ХИМИЯ ВЫСОКИХ ЭНЕРГИЙ, 2013, том 47, № 5, с. 343-350
ФОТОХИМИЯ
EOSIN PHOTODEGRADATION OVER TiO2 MODIFIED BY y-RAY IRRADIATION
© 2013 R. Hazem", M. Doulache4, M. Izerroukenc, M. Trari4
aLaboratory of Nuclear Sciences, Faculty of Physic (USTHB) BP 32 16111 Algiers, Algeria bLaboratory of Storage and Valorization of Renewable Energies, Faculty of Chemistry (USTHB)
BP 32 16111 Algiers, Algeria cNuclear Research Centre (CRND)
Algiers BP 43 Draria, Algeria E-mail: solarchemistry@gmail.com Поступила в редакцию 24.02.2013 г. В окончательном виде 12.05.2013 г.
TiO2 thin films are elaborated by sol gel on glass substrates and irradiated with 60Co y-rays. The X-ray diffraction, UV-Visible spectroscopy and transport properties are investigated. The films are nominally non stochi-ometric and the conductivity occurs by thermally activated hopping of lattice polaron. The oxygen vacancies induced by y-ray irradiation at lower dose (<10 kGy) generate mixed valences Ti4+/3+, thus altering the transport properties. The photo-electrochemical characterisation is undertaken to evaluate the photo catalytic performance. The Mott-Schottky plots are characteristic of n type conduction from which a flat band potential of—0.62 VSCE and a donor density of 5 x 1017 cm-3 are determined for the most active film. The Nyquist plot exhibits a semi-circular arc whose center lies below the real axis, due to the constant phase element (CPE). The energy band diagram shows the potentiality of the films for the eosin photodegradation. 68% of initial concentration (10 mg L-1) disappears after 2 h of exposure to the solar light. TiO2 irradiated with gamma dose of10 kGy shows the best efficiency, due to the resistance decrees and high electron mobility (25 cm2 V-1 s-1). The eosin oxidation follows a first order kinetic with a rate constant of 6 x 10-2 min-1.
DOI: 10.7868/S0023119713050049
TiO2 is one of the most popular photocatalysts owing to its superior properties, low cost and non toxicity [1, 2]. It is actively used in both the environmental and energetic applications [3]. For the photo-electrochemical (PEC) cells and photovoltaic devices, it would be advantageous for economic reasons to use thin films where the transport properties behave rather differently from those of dense materials [4]. A number of methods have been used for preparing thin films such as hydrothermal route [5], anodic oxidation [6] and chemical vapour deposition [7]. However, they are expensive and often present low adherence and consequently a non reproducibility of the physical properties. So, the settled question is to what extent the film becomes well adhered and pores free. In this regard, the sol gel method would be of practical interest since it is relatively simple and appropriate for large area film deposition [8, 9]. The advantage is that the starting reagents are mixed at atomic level and this should catalyze the reaction rate and decreases both the temperature and duration.
Up to now, little is known about the PEC properties of y-irradiated TiO2 which is seldom investigated [10] and to our knowledge, fewer photocatalytic applica-
tions are reported [11]. Thus, it is of great concern to answer to the question on does the y irradiation enhance the photocatalytic properties. With the aim of preparing efficient devices for the solar energy conversion, the present work deals with TiO2 thin films grown by sol gel onto glass substrates. The films are subjected to y-rays at various doses and the optical properties and photo-electrochemical (PEC) characterization are reported. The homogeneity and stoichiometry are difficult to achieve and this turns to advantage since it offers the opportunity to characterize the oxides pho-to-electrochemically. The electrochemical impedance spectroscopy is also undertaken.
On the other hand, many dyes are discharged in water as industrial effluents without any restriction and are responsible of the large scale pollution [12]. They disturb the ecologic system and attenuate the light flux, thus affecting dramatically the aquatic life. Eosin is used as model molecule for testing the pho-
toactivity on modified TiO2 [1]. The O2- and/or OH' radicals are oxidative species capable to mineralize organic molecules and as application, the eosin degradation is successfully achieved over TiO2 under sunlight.
24 25 26 27 28 29 29, degree
Fig. 1. GAXRD pattern of T1O2 thin film elaborated by sol-gel.
the reference electrode is changed when necessary to prevent contamination. The current density is maintained uniform by positioning the working electrode midway between the counter electrode and SCE. Unless stated otherwise, the potential scan is 10 mV s-1. The capacitance is determined as a function of the potential with a scan rate of10 mV step-1 and a frequency of 10 kHz. The electrochemical impedance spectroscopy (EIS) is performed using small amplitude sine wave signals with frequencies ranging from 1 mHz to 100 kHz.
The photocatalytic tests are done under direct solar irradiation in a sunny day where the flux intensity fluctuates between 105 and 112 mW cm-2. For each experiment, 20 mL of solution (eosin, 10 mg L-1) are used. The solutions are freshly prepared from analytical substances in twice distilled water. The eosin analysis is performed with a UV-vis spectrophotometer Shimad-zu 1800 (Xmax = 516 nm).
EXPERIMENTAL
RESULTS AND DISCUTION
Sol gel has gained popularity in the materials science and has succeeded in the synthesis of oxides [13]. The nature of the substrate influences the nucleation and adherence ofthe films. Tetra iso-propoxide ortho-titanate is mixed to 2-propanol as solvent, acetylace-tone as ligand and acetic acid as buffer (pH ~ 4). After addition of water, the mixture is stirred at room temperature (2 h). The glass substrate is cleaned ultrasoni-cally and dip-coated on both sides at a pulling rate of 5.5 cm min-1. Then, the films are heated at 500°C (1 h) in air oven.
The irradiation with y-rays is performed at room temperature in the range (102-106 Gy) at the Nuclear Research Centre, Algiers (CRNA) using 60Co as source with an average energy of1.25 MeV. The X-ray diffraction (XRD) patterns are recorded before and after irradiation with a grazing incidence angle X-ray dif-fractometer (GAXRD model: Philips X'PERT PRO MPD) using Cu Ka radiation (X = 0.154178 nm). The transmittance spectra are obtained with Cintra 40 UV-Visible spectrophotometer. The resistivity measurements are performed by the two probe technique at room temperature.
The electrochemical study is conducted using a standard cell. The electrical contact is established by soldering copper wires onto the surface of the substrates which are enrobed with epoxy resin leaving an exposed surface area of 1 cm2. The intensity-potential J(E) curves are plotted in the working solution (eosin 10 mg L-1, Na2SO4 10-2 M) in aerated atmosphere using a potentiostat/galvanostat (PGZ 301, Radiometer analytical). The auxiliary electrode consists of a Pt electrode (Tacussel, 1 cm2) and the potentials are given relative to a saturated calomel electrode (SCE) and uncorrected for junction potentials. The solution in
As mentioned above, one of the major problems in elaborating thin films is the bad adherence and consequently the non reproducibility of the output parameters. The films prepared by sol gel adhere tightly on the glass substrate. The method is attractive for preparing homogeneous layers with improved physical properties. The phase purity is checked by XRD (Fig. 1), which clearly shows a mixture of both varieties with a preferential orientation (101, anatase ~80%) and (110, rutile). The weight fraction of the anatase phase w(A) is calculated from the relation [14]:
w
(A )%
1
1 +1.265-
(1)
where IA and IR are respectively the peaks intensity of anatase and rutile planes. The crystallite size (Table 1), is determined from the full width at half maximum (=0.94 X/P cos 9), P (rd.) being the broadening of the intense XRD peak. The y-ray irradiation does not affect significantly the crystallite size which averages 25 nm (Fig. 2, Table 1); similar results have already been obtained by others [3] on TiO2 films irradiated by electron beam (1.14 MeV).
Both the optical and transport properties of the films are altered by y-rays through the oxygen deficiency. The optical transmittance spectra of TiO2 films as well as the substrate are presented in Fig. 3a. After irradiation, the transmittance edge is slightly shifted toward higher wavelengths (red shift) (Fig. 3b) due to the defects formation as observed by others after ion implantation [15]. The intercept of the linear plot of (ahv)m with the hv-axis (Fig. 4) gives the gap Eg (Table 1 and Fig. 5), a is the optical absorption coefficient. The transition is indirectly allowed (m = 1/2) and corre-
The main physical parameters of TiO2 thin films irradiated at different y doses
Dose (Gy) Untreated 100 500 104 105 5 x 105 106
Resistance (Q) 27 x 104 2 x 104 4.1 x 103 500 6 x 104 5 x 106 5 x 106
Nd x 1017 (cm3) 1.7 3.4 6.5 5 5.9 - -
f (V) -0.66 -0.65 -0.39 -0.62 -0.38 - -
Eg (eV) 2.74 2.86 2.90 2.76 2.73 2.61 2.76
Cristallite size (nm) 25 - - 23 30 29 24
x 105 (cm2 V-1 s-1) 0.14 0.92 2.36 25.20 0.18 - -
W (nm) 90 64 46 53 48 - -
sponds to the charge transfer. As can be seen, the band gap is not significantly affected after y-rays radiation. The (Ti—O)n valence band is made up mainly of O2-: 2p6 orbital with little admixture of metal while the Ti4+: 3d0 orbital predominantly accounts for the (Ti-O)n* conduction band.
The transport properties of TiO2 remain controversial despite many works in the literature and depend on the extent of deviation from the stoichiometry. It has been reported that TiO2 is an amphoteric semiconductor and occurs in n- as well as p-types [16]. In the present case and based on the resistance values, TiO2 behaves as a non degenerate semiconductor with large activation energy. The lowe
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