211) for rutile (ICDD File No.: 03-065-1118) were considered while calculating the lattice parameters values. The content ratio of the anatase to rutile phase in titanium dioxide was roughly estimated using the following equations [36,37]: CA = 100 / (1 + 1.265 IA/IR)

(1)

CR = 1−100 / (1 + 1.265 IA/IR)

(2)

where CA is content ratio of anatase (%) and CR content ratio of rutile (%). The micrographs of the powders were examined using a scanning electron microscope (JSM– 5410, JEOL, Japan) with an energy dispersive scanning (Sigma 3.42 Quaser, Kevex, USA) attachment for qualitative and quantitative micro analysis. The FT-IR spectra were recorded on an FT-IR 1650 Perkin–Elmer Spectrometer (4000–200 cm–1) using KBr pellets. The FT-Raman spectra were recorded on a triple subtractive Jobin Yvon T64000 Raman spectrometer equipped with a liquid-nitrogen-cooled charge-coupled device (CCD) detector. Diffuse reflectance spectra (DRS) of the powders were recorded on a Shimadzu UVPC 3101 spectrophotometer equipped with an integration sphere. The powder samples were directly placed into a cylindrical sample port holder (3 cm diameter, 1 mm deep). The zeta-potentials of powders in 10–3 M KCl aqueous solutions were measured on a Zeta meter (Zeta Meter Inc., USA). The band gap energies of Fe-doped TiO2 powders were calculated using the absorbance data obtained from DRS study following the Tauc’s relation [(α·h·ν) = C(h·ν – Eg)n], where C is a constant, α is absorption coefficient, Eg is the average band gap of the material and n depends on type of transition (2 for indirect band gap and ½ for direct band gap), h is the Plank’s constant (6.626 × 10–34 J·s), v is the frequency of photons [38,39]. The direct and indirect average band gap transition energies were estimated from the intercepts of linear portion of the (α·h·ν)2 or (α·h·ν)1/2 vs. h·ν of plots, respectively.

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III. Results and discussion X-ray diffraction patterns of the Fe-doped TiO2 (with 0 to 0.9, and 1 to 10 wt.% Fe) powders formed at 550 °C for 6 h are presented in Fig. 1. For sake of an easy interpretation, the XRD patterns reported in ICDD files for anatase TiO2 (ICDD File No.: 03-065-5714) and rutile TiO2 (ICDD File No.: 03-065-1118) are also presented in these figures. It can be seen that, in general, all the powders are well crystalline materials. The pure un-doped TiO2 is in the form of rutile and all the Fedoped TiO2 powders are mainly in anatase phase together with some minor phases including α-Fe2O3, TiFeO3, and Fex(Ti)1−0.75xO2−δ; 0.01