2009_Central_European_Journal_of_Chemistry.pdf

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Central European Journal of Chemistry

Photocatalytic properties of Ru-doped titaniaprepared by homogeneous hydrolysis

* E-mail: [email protected]

Received 03 July 2008; Accepted 06 January 2009

Abst ra ct : Nanocrystalline titania particles doped with ruthenium oxide have been prepared by the homogenous hydrolysis of TiOSO4 in aqueoussolutions in the presence of urea. The synthesized particles were characterized by X-ray diffraction (XRD), Scanning ElectronMicroscopy (SEM), High Resolution Transmission Electron Microscopy (HRTEM), Selected Area Electron Diffraction (SAED) andNitrogen adsorptiondesorption was used for surface area (BET) and porosity determination (BJH). The photocatalytic activity of theRu-doped titania samples were determined by photocatalytic decomposition of Orange II dye in an aqueous slurry during irradiationat 365 nm and 400 nm wavelengths.

Versita Warsaw and Springer-Verlag Berlin Heidelberg.

Keywords: TiO 2 RuO 2 Urea Photocatalytic activity

Institute of Inorganic Chemistry, Academic of Sciences of the Czech Republic, Rez-Husinec 250 68, Czech Republic

Vendula Houkov*, Vclav tengl, Snejana Bakardjieva, Nataliya Murafa,Vclav Tyrpekl

SSC-2008

1. IntroductionPhotocatalytic degradation processes have been widelyapplied as techniques for the destruction of organicpollutants in wastewater and ef uents. TiO2 is widely usedfor photocatalytic air and water puri cation and manyother purposes based on photocatalytic oxidation anddecomposition of organic pollutants [1-4 ]. The materialcan also be used for solar energy storage and conversion[5] and organic syntheses [6]. Titanium dioxide is oneof the most popular and promising materials for thesepurposes, because of its stability, commercial availabilityand ecological safety. According to the literature [7-9 ],the photocatalytic activity of suspended TiO2 in solutionstrongly depends on its physical properties ( e.g ., crystalstructure, surface area, surface hydroxyls, and particlesize).

In order to enhance its activity as a catalyst, mucheffort has been expended attempting to modify TiO2 bydoping with Ru, S, Te, Si, Ag and other materials [10 ,11 ].Ohno et al . [12 ] have reported on ruthenium-dopedtitania and its photocatalytical activity under visiblelight. Ru-based oxides such as RuZr, RuTi, RuTa,

and RuSn binary oxides, have been widely used asthe dimensionally stable anodes (DSA) for oxygen andchlorine evolution reactions [13 ,14 ].

There has also been considerable interest in usingRuO2 electrodes for hydrogen evolution [15 ,16 ], andexploratory work has also been done on the use ofRuO2-based electrodes for the oxidation (destruction) oforganic waste [17 ].

Homogeneous hydrolysis with urea as a precipitatingagent can be used in the preparation of oxo-compounds,such as metal oxides and hydroxides or precursors ofbase mixed oxo-hydroxides. The urea method is basedon the thermal decomposition of urea at temperaturesgreater than 60C [18 ,19 ].

In this contribution, a Ru doped titania was preparedby the homogeneous hydrolysis of ruthenium chloride,titanium oxo-sulphate (TiOSO4) and urea at 100Cand followed by controlled annealing in an oxygenatmosphere. The photocatalytic activity of the Ru-dopedTiO2 composites were tested using photodegradation ofan aqueous solution of 0.02 M Orange II (OII) dye atwavelengths of 365 nm and 400 nm. Under the same

Cent. Eur. J. Chem. 7(2) 2009 259-266DOI: 10.2478/s11532-009-0019-x

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conditions, the photocatalytic activity of the undopedTiO2 nanoparticles were also examined (sample denotedas TIT 300).

2. Experimental Procedures2.1. Synthesis of Ru-doped titania All the chemicals used in this study,ruthenium chloride (RuCl3), titaniumoxo-sulphate (TiOSO4) and urea ((NH 2)2CO), were ofanalytical grade and were supplied by Fluka and Sigma- Aldrich Ltd. TiOSO4 (100 g) and 1 g (0.75; 0.5; 0.25; 0.1;0.05; 0.01; 0.005 or 0.001 g) of RuCl3 were dissolved in4 L of distilled water and 300 g of urea was added. Thereaction mixture was adjusted to a pH = 2 with sulfuricacid and then heated to 100C while stirring for 8 hours.

The resulting titania powders were washed with distilledwater, decanted, ltered off and then dried at 105Cin a furnace. After annealing at 600C in an oxygenatmosphere for 2 hours the synthesis of the Ru-dopedtitania powders denoted as TiRu2_600, TiRu3_600,TiRu4_600, TiRu5_600, TiRu6_600, TiRu7_600,TiRu8_600, and TiRu9_600 were complete.

2.2. Characterization methodsX-ray diffraction (XRD) patterns were obtained witha Siemens D5005 instrument using Cu-K radiation(40 kV, 30 mA) and a diffracted beam monochromator.Qualitative analysis was performed with the DiffracPlusEva Application (Bruker AXS) using the JCPDS PDF-2database [ 20 ]. For the quantitative phase analysis, meancoherence length analysis, and structural re nement,Rietveld analysis with DiffracPlus Topas (Bruker ASX)and structural models from the ICSD database wereused [21 ]. The sample of TiRu1 was studied by in-situheating in air on a PANalytical XPert PRO diffractometerusing CoK radiation (40 kV, 30 mA) and a multichanneldetector XCelerator with an anti-scatter shield, equippedwith a high temperature chamber (HTK 16, Anton Paar,Graz, Austria). The measurements started at roomtemperature and nished at 1000C.

The surface areas of the samples were determinedfrom nitrogen adsorptiondesorption isotherms atliquid nitrogen temperature using a Coulter SA3100instrument with outgas for 15 minutes at 120C. TheBrunauer-Emmett-Teller (BET) method was used forthe surface area calculation [22 ], and the pore sizedistribution (pore diameter and pore volume of thesamples) was determined by the Barrett-Joyner-Halenda(BJH) method [23 ].

Scanning electron microscopy (SEM) studies wereperformed using a Philips XL30 CP microscope equippedwith an energy-dispersive X-ray (EDX), Robinson, SEand BSE detectors. The sample was placed on anadhesive C slice and coated with 10 nm thick layer of AuPd alloy.

Transmission electron micrographs (TEM andHRTEM) were obtained using two instruments, namely aPhilips EM 201 operated at 80 kV and a JEOL JEM 3010operated at 300 kV (LaB6 cathode). A copper grid coatedwith a holey carbon support lm was used to preparethe samples for TEM observation. A powdered samplewas dispersed in ethanol and then the suspension wastreated in an ultrasonic bath for 10 minutes.

The diffuse re ectance UV/VIS spectra for theevaluation of photophysical properties were recorded inthe diffuse re ectance mode (R) in a wavelength range

of 250 800 nm and transformed to absorption spectrathrough the Kubelka-Munk function [24 ]. A Perkin ElmerLambda 35 spectrometer equipped with a LabsphereRSAPE-20 integration sphere with BaSO4 was used asthe standard. The band gap was obtained from a plot ofmodi ed KubelkaMunk function versus the energy ofexciting light by converting the scanning wavelength ()into photon energies (E bg).

The photocatalytic activity of the samples wasassessed from the kinetics of the photocatalyticdegradation of OII dye in aqueous slurries. Thephotocatalytic degradation of aqueous OII dye solutionwas determined in a stainless steel photoreactor [25 ,26 ].The photoreactor consists of a stainless-steel cover andquartz tube with orescent lamp (365 nm and 400 nm)with 8 W of power. OII dye solution was circulated bymeans of a membrane pump through a ow curvette. Theconcentration of OII dye was determined by measuringthe absorbance at 480 nm with a VIS spectrofotometerColorQuestXE.

3. Results and Discussion3.1. X-Ray Diffraction (XRD)The high temperature XRD diffractogram of the sampleTiRu1 (sample containing 3.83 mol. rutheniumcontent) is shown in Fig. 1 . The anatase phase ispresent in the temperature range of 25 to 1000C andstarts to decline at the temperature of the anataserutiletransition. The transition from the anatase to the rutilemodi cation takes place around 400C, unlike undopedTiO2 which shows the anataserutile transition at 800C[27 ]. The absence of peaks from the ruthenium oxidephase, in case of the samples with lower Ru doping

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levels, may be attributed to their ultra ne dispersion ofTiO2 particles as very small clusters, or due to the verylow dopant content.

Fig. 2 shows the percentage content of each phase(anatase, rutile, RuO2) as a function of the heatingtemperature of the sample TiRu1 (model sample).Up to 600C, the crystallinity of RuO2 increases. Attemperatures > 600C, RuO2 decreases because ofthe inclusion of Ru ions in the TiO2 lattice [12 ]. This ispossibly due to the comparable lattice parameters ofRuO2 and rutile. Table 1 shows the average particlediameters of the TiRu1 sample calculated from XRDpeak broadening. Clearly, the particle sizes of anataseand rutile phases increase with increasing temperature.The crystallite sizes of the annealed samples at 600Care shown in Table 2 . Table 2 clearly shows that thecontent of RuO2 has an in uence on the particle sizeof anatase, which decreases with increasing amountsof RuO2.

3.2. Surface area and porosityTable 2 shows the surface area (BET), the pore radiusand the total volume of pores related to the mass(porosity) of samples with various contents of RuO2. All of the prepared samples displayed a type I isothermwith a desorption hysteresis loop A [28 ]. The nature ofhysteresis is primarily due to the cylindrical pores openat both ends. The result shows that the speci c surfaceareas of the titania samples grow with increasingcontent of RuO2. The pore radius and total pore volumeof samples also increases with increasing content ofRuO2.

The pore distribution was calculated using the BHJmethod (see Fig. 3) and con rms that the Ru-dopedtitania samples are microporous with pore sizes rangingfrom 2 to 4 nm.

Figure 1. XRD pattern of a sample of TiRu1 as a function of temperature (25 - 1000C)

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It is apparent that the product of homogeneousprecipitation of urea and titanium oxo-sulphate consistsof approximately spherical round particles; agglomeratesof diameter ca. 1 to 2 m. Fig.4 shows a representativeSEM micrograph of the doped titania particles. SEMdid not illustrate any signi cant difference betweenmorphological features of the doped and the undopedparticles.

Image processing analysis of HRTEM micrograph isa useful method for re ning microstructures because itpermits the study of grain sizes and grain boundaries moreaccurately. From Fourier transform (FFT) spectroscopyit is possible to determine and index crystallographicplanes and nd the orientations of nanoparticles.Fig. 5 shows HRTEM micrograph of the TiRu3_600sample. From an archived database the values (PDF21-1272, JCPDS PDF2, 2001) of the lattice spacing0.352 nm (Fig. 5d ) corresponds to the [101] diffraction

Table 2. Characteristics of the prepared titania samples doped with variable amounts of RuCl 3.

Sample RuCl 3 [g] Ru content[mol.]

Anataseby XRD

[%]

RuO 2 by XRD

[%]

AnataseCrystallite size

[nm]

Surface areaBET

[m 2 g -1 ]

Poreradius[nm]

Total pore volume[cm 3 g -1 ]

TiRu2_600 0.75 2.878 99.23 0.77 28.4 82.12 33.35 0.134

TiRu3_600 0.5 1.920 99.61 0.39 35.5 50.11 33.07 0.099TiRu4_600 0.25 0.961 99.37 0.63 36.4 55.29 33.22 0.090TiRu5_600 0.1 0.385 99.39 0.61 38.8 62.15 21.77 0.075TiRu6_600 0.05 0.192 99.93 0.07 39.3 50.99 28.32 0.080TiRu7_600 0.01 0.038 100 -* 39.5 54.13 24.61 0.071TiRu8_600 0.005 0.019 100 -* 35.4 57.28 24.51 0.077TiRu9_600 0.001 0.004 100 -* 44.9 47.81 28.64 0.074

* XRD cannot assign such small amount of RuO 2

Figure 3. Pore size distributions of the doped titania samples

(BJH method).

3.3 Scanning Electron Microscopy (SEM), HighResolution Transmission ElectronMicroscopy (HRTEM) and selected areaelectron diffraction (SAED)

Table 1. Crystallite sizes of anatase, rutile and RuO 2 phases insample TiRu1 (determined by XRD).

Heat. Temperature

[C]

AnataseCrystallitesize [nm]

RutileCrystallitesize [nm]

RuO 2 Crystallitesize [nm]

1000 46.6 104 82.5950 39.4 102.8 51.7

900 40.5 96.5 57.5

850 42.6 88.7 63.7

800 55.7 73.9 62.1

750 45.1 57.8 57.2

700 37.1 44.5 53.9

650 28.4 37.1 53.2

600 20.9 37.2 55.4

550 11.7 31.4 53.1

500 8.4 28.4 50.1

400 6.6 27 50

300 5.2 - 50

200 4.6 - -

100 4.5 - -

25 4.5 - -

Figure 2. The dependence of crystal phase evolution of TiRu1 ontemperature.

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line of anatase and (PDF 40-1290, JCPDS PDF2, 2001)the lattice spacing 0.25 nm (Fig.5c ) corresponds to the[101] diffraction line of ruthenium oxide.

The selected area of the SAED electron diffractionpatterns con rms the presence of the anatase and theRuO2 phase (see Fig. 6).

3.4 UV/VIS spectra and band-gap energyThe UV/VIS diffuse re ectance spectroscopy methodwas employed to estimate the band-gap energies ofthe prepared Ru-doped titania samples. To establishthe type of band-to-band transition in these synthesizedparticles, the absorption data were tted to equations fordirect band-gap transitions. The minimum wavelengthrequired to promote an electron depends upon theband-gap energy E bg of the photocatalyst and is givenby Eq. 1 .

E bg = 1240 -1 [eV] (1)

where is the wavelength in nanometers [29 ]. Theintensity of the absorbance in the visible regionincreases with the concentration of the doped Ru ion(see Fig. 7). A red-shift on the absorption edge in VISabsorption spectra is observed and increases withincreasing content of RuO2.

Fig. 8 shows the (Ebg)2 versus E bg for an indirectbandgap transition, where is the absorption coef cientand E bg is the photon energy. The value of Ebg extrapolatedto = 0 gives an absorption energy, which correspondsto a band-gap energy. The value of 3.20 eV for sampledenoted as TIT 300 is reported in the literature for pureanatase nanoparticles [ 30 ]. The band-gap energy isalso dependent on the extent of doping and decreaseswith increasing content of RuO2 (see Table 3).

3.5 Photocatalytic activityThe photocatalytic activity of the prepared sampleswas determined by the degradation of Orange II dyeaqueous solutions under UV irradiation (at 365 nm)and VIS irradiation (over 400 nm). In regions whereLamber-Beer law (2) is signi cant, the concentration ofOrange II dye is proportional to absorbance.

A = c l (2)

where A is the absorbance, c is the concentration ofabsorbing component, l is the length of absorbing

Figure 5. HRTEM micrographs of (a), (b) TiRu3_600, (c) RuO 2phase, (d) anatase phase.

Figure 6. Electron diffraction pattern of TiRu3_600.

Figure 4. SEM micrograph of TiRu4_600

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layer and is the molar absorbing coef cient. The timedependences of Orange II dye decomposition can be

described using Eq. 3 for the rst kinetics reaction [31 ]:

where [OII] is concentration of Orange II dye, a0 is theinitial concentration of Orange II dye and k is the rst

order rate constant. The photocatalytic degradation ofOrange II dye is shown in Table 3 and the kinetics of

the degradation on sample TiRu3_600 and the undopedtitania sample (TIT 300) is shown in Fig.9. Based on theresults in Table 3 we have found that the rate constant kof the mineralization of Orange II dye is the highest forthe sample with 1.920 mol. of ruthenium (TiRu3_600)

Table 3. Band-gap energies and rate constants of the Ru-doped titania samples.

Sample Band-gap energy [eV] Rate constant at 365 nm[min-1]Rate constant at 400 nm

(warm white)[min -1]Rate constant at 400 nm

(bio light) [min -1]

TiRu2_600 2.9 0.0177 0.0067 0.0044

TiRu3_600 3.0 0.0494 0.0074 0.0079TiRu4_600 3.0 0.0277 0.0058 0.0064TiRu5_600 3.0 0.0229 0.0061 0.0049TiRu6_600 3.1 0.0238 0.0081 0.0062TiRu7_600 3.1 0.0233 0.0063 0.0043TiRu8_600 3.15 0.0272 0.0074 0.0063TiRu9_600 3.15 0.0094 0.0049 0.0038

TIT 300 3.2 0.0397 0.0025 0.0033

Figure 9. Photodegradation of Orange II during irradiation at 365 nm, 400 nm (warm white light) and 400 nm(bio light); (a) TiRu3_600, (b) TIT 300.

Figure 7. Absorbance UV/VIS spectra of prepared Ru-doped titaniasamples and undoped titania (TIT 300).

Figure 8. Band-gap energy of Ru-doped titania samples andundoped titania (TIT 300).

d [OII ]dt

= k a 0 [ OII ] (3)

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and with 0.192 mol. of ruthenium (TiRu6_600). Theseresults suggest that doping with Ru ions increases thephotocatalytic activity under visible light (at 400 nm).Furthermore, from Table3 the Ru-doped titania samplesdiffer in photocatalytic activity at 400 nm under warmwhite light (more suitable for sample TiRu6_600) andat 400 nm under Bio light (more suitable for sampleTiRu3_600) from the undoped titania, which haslower activity under warm white light. These resultsare in agreement with the red-shift in the absorbancespectra and with the diminished band-gap energy.

4. ConclusionThis paper describes the synthesis of TiO2 containingvarying amounts of ruthenium oxide by homogeneous

hydrolysis. Ruthenium oxide causes the anatase torutile transformation for ruthenium doped titania to occurat lower temperatures with increased ruthenium doping.Futhermore, Ru-doping results in increasing the titaniasurface area, decreasing of anatase particle size anddiminished band-gap energy.

Doping with Ru ions is accompanied by animprovement in photocatalytic activity using visiblelight. The best degradation rate constant of 0.008 min1 was observed for sample with 1.920 mol. and 0.192mol. content of ruthenium (samples TiRu3_600 andTiRu6_600).

AcknowledgementsThe work was supported by the Academy of Sciencesof the Czech Republic (Project No. AV0Z40320502) andGAR (Project No. 203 334).

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