Keywords: Titanium dioxide; metal doping, metal ions, band gap, photodegradation, hydrothermal method, sol-gel, photocatalysis.
Abstract: Titanium dioxide (Titania; TiO2) is one of the most widely used metal oxide semiconductor in the field of photocatalysis for removal of pollutants. It has been noted that titanium dioxide is a research friendly material as its physico-chemical and catalytic properties can be easily altered as per specific application. Since many years, researchers have tried to modify the properties of titanium dioxide by means of doping with metals and non-metals to improve its performance for photocatalytic degradation (PCD) applications. The doping of various metal ions like Ag, Ni, Co, Au, Cu, V, Ru, Fe, La, Pt, Cr, Ce, etc. in titanium dioxide have been found to be influencing the band gap, surface area, particle size, thermal property, etc. and therefore the photocatalytic activity in PCD. Moreover, photocatalytic activity of doped titanium dioxide has been observed in visible light range (i.e., at wavelength >400 nm). In this review, different synthesis route for doping of metal ions in titanium dioxide have been emphasised. The effect of metal dopant on the structural, textural and photocatalytic properties of titanium dioxide has been reviewed.
Introduction
Titanium dioxide (TiO2) is widely used as photocatalyst because of its low cost, high photocatalytic activity, chemical stability and non-toxicity [1, 2]. When titanium dioxide particles are irradiated with near-ultraviolet light of energy equal to or greater than the band gap energy (λ <380 nm), electrons from valence band migrate to the conduction band creating positively charged holes (h+vb) in the valence band and negatively charged electrons (e-cb) in conduction band (Eq. 1). Holes and electrons migrate to the surface of the photocatalyst where they can initiate oxidation and reduction reactions with adsorbed species. In aqueous titanium dioxide suspensions, the adsorbed organic molecules (Dads), hydroxyl ions (HOads
-) and water molecules (H2Oads) are oxidisable species. Hydroxyl ions and water molecules are oxidized to hydroxyl radicals (HO●) with the help of h+vb (Eq. 2 to 4). Valence band electrons (e-cb) can reduce dissolved oxygen molecules to superoxide radicals (O2
●-) (Eq. 5). The HO● and O2●- radicals are also involved in photo oxidation of the
organic molecules. The e-cb can also reduce organic species as the potential of the conduction band electron is sufficient to reduce many organic molecules [3]. The recombination of holes and electrons can reduce their availability for participation in redox reactions (Eq. 6).
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The efficiency of photocatalytic reactions is governed by how effectively electrons and holes are channelled into oxidation and reduction reactions before recombination takes place [4]. Typically, the activity of titanium dioxide in photocatalytic reactions is quite low due to the recombination of holes and electrons [5]. Material scientists are continuously looking for more active materials that can perform better in various conditions. Increasing the efficiency of titanium dioxide based photocatalytic processes is important for industrial scale application of this technique [6]. To improve the efficiency of photocatalytic reactions, the lifetimes of electrons and holes in TiO2 particles must be increased before recombination takes place [7]. One approach that has been applied to improve the photocatalytic activity of titanium dioxide in organic oxidation reactions is the doping with metals. Plenty of metal ions have been inspected as potential dopants and they have been studied for photocatalytic degradation reactions. Different metals like silver (Ag), copper (Cu), cerium (Ce), vanadium (V), iron (Fe), chromium (Cr), aluminium (Al), nickel (Ni), ruthenium (Ru), lanthanum (La), rhodium (Rh), gold (Au), platinum (Pt), Lithium (Li), Magnesium (Mg), Palladium (Pd), and Strontium (Sr) etc. have been used for doping in titanium dioxide [8-16]. Doping is the process in which guest atom is incorporated into structure of host material. By doping, one can have advantage of vacant sites and defects present into atomic structure of host material. Modifying a material never means the change in base properties entirely but to improve material’s properties for various applications. The doping of metal in the titanium dioxide is said to accelerate both the removal of electrons (e-cb) from the particles and their transfer to molecular oxygen [17]. According to Gerischer and Heller [18], the reduction of oxygen to superoxide radicals is proposed to be the rate-limiting step in the photocatalytic oxidation of organics. In undoped titanium dioxide, surface electrons undergo electron–hole recombination with fast rate [19]. In the presence of metal ions, the efficiency of the photocatalytic reactions may be increased by improving the separation of charge carriers and the rate of oxygen reduction. The doping of metals in titanium dioxide serves the purpose of mediating the electrons away from the TiO2 surface, hence preventing them from recombining with valence band positive holes. The methods of metal doping in titanium dioxide, nature, size, concentration of dopant, etc. are the major factors influencing the characteristics (particle size, surface area, thermal property, band gap, etc.) and the photocatalytic properties of doped titanium dioxide. Therefore, the present review was focused on the various methods of metal doping in titanium dioxide and the effect of the metal dopants (nature and concentration) on the structural, textural and photocatalytic properties of doped titanium dioxide.
Synthesis of metal doped titanium dioxide
The metal doping in titanium dioxide has been carried out by physical as well as chemical routes; however, chemical route is the more appropriate way for doping. Mechanical Alloying is one of physical routes for doping of metal in TiO2. Nanocrystalline Fe, Ni and Cu doped TiO2 powders were synthesized by mechanical alloying of anatase and rutile TiO2 with metal powders [20-24]. TiO2 anatase as well as rutile phase were prepared by homogeneous spontaneous precipitation process (HSPPLT) and TiO2 powder was mechanically alloyed for 14 hours by planetary ball mill with nickel powder in various concentrations at ball milling speed of 150 rpm. Because of cold working type of process on material, mechanical strain may be produced that can lower the performance. Moreover, it is well known that choice of balls for milling is most important thing in this technique. Balls must not react with host material while milling process to avoid contamination problems.
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The widely used chemical synthesis techniques for metal doping in titanium dioxide are sol-gel and modified sol-gel methods, wet impregnation, hydrothermal, chemical vapour deposition (CVD), ion-assisted sputtering, metal ion implantation and water in oil (W/O) microemulsion, etc. All these methods have their own identity for different applications. The sol-gel technique has been excessively used by different researchers for the doping of metal ions in titanium dioxide by adding metal dopant during the hydrolysis of titanium alkoxide. The sol-gel route brought a new approach to synthesize nano materials having better physical and chemical properties with homogeneity in appropriate way. This is an emerging synthetic method to synthesize nano metal oxides with high porosity and specific surface area.
The sol-gel process involves the evolution of inorganic network in a continuous liquid phase
(solvent). Sol-gel process refers to the hydrolysis and condensation of alkoxide based precursor (metal alkoxides) [25]. The alkoxide precursor is hydrolyzed with water or dilute acid to form a dispersible solid, called sol. The sol particles undergo condensation to form the gel. The calcinations of the dried gel produce the metal oxide [26]. During the synthesis, at particular level the pH of system must be maintained in order to get uniformity in the properties.
A series of vanadium-doped TiO2 catalysts were synthesized by two modified sol–gel
methods [27]. In the first method, vanadyl acetylacetonate dissolved in n-butanol was added in a solution of acetic acid - titanium butoxide. The hydrolysis of titanium butoxide took place by the water generated during the esterification of acetic and butanol in 24 h. The solution was dried (at 150°C) and pulverized to powder, which was calcined (at 400 °C for 30 min.) to get V4+ doped TiO2 catalysts. The second method was carried out in a cooled acidic aqueous solution by mixing ethanolic solution of vanadium chloride into titanium butoxide-ethanol solution (the volume of ethanol was 10 times of titanium butoxide). The mixed solution was added drop wise in a 0.1 M HCl solution (volume was 10 times of ethanol/titanium butoxide solution) and quickly hydrolyzed due to large amount of water. During this process, the solution temperature was kept at near 0°C. The solution was dried (at 110 °C) and then calcined to V4+ doped TiO2.
The chromium was also doped in TiO2 by sol-gel method in cooled acidic aqueous solution
[28]. Chromium chloride dissolved in ethanol was added in then titanium butoxide (volume of ethanol was 10 times of titanium butoxide). This mixed solution was added dropwise in 0.1M HCl solution (10 times of ethanol/titanium butoxide solution volume). During this process the solution was kept cooled at near 0 °C. The hydrolysis was completed after 8 h, and then the sol was dried at 110 °C and pulverized to powder. The dried gel was then calcined in air to obtain chromium doped titanium dioxide. The X-ray absorption near edge spectroscopy (XANES) indicated the possibility of either Cr3+ or Cr4+ in TiO2.
Lee et al. [29] reported the synthesis of Ag doped titanium dioxide via sol-gel route using
titanium tetra iso-propoxide precursor, silver nitrate as source of dopant and sodium citrate tribasic dihydrate as reducing agent. A visible light active platinum ion doped TiO2 photocatalyst was synthesized by a sol-gel method and its photocatalytic activities were demonstrated for the oxidative and reductive degradation of chlorinated organic compounds in visible light [30]. The Nb doped titanium dioxide has also been synthesised by sol-gel route from alkoxide precursors [31].
Zhang et al. [32] synthesized europium doped titanium dioxide by sol–gel method using europium oxide as precursor of europium, which was dissolved in nitric acid and then evaporated to dryness to get europium nitrate. The dried europium nitrate was dissolved in butanediol and titanium tetra butoxide solution was added into the solution at room temperature under stirring. This solution was exposed to air at room temperature for one week resulting into a dry solid gel, which was further heated at 120 °C and then calcined at different temperatures above 400 °C to get europium doped TiO2. Similarly, for lanthanum doping by sol-gel method, La2O3 was dissolved in
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HNO3, which was then added to the mixture of water and ethanol solution. This prepared system was then mixed with a solution of titanium tetra butoxide and ethanol for hydrolysis under constant stirring [33].
A series of lanthanide-incorporated mesoporous TiO2 catalysts were synthesized by a
modified sol–gel method using dodecylamine template in a solution of titanium tetra butoxide and ethanol at ambient temperature [34]. This solution was slowly added into a solution containing a desired amount of lanthanide precursor (Ln(NO3)3.6H2O), ethanol and acetic acid under vigorous stirring. The composition molar ratio of titanium tetra butoxide, surfactant, acid and alcohol was 1:0.05:5:30. The resulting suspension was then stirred for 2 h and aged at room temperature for 48 h. The gel was dried and then calcined at 500 °C to Ln doped TiO2.
Using inorganic precursor, the cobalt doped titanium dioxide was prepared by hydrolysis of
titanyl sulphate- cobalt acetate tetrahydrate in ethanol-water solution with ethanolic NaOH solution followed by stirring for 1 h at 78 °C. The reaction mixture was cooled to room temperature giving precipitate, which was separated by centrifugation, washed, dried and calcined to get cobalt doped titanium dioxide [35].
The photo reduction method has also been employed for the synthesis of metal (in reduced
form) doped titanium dioxide [36-38]. With an attempt to excite TiO2 photocatalyst in the visible light range and eliminate the rapid recombination of excited electrons and holes, a new type of photocatalysts Au3+-TiO2 and Au-TiO2 catalysts with different gold content (0.25 to 5 mol %) were synthesized by a sol-gel and photoreduction method [37]. The titanium tetra butoxide dissolved in absolute ethanol was added drop wise under vigorous stirring to the mixture solution of ethanol - 0.1 M tetrachloroauric acid - acetic acid. The obtained transparent colloidal suspension was stirred for 2 h and aged for 2 days to form the gel, was dried and then calcined (650 °C) to get Au3+-TiO2. They prepared gold-doped TiO2 samples (Au-TiO2) by reducing Au3+-TiO2 in flow of H2 as well as by photoreduction method. In photoreduction method, the weighted amount of TiO2 was suspended in a mixture solution containing the required concentration of tetrachloroauric acid and 0.1 M methanol solution as a hole scavenger. The suspension was irradiated with a 125 W mercury lamp for 60 min. giving Au-TiO2, which was filtered, washed and dried. Similarly, the silver nanoparticles doped TiO2 (with 0.5, 1.0, 1.5 and 2.0 atom % of Ag) were prepared by photoreducing Ag+ ions to Ag metal on the TiO2 surface [38]. In this method, the aqueous solution of silver nitrate was added in TiO2 suspension at a pH of 3 (maintained using perchloric acid). The mixture was then irradiated with UV light from eight mercury lamps (8 W) for 3 h with continuous air supply. The suspensions were then filtered, washed and dried to obtain Ag doped TiO2 catalysts. The wet impregnation of TiO2 has been used for surface deposition/ doping of metal ions in TiO2. In wet impregnation method, the titanium dioxide, synthesized by either sol-gel or precipitation method, are impregnated with aqueous/ non-aqueous solution of metal precursors like nitrate, chloride, acetate, complexes, etc. In many cases, different metal chlorides and nitrates were taken as precursor for dopant, mixed with specific solvent [27, 28, 39].
For ruthenium doping, RuCl3 was used as precursor of ruthenium dissolved in alcoholic
solution maintaining pH of solution at 1.5 with nitric acid [8,40]. The Ru-doped titanium dioxide samples with different Ru loading were synthesized by adding TiO2 powder in to aqueous solutions of RuCl3 to obtain RuCl3-loaded TiO2 particles. A series of various metal (Co, Cr, Cu, Fe, Mo, V and W) ion doped titanium dioxide catalysts were prepared by wet impregnation method [41]. The aqueous solution of TiCl3 was hydrolyzed with ammonia titanium hydroxide. The obtained titanium hydroxide was calcined at 500°C to get TiO2. The TiO2 was impregnated with aqueous solution of
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respective transition metal ions (Co(NO3)2•6H2O, Cr(NO3)3•9H2O, Cu(NO3)2•3H2O, Fe(NO3)3•9H2O, (NH4)6Mo7O24•4H2O, NH4VO3 and (NH4)6W12O39•xH2O) to prepare metal ion doped titanium dioxide.
Sol- Gel method has also been modified in various ways; even it can be combined with other techniques in order to get appropriate properties. To dope Fe3+ in titanium dioxide, Zhu et al. [42] used sol-gel technique combined with hydrothermal method. They used mixture of water and nitric acid to hydrolyse titanium tetra iso-propoxide. The obtained gel was mixed with aqueous solution of Fe(NO3)3.9H2O and the mixture was autoclaved at ~200 °C. A modified sol-gel method with post-hydrothermal treatment was also employed to synthesize metal (Ni, Ga, Fe, Cd and Ag) doped TiO2 with varied dopant concentrations using titanium butoxide and metal nitrate hydrate as the metal precursors [43].
Hydrothermal synthesis of metal oxides is well known method in material science fraternity
as this synthesis route is widely used to synthesize high surface area mesoporous metal oxides with controlled shapes and size. As name suggests, it concerns with synthesis under pressure and temperature. By this technique people have also synthesized nanorods, nanotubes and nano ribbons of TiO2 [44]. For metal doping in titanium dioxide, a mixture of titanium and metal precursors, solvent (water) and hydrolyzing agent are heated under pressurized (using an inert gas) condition in an autoclave. Liu et al [45] has reported hydrothermal synthesis for iron doped titanium dioxide by taking a mixture of TiOSO4, CO(NH2)2, CN3H5.HCl and Fe(NO3)3 in certain ratio into a Teflon-lined autoclave. The whole system was pressurized and heated at temperature of 100-200 °C under constant stirring for 3-12 hours. The synthesised material was then washed properly with distilled water, dried and calcined.
The W/O microemulsion contains inverted micellar aggregates consisting of water droplets
surrounded by a continuous nonpolar system. The W/O microemulsion have successfully used for the synthesis of various inorganic nanoparticles. The particle sizes growth can be controlled by the nanodroplet size of the inner phase of the microemulsion. The Cu-TiO2 and Ag/Cu-TiO2 nanoparticles have been prepared using a W/O microemulsion system having water/AOT/cyclohexene [46]. The synthesized Cu-TiO2 and Ag/Cu-TiO2 nanoparticles were found to be highly active in photocatalysis (phenol decomposition under visible light irradiation) as well as in antimicrobial activities (for bacteria Escherichia coli and Staphylococcus aureus, yeast Saccharomyces cerevisiae and pathogenic fungi belonging to Candida family).
MOCVD is versatile technique to synthesize inorganic materials, which is actually extension
of chemical vapour deposition (CVD) technique. MOCVD technique has been adopted for the synthesis of inorganic nano materials in powder as well as in film form. Zhang and Lie [47] studied visible light response of iron doped TiO2 prepared by MOCVD. The fixed amount of activated carbon as a support material in silica reactor, inert gas was passed to avoid the condensation of precursor. The temperature of the reactor was adjusted to 500°C – 700°C. Deposition of TiO2 was started by switching the tetrabutyltitnate with carrier gas. Ferrosine was taken as an iron precursor which was introduced into reactor with source temperature of 120 °C, after certain time nitrogen gas was purged. Sarah Klosek and Daniel Raftery also synthesized V-TiO2 by modified CVD method [48].
The reactive magnetron sputtering method was employed to synthesize Fe-doped titanium
dioxide thin films deposited onto microscope glass slides and polycarbonate plates at different total pressure and Fe-doping concentrations [49]. On glass substrates a polycrystalline anatase TiO2 was obtained and at the highly Fe-doped samples, an iron phase also appeared. Whereas, an amorphous TiO2 structure was present on the polycarbonate substrate for all concentration of Fe. Both films prepared at the total pressure of 0.5 Pa and a low iron concentration gave better photocatalytic activity than the pure TiO2 films prepared under the same deposition conditions.
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The V4+ doped TiO2 films were synthesized using sol-gel technique by hydrolysis of titanium tetra iso-propoxide or tetra butoxide precursor, dissolved in a suitable solvent, with a acidic solution containing vanadium (IV) oxyacetylacetonate as source of dopant [27, 39, 50]. A starting material of vanadium (IV) compound was used for doping of V4+ species into TiO2 to prevent the formation of V5+, which detriments the photoreactivity [8, 40]. The vanadium (IV) oxyacetylacetonate, water, nitric acid, and tetrahydrofuran (THF)) mixed solution was drop wise added into titanium isopropoxide – THF solution at room temperature for hydrolysis. The obtained sol was used for making thin films by spin-coating on pyrex glass substrates. The coated substrates were pre-heated (100°C for 1 h) followed by calcination (500 °C for 1 h). The four times repetitive coating, preheating and calcination processes gave thin films with a thickness of ~800 nm.
Effect of metal doping in TiO2 on physico-chemical properties
The characterization of metal doped titanium dioxide involves mainly the examination of textural, structural and catalytic properties. The textural properties include the surface area, pore volume, pore size and their distribution, pore shape, etc. of the material. Structural properties are the crystal phase, crystal structure, crystallinity and crystallite size. Besides these, the elemental analysis to get metal content, the thermal analysis for thermal stability and phase transformation temperature are also important for complete evaluation of the metal doped titanium dioxide. The femtosecond pump-probe diffuse reflectance spectroscopy technique has been used to estimate the relative rate constants of electron–hole recombination process. UV-Vis spectroscopic study reveals the working wavelength range for the catalyst.
Material’s crystal structure related information like crystalline phase and composition, crystallinity, crystallite size, etc. is obtained by X-ray diffraction (XRD) analysis. XRD study has been highly useful for examining the doping of metal ions in TiO2 structure. The as synthesized doped material (titanium hydroxide), before calcination is amorphous in nature, which on calcination results to crystalline TiO2 primarily as anatase phase. However, for crystallization, calcination temperature ranges from 200-700 °C. Rutile phase starts to appear on high temperature calcination. The amorphous to anatase and anatase to rutile phase transformation temperatures are observed to be significantly influenced by the presence of metal ion doped in TiO2. Furthermore, the nature (electronic status and size) of metal ion also affects these transformation temperatures. Choi et al. [52] reported that pure TiO2 calcined at 400 °C possessed only anatase phase and calcination at 700 °C resulted to little amount of rutile phase formation. They noticed that metal ion doping to host structure of TiO2 influenced the temperature of anatase to rutile phase transformation. They concluded that certain dopants (Pt, Cr, V, Fe, Y, and Rb) lowered the anatase to rutile phase transformation temperature of TiO2. The Ru doping increased the temperature for anatase to rutile phase transformation. Similarly, La doped TiO2 calcined at 400°C also contained only anatase phase and after calcinations at 700 °C very small fraction of rutile phase was observed showing the stability of anatase phase [33, 52]. The europium dopant (1 at. %) significantly increase the anatase-to-rutile phase transition temperature as compared to pure titanium dioxide [32]. The rutile phase was observed until the temperature of 1000 °C, showing a strong inhibition effect of europium for the phase transformation. This remarkable shifting in the phase transformation temperature towards high temperatures was stated to be attributed to the substitution of titanium (IV) ions by europium ions within the structural framework. Further increase in the europium content causes a slight decrease in the anatase-to-rutile phase transition temperature due to increased number of defects inside the anatase phase. The inhibition of anatase to rutile phase transformation in Ce, La and Y ions doped titanium dioxide was explained to be attributed to Ti-O-metal bond formation at the interface because of relatively high ionic radii, they could be located primarily on the surface of TiO2 [33, 51, 53, 54] Doping level also affects the anatase to rutile phase transformation [35]. Choi et al. [8] reported that in case of Pt doped titanium dioxide, the
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rutile fraction increased on increasing the concentration of Pt from 0.1 to 0.3 % and then decreased with increase in Pt concentration from 0.5 to 1.0 % [52]. They stated that these variations indicate the effect of metal ion doping in TiO2 on anatase to rutile phase transformation is dependent of not only the intrinsic physicochemical properties of doping metal ion but also the concentration of the dopant.
The transformation behaviors of Sn2+-doped titanium dioxide, such as transformation temperature, rate and the crystal size were studied by using X-ray diffraction and thermo-gravimetric/ differential thermal analysis [55]. It was observed that the anatase, rutile and SnO2 phases can be formed by varying Sn2+ content and calcinations temperature. Sn2+-doping greatly reduced the growth of anatase and rutile crystals giving nano crystals. With increase of Sn2+ content, the transformation temperatures of amorphous to anatase and anatase to rutile first decrease and then increase. The transformation rates are dependent on Sn2+ content, change in its valence and the calcinations temperature.
The doping effect on anatase to rutile phase transformation has been explained on the basis
of the oxygen vacancies created during the anatase to rutile phase transformation, which causes a contraction or shrinking of the oxygen structure. The doping can also affect the rate of the phase transformation by modifying the defect structures of TiO2. The increase in oxygen vacancies accelerates the anatase to rutile phase transformation [56]. An increase of oxygen vacancy concentration reduces the strain energy, which is overcome before the rearrangement of Ti-O octahedral occurs. The ionic status (valency) of metal ion also influences the anatase to rutile phase transformation. The monovalent ions are more effective in accelerating the anatase to rutile phase transformation than divalent or trivalent ions as the doping of monovalent ions would create more oxygen vacancies as compared to divalent or trivalent ions [57].
Generally no characteristic peaks attributed to doping of metal ion species (metal oxide) in
titanium dioxide are observed at the low dopant concentration [43]. It is reported that the doped TiO2 with high metal dopant concentration can show a small peak attributable to a separate oxide phase in addition to the lines of anatase TiO2 [58]. The doped metal ions having the ionic radii similar to that of Ti4+ (0.75 Å) for a coordination number of 6, can likely to be incorporated into the TiO2 structure substituting the Ti4+ in lattice [59]. Choi et al. [52] stated that doping levels or thermal treatment did not induce the formation of discrete impurity phases and metal ion integrates into the basic structure of TiO2. An additional peak was observed for chromium doping at higher concentration of dopant, which is attributed to chromium oxide [60]. The dopants like Cr3+, Pt4+ and V3+ are most likely to be substituted at Ti4+ sites within TiO2 because ionic radii of these dopants are ~ 0.75 Å, which is almost similar to ionic radii of Ti4+ (0.745 Å). In the case of Co2+, Cu2+ and Pt2+ ions are possibly located in interstitial position of lattice because of relatively larger ionic size of the dopant ions (~ 0.89 Å). The Ag+, Rb+, Y3+ and La3+ ions are too large to be incorporated in TiO2 lattice and hence they are more likely to be found as dispersed metal oxide on surface of TiO2. From the XRD data, the crystallite size of anatase phase is calculated by Scherer’s formula using anatase (1 0 1) peak [28, 33, 35]. The crystallite size usually increases with increasing calcinations temperature for undoped as well as doped titanium dioxide showing the intense and narrow diffraction peaks due to sintering. Usually heavy sintering is observed in pure undoped titanium dioxide. The doping reduces the sintering giving smaller crystallite size than undoped titanium dioxide. The particle size of the europium doped titanium dioxide was much more uniform and much smaller than pure titanium dioxide showing the control of particle growth by doped europium [53]. Arbiol et al. [61] studied the effect of Nb doping in titanium dioxide synthesized by induced laser pyrolisis. They observed that presence of Nb substitutional ions in the anatase structure hinders the particles growth and transformation from anatase to rutile in TiO2 nano particles. The effect of Nb and Ag ions in the TiO2 nanostructure on the transformation of anatase to rutile phase
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and grain growth was compared [62]. The transformation of anatase to rutile phase was observed to be delayed in Nb doped titanium dioxide on increasing the Nb content (from 0.5 at. % to 1.5 at. %) due to decrease of oxygen vacancies. The grain growth in niobium doped titanium dioxide was lesser than pure titanium dioxide at above 500 °C. It was found that the Ag dopant promotes the TiO2 anatase to rutile transformation due to an increase of oxygen vacancies at the surface of anatase grains. The silver doping also decreases particle size with increasing doping concentration. The BET surface area of Degusa P25 TiO2 is reported to be 50 ± 5 m2/g [27]. As far as photocatalytic action is concerned, surface area plays a major role. The surface area is also dependent of concentration and ionic size of dopant, calcination temperature, etc. BET surface area of metal ion doped TiO2 is observed to be higher than pure TiO2 and the surface area increases with dopant concentration till an optimum concentration [27, 35, 42] Further increase in dopant concentration reduces the surface area. With increasing the calcinations temperature, the surface area of doped as well as undoped titanium dioxide decreases. The surface area of europium doped titanium dioxide was found to be higher than that of pure TiO2 and the surface area increases with doping concentration [32]. Europium dopants stabilize the nanocrystalline titanium dioxide and arresting the sintering process, which results to high surface area. The surface areas of different metal-doped TiO2 were found to be almost twice than that of the undoped TiO2 or Degussa P25 [43]. For Ga-doped TiO2, the surface areas increase with the increase of Ga dopant concentration up to a maximum of 0.1 wt % and then remain constant with the increase of Ga dopant concentration. In case of 0.3 % Pt doped TiO2 calcined at 200°C has surface area 150 m2/g [52]. At 700°C for
same material surface area was 57 m2/g. It because of anatase to rutile phase transformations. M.
Bettinelli et al. has reported Boron doped TiO2 has surface area 139 m2/g and vanadium doped TiO2
has surface area of 107 m2/g, which was higher than undoped TiO2 [63].
BET surface area of the Ni-doped powder, synthesized by mechanical alloying was also larger than the undoped powders, which was responsible for highest photocatalytic activity of Ni-doped catalyst [24]. For a series of transition metal (Cr, Mn, Fe, Co, Ni, Cu and Zn) doped titanium dioxide synthesized by sol-gel method, the higher surface area and lower mean pore diameter were observed as compared to pure titanium dioxide [64]. This was explained to be attributed to high sintering resistance and delayed amorphous to crystalline phase transformation induced by incorporated metal ions.
The relative rate constants of electron–hole recombination process are estimated by using
the femtosecond pump-probe diffuse reflectance spectroscopy technique [65]. In a semiconductor photocatalyst, band-gap photo excitation generates electrons and holes, which can migrate to the surface to perform redox reactions with adsorbed species. Their disappearance due to mutual recombination reduces the activity in photocatalysis. The doped metal ions can trap the photoexcited electrons before their recombination with the holes. The lifetime of trapped electrons were correlated with the photocatalytic activity of pure and metal-doped TiO2 [65]. It was stated that the lower the electron–hole recombination rate, the higher the photocatalytic activity of the samples. The electron–hole recombination rate constant values of the metal (Co, Cr, Cu, Fe, Mo, V and W) doped titanium dioxide catalysts was higher than that of pure titanium dioxide, indicating that doping enhances the electron–hole recombination [41]. However, these doped titanium dioxide samples showed high activity in carboxylic acid photo decomposition. It was explained that the photocatalytic activity of these doped titanium dioxide samples in carboxylic acid photo decomposition was influenced by point of zero charge (PZC).
The UV-Vis spectroscopic study has been carried out to measure the band gap energy of the
doped and undoped titanium dioxide to know the working wavelength range for catalyst. The band gap energy of anatase is about 3.2 eV with the threshold wavelength 387.5 nm [52]. Most of the applications are so far limited to UV- light irradiation because of adsorption edge of it is less than
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387 nm. A well proficient way to extend the absorbance of TiO2 to visible light is doping of transition metals which can decrease band gap of TiO2 [47]. The extension in absorption at 500 nm, which is above fundamental absorption of TiO2, was observed when Fe was doped in TiO2 [47]. Substantial and broader absorption shoulder up to 700 nm was observed when titanium dioxide was doped with ruthenium ions [52]. In case of Cr-TiO2, increase in dopant amount results into increase in absorption shoulder because of chromium doping created an occupied level in band gap [28]. To develop TiO2 based catalysts working under visible range, one must consider atomic radius, calcination temperature and concentration of dopant. Au/Au3+ doped TiO2 is having better absorption ranging from 480-600 with different concentration of doping [37]. The Fe, Cu and Ni doped TiO2 shows relatively less absorption between 400-550 nm. Whereas Co, Os and Ru doped TiO2 indicate broader absorption range up to 700 nm [52]. In case of V- TiO2 prepared via sol-gel route, broad shoulder of absorbance 450 to 700 nm is observed. It is also concluded that crystalline size is increased as vanadium concentration is increased [27].
Photocatalytic activity of metal doped TiO2
The positive effect of metal doping on the photoactivity of TiO2 for photocatalytic degradation may be explained by its ability to trap electrons and transfer to the electron acceptor (O2). This process reduces the recombination rate of electron-hole pair and facilitates the electron transfer at TiO2
surface [66]. Therefore, a more effective electron transfer occurs to the electron acceptors and from donors adsorbed on the surface of the particle than in the case of undoped TiO2. Oxygen adsorbed on photocatalyst surface traps the electrons and produces superoxide anion [67]. On the other hand, the holes at the TiO2 surface can oxidize adsorbed water/ hydroxide ions to hydroxyl radicals as well as adsorbed organic molecules [68].
Ghasemi et al. [64] compared the photocatalytic activity of pure and transition metal ions
(Cr, Mn, Fe, Co, Ni, Cu and Zn) doped TiO2 for the degradation of Acid Blue 92 dye. The results showed that photocatalytic activity of titanium dioxide was significantly increased by the presence of transition metal ions and the most active photocatalyst was Fe doped TiO2. The increase in photocatalytic activity of transition metal doped titanium dioxide was attributed to smaller crystallite size, higher surface area, reduced band gap energy, lower electron-hole recombination rate and higher efficiency for the electron-hole generation than pure titanium dioxide.
Sobana et al. [38] investigated the photocatalytic degradation of Direct red 23 (DR 23) and
Direct blue 53 (DB 53) dyes using silver deposited titanium dioxide by wet impringnated method under UV-A light irradiation. The presence of silver in TiO2 was found to enhance the photocatalytic degradation of DR 23 and DB 53. The higher activity of silver doped TiO2 was due to the enhancement of electron–hole separation by the electron trapping of silver particles. Whang et al. [69] found similar results for photocatalytic degradation of Methylene Blue (MB) with silver doped titanium dioxide. The highest MB degradation was obtained with 2.0 wt.% silver doped titanium dioxide photocatalyst under 2 h illumination with a halogen lamp. The silver nanoparticles deposited on TiO2 acts as electron traps of the matrix, reducing recombination of electron-hole pairs on the surface of titanium dioxide and improving charge transfer processes. The Au/Au3+-TiO2 catalyst synthesized by a sol-gel and photoreduction method with optimum 0.5 mol % of gold also showed significantly higher photooxidation efficiency (in photodegradation of methylene blue in aqueous solutions under visible light irradiation) than that of undoped TiO2 powder [37].
Choi et al. [8] have studied degradation of chloroform by using different metal ions (Fe3+,
Mo5+, Ru3+, Os3+, Re5+, V4+,Rh3+, Co3+and A13+) doped titanium dioxide. In general, in all case they got maximum yield at dopant concentration of 0.5% to titanium dioxide. In terms of chloroform oxidation, Fe3+ has shown highest photocatalytic activity, whereas Co3+ and A13+ reduced the activity. Metal ion dopant’s relative efficiency depends upon whether it serves as a recombination
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center or as a mediator of interfacial charge transfer. This ability of a dopant for functioning of effective trapping is related to dopant concentration, the incident light intensity, their d-electronic configuration, the distribution of dopant within the particles, the energy level of dopants within the titanium dioxide lattice and the electron donor concentration [8]. For enhancement of photo activity, enhanced interfacial charge transfer in the presence of effective dopants appears to be most important factor.
Hussain and siddiqa [70] reported remarkable improvement in photocatalytic activity of Fe
and Cr doped titanium dioxide nanotubes (TNTs) prepared by hydrothermal method as compared to undoped TNTs for the degradation of phenol. This may be owing to the inhibition of a recombination rate between the hole and electron occurring on the surface of excited TNTs by the metal dopants [71]. The photodegradation efficiency of TNTs was doubled with doped TNT’s under similar experimental condition.
The photocatalytic activity of cerium doped titanium dioxide (Ce-TiO2) was investigated for the degradation of Rhodamine B (RB) in aqueous solution under UV as well as visible light irradiation [72]. Doping of cerium in titanium dioxide showed maximum photocatalytic activity with an optimum doping concentration in the range of 0.2 to 0.4%. Doping of cerium accelerate the formation of super oxide anion radicals and improve the effective separation of electrons and holes. However, excess amount of dopant could become the center of recombination for electrone-hole pairs which adversely affect the photocatalytic activity of doped titanium dioxide. Iwasaki et al. [35] found similar observation for the cobalt doped titanium dioxide for photocatalytic degradation of acetaldehyde under UV and visible light irradiation. The photocatalytic activity of cobalt doped titanium dioxide strongly depends on the valence state of Co ions in the dopant and its concentration.
Although some conflictions in results are observed as far as effect of doping is concern.
Metal ion in host material’s atomic structure can serve as either recombination center or mediator of interfacial charge transfer. The important reason for these conflictions would be specific preparation method, size of guest atom, concentration of dopant, electronegativity, electronic state and configuration of both host and guest atoms. The europium doped titanium dioxide prepared by impregnation method results in diminished activity, showing that the preparation method and distribution of europium play a key role in the photoactivity [32]. The europium doping by sol-gel method could bring about remarkable improvement in the photoactivity for the degradation of Rhodamine B and the optimum enhancement of photoactivity was found with 1 at.% europium doped titanium dioxide.
The Fe-TiO2 showed better activity in phenol degradation as compared to P25 Degussa
titanium dioxide [73]. Fe doped TiO2 was responsive to visible light as well as it showed elevated activity under UV light. For Fe doped TiO2, it is reported that UV activity is around 4 times higher than visible light activity but overloading of Fe in to titanium dioxide decreases the photoactivity. The surface doped TiO2 with Cr3+ ion precipitated from H2SO4 solution [74] was reported to be highly active for water cleavage under visible light, while the photocatalytic activity of Cr-doped TiO2 prepared by flame reactor method was less active in photocatalysis [75]. Doping of Cr3+ and Mo5+ in TiO2 did not show remarkable improvement in photocatalytic activity over undoped TiO2 [76]. Similarly, V5+ doped TiO2 prepared by coprecipitation mthod led to reduced activity [8, 51], while increased photocatalytic activity was observed with V4+-doped TiO2 [8].
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Conclusion
The metal doping in titanium dioxide have been carried out by various physical and chemical methods such as sol-gel, wet impregnation of titanium dioxide with solution of metal precursor, hydro thermal, chemical vapour deposition (CVD), ion-assisted sputtering, water in oil (W/O) microemulsion, etc. The sol-gel technique has been excessively used for the doping of metal ions in titanium dioxide by adding metal dopant during the hydrolysis. The synthesis method, size and concentration of dopant, electronegativity and electronic configuration of both support and dopant atoms influence the physico-chemical and photocatalytic properties of doped titanium dioxide. The photocatalytic efficiency of the metal doped titanium dioxide is significantly enhanced by doping of a suitable metal ion. However the economic, effective and recyclable visible light activated photocatalyst is still under investigation.
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