Journal of Colloid and Interface Science 439 (2015) 54–61
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Regular Article
Enhanced visible light photocatalytic activity of Gadolinium doped nanocrystalline titania: An experimental and theoretical study Susmita Paul, Pawan Chetri, Biswajit Choudhury, Gazi Ameen Ahmed, Amarjyoti Choudhury ⇑ Department of Physics, Tezpur University, Napaam-784028, Tezpur, Assam, India
a r t i c l e
i n f o
Article history: Received 9 June 2014 Accepted 30 September 2014 Available online 17 October 2014 Keywords: Surface area Visible light Photoluminescence Band gap Photocatalytic activity Phenol Gd 4f states Density functional theory Trap centers Free charge carriers
a b s t r a c t Hypothesis: Undoped TiO2 nanoparticles are considered as a poor photocatalytic candidate in visible light due to the wide band gap. Incorporation of Gd ions is expected to modulate the electronic structure of the material and thereby enhance the photocatalytic properties of the material. Experiments: Gadolinium doped TiO2 nanoparticles were fabricated via a simple sol–gel method. Findings: The surface area of Gd doped TiO2 (225 m2/g) nanoparticles is much higher than that of undoped TiO2 (95 m2/g). Doping of Gadolinium enhances the visible light absorption property of TiO2 nanoparticles. Photoluminescence intensity increases at 0.03 and 0.05 mol and thereafter reduces at 0.07 mol. The photocatalytic efficiency of these nanoparticles is evaluated by observing degradation of phenol in aqueous solution under visible light. The doped nanoparticles are found to exhibit better photocatalytic activity. This enhancement has been attributed to the introduction of the Gd 4f energy levels in the band gap of TiO2. The presence of these states has been further confirmed by theoretical study based on density functional theory (DFT). It is speculated that the 4f states of Gd act as efficient electron trap centers. These 4f states facilitate electron migration to the surface making available free carriers to take part in photocatalysis. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Nanocrystalline titania (TiO2) are of intense interest due to their unique properties such as high chemical stability when exposed to acidic and basic compounds, its non-toxicity, low cost, and strong oxidizing power that makes it a competitive candidate for many photocatalytic applications [1–3]. However, the wide band gap (3.2 eV) and easy recombination of photo induced electron and holes results in a low efficiency of photocatalysis and seriously limit its practical applications [4,5]. Recently, some studies have been conducted with a view to improving the visible light photoactivity of TiO2 nanoparticles. One of the major strategies for inducing visible light response is by chemical doping with transition metals containing partially filled d orbitals such as V, Ni, Cr, Mo, Mn [6–10]. More recently TiO2 doped with lanthanide ions has also provoked great interests [11–14]. Lanthanide ions are known for their stability to form complexes with various Lewis bases. They have special electronic structure of 4f n5dn (n being the number of electrons) that leads to different optical properties and dissimilar ⇑ Corresponding author. Fax: +91 371222345. E-mail address: [email protected] (A. Choudhury). http://dx.doi.org/10.1016/j.jcis.2014.09.083 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
catalytic capacity [15]. The formation of redox couple of Lnn+/ Lnn+1 enables the formation of stable oxygen vacancies compared to the relatively high mobility of bulk oxygen vacancies. Inclusion of lanthanide ions in the TiO2 lattice provides a way to concentrate the organic pollutants on the semiconductor surface [15]. Doping has been well accepted as an efficient mode of modifying the band structure of TiO2 to make it efficient in the visible region for practical purpose. However, the doped oxides are prone to suffer from thermal instability, together with the decrease in UV light photocatalytic effectiveness and in redox potential that are caused by the states in the band gap. These drawbacks are attributed to the presence of additional extrinsic electronic levels introduced by doping that serves as the electron–hole recombination centers, especially for heavily doped materials [16–18]. In the present work the influence of Gadolinium doping on structural, optical and photocatalytic properties of TiO2 nanoparticles is reported. The doped photocatalysts are found to exhibit enhanced photocatalytic activity compared to undoped TiO2. The abinitio calculation based on density functional theory shows that the Gd ions are localized, indicating that they mainly act as charge carrier trapping centers and delay the electron–hole recombination rate beneficial for enhanced photocatalytic activity.
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2. Experimental details
2.4. Photocatalytic activity
2.1. Preparation of Gd doped TiO2 nanoparticles
Photocatalytic activity for the undoped and Gadolinium doped TiO2 nanoparticles were studied by examining the degradation of aqueous solution of phenol. For visible light irradiation a 25 watt white lamp was used. In order to carry out the process 50 mg of the photocatalyst was added to 50 ml water in a 100 ml beaker. To it 5 mg of phenol was added and the solution was stirred for about half an hour to obtain the absorption–desorption equilibrium. The catalyst loaded phenol solution was then irradiated for 20, 40, 60, 80 min. After completion of the irradiation process the samples were centrifuged at 10,000 rpm to make it free from any catalyst. 5 ml of the aliquot was taken to measure the absorbance. The degradation efficiency was calculated by using the equation:
The reagents used for the synthesis were titanium isopropoxide (TTIP, purity 97%) and isopropanol (99%) and Gadolinium nitrate hexahydrate. Gd doped TiO2 nanoparticles with three different nominal concentrations of Gd (0.03 mol, 0.05 mol and 0.07 mol) were synthesized employing a simple sol–gel method. The precursors for dopant and host were taken to be Gadolinium nitrate hexahydrate and titanium isopropoxide respectively. 5 ml of titanium isopropoxide and 15 ml of 2 propanol were added to a 100 ml conical flask under constant stirring. This is followed by the addition of 1 ml of water to initiate the hydrolysis reaction. To the white sol of titanium isopropoxide, the dopant precursor solution were added and stirred for 7–8 h. During such process first a sol was formed which ultimately transformed into a gel. The gel was then centrifuged in water followed by ethanol for 4 times. The centrifuged product was dried at 80 °C. The resulting product was finally annealed at 450 °C to obtain crystalline Gadolinium doped anatase TiO2 nanoparticles. 2.2. Characterization details The structure of all the samples are determined using Rigaku Miniflex CD 10,041 XRD unit with copper target and k = 0.154 nm at a scanning rate of 1°/min and in the scanning range of 10–80°. High resolution transmission electron microscope images for morphology and particle size determination are observed with JEM2100, 200 kV JEOL. The oxidation state of the samples are studied with the help of X-ray photoelectron spectroscopy (XPS). The spectra is recorded on KRATOS-AXIS 165 instrument equipped with dual aluminium–magnesium anodes using the Mg Ka radiation (hm = 1253.6 eV) operated at 5 kV. Nitrogen adsorption–desorption isotherms are measured at 77 K in a Quantochrome Qautosorb autolyzer. Surface area is determined using multipoint Brunauer– Emmett–Teller (BET) method. The pore size distributions of the prepared samples are determined based on Barett–Joyner–Halenda (BJH) model. Diffuse Reflectance Spectra (DRS) of all the samples are taken with Shimadzu-2450 UV–Vis spectrometer. The photoluminescence (PL) measurements at room temperature are recorded with PERKIN ELMER LS 55 fluorescence spectroscopy.
%D ¼
A0 At 100 A0
ð1Þ
A0 = initial absorbance and At = absorbance after time t. 3. Results and discussions 3.1. Determination of Gd doping and morphological analysis of the samples. Fig. 1a shows the XRD pattern of Gd doped TiO2 samples. All the peaks in the samples are well indexed to the tetragonal anatase phase (JCPDS-782486) and no hint of Gadolinium containing oxide phases are resolved. The average crystallite size are calculated by using the Scherrer’s formula
d¼
0:9k b cos h
2.3. Computational details
where k = wavelength of the X-ray source used; b = full width at half maximum (FWHM); h = Bragg diffraction angle. The average crystallite size for pristine TiO2 (0.00 mol) is calculated to be 7.8 nm and 5.9 nm, 5.45 nm, 5.42 nm for 0.03, 0.05 and 0.07 mol of Gd respectively. The decrease in the size after doping is attributed to the repulsive interactions between dopant ions that prevents coalescence of the nanocrystallites and inhibits the growth by formation of Gd–O–Ti bond [19]. The width of the diffraction peak increases with the increase in doping concentration indicating systematic decrease in grain size and degradation of the structural quality after doping.
A computer model of 2 1 1 supercell is built with the unit cell of TiO2 in anatase form. The undoped supercell contains 8 atoms of Ti and 16 atoms of oxygen which gives Ti8O16. The Gd doped TiO2 takes the form as Ti7GdO16. The minimum energy state is computed by varying the internal position of atoms until the residual force is 0.01 eV/A°. The cut off kinetic energy for the structural optimization is set to 400 eV along with convergence criterion of 105 eV. A 3 3 3 K-mesh is used which correspond to spacing less than 0.3(A°)1 in reciprocal space to calculate Density of states (DOS) and band structure; Fermi level is considered to be at 0 eV. The effect of Gadolinium ions (Gd) on the TiO2 system is understood by substituting one center titanium (Ti) atom by Gd atom and allowing it to relax in all directions. Density functional calculations using generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) theory is performed to describe the electron–electron exchange and correlation effects. The density functional theory (DFT) equations are solved via projector augmented wave (PAW) method as implemented in Vienna Ab Initio Simulation Package (VASP) and interfaced with MedeA technology platform.
Fig. 1. X-ray diffraction pattern of (a) undoped and (b) Gd doped TiO2 nanoparticles.
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The XPS spectra have been used to determine the chemical composition of the prepared samples so as to determine the existing states of the different elements. The sample with 0.03 mol has been used for XPS study. Fig. 2(a) shows the Ti 2p core XPS result of undoped and Gd doped TiO2 nanoparticles. The core level binding energies of Ti 2p3/2 and Ti 2 p1/2 are roughly at 459.31 eV and 464.90 eV. The variation of 5.59 eV ensure the characteristic binding energies of Ti4+ in TiO2 matrix [20]. The core level XPS O 1s spectra are also analyzed. Fig. 2b display the XPS spectrum of O 1s. The spectra specifies presence of at least two kinds of chemical states that include crystal lattice oxygen (OL) and chemisorbed oxygen (OH). The OL peak appears around 530.08 eV and the OH peak at 531.32 eV [21]. The amount of OH in both pure and doped sample is much more than that of OL (Table 1). The presence of OH peak is developed with increasing oxygen vacancy [22]. During the photo-excitation process these oxygen vacancies act as trap centers. The electrons trapped in these centers migrate to the surface, react with oxygen molecule and convert it to superoxide radical (O 2 ). This superoxide radical act as potential candidate in the photocatalytic degradation, explicated later in this chapter. In Fig. 2c, the Gd 4d3/2 peak at 144 eV corresponds to +3 oxidation state of Gd ion [19]. The size, shape and overall morphology of the samples have been characterized using transmission electron microscopy. The TEM images of the samples are shown in Fig. 3a and b. As observed from the micrographs both the doped and the undoped system consist of nearly spherical size particles. The particle size of the undoped and doped system are of the order of 21 nm (Fig. 3a) and 12 nm (Fig. 3b) respectively. The high resolution TEM images of the samples are shown in Fig. 3c and d. The lattice fringes are clearly visible with inter planar spacing of 0.345 nm for undoped (Fig. 3c) and 0.357 nm for doped sample (Fig. 3d). The fringes agrees well with the (1 0 1) plane of anatase TiO2 [23]. With the incorporation of Gd ions the fringe spacing is slightly increased as compared to undoped TiO2. The ionic radii of Gd3+ (0.94 Å) being larger than Ti4+ (0.68 Å), it is possible for the Gd ions to substitute and occupy Ti4+ sites. That is to say, substitutional doping occurs, and one Gd3+ ions replaces one Ti4+ leading to the formation of Gd with one negative charge and oxygen vacancy with one or two positive charges (Vo ; V o ). =
Gd2 O3 () 2GdTi þ 3Oo þ Vo =
2½GdTi ¼ ½Vo =
Gd2 O3 () 2GdTi þ 3Oo þ 2V o =
½GdTi ¼ ½Vo
Table 1 Quantitative analysis of OH and OL from XPS spectra. Samples
Detailed study
OL
OH
Position
530.2 eV
531.43 eV
At% Position
47.8 530.08 eV
52.8 532.62
At%
44.3
55.7
TiO2
TiO2:Gd
3.2. Surface area measurements Fig. 4 shows the nitrogen adsorption–desorption isotherm of pure and 0.03 mol Gd doped TiO2nanoparticles. The isotherms are of type IV with H1-type hysteresis loop in the relative pressure range of 0.4–0.8. The type indicates presence of mesopores and are the characteristics of materials containing agglomeration [24]. The surface area for 0.03 mol Gd doped TiO2 nanoparticles is found to be 225 m2/g which is about 2.2 times the surface area of undoped TiO2 (95 m2/g). From the pore size distribution the pore diameter is determined to be 6 nm and 4 nm for undoped and doped sample respectively. The decrease in pore size after doping might be due to the larger size Gadolinium ions that causes higher pore filling effect [25]. Table 2 shows the surface area and pore size of the doped and undoped samples calculated by BET and Barret–Joyner–Halenda methods (BJH). Thus introduction of dopant ion causes a considerable increase in surface area and a decrease in pore size, thereby favoring the criteria for enhanced photocatalytic activity. 3.3. Optical property and electronic structure study via DFT To investigate the influence of Gd concentration on the absorption of TiO2 nanoparticles, the absorption spectra of all the samples measured at room temperature is shown in Fig. 5. The absorption band edge of TiO2 nanoparticle appears at 340 nm. Gd doping shifts the absorption edge towards the visible region. This red shift is most likely due to the transition of electrons from the valence band to the 4f levels of Gd and the observation is in accordance with the absorbance spectra of other rare earth doped TiO2 nanoparticles [26]. The photoluminescence spectra owing to the recombination of charge carriers is a useful tool to explore the effectiveness of charge carrier trapping, migration, alteration and separation of charge carriers [27]. Fig. 6a shows the PL spectra of all the samples.
Fig. 2. Core level X-ray photoelectron spectrum (XPS) of (a) Ti 2p, (b) O 1s and (c) Gd d3/2 for 0.07 mol Gd doped TiO2.
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Fig. 3. TEM image of: (a) undoped and (b) Gd3+ doped TiO2 nanoparticles. High resolution images of (c) undoped and (d) Gd3+ doped TiO2 nanoparticles.
Fig. 4. Nitrogen- adsorption and desorption studies of (a) undoped and (b) 0.03 mol Gd doped TiO2 nanoparticles.
at an excitation wavelength of 320 nm. Undoped TiO2 shows near band edge emission peak at 398 nm. The intensity of this band edge emission peak is suppressed with the incorporation of Gadolinium ions. This reduction in the intensity is attributed to the band gap narrowing introduced by the formation of f states of the Gadolinium ions. Fig. 6b shows variation of band gap with the increase in dopant concentration. Doping of Gd3+ ions causes loss of oxygen atoms and results in the generation of free electrons. These electrons are trapped in the oxygen vacancy giving F center or color Table 2 Parameters as obtained from the BET surface area analysis. Samples
Surface area (m2/g)
Pore size (nm)
X = 0.00 X = 0.03
95 225
6 4
centers. Depending on the number of trapped electrons, these centers are referred as F (two electrons), F+ (one electron) and F2+ (devoid of electrons). The emission peaks at 490 nm and 535 nm are attributed respectively to the transition from oxygen vacancies with two trapped electrons and one trapped electron to the valence band of TiO2 [28]. Another peak at 429 nm is attributed to self trapped excitons (STE) originated by interactions of conduction band electrons localized on the Ti 3d orbital with holes present in the O 2p orbital of TiO2 [29]. In the photoluminescence process the electrons in the conduction band gets trapped into the oxygen vacancies via a non-radiative process and than recombines with the photogenerated holes in the valence band accompanied by fluorescence emission. It is to be noted that in the PL spectra no Gd3+ ions related emission peaks are observed. Only changes in the emission intensity are noticed after doping. The spectra presents enhanced emission intensity on incorporation of 0.03 mol
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Fig. 5. UV–Vis spectra of all the samples. Fig. 7. TDOS (Total Density of State) of undoped TiO2.
and 0.05 mol Gd and sudden quenching of the emission intensity on incorporation of 0.07 mol Gd. At lower dopant concentration the interactions between ions are too weak to have an effect on the energy levels of each dopant ion. But with the increase in dopant concentration the distance between the ions decreases and density of defects increases. This increases the interactions between the ions and the emission energy gets transferred from one ion to another ions or to another defects. The electrons being localized in the oxygen vacancies and attached to the nearby Gd3+ ions, are not easily available to undergo radiative recombination with holes. The energy is dissipated non-radiatively resulting in the reduction of emission intensity. We have calculated the average shortest distance (known as the critical distance) between ions at which the emission energy transfer occurs, using the equation [30].
RC ¼ 2
3V 4pxc N
1=3
V = volume of the unit cell = a2c = 0.133 nm3, where a = 0.3784 nm; c = 0.9420 nm. xc = critical concentration of the dopant (=0.07), N = number of Zions in the unit cell, Rc = the critical transfer distance. Putting the values the critical distance (Rc) for 0.07 mol is calculated to be 0.9682 nm. Therefore, Gadolinium doping is responsible for the red shift of the absorption band edge and band gap narrowing owing to the introduction of f states. These f states can largely delay the electron–hole recombination
rate and contribute to the enhancement of the photocatalytic activity. We have further carried out theoretical study based on density functional theory (DFT) to understand the electronic structure of Gd doped TiO2. Theoretical study is carried out considering two supercell Ti8O16 and Ti7GdO16 along with spin polarized calculation. Density of states (DOS) is calculated for both the systems as shown in Figs. 7 and 8 respectively. Although standard DFT calculation may not determine the actual band gap, it could frame out the changes in optical properties of Gd doped TiO2 system with respect to pristine TiO2 system. Fermi level in DOS is considered at 0 eV. If we compare the TDOS (Total Density of States) of Gd doped TiO2 and TiO2, we observe presence of states near the Fermi level in Gd doped TiO2 while in pure TiO2 there are no such levels near the Fermi level. Our interest is to identify the atoms responsible for the states near the Fermi level in Gd doped TiO2 and hence plotted PDOS (Partial Density of States) for Gd, Ti and O as shown in Fig. 8. PDOS plots shows that Gd 4f state is the most prominent near the Fermi level whereas Ti and O have lesser impact. We have also observed that other states of Gd i.e., s, p and d has almost no contribution in comparison to the f state. This very much indicates that f state of Gd is the key factor to tune the optical property of TiO2. On the other hand d and p states are the most effective states for Ti and O near the Fermi level respectively. The formation of these states near the Fermi level is due to the instability in structure as the system (TiO2) is suffering from foreign entity (Gd).
Fig. 6. (a) Photoluminescence spectra of the samples with an excitation wavelength of 320 nm. (b) Band gap analysis of the samples as calculated from the Kubelka Munk plot.
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Fig. 8. TDOS and PDOS (Partial Density of State) for Gd doped TiO2 nanoparticles.
The irregular bonding and unstable charge transfer among Gd, Ti and O leads to the splitting of states near the Fermi level. The instability in charge transfer occurs after substitution of Ti4+ ion by Gd3+ ion. A weak hybridization is observed among Gd 4f, O 2p and Ti 3d states below the Fermi level. Chen et al. [31] found that the Gd doping in TiO2 does not produce any states near the Fermi level. But in our case we have observed a prominent Gd 4f state at and above Fermi level. These states might act as trapping state. Illumination of a solid with UV/Vis light excites electrons to the conduction band leaving holes behind in the valence band. During the period of this photo excitation process few electrons move to the conduction band while few others are trapped in the levels created due to Gd 4f state. These trapped electrons easily migrate to the surface and react with the atmospheric O2 molecules to convert it to superoxide (O 2 ) radicals [32]. These superoxide radicals act as a potential candidate in the photocatalysis process. 3.4. Photocatalytic activity study Using aqueous phenol as the targeting medium the visible light photocatalytic activity of undoped and Gd3+ ion doped TiO2 nanoparticles were studied. Prior to the photocatalyst addition the phenol solution was irradiated for 20, 40, 60 and 80 min. No appreciable self degradation is noticed as evident from the degradation curve, Fig. 9. Pure TiO2 exhibits very low visible light photoactivity due to its large band gap. The photocatalytic activity has been found to be largely improved on incorporating Gadolinium ions. Under visible light irradiation electrons are excited from the valence band to the conduction band. Gadolinium dopants in TiO2 effectively scavenge electrons and prevent their recombina-
Fig. 9. Phenol degradation curve of the samples under visible light illumination.
tion with holes due to the existence of their half filled f orbitals. Presence of these f orbitals have been supported with the help of DFT based calculations as mentioned above. The Gd3+ ions with half filled f orbitals are very stable. When these Gd ions trap electrons the electronic configuration is largely destroyed and their
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Fig. 10. Schematic illustration of the photocatalytic process by Gd doped TiO2 nanoparticles.
stability decreases, the trapped electrons are easily transferred to the oxygen molecules adsorbed on the surface of the catalysts, and the Gd3+ ions return to the original stable half-filled electronic structure. This in turn promotes charge transfer and proficiently disperse the electrons and holes by superficially trapping the electrons [33]. Fig. 10 shows the schematic illustration of the photocatalytic activity of Gd doped TiO2 nanoparticles. The mechanism of trapping and detrapping can be represented with the following mechanisms: 3þ
þ e ! Gd
2þ
þ O2adbs ! Gd
3þ
þ h ! Gd
Gd Gd Gd
4þ
Gd
2þ 3þ
þ
þ O2
4þ
þ OH ! Gd
3þ
þ OH
The Gd4+ can also trap conduction band electron or Gd2+ can trap valence band hole to retain half filled electronic structure of Gd3+. 4þ
3þ
Gd þ e ! Gd 2þ þ 3þ Gd þ h ! Gd The enhanced photocatalytic activity is owing to the presence of half filled orbitals of the Gadolinium ions that serve as shallow trap for the charge carriers to accelerate interfacial charge transfer process [34]. The processes not only accelerate interfacial charge transfer but also enhance the generation of highly reactive oxidative species like superoxide and hydroxyl radicals. 3þ hm
TiO2 =Gd
! e þ h
þ
þ hm
OH þ h ! OH O2 þ e ! O2 Phenol þ O2 ! Products Phenol þ OH ! Products Due to difference in charge state substitution of Gd3+ ions leads to creation of oxygen vacancies. These oxygen vacancies promote the adsorption of O2 molecules and favors the adsorbed oxygen to arrest the photoelectrons simultaneously producing free radicals [35]. The presence of these oxygen vacancies have been testified from the XPS and the photoluminescence spectra. In the
photoluminescence process oxygen vacancies and defects bind the photoinduced electrons resulting in free or binding excitons for the photoluminescence process to easily occur, the larger the amount of oxygen vacancies the stronger the PL intensity. But in the photocatalytic process these oxygen vacancies and defects become centers to capture the photo induced electrons so that the recombination of photoinduced electrons and holes is effectively inhibited. In our work the photoluminescence intensity has been found to be maximum at 0.03 mol dopant concentration and at this concentration we are getting higher photocatalytic activity. The results thus, demonstrates certain kind of correlation between the photoluminescence process and photocatalytic activity, the stronger the PL intensity the larger the content of oxygen vacancies and defects, the higher the photocatalytic activity. Further the surface area of the nanoparticle has a profound role to enhance the photocatalytic activity. After 60 min, the degradation curve shows a decrease in the degradation trend. Although longer irradiation period produces higher number of charge carriers but only limited number of them reaches the semiconductor–liquid interface region for dye degradation, the rest undergo volume recombination dominating the reaction in accordance with the equation [36].
K R / exp
2R ao
Here KR is the rate of recombination, R is the distance separating the electron and the hole pair, ao is the hydrogenic radius of the wave function of the charge carriers. The recombination rate increases due to the decrease in the number of the trap sites. Thus it is seen that after 60 min of irradiation and production of charge carriers on the surface of Gd doped TiO2 nanoparticles, the rate of recombination is dominated to the rate of other reactions in the solution [36]. Dopant concentration also has some important influence in regulating the photocatalytic reaction rate. In the degradation curve it is observed that dopant concentration of 0.07 mol is showing the lowest photocatalytic activity. On this basis it is surmised that there exists an optimum dopant concentration in the TiO2 matrix to protract the separation of photogenerated charge carriers. The decrease in the activity can be correlated to the thickness of the space charge layer that is inversely proportional to the dopant con-
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centration. The thickness of the space charge layer is affected by dopant concentration according to the equation [36].
W¼
1=2 2eeo V eNd
W is the thickness of the space charge layer. e and eo are the dielectric constants of the semiconductor and vacuum. With the increase in dopant concentration the thickness of the space charge layer becomes less than the penetration depth of light and this greatly increases the electron hole recombination rate. Moreover, at higher concentration charge carriers undergo multiple trapping. This behavior decreases the mobility rate as a result of which they are absorbed before they can reach the surface. The increased recombination rate with increase in dopant concentration is attributed to the concomitant decrease in the average distance between the trap sites [37]. 4. Conclusion In this work Gd doped TiO2 nanoparticles have been prepared successfully by a simple sol–gel method. XPS spectra reveals presence of trivalent Gadolinium ions and presence of oxygen vacancies. Both XRD and TEM analysis indicates decrease in size after doping. Owing to the presence of Gadolinium ions, there is a substantial increase in the surface area. The PL spectra depicts increased peak intensity at 0.03 mol Gd ions and at this concentration the sample is exhibiting maximum photocatalytic activity. Using density functional theory the f states generated due to the presence of Gadolinium ions in the host matrix are identified and these states are believed to contribute to the enhanced photocatalytic activity. Acknowledgments We are thankful to, Sophisticated Analytical Instrument Facility (SAIF), NEHU, Shillong for the HRTEM images and IICT Hyderabad for XPS data. We are also thankful to UGC for financial support to the Project F.No.42-785/2013 (SR). References [1] A.L. Linsebigler, G. Lu, J.T. Yates Jr., Chem. Rev. 95 (1995) 735–758. [2] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemannt, Chem. Rev. 95 (1995) 69–96.
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Regular Article
Enhanced visible light photocatalytic activity of Gadolinium doped nanocrystalline titania: An experimental and theoretical study Susmita Paul, Pawan Chetri, Biswajit Choudhury, Gazi Ameen Ahmed, Amarjyoti Choudhury ⇑ Department of Physics, Tezpur University, Napaam-784028, Tezpur, Assam, India
a r t i c l e
i n f o
Article history: Received 9 June 2014 Accepted 30 September 2014 Available online 17 October 2014 Keywords: Surface area Visible light Photoluminescence Band gap Photocatalytic activity Phenol Gd 4f states Density functional theory Trap centers Free charge carriers
a b s t r a c t Hypothesis: Undoped TiO2 nanoparticles are considered as a poor photocatalytic candidate in visible light due to the wide band gap. Incorporation of Gd ions is expected to modulate the electronic structure of the material and thereby enhance the photocatalytic properties of the material. Experiments: Gadolinium doped TiO2 nanoparticles were fabricated via a simple sol–gel method. Findings: The surface area of Gd doped TiO2 (225 m2/g) nanoparticles is much higher than that of undoped TiO2 (95 m2/g). Doping of Gadolinium enhances the visible light absorption property of TiO2 nanoparticles. Photoluminescence intensity increases at 0.03 and 0.05 mol and thereafter reduces at 0.07 mol. The photocatalytic efficiency of these nanoparticles is evaluated by observing degradation of phenol in aqueous solution under visible light. The doped nanoparticles are found to exhibit better photocatalytic activity. This enhancement has been attributed to the introduction of the Gd 4f energy levels in the band gap of TiO2. The presence of these states has been further confirmed by theoretical study based on density functional theory (DFT). It is speculated that the 4f states of Gd act as efficient electron trap centers. These 4f states facilitate electron migration to the surface making available free carriers to take part in photocatalysis. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Nanocrystalline titania (TiO2) are of intense interest due to their unique properties such as high chemical stability when exposed to acidic and basic compounds, its non-toxicity, low cost, and strong oxidizing power that makes it a competitive candidate for many photocatalytic applications [1–3]. However, the wide band gap (3.2 eV) and easy recombination of photo induced electron and holes results in a low efficiency of photocatalysis and seriously limit its practical applications [4,5]. Recently, some studies have been conducted with a view to improving the visible light photoactivity of TiO2 nanoparticles. One of the major strategies for inducing visible light response is by chemical doping with transition metals containing partially filled d orbitals such as V, Ni, Cr, Mo, Mn [6–10]. More recently TiO2 doped with lanthanide ions has also provoked great interests [11–14]. Lanthanide ions are known for their stability to form complexes with various Lewis bases. They have special electronic structure of 4f n5dn (n being the number of electrons) that leads to different optical properties and dissimilar ⇑ Corresponding author. Fax: +91 371222345. E-mail address: [email protected] (A. Choudhury). http://dx.doi.org/10.1016/j.jcis.2014.09.083 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
catalytic capacity [15]. The formation of redox couple of Lnn+/ Lnn+1 enables the formation of stable oxygen vacancies compared to the relatively high mobility of bulk oxygen vacancies. Inclusion of lanthanide ions in the TiO2 lattice provides a way to concentrate the organic pollutants on the semiconductor surface [15]. Doping has been well accepted as an efficient mode of modifying the band structure of TiO2 to make it efficient in the visible region for practical purpose. However, the doped oxides are prone to suffer from thermal instability, together with the decrease in UV light photocatalytic effectiveness and in redox potential that are caused by the states in the band gap. These drawbacks are attributed to the presence of additional extrinsic electronic levels introduced by doping that serves as the electron–hole recombination centers, especially for heavily doped materials [16–18]. In the present work the influence of Gadolinium doping on structural, optical and photocatalytic properties of TiO2 nanoparticles is reported. The doped photocatalysts are found to exhibit enhanced photocatalytic activity compared to undoped TiO2. The abinitio calculation based on density functional theory shows that the Gd ions are localized, indicating that they mainly act as charge carrier trapping centers and delay the electron–hole recombination rate beneficial for enhanced photocatalytic activity.
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2. Experimental details
2.4. Photocatalytic activity
2.1. Preparation of Gd doped TiO2 nanoparticles
Photocatalytic activity for the undoped and Gadolinium doped TiO2 nanoparticles were studied by examining the degradation of aqueous solution of phenol. For visible light irradiation a 25 watt white lamp was used. In order to carry out the process 50 mg of the photocatalyst was added to 50 ml water in a 100 ml beaker. To it 5 mg of phenol was added and the solution was stirred for about half an hour to obtain the absorption–desorption equilibrium. The catalyst loaded phenol solution was then irradiated for 20, 40, 60, 80 min. After completion of the irradiation process the samples were centrifuged at 10,000 rpm to make it free from any catalyst. 5 ml of the aliquot was taken to measure the absorbance. The degradation efficiency was calculated by using the equation:
The reagents used for the synthesis were titanium isopropoxide (TTIP, purity 97%) and isopropanol (99%) and Gadolinium nitrate hexahydrate. Gd doped TiO2 nanoparticles with three different nominal concentrations of Gd (0.03 mol, 0.05 mol and 0.07 mol) were synthesized employing a simple sol–gel method. The precursors for dopant and host were taken to be Gadolinium nitrate hexahydrate and titanium isopropoxide respectively. 5 ml of titanium isopropoxide and 15 ml of 2 propanol were added to a 100 ml conical flask under constant stirring. This is followed by the addition of 1 ml of water to initiate the hydrolysis reaction. To the white sol of titanium isopropoxide, the dopant precursor solution were added and stirred for 7–8 h. During such process first a sol was formed which ultimately transformed into a gel. The gel was then centrifuged in water followed by ethanol for 4 times. The centrifuged product was dried at 80 °C. The resulting product was finally annealed at 450 °C to obtain crystalline Gadolinium doped anatase TiO2 nanoparticles. 2.2. Characterization details The structure of all the samples are determined using Rigaku Miniflex CD 10,041 XRD unit with copper target and k = 0.154 nm at a scanning rate of 1°/min and in the scanning range of 10–80°. High resolution transmission electron microscope images for morphology and particle size determination are observed with JEM2100, 200 kV JEOL. The oxidation state of the samples are studied with the help of X-ray photoelectron spectroscopy (XPS). The spectra is recorded on KRATOS-AXIS 165 instrument equipped with dual aluminium–magnesium anodes using the Mg Ka radiation (hm = 1253.6 eV) operated at 5 kV. Nitrogen adsorption–desorption isotherms are measured at 77 K in a Quantochrome Qautosorb autolyzer. Surface area is determined using multipoint Brunauer– Emmett–Teller (BET) method. The pore size distributions of the prepared samples are determined based on Barett–Joyner–Halenda (BJH) model. Diffuse Reflectance Spectra (DRS) of all the samples are taken with Shimadzu-2450 UV–Vis spectrometer. The photoluminescence (PL) measurements at room temperature are recorded with PERKIN ELMER LS 55 fluorescence spectroscopy.
%D ¼
A0 At 100 A0
ð1Þ
A0 = initial absorbance and At = absorbance after time t. 3. Results and discussions 3.1. Determination of Gd doping and morphological analysis of the samples. Fig. 1a shows the XRD pattern of Gd doped TiO2 samples. All the peaks in the samples are well indexed to the tetragonal anatase phase (JCPDS-782486) and no hint of Gadolinium containing oxide phases are resolved. The average crystallite size are calculated by using the Scherrer’s formula
d¼
0:9k b cos h
2.3. Computational details
where k = wavelength of the X-ray source used; b = full width at half maximum (FWHM); h = Bragg diffraction angle. The average crystallite size for pristine TiO2 (0.00 mol) is calculated to be 7.8 nm and 5.9 nm, 5.45 nm, 5.42 nm for 0.03, 0.05 and 0.07 mol of Gd respectively. The decrease in the size after doping is attributed to the repulsive interactions between dopant ions that prevents coalescence of the nanocrystallites and inhibits the growth by formation of Gd–O–Ti bond [19]. The width of the diffraction peak increases with the increase in doping concentration indicating systematic decrease in grain size and degradation of the structural quality after doping.
A computer model of 2 1 1 supercell is built with the unit cell of TiO2 in anatase form. The undoped supercell contains 8 atoms of Ti and 16 atoms of oxygen which gives Ti8O16. The Gd doped TiO2 takes the form as Ti7GdO16. The minimum energy state is computed by varying the internal position of atoms until the residual force is 0.01 eV/A°. The cut off kinetic energy for the structural optimization is set to 400 eV along with convergence criterion of 105 eV. A 3 3 3 K-mesh is used which correspond to spacing less than 0.3(A°)1 in reciprocal space to calculate Density of states (DOS) and band structure; Fermi level is considered to be at 0 eV. The effect of Gadolinium ions (Gd) on the TiO2 system is understood by substituting one center titanium (Ti) atom by Gd atom and allowing it to relax in all directions. Density functional calculations using generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) theory is performed to describe the electron–electron exchange and correlation effects. The density functional theory (DFT) equations are solved via projector augmented wave (PAW) method as implemented in Vienna Ab Initio Simulation Package (VASP) and interfaced with MedeA technology platform.
Fig. 1. X-ray diffraction pattern of (a) undoped and (b) Gd doped TiO2 nanoparticles.
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The XPS spectra have been used to determine the chemical composition of the prepared samples so as to determine the existing states of the different elements. The sample with 0.03 mol has been used for XPS study. Fig. 2(a) shows the Ti 2p core XPS result of undoped and Gd doped TiO2 nanoparticles. The core level binding energies of Ti 2p3/2 and Ti 2 p1/2 are roughly at 459.31 eV and 464.90 eV. The variation of 5.59 eV ensure the characteristic binding energies of Ti4+ in TiO2 matrix [20]. The core level XPS O 1s spectra are also analyzed. Fig. 2b display the XPS spectrum of O 1s. The spectra specifies presence of at least two kinds of chemical states that include crystal lattice oxygen (OL) and chemisorbed oxygen (OH). The OL peak appears around 530.08 eV and the OH peak at 531.32 eV [21]. The amount of OH in both pure and doped sample is much more than that of OL (Table 1). The presence of OH peak is developed with increasing oxygen vacancy [22]. During the photo-excitation process these oxygen vacancies act as trap centers. The electrons trapped in these centers migrate to the surface, react with oxygen molecule and convert it to superoxide radical (O 2 ). This superoxide radical act as potential candidate in the photocatalytic degradation, explicated later in this chapter. In Fig. 2c, the Gd 4d3/2 peak at 144 eV corresponds to +3 oxidation state of Gd ion [19]. The size, shape and overall morphology of the samples have been characterized using transmission electron microscopy. The TEM images of the samples are shown in Fig. 3a and b. As observed from the micrographs both the doped and the undoped system consist of nearly spherical size particles. The particle size of the undoped and doped system are of the order of 21 nm (Fig. 3a) and 12 nm (Fig. 3b) respectively. The high resolution TEM images of the samples are shown in Fig. 3c and d. The lattice fringes are clearly visible with inter planar spacing of 0.345 nm for undoped (Fig. 3c) and 0.357 nm for doped sample (Fig. 3d). The fringes agrees well with the (1 0 1) plane of anatase TiO2 [23]. With the incorporation of Gd ions the fringe spacing is slightly increased as compared to undoped TiO2. The ionic radii of Gd3+ (0.94 Å) being larger than Ti4+ (0.68 Å), it is possible for the Gd ions to substitute and occupy Ti4+ sites. That is to say, substitutional doping occurs, and one Gd3+ ions replaces one Ti4+ leading to the formation of Gd with one negative charge and oxygen vacancy with one or two positive charges (Vo ; V o ). =
Gd2 O3 () 2GdTi þ 3Oo þ Vo =
2½GdTi ¼ ½Vo =
Gd2 O3 () 2GdTi þ 3Oo þ 2V o =
½GdTi ¼ ½Vo
Table 1 Quantitative analysis of OH and OL from XPS spectra. Samples
Detailed study
OL
OH
Position
530.2 eV
531.43 eV
At% Position
47.8 530.08 eV
52.8 532.62
At%
44.3
55.7
TiO2
TiO2:Gd
3.2. Surface area measurements Fig. 4 shows the nitrogen adsorption–desorption isotherm of pure and 0.03 mol Gd doped TiO2nanoparticles. The isotherms are of type IV with H1-type hysteresis loop in the relative pressure range of 0.4–0.8. The type indicates presence of mesopores and are the characteristics of materials containing agglomeration [24]. The surface area for 0.03 mol Gd doped TiO2 nanoparticles is found to be 225 m2/g which is about 2.2 times the surface area of undoped TiO2 (95 m2/g). From the pore size distribution the pore diameter is determined to be 6 nm and 4 nm for undoped and doped sample respectively. The decrease in pore size after doping might be due to the larger size Gadolinium ions that causes higher pore filling effect [25]. Table 2 shows the surface area and pore size of the doped and undoped samples calculated by BET and Barret–Joyner–Halenda methods (BJH). Thus introduction of dopant ion causes a considerable increase in surface area and a decrease in pore size, thereby favoring the criteria for enhanced photocatalytic activity. 3.3. Optical property and electronic structure study via DFT To investigate the influence of Gd concentration on the absorption of TiO2 nanoparticles, the absorption spectra of all the samples measured at room temperature is shown in Fig. 5. The absorption band edge of TiO2 nanoparticle appears at 340 nm. Gd doping shifts the absorption edge towards the visible region. This red shift is most likely due to the transition of electrons from the valence band to the 4f levels of Gd and the observation is in accordance with the absorbance spectra of other rare earth doped TiO2 nanoparticles [26]. The photoluminescence spectra owing to the recombination of charge carriers is a useful tool to explore the effectiveness of charge carrier trapping, migration, alteration and separation of charge carriers [27]. Fig. 6a shows the PL spectra of all the samples.
Fig. 2. Core level X-ray photoelectron spectrum (XPS) of (a) Ti 2p, (b) O 1s and (c) Gd d3/2 for 0.07 mol Gd doped TiO2.
S. Paul et al. / Journal of Colloid and Interface Science 439 (2015) 54–61
57
Fig. 3. TEM image of: (a) undoped and (b) Gd3+ doped TiO2 nanoparticles. High resolution images of (c) undoped and (d) Gd3+ doped TiO2 nanoparticles.
Fig. 4. Nitrogen- adsorption and desorption studies of (a) undoped and (b) 0.03 mol Gd doped TiO2 nanoparticles.
at an excitation wavelength of 320 nm. Undoped TiO2 shows near band edge emission peak at 398 nm. The intensity of this band edge emission peak is suppressed with the incorporation of Gadolinium ions. This reduction in the intensity is attributed to the band gap narrowing introduced by the formation of f states of the Gadolinium ions. Fig. 6b shows variation of band gap with the increase in dopant concentration. Doping of Gd3+ ions causes loss of oxygen atoms and results in the generation of free electrons. These electrons are trapped in the oxygen vacancy giving F center or color Table 2 Parameters as obtained from the BET surface area analysis. Samples
Surface area (m2/g)
Pore size (nm)
X = 0.00 X = 0.03
95 225
6 4
centers. Depending on the number of trapped electrons, these centers are referred as F (two electrons), F+ (one electron) and F2+ (devoid of electrons). The emission peaks at 490 nm and 535 nm are attributed respectively to the transition from oxygen vacancies with two trapped electrons and one trapped electron to the valence band of TiO2 [28]. Another peak at 429 nm is attributed to self trapped excitons (STE) originated by interactions of conduction band electrons localized on the Ti 3d orbital with holes present in the O 2p orbital of TiO2 [29]. In the photoluminescence process the electrons in the conduction band gets trapped into the oxygen vacancies via a non-radiative process and than recombines with the photogenerated holes in the valence band accompanied by fluorescence emission. It is to be noted that in the PL spectra no Gd3+ ions related emission peaks are observed. Only changes in the emission intensity are noticed after doping. The spectra presents enhanced emission intensity on incorporation of 0.03 mol
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Fig. 5. UV–Vis spectra of all the samples. Fig. 7. TDOS (Total Density of State) of undoped TiO2.
and 0.05 mol Gd and sudden quenching of the emission intensity on incorporation of 0.07 mol Gd. At lower dopant concentration the interactions between ions are too weak to have an effect on the energy levels of each dopant ion. But with the increase in dopant concentration the distance between the ions decreases and density of defects increases. This increases the interactions between the ions and the emission energy gets transferred from one ion to another ions or to another defects. The electrons being localized in the oxygen vacancies and attached to the nearby Gd3+ ions, are not easily available to undergo radiative recombination with holes. The energy is dissipated non-radiatively resulting in the reduction of emission intensity. We have calculated the average shortest distance (known as the critical distance) between ions at which the emission energy transfer occurs, using the equation [30].
RC ¼ 2
3V 4pxc N
1=3
V = volume of the unit cell = a2c = 0.133 nm3, where a = 0.3784 nm; c = 0.9420 nm. xc = critical concentration of the dopant (=0.07), N = number of Zions in the unit cell, Rc = the critical transfer distance. Putting the values the critical distance (Rc) for 0.07 mol is calculated to be 0.9682 nm. Therefore, Gadolinium doping is responsible for the red shift of the absorption band edge and band gap narrowing owing to the introduction of f states. These f states can largely delay the electron–hole recombination
rate and contribute to the enhancement of the photocatalytic activity. We have further carried out theoretical study based on density functional theory (DFT) to understand the electronic structure of Gd doped TiO2. Theoretical study is carried out considering two supercell Ti8O16 and Ti7GdO16 along with spin polarized calculation. Density of states (DOS) is calculated for both the systems as shown in Figs. 7 and 8 respectively. Although standard DFT calculation may not determine the actual band gap, it could frame out the changes in optical properties of Gd doped TiO2 system with respect to pristine TiO2 system. Fermi level in DOS is considered at 0 eV. If we compare the TDOS (Total Density of States) of Gd doped TiO2 and TiO2, we observe presence of states near the Fermi level in Gd doped TiO2 while in pure TiO2 there are no such levels near the Fermi level. Our interest is to identify the atoms responsible for the states near the Fermi level in Gd doped TiO2 and hence plotted PDOS (Partial Density of States) for Gd, Ti and O as shown in Fig. 8. PDOS plots shows that Gd 4f state is the most prominent near the Fermi level whereas Ti and O have lesser impact. We have also observed that other states of Gd i.e., s, p and d has almost no contribution in comparison to the f state. This very much indicates that f state of Gd is the key factor to tune the optical property of TiO2. On the other hand d and p states are the most effective states for Ti and O near the Fermi level respectively. The formation of these states near the Fermi level is due to the instability in structure as the system (TiO2) is suffering from foreign entity (Gd).
Fig. 6. (a) Photoluminescence spectra of the samples with an excitation wavelength of 320 nm. (b) Band gap analysis of the samples as calculated from the Kubelka Munk plot.
S. Paul et al. / Journal of Colloid and Interface Science 439 (2015) 54–61
59
Fig. 8. TDOS and PDOS (Partial Density of State) for Gd doped TiO2 nanoparticles.
The irregular bonding and unstable charge transfer among Gd, Ti and O leads to the splitting of states near the Fermi level. The instability in charge transfer occurs after substitution of Ti4+ ion by Gd3+ ion. A weak hybridization is observed among Gd 4f, O 2p and Ti 3d states below the Fermi level. Chen et al. [31] found that the Gd doping in TiO2 does not produce any states near the Fermi level. But in our case we have observed a prominent Gd 4f state at and above Fermi level. These states might act as trapping state. Illumination of a solid with UV/Vis light excites electrons to the conduction band leaving holes behind in the valence band. During the period of this photo excitation process few electrons move to the conduction band while few others are trapped in the levels created due to Gd 4f state. These trapped electrons easily migrate to the surface and react with the atmospheric O2 molecules to convert it to superoxide (O 2 ) radicals [32]. These superoxide radicals act as a potential candidate in the photocatalysis process. 3.4. Photocatalytic activity study Using aqueous phenol as the targeting medium the visible light photocatalytic activity of undoped and Gd3+ ion doped TiO2 nanoparticles were studied. Prior to the photocatalyst addition the phenol solution was irradiated for 20, 40, 60 and 80 min. No appreciable self degradation is noticed as evident from the degradation curve, Fig. 9. Pure TiO2 exhibits very low visible light photoactivity due to its large band gap. The photocatalytic activity has been found to be largely improved on incorporating Gadolinium ions. Under visible light irradiation electrons are excited from the valence band to the conduction band. Gadolinium dopants in TiO2 effectively scavenge electrons and prevent their recombina-
Fig. 9. Phenol degradation curve of the samples under visible light illumination.
tion with holes due to the existence of their half filled f orbitals. Presence of these f orbitals have been supported with the help of DFT based calculations as mentioned above. The Gd3+ ions with half filled f orbitals are very stable. When these Gd ions trap electrons the electronic configuration is largely destroyed and their
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Fig. 10. Schematic illustration of the photocatalytic process by Gd doped TiO2 nanoparticles.
stability decreases, the trapped electrons are easily transferred to the oxygen molecules adsorbed on the surface of the catalysts, and the Gd3+ ions return to the original stable half-filled electronic structure. This in turn promotes charge transfer and proficiently disperse the electrons and holes by superficially trapping the electrons [33]. Fig. 10 shows the schematic illustration of the photocatalytic activity of Gd doped TiO2 nanoparticles. The mechanism of trapping and detrapping can be represented with the following mechanisms: 3þ
þ e ! Gd
2þ
þ O2adbs ! Gd
3þ
þ h ! Gd
Gd Gd Gd
4þ
Gd
2þ 3þ
þ
þ O2
4þ
þ OH ! Gd
3þ
þ OH
The Gd4+ can also trap conduction band electron or Gd2+ can trap valence band hole to retain half filled electronic structure of Gd3+. 4þ
3þ
Gd þ e ! Gd 2þ þ 3þ Gd þ h ! Gd The enhanced photocatalytic activity is owing to the presence of half filled orbitals of the Gadolinium ions that serve as shallow trap for the charge carriers to accelerate interfacial charge transfer process [34]. The processes not only accelerate interfacial charge transfer but also enhance the generation of highly reactive oxidative species like superoxide and hydroxyl radicals. 3þ hm
TiO2 =Gd
! e þ h
þ
þ hm
OH þ h ! OH O2 þ e ! O2 Phenol þ O2 ! Products Phenol þ OH ! Products Due to difference in charge state substitution of Gd3+ ions leads to creation of oxygen vacancies. These oxygen vacancies promote the adsorption of O2 molecules and favors the adsorbed oxygen to arrest the photoelectrons simultaneously producing free radicals [35]. The presence of these oxygen vacancies have been testified from the XPS and the photoluminescence spectra. In the
photoluminescence process oxygen vacancies and defects bind the photoinduced electrons resulting in free or binding excitons for the photoluminescence process to easily occur, the larger the amount of oxygen vacancies the stronger the PL intensity. But in the photocatalytic process these oxygen vacancies and defects become centers to capture the photo induced electrons so that the recombination of photoinduced electrons and holes is effectively inhibited. In our work the photoluminescence intensity has been found to be maximum at 0.03 mol dopant concentration and at this concentration we are getting higher photocatalytic activity. The results thus, demonstrates certain kind of correlation between the photoluminescence process and photocatalytic activity, the stronger the PL intensity the larger the content of oxygen vacancies and defects, the higher the photocatalytic activity. Further the surface area of the nanoparticle has a profound role to enhance the photocatalytic activity. After 60 min, the degradation curve shows a decrease in the degradation trend. Although longer irradiation period produces higher number of charge carriers but only limited number of them reaches the semiconductor–liquid interface region for dye degradation, the rest undergo volume recombination dominating the reaction in accordance with the equation [36].
K R / exp
2R ao
Here KR is the rate of recombination, R is the distance separating the electron and the hole pair, ao is the hydrogenic radius of the wave function of the charge carriers. The recombination rate increases due to the decrease in the number of the trap sites. Thus it is seen that after 60 min of irradiation and production of charge carriers on the surface of Gd doped TiO2 nanoparticles, the rate of recombination is dominated to the rate of other reactions in the solution [36]. Dopant concentration also has some important influence in regulating the photocatalytic reaction rate. In the degradation curve it is observed that dopant concentration of 0.07 mol is showing the lowest photocatalytic activity. On this basis it is surmised that there exists an optimum dopant concentration in the TiO2 matrix to protract the separation of photogenerated charge carriers. The decrease in the activity can be correlated to the thickness of the space charge layer that is inversely proportional to the dopant con-
S. Paul et al. / Journal of Colloid and Interface Science 439 (2015) 54–61
centration. The thickness of the space charge layer is affected by dopant concentration according to the equation [36].
W¼
1=2 2eeo V eNd
W is the thickness of the space charge layer. e and eo are the dielectric constants of the semiconductor and vacuum. With the increase in dopant concentration the thickness of the space charge layer becomes less than the penetration depth of light and this greatly increases the electron hole recombination rate. Moreover, at higher concentration charge carriers undergo multiple trapping. This behavior decreases the mobility rate as a result of which they are absorbed before they can reach the surface. The increased recombination rate with increase in dopant concentration is attributed to the concomitant decrease in the average distance between the trap sites [37]. 4. Conclusion In this work Gd doped TiO2 nanoparticles have been prepared successfully by a simple sol–gel method. XPS spectra reveals presence of trivalent Gadolinium ions and presence of oxygen vacancies. Both XRD and TEM analysis indicates decrease in size after doping. Owing to the presence of Gadolinium ions, there is a substantial increase in the surface area. The PL spectra depicts increased peak intensity at 0.03 mol Gd ions and at this concentration the sample is exhibiting maximum photocatalytic activity. Using density functional theory the f states generated due to the presence of Gadolinium ions in the host matrix are identified and these states are believed to contribute to the enhanced photocatalytic activity. Acknowledgments We are thankful to, Sophisticated Analytical Instrument Facility (SAIF), NEHU, Shillong for the HRTEM images and IICT Hyderabad for XPS data. We are also thankful to UGC for financial support to the Project F.No.42-785/2013 (SR). References [1] A.L. Linsebigler, G. Lu, J.T. Yates Jr., Chem. Rev. 95 (1995) 735–758. [2] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemannt, Chem. Rev. 95 (1995) 69–96.
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