Molybdenum-Doped Anatase and Its Extraordinary Photocatalytic

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Molybdenum-Doped Anatase and Its Extraordinary Photocatalytic Activity in the Degradation of Orange II in the UV and vis Regions Vaćlav Sˇtengl* and Snejana Bakardjieva Institute of Inorganic Chemistry AS CR V.V.i., 250 68 Rˇezˇ, Czech Republic ReceiVed: May 11, 2010; ReVised Manuscript ReceiVed: September 27, 2010

Molybdenum-doped anatase was prepared by thermal hydrolysis of peroxotitanium complex aqueous solutions containing a molybdenum peroxo-complex. The synthesized samples were characterized by X-ray diffraction, high-resolution transmission electron microscopy, selected area electron diffraction, and surface area (BET) and porosity (BJH) determination. Molybdenum doping caused the increase of unit cell constants of anatase and changes in the morphology of particles from spindle-like shapes to the shapes with rectangular or square cross sections. The presence of Mo5+/Mo6+ ion doping in the TiO2 nanostructure has no significant effect on the transformation of anatase to rutile. In the visible region, the photocatalytic activity is substantially enhanced in the molybdenum concentration of about 1.38%. The photocatalytic activity of doped titania samples was determined by the decomposition of Orange II dye during irradiation at 365 and 400 nm. The titania sample with 1.38% Mo has the highest catalytic activity during the photocatalyzed degradation of Orange II dye in an aqueous suspension in the UV and visible regions. 1. Introduction Titanium dioxide (anatase modification) is one of the most efficient photocatalysts for the detoxification of organically polluted wastewater. However, this material suffers from the drawback of poor absorption properties because of a band gap of 3.2 eV. Thus, wavelengths shorter than 400 nm are needed for light-induced generation of e-/h+ pairs. Therefore, doping with transition-metal ions is interesting for inducing a red shift of the band gap. However, this doping changes other physical properties, such as the lifetime of e-/h+ pairs and absorption characteristics. The substitution effects of Ti by Mo and W ions on the stability of anatase in commercially available TiO2 powders have been studied by X-ray diffraction, transmission electron microscopy, and surface area measurements in the 300-1000 K temperature range.1 Photocatalytic degradation of trichloroethylene with TiO2 and molybdenum-doped TiO2 (Mo/ Ti) mixed oxides was studied in ref 2 using a tubular quartz reactor packed with photocatalyst-coated glass beads. Nanosized molybdenum-doped TiO2 mixed oxide photocatalysts were prepared with the Mo5+ content varying from 0 up to 2.5 mol %, to shift the absorption onset into the visible region and to enhance the efficiency of the photocatalytic activity by retarding the e-/h+ recombination.3 The thin films of photocatalyst, TiO2 doped with Mo6+ in different forms, were prepared by a sol-gel method. The results showed that the TiO2 doped with Mo6+ extends the absorption edge and decreases the interfacial charge transfer resistance, and the photocatalytic activity of TiO2 doped with Mo6+ is much better than that of pure TiO2.4 Sulfate- and/ or molybdate-modified titania catalysts were prepared by incipient wet impregnation of nanosized (5-10 nm crystallite size) samples.5 Nanocrystalline samples of titanium dioxide monodoped or codoped with molybdenum (Mo) and nitrogen (N) were prepared by a sol-gel method. The codoped sample exhibited better absorption performance to visible light from * To whom correspondence should be addressed. Tel: 420 2 6617 3534. Fax: 420 2 2094 0157. E-mail: [email protected].

the absorption spectra, which was understood by the synergistic effect due to Mo and N doping.6 TiO2 photocatalysts were prepared by doping by transitionmetal ions, such as V5+ and Mo6+, and an inner transition metal, Th4+, in the concentration range of 0.02-0.1% and were characterized by various analytical techniques. Their photocatalytic activities were studied for the mineralization of chlorpyrifos as a probe molecule.7 TiO2 doped with p-type (Mn2+) and n-type (Mo6+) dopants and their photocatalytic activity were studied under solar light, choosing the degradation reaction of amaranth. The Mo6+-TiO2 sample showed the largest red shift in the band gap owing to its higher electronegativity. Mn2+-TiO2 (0.06%) showed better activity due to the synergetic effect of the mixed phase.8 The degradation of synthetic dyes, such as Methyl Orange, p-amino azobenzene, Congo Red, Brilliant Yellow, Rhodamine-B, and Methylene Blue, under solar light was carried out using TiO2 doped with Mo6+ ions. Among the photocatalysts used, Mo6+-TiO2 (0.06%) showed enhanced activity due to the effective separation of charge carriers.9 The paper in ref 10 deals with doping titania by Cr3+ and Mo5+ ions. If the molybdenum concentration is increased beyond 1 atom %, the influence of adsorption becomes predominant and both adsorption and photodegradation increase. Kubacka et al.11 investigated the structure-activity link of anatase-type Ti-M (M ) V, Mo, Nb, and W) mixed oxides for toluene photooxidation under sunlight-type excitation. The MoO3/TiO2 and MoO3/TiO2-CeO2 samples were prepared at the 8 wt % Mo loading by an incipient wetness impregnation method. The UV-vis spectra indicated that the dispersed Mo ions in the MoO3/TiO2 sample are in tetrahedral coordination, whereas it is in octahedral coordination in the MoO3/TiO2-CeO2 sample that showed discrete amounts of bulk MoO3 species.12 A series of TiO2-MoO3 nanocomposite photocatalysts were prepared by a supercritical fluid dry method and an impregnation technique with TiCl4 and (NH4)6Mo7O24 · 4H2O as the starting materials. The presence of a small amount of Mo in the composite catalyst gives rise to the red shift of its absorbance

10.1021/jp104271q  2010 American Chemical Society Published on Web 10/22/2010

Molybdenum-Doped Anatase

J. Phys. Chem. C, Vol. 114, No. 45, 2010 19309

TABLE 1: Characteristics of Prepared Titania Samples Doped with Variable Amounts of Mo

sample

(NH4)6Mo7O24 · 4H2O [g]

EDX Mo [wt %]

crystallite size [nm]

aspect ratio l/d

BET [m2 g-1]

total pore volume [cm3 g-1]

TiMo_1 TiMo_2 TiMo_3 TiMo_4 TiMo_5 TiMo_6

0.05 0.10 0.15 0.25 0.50 1.00

0.56 0.93 1.38 1.85 3.61 4.76

26.2 26.2 25.7 25.6 24.2 15.6

3.1 4.6 3.6 2.5 1.2 1.0

64.4 77.2 56.4 109.4 107.7 153.2

0.365 0.235 0.293 0.286 0.302 0.301

wavelength, decrease of its energy gap, and increase of the utility of visible light.13 The aim of this paper is to study the influence of incorporation of Mo6+ ions to monodispersed anatase. The catalysts were prepared by a very simple “one-pot” method using inexpensive chemicals and a nondemanding technique. Six samples labeled as TiMo_1-TiMo_6 were prepared using this method. The photocatalytic activity of the as-prepared titania-modified samples was assessed by the photocatalytic decomposition of Orange II dye in an aqueous slurry under irradiation of 365 and 400 nm wavelengths. 2. Experimental Section 2.1. Synthesis of Mo-Doped Titania. All chemicals used, titanium oxo-sulfate (TiOSO4), ammonium heptamolybdate tetrahydrate (NH4)6Mo7O24 · 4H2O, ammonium hydroxide (NH4OH), and hydrogen peroxide (H2O2) were of analytical grade and were supplied by Sigma-Aldrich Ltd. In a typical experiment, titanyl oxo-sulfate was dissolved in 100 mL of distilled water and hydrolyzed by slow addition of ammonium hydroxide solution (10%) under constant stirring at a temperature of 0 °C in an ice bath, until the reaction mixture reached pH 8.0. The obtained white precipitate was separated by filtration and was washed until it was free of sulfate ions (confirmed by the BaCl2 test) with distilled water. The obtained wet precipitate was mixed with 100 mL of 30% hydrogen peroxide solution, and a yellow gelatinous mass was obtained. The yellow gelatin-like product obtained by the foregoing reaction was mixed with a defined amount of ammonium heptamolybdate tetrahydrate (NH4)6Mo7O24 · 4H2O dissolved in 10 mL of H2O2 (see Table 1) and subsequently heated in a heating mantle in a round-bottom flask under reflux cooling. During the heating, a yellow-white precipitate was formed. The as-obtained precipitate was dried in an oven at 105 °C. Annealing was continued until the precipitate changed its color to white or pale bluish (∼36 h). The as-prepared samples have weak photochromic properties: on light, its color slightly changed. In the case of molybdenum doping, it has been shown that Mo5+ and Mo6+ can coexist simultaneously during illumination.14 2.2. Characterization Methods. Diffraction patterns were collected with a PANalytical X’Pert PRO diffractometer equipped with a conventional X-ray tube (Cu KR radiation, 40 kV, 30 mA) and a linear position sensitive detector PIXcel with an antiscatter shield. A programmable divergence slit set to a fixed value of 0.5°, Soller slits of 0.02 rad, and a mask of 15 mm were used in the primary beam. A programmable antiscatter slit set to a fixed value of 0.5°, a Soller slit of 0.02 rad, and a Ni beta filter were used in the diffracted beam. Qualitative analysis was performed with the DiffracPlus Eva software package (Bruker AXS, Germany) using the JCPDS PDF-2 database.15 For quantitative analysis of XRD patterns, we used Diffrac-Plus Topas (Bruker AXS, Germany, version 4.1) with structural models based on the ICSD database.16 This program

micropore surface area [m2 g-1]

micropore volume [cm3 g-1]

1.556

0.00021

3.647

0.00096

cell param a [Å]

cell param c [Å]

3.7949 3.7952 3.7954 3.7960 3.7974 3.8001

9.5074 9.5075 9.5079 9.5077 9.5065 9.5014

permits estimating the weight fractions of crystalline phases and mean coherence length by means of a Rietveld refinement procedure. Sample TiMo_6 was studied by in situ hightemperature XRD in air with a PANalytical XPertPRO diffractometer using Co K radiation (40 kV, 30 mA) and a multichannel detector X’Celerator with an antiscatter shield, equipped with a high-temperature chamber (HTK 16, Anton Paar, Graz, Austria). The measurements started at room temperature and finished at 1200 °C. Transmission electron micrographs (TEM and HRTEM) were obtained using a JEOL JEM 3010 microscope operated at 300 kV (LaB6 cathode). A copper grid coated with a holey carbon support film was used to prepare samples for TEM observation. The powdered sample was dispersed in ethanol and the suspension treated in an ultrasonic bath for 10 min. The surface areas of samples were determined from nitrogen adsorption-desorption isotherms at liquid nitrogen temperature using a Coulter SA3100 instrument with outgassing for 15 min at 150 °C. The Brunauer-Emmett-Teller (BET) method was used for surface area calculation,17 and the pore size distribution (pore diameter, pore volume, and micropore surface area of the samples) was determined by the Barrett-Joyner-Halenda (BJH) method.18 Diffuse reflectance UV/vis spectra for evaluation of photophysical properties were recorded in the diffuse reflectance mode (R) and transformed to absorption spectra through the KubelkaMunk function.19 A PerkinElmer Lambda 35 spectrometer equipped with a Labsphere RSAPE-20 integration sphere with BaSO4 as a standard was used. The photocatalytic activity of samples was assessed from the kinetics of the photocatalytic degradation of 0.02 M Orange II dye (sodium salt of 4-[(2-hydroxy-1-naphtenyl)azo]-benzenesulfonic acid) in aqueous slurries. The azo dyes (Orange II, Methyl Red, Congo Red, etc.) are not absorbed on titania surfaces, in contrast to Methylene Blue. For the azo-dye degradation, the complete mass balance in nitrogen indicated that the central -NdN- azo group was converted into gaseous dinitrogen, which is ideal for the elimination of nitrogencontaining pollutants, not only for environmental photocatalysis but also for any physicochemical method.20 Direct (noncatalyzed) photolysis by artificial UV light or a solar energy source cannot mineralize Orange II.21 Kinetics of the photocatalytic degradation of the aqueous Orange II dye solution was measured by using a self-constructed photoreactor.22 The photoreactor consists of a stainless steel cover and quartz tube with a Narva fluorescent lamp with a power of 13 W and a light intensity of ∼3.5 mW cm-2. “Black light” (365 nm) for UV and “warm white” (upon 400 nm) for visible light irradiation were used. Orange II dye solution was circulated by means of a membrane pump through a flow cuvette. The concentration of the Orange II dye was determined by measuring the absorbance at 480 nm with a ColorQuest XE VIS spectrophotometer. The 0.5 g of titania sample was sonicated for 10 min with an ultrasonic bath (300 W, 35 kHz) before use. The pH of the resulting suspension

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Sˇtengl and Bakardjieva

Figure 1. XRD spectra of Mo5+/Mo6+-doped titania: (a) TiMo_1, (b) TiMo_2, (c) TiMo_3, (d) TiMo_4, (e) TiMo_5, (f) TiMo_6.

was taken as the initial value for neutral conditions, and during the experiment, it was kept at a value of 7.0. 3. Results and Discussion The XRD patterns of the Mo5+/Mo6+-doped titania samples are presented in Figure 1, and the high-temperature diffractograms of sample TiMo_6 are shown in Figure 2. The measurement started at room temperature and finished at 1200 °C with a step of 100 °C from 25 to 550 °C and a step of 25 °C in the remaining ranges. The phase composition of the prepared samples, unit cell parameters, and crystallite sizes were calculated by the Rietveld method and are presented in Table 1 and Figure 3. Only anatase (ICDD PDF 21-1272) and rutile (ICDD PDF 21-1276) were observed during the heating of the samples. No diffraction lines of Mo-containing or other phases, with the exception of the Pt sample holder, were observed, which means that Mo is incorporated into the TiO2 structure. The phase transformation of anatase-rutile proceeds at approximately the same temperature as nondoped anatase: it starts at 800 °C and continues up to 950 °C,23 and the crystallite size increases with temperature (see Figure 3b). Unit cell parameters of the TiMo_6 sample prepared by in situ heating experiments increase almost linearly, which reflects the thermal expansion of the structures (see Figure 3c,d). An exception is the unit cell parameter of anatase, a, which slightly decreases with increasing the temperature up to 400 °C. This reduction is probably caused by the escape of physically and chemically bonded water. It points to crystal water (H2O or OH groups) in the anatase structure, which is quite common for transition-metal oxides prepared from an aqueous environment; it is responsible for the slightly smaller lattice size of anatase with respect to stoichiometric database specimens. In the Mo-doped titania, Mo could be in the +6 or +5 oxidation state. The ionic radius of Mo6+ is 0.062 nm,24 and that of Ti4+ is 0.0605 nm.25 Because of the similarity in their ionic sizes, Mo can easily be incorporated into the TiO2 lattice. Isostructural substitution can be confirmed by the examination of the lattice size of doped structures. In the simplest case, the

unit cell dimensions should linearly change with the dopant percentage (Vegard rule). This would confirm the possibility of incorporation of the metal ions as a substituent in the TiO2 lattice. Considering the ionic size and the oxidation state of the Mo6+ dopant, the following defect states can be proposed in accordance with the Kro¨ger-Vink notation26 when (eq 1) the incorporation of dopants creates conduction band electrons or (eq 2) instead of creating conduction electrons, charge compensation can also be achieved by defects, such as interstitial oxygens, or (eq 3) oxides are completely soluble, and then the dopants occupy substitution sites, creating oxygen vacancies.27 •• MoO3 T MoTi + 2OO + 2e- + 1/2O2

(1)

•• MoO3 T MoTi + Ooo + 2OO

(2)

•• MoO3 T MoTi + VOoo + 2OO

(3)

The imbalance in the charge created on doping can also be compensated by titanium vacancies •• oooo 2MoO3 T 2MoTi + VTi + 6OO

(4)

where MoTi is a molybdenum ion at a titanium lattice site, Oo is oxygen occupying an oxygen lattice site, Vti is the titanium vacancy, VO is oxygen vacancy, and the (•) represents the excess charge, whereas the (O) represents the deficiency in the charge. The specific surface area of the sample, calculated by the multipoint Brunauer-Emmett-Teller (BET) method, total pore volume, micropore surface area, and micropore volume are listed in Table 1. Barrett-Joyner-Halenda (BJH) pore size distribution plots and nitrogen adsorption/desorption isotherms (insets) of prepared samples are shown in Figure 4. According to IUPAC convention,28 microporous materials have pore diameters of less



Figure 2. High-temperature XRD diffractogram of sample TiMo_6.

than 2 nm and macroporous materials greater than 50 nm; the mesoporous category thus lies between the middle. The samples denoted as TiMo_1 and TiMo_3 have a hysteresis loop type H3 shape, which may represent interparticle pores within the bulk sample. For the remaining samples, the isotherm of a type IV characteristic of mesoporous material with a type H2 hysteresis is typically observed, which is assigned to large-pore mesoporous materials and can be ascribed to capillary condensation in mesopores.28 The high steepness of the hysteresis indicates the high order of mesoporosity. All samples, except for TiMo_5, are characteristic of a Type A hysteresis loop according de Bore’s characterization.29 This hysteresis type is connected with pores in the form of capillary tubes open at both ends, wide ink-bottle pores, and wedge-shaped capillaries. The sample denoted as TiMo_5 has Type E hysteresis loops, which have a closer resemblance to Type A hysteresis loops and may be connected with ink-bottle pores or with interconnected capillaries. The samples denoted as TiMo_2 and TiMo_4 are microporous and have micropore surface areas of 1.5 and 3.6 m2 g-1, respectively. HRTEM images of Mo5+/Mo6+ nanocrystals incorporated into anatase particles are shown in Figure 5. The aspect ratio of anatase particles decreased gradually with the addition of molybdenum, which changes the morphology of the particles from spindle-like shapes to rectangular or squarelike shapes. The HRTEM image confirms that all particles possess an anatase structure with the c axis of the tetragonal symmetry collinear with the long axis of the spindle, as indicated by the crystal planes. The interlayer spacing, d, along the [101] direction of

the anatase crystal linearly increased with the amount of Mo. These images clearly show very well crystalline materials, which contain no amorphous domains. The absence of amorphous domains is an essential presumption for good photocatalytic properties. The selected area electron diffraction patterns (SAED) analyzed by the Process Diffraction program showed that the structure of all samples is anatase (ICDD PDF 21-1272). Figure 6 presents UV/vis absorption spectra of the as-prepared samples and samples heated to a temperature of 600 °C. Anatase samples with Mo5+/Mo6+ ion incorporation exhibit new optical properties, which are different from those of both titania and molybdenum oxide. The anatase has a wide absorption band in the range from 200 to 385 nm, and the MoO3 has a UV absorption band in the range from 200 to 750 nm.30 The Kubelka-Munk theory is generally used for the analysis of diffuse reflectance spectra obtained from weakly absorbing samples. It provides a correlation between reflectance and concentration. The concentration of an absorbing species should be proportional to the absorbance according to the KubelkaMunk formula

f(R) ) (1 - R)2 /2R ) k/s ) Ac/s

(5)

where R is the reflectance, s is the scattering coefficient, k is the molar absorption coefficient, c is the concentration of the absorbing species, and A is the absorbance.31 Compared with the pure sample, obvious absorption edge red shifts are observed as the result of the doping. For the Mo-

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Figure 3. Results of the Rietveld refinement of sample TiMo_6: (a) phase composition, (b) crystallite size, (c) unit cell parameters a and c of anatase, (d) unit cell parameters a and c of rutile.

doped TiO2, an absorption edge is red shifted and the absorption tail extends to ∼450 nm, as Figure 6 shows. The UV/vis diffuse reflectance spectroscopy method was employed to estimate band-gap energies of the prepared Mo5+/ Mo6+-TiO2 samples. First, to establish the type of band-toband transition in these synthesized particles, the absorption spectra were fitted to equations for direct band-gap transitions. The minimum wavelength required to promote an electron depends upon the band-gap energy, Ebg, of the photocatalyst and is given by

Ebg ) 1240/λ

[eV]

(6) A ) εcl

where λ is the wavelength in nanometers.32 The band-gap values were calculated using the UV-vis spectra from the following equation

R(hν) ) A(hν - Ebg)n

value of hν extrapolated to R ) 0 gives an absorption energy, which corresponds to a band-gap energy (see Tables 2 and 3). The value of 3.20 eV for the sample denoted as TiMo_0 is reported in the literature for pure anatase nanoparticles.36 The value of the bandgap energy decreases with the increasing content of the Mo dopant. The photocatalytic activity of the prepared samples was determined using the degradation of 0.02 M Orange II dye aqueous solutions under UV radiation at 365 nm (UV-A, “black light” lamp) and up to 400 nm (“warm white” lamp). In regions in which the Lambert-Beer law is valid, the concentration of the Orange II dye is proportional to the absorbance

(7)

where R is the absorption coefficient and hν is the photon energy. In the case that the fundamental absorption of the titania crystal possesses an indirect transition between bands,33,34 then n ) 2; for direct transition between bands, n ) 1/2. The energy of the band gap is calculated by extrapolating a straight line to the abscissa axis; when R is 0, then Ebg ) hν.35 Figure 7 shows the (Rhν)2 versus Ebg for a direct band-gap transition, where a is the absorption coefficient and Ebg is the photon energy. The

(8)

where A is the absorbance, c is the concentration of the absorbing component, l is the length of the absorbing layer, and ε is the molar absorption coefficient. Orange II dye does not undergo a direct photolysis, and any change in the Orange II dye concentration can be attributed only to the heterogeneous photocatalysis. Photodegradation experiments with the Orange II dye and catalysts process are described by the first-order kinetics with respect to the concentration of the organic compound. The time dependence of the Orange II dye decomposition can be described using eq 9 for a reaction following first-order kinetics

dC ) k(C0 - C) dt

(9)



Figure 4. BJH pore size distribution plots of desorption pore area vs pore radius. The insets are hysteresis loops.

where C is the concentration of the Orange II dye, C0 is the initial concentration of the Orange II dye, and k is the rate constant. The rate constants k of the photocatalytic degradation of Orange II are shown in Tables 2 and 3, and the kinetics of the degradation is presented in Figures 8 and 9. According to the degradation pathway proposed by ref 37, the main byproducts formed by the ozonation of azo dye are organic acids, aldehydes, ketones, and carbon dioxide. Meanwhile, Demirev and Nenov38 suggested that the eventual degradation products of an azo dye in the ozonation system would be acetic, formic, and oxalic acids. The reaction pathway for the visible-light-driven photocatalytic degradation of Orange II dye in aqueous TiO2 suspensions to products not absorbing visible light has been described in detail by Stylidi et al.39 The calculated degradation rate constants k (min-1) are shown in Tables 2-3, and the kinetics of the degradation of Orange II dye at 365 nm (“black light”) and 400 nm (“warm white”) on samples TiMo_1-TiMo_6 and heated samples at a temperature of 600 °C are illustrated in Figures 8 and 9. As can be seen from Table 2, doping of molybdenum increases the photocatalytic activity in the UV region. The low activity of the nondoped sample is due to its low crystallinity and the presence of amorphous domains.40 The best photocatalytic activity (k ) 0.04614 min-1) is the sample marked TiMo_3, which contains

1.38 wt % molybdenum, according to EDX analysis. Further incorporation of Mo ions into the TiO2 lattice decreases the photocatalytic efficiency under UV light. This decline could be explained by the generation rate of the electron conduction band and valence band holes and their lifetime. Usually electron and hole recombination lowers the rate of degradation. In the case of high Mo ion doped titania, the mid-band gaps created due to the doping may act as recombination centers under UV light.41 As indicated in Table 2, nondoped anatase (sample TiMo_0) exhibits very low visible-light photocatalytic activity for the degradation of Orange II dye. This is caused by the higher bandgap energy of pure anatase (3.2 eV, λbg ) 388 nm) and rutile (3.0 eV, λbg ) 413 nm).35 In the visible light spectrum, all prepared samples show higher activity than the nondoped sample, labeled TiMo_0. The photocatalytic activity under the visible spectrum is caused by a decreasing band-gap energy (see Table 2.). The best photocatalytic activity (k ) 0.00969 min-1) has been achieved with the sample labeled TiMo_3. This may be due to the shift in the band gap to the visible region by the creation of mid-band gaps by the dopant. This enhances the absorption of more photons from visible light illumination. It is very clear that the degradation efficiency increased with increasing the Mo5+/Mo6+ content from 0.56 to 1.38 wt %. This

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Figure 6. UV-vis absorbance spectra of Mo5+/Mo6++-doped titania: (a) unheated and (b) heated at 600 °C.

Figure 5. HRTEM photographs of Mo5+/Mo6+-doped titania: (a) TiMo_1, (b) TiMo_2, (c) TiMo_3, (d) TiMo_4, (e) TiMo_5.

intermediate concentration of dopant narrows the band gap and shifts the absorption band to 480 nm, making the catalyst effective for absorbing visible light. Further, the degradation efficiency decreased with an increase in the Mo5+/Mo6+ content above 1.38 wt %. The enhanced photoresponse of Mo-doped titania under visible light irradiation can be accounted for in the following

ways:24 (i) fast interfacial electron transfer rate due to the creation of new energy levels at 0.40 eV, from which electrons can be easily promoted to the conduction band; (ii) slower recombination rate of electron-holes; and (iii) the MoTi••-VoOO state is acting as an electron donor below the conduction band edge. With respect to the dependence on Mo concentration, the samples annealed at the temperature of 600 °C behave like the nonannealed samples; the sample labeled TiMo_3_600 has again the highest activity. Lower levels of activity of the samples annealed at a temperature of 600 °C are due to reduction of surface area, as shown in Table 3. Annealing the samples at 600 °C increased almost twice the size of the crystallites (see Table 3). This increase of the crystallite size decreases the photocatalytic activity of annealed samples. Two basic factorsssurface area and crystallinityshave a major influence on the photocatalytic activity of TiO2. The larger the surface area of the photocatalyst is, the more reaction sites there are, which is in favor of the activity. The rate constants were, hence, normalized to the surface area of the catalyst in Figure 10 to allow comparison of the real surfaces of the catalysts. Crystallinity is another important factor affecting the photocatalytic efficiency because poorly crystalline defects in a crystal always lead to the recombination of electrons and holes at defect positions. To obtain high photocatalytic activity, it is necessary to have a high surface area and good crystallinity.



Figure 8. Orange II dye degradation on Mo5+/Mo6+-doped titania at wavelengths of (a) 365 and (b) 400 nm. Figure 7. Band-gap energy of Mo5+/Mo6+-doped titania: (a) unheated and (b) heated at 600 °C.

TABLE 2: Band Gap and Rate Constant k of Samples TiMo sample

band gap [eV]

k 365 nm [min-1]

k 400 nm [min-1]

TiMo_0 TiMo_1 TiMo_2 TiMo_3 TiMo_4 TiMo_5 TiMo_6

3.2 3.0 2.8 2.8 2.8 2.8 2.6

0.00730 0.01893 0.02204 0.04614 0.03138 0.01705 0.01000

0.00200 0.00579 0.00790 0.00969 0.00322 0.00380 0.00354

TABLE 3: BET, Band Gap, and Rate Constant k of Heated Samples TiMo sample TiMo_1_600 TiMo_2_600 TiMo_3_600 TiMo_4_600 TiMo_5_600 TiMo_6_600

BET crystallite band k 365 nm k 400 nm [m2 g-1] size [nm] gap [eV] [min-1] [min-1] 54.2 64.5 50.2 64.2 65.8 70.5

59.6 41.0 41.0 41.0 38.1 31.8

3.00 2.95 2.90 2.85 2.80 2.70

0.00593 0.01140 0.02004 0.02038 0.01097 0.00742

0.00441 0.00339 0.00542 0.00372 0.00236 0.00469

However, there is a need to find a compromise because the increase in crystallinity reduces the surface area. Mo modification of titania has mostly been performed by impregnation of existing titania particles by a solution of molybdate..42,43,7 Di Paola et al. found that most of the Mo was retained on the surface of the titania particles. The method of

homogeneous precipitation, which we employed, has a better chance then wet impregnation methods to obtain titania modified in the bulk. Mo can, in principle, be present as Mo6+ ions, Mo5+ ions, or MoO3+,44 and these ions can either replace Ti4+ ions or enter into the interstitial positions.1 To estimate the actual Mo species present in the catalysts, their careful characterization must be performed, yet is missing. HRTEM imaging revealed neither nanosized inclusions inside the titania crystals nor surface films with different structures on their outer surface. Neither HRTEM nor XRD found a trace of crystalline Mo admixtures. EDX analysis showed an increase of the observed Mo percentage with respect to the bulk concentration, which would mean an enhanced surface concentration of Mo species. The changes in the lattice parameters (Figure 10b) and the position of the [101] diffraction of titania (Figure 5) clearly show that Mo ions really entered the titania lattice. The pale bluish-green color of Mo-doped titania and the corresponding vis absorption band show that at least a part of Mo is present as Mo5+. The same vis absorption band was obvious in Mo-doped titania prepared by previous researchers.43,7 According to the characteristics plotted in Figure 10, the Mo doping proceeded in two stages. At concentrations up to 1.4% Mo, the lattice of titania was slightly isotropically expanded, the [101] spacing (Figure 10b.) was accordingly increased, and visible absorption of Mo5+ species continuously increased. According to EDX, a part of the Mo was incorporated near (or directly on) the titania surface. The Mo species formed in this stage are clearly extremely beneficial for catalysis as it is clear from the sharp maximum of the surface-area-normalized reaction

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Figure 9. Orange II dye degradation on Mo5+/Mo6+-doped titania heated at 600 °C at wavelengths of (a) 365 and (b) 400 nm.

rate in Orange II degradation (Figure 10c). The simultaneously increasing a and c lattice parameters of titania are consistent with normal Vegardian behavior, assuming that the dopant ions are slightly larger than Ti4+. Although the Mo6+ ion is of a similar size as Ti4+ or smaller (depending on their actual coordination numbers), actually the Mo5+ cation is slightly larger. With the further growing concentration of Mo in the second stage of doping, the titania structure started to distort: c lattice constants decreased, and a lattice constants increased. These changes are more substantial than in the first doping stage, which is in agreement with more Mo incorporated in the bulk titania, proven by the decreasing relative enhancement of EDX content of Mo with respect to its bulk concentration. The growth of visible light absorption of Mo5+ species was stopped: the prevailing Mo valence in the second stage of doping is Mo6+. The local maximum in the c lattice parameter at growing Mo doping has also been observed by Gomatha Devi et al.7 Clearly, there are at least two different major Mo species formed on doping, one prevailing at lower concentrations, preferentially close to the particle surface, at least partly pentavalent, and being highly favorable for photocatalysis, and the other being formed at larger concentrations, with Mo in the hexavalent stage and a dramatically worsened catalyst performance. 4. Conclusions The doped titania samples were prepared by thermal hydrolysis of Mo and Ti peroxo-complexes from aqueous solutions. The thermal hydrolysis of a peroxo-complex has one great

Figure 10. Dependence of (a) k1 (k1 ) k/BET), (b) cell parameters a and c, and (c) absorbance at 550 nm on the content of Mo [wt %].

advantage over other reaction processes, namely, that the reaction takes place in a one-step reaction and the side product is pure water. Another great advantage of this procedure could be its easy transfer to low-cost manufacturing of photocatalytic pigment. The prepared monodispersed particles have a mesoporous character with a pore size distribution of 10-15 nm. Incorporation of molybdenum ions into the crystal lattice of anatase TiO2 changes the morphology of particles from spindlelike shapes to rectangular or squarelike. Structural investigation based on X-ray diffraction as well as SAED in high-resolution transmission electron microscopy confirms a well-developed crystal structure with an interlayer distance d[101] ) 0.344-0.366 nm, which indicates crystal lattice expansion due to the incorporation of Mo dopants. The molybdenum addition also increases the anatase phase stability to above 900 °C by inhibiting the growth of the crystallite size of anatase, the increase of lattice constants of anatase, and the red shift of

Molybdenum-Doped Anatase the optical absorption. The molybdenum addition increases the photocatalytic activity of titania in the visible light region. Acknowledgment. This work was supported by the Academy of Sciences of the Czech Republic (Project No. AV OZ 40320502) and the Czech Science Foundation (Project No. 203/ 08/0334). The authors gratefully thank N. Murafa for HRTEM measurements and P. Bezdicˇka for XRD measurements. T. Matys Grygar contributed to the result evaluation and discussion. References and Notes (1) Bregani, F.; Casale, C.; Depero, L. E.; Natali-Sora, I.; Robba, D.; Sangaletti, L.; Toledo, G. P. Sens. Actuators, B 2004, 31, 25–28. (2) Yoon, J. K.; Yoon, W. S.; Joo, H. K.; Jeon, M. Sv.; Lee, T. K. J. Korean Inst. Chem. Eng. 2000, 38, 288–29. (3) Jeon, M. S.; Yoon, W. S.; Joo, H.; Lee, T. K.; Lee, H. Appl. Surf. Sci. 2004, 165, 209–216. (4) Yang, Y.; Li, X.; Chen, J.; Wang, L. J. Photochem. Photobiol., A 2004, 163, 517–522. (5) Ciambelli, P.; Sannino, D.; Palma, V.; Vaiano, V. Int. J. Photoenergy 2008, 1, 1–8. (6) Tan, K.; Zhang, H.; Xie, C.; Zheng, H.; Gu, Y.; Zhang, W. F. Catal. Commun. 2000, 11, 331–335. (7) Devi, L. G.; Murthy, B. N.; Kumar, S. G. J. Mol. Catal. A: Chem. 2009, 308, 174–181. (8) Devi, L. G.; Kumar, S. G.; Murthy, B. N.; Kottam, N. Catal. Commun. 2009, 10, 794–798. (9) Devi, L. G.; Murthy, B. N.; Kumar, S. G. Chemosphere 2009, 76, 1163–1166. (10) Wilke, K.; Breuer, H. D. J. Photochem. Photobiol., A 1999, 121, 49–53. (11) Kubacka, A.; Coloń, G.; Fernańdez-Garcıá, M. Catal. Today 2009, 143, 286–292. (12) Mohamed, M. M. Appl. Catal., A 2004, 267, 135–142. (13) Zhang, J.; Li, Q.; Cao, W. J. EnViron. Sci. 2005, 17, 350–352. (14) Gra¨tzel, M.; Howe, R. F. J. Phys. Chem. 1990, 94, 2566–2572. (15) JCPDS PDF-2 Database, release 54; International Centre for Diffraction Data: Newtown Square, PA, 2004. (16) ICSD Database, release 2009/1; FIZ Karlsruhe: Karlsruhe, Germany, 2009. (17) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309–319. (18) Barret, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373–380. (19) Orel, Z. C.; Gunde, M. K.; Orel, B. Prog. Org. Coat. 1997, 30, 59–66.

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Molybdenum-Doped Anatase and Its Extraordinary Photocatalytic

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