Photocatalytic activity of TiO2 doped with Zn2 and V5 transition metal ions: Influence of crystallite size and dopant electronic configuration on photocatalytic activity

Materials Science and Engineering B 166 (2010) 1–6 Contents lists available at ScienceDirect Materials Science and Engineering B j our nal homepage: www. el sevi er . com/ l ocat e/ mseb Photocatalytic activity of TiO 2 doped with Zn 2+ and V 5+ transition metal ions: Influence of crystallite size and dopant electronic configuration on photocatalytic activity L. Gomathi Devi ∗ , B. Narasimha Murthy, S. Girish Kumar Department of Post Graduate Studies in Chemistry, Central College City Campus
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  Materials Science and Engineering B 166 (2010) 1–6 Contents lists available at ScienceDirect MaterialsScienceandEngineeringB  journal homepage: Photocatalytic activity of TiO 2  doped with Zn 2+ and V 5+ transition metal ions:Influence of crystallite size and dopant electronic configuration onphotocatalytic activity L. Gomathi Devi ∗ , B. Narasimha Murthy, S. Girish Kumar Department of Post Graduate Studies in Chemistry, Central College City Campus, Dr. Ambedkar Street, Bangalore University, Bangalore 560001, India a r t i c l e i n f o  Article history: Received 6 March 2009Received in revised form 6 August 2009Accepted 5 September 2009 Keywords: V 5+ doped TiO 2 Zn 2+ doped TiO 2 Photocatalytic activity under solar lightCrystallite sizeDopant electronic configurationCongo Red a b s t r a c t AnataseTiO 2  waspreparedbysol–gelmethodthroughthehydrolysisoftitaniumtetrachlorideanddopedwith transition metal ions like V 5+ and Zn 2+ . The photocatalysts were characterized by various analyt-ical techniques. Powder X-ray diffraction studies revealed only anatase phase for the doped samples.The band gap absorption for the doped samples showed red shift to the visible region ( ∼ 456nm) asconfirmed by UV–vis absorption spectroscopy and diffuse reflectance spectral studies. The surface areaof the Zn 2+ doped samples were higher than the V 5+ doped samples as observed by BET surface areameasurements due to their smaller crystallite size. Scanning electron microscopy showed almost sim-ilar morphology, while energy dispersive X-ray analysis confirmed the presence of dopant in the TiO 2 matrix.ThephotocatalyticactivitiesofthesecatalystsweretestedforthedegradationofCongoRedundersolar light. Although both the doped samples showed similar red shift in the band gap, Zn 2+ (0.06at.%)doped TiO 2  showed enhanced activity and its efficiency was five fold higher compared to Degussa P-25TiO 2 . This enhanced activity was attributed to smaller crystallite size and larger surface area. Furthercompletely filled stable electronic configuration (d 10 ) of Zn 2+ shallowly traps the charge carriers anddetraps the same to the surface adsorbed species thereby accelerating the interfacial charge transferprocess. © 2009 Elsevier B.V. All rights reserved. 1. Introduction TheuseofTiO 2  photocatalystforthedegradationoforganicpol-lutant has been studied extensively [1–5]. However the large band gap of TiO 2  requires higher energy artificial UV light for activation.TheeffectivewaytoshifttheabsorptionbandgapofTiO 2  tothevis-ible region of the solar spectrum can be significantly achieved bytransition metal ion doping. Although impressive publications areavailable in this regard [6–10], least attempt is made towards the correlatingthenatureofdopant,oxidationstate,crystallitesizeandits electronic configuration with the photocatalytic activity. In thisview, the present research work focuses on the doping of Zn 2+ (p-type dopant) and V 5+ (n-type dopant) which has completely filledand vacant ‘d’ orbitals respectively into the TiO 2  lattice. The pho-tocatalytic activities were studied under natural solar light for thedegradation of Congo Red (CR) as a probe reaction. The influenceof crystallite size and the dopant electronic configuration on thephotocatalytic activity is explored. ∗ Corresponding author. Tel.: +91 080 22961336; fax: +91 080 22961331. E-mail address:  gomatidevi (L.G. Devi). 2. Materials and methods TiCl 4 wassuppliedfromMerckchemicals.Zincoxalate(ZnC 2 O 4 ),ammoniumvanadate(NH 4 VO 3 ),aqueousNH 3  concentratedH 2 SO 4 and CR were of analytical grade. The molecular formula of CR isC 32 H 22 N 6 O 6 S 2 Na 2  and molecular weight is 697 and has   max  at497nm. The structure of the CR is shown in Fig. 1.  2.1. Catalyst preparation AnataseTiO 2  ispreparedbysolgelmethodthroughthehydrol-ysis of TiCl 4  [11]. 25ml of diluted TiCl 4  with 1ml of concentratedH 2 SO 4  is taken in a beaker and it is further diluted to 1000ml. ThepH of the solution was maintained at 7–8 by adding liquor ammo-nia. The gel obtained was allowed to settle down. The precipitateis washed free of chloride and ammonium ions. The gelatinousprecipitate is filtered and oven dried at 100 ◦ C. The finely groundpowder was then calcined at 550 ◦ C for 4.5h. A known concentra-tion of the metal ion solution was added to the calculated amountofTiO 2 togetthedopantconcentrationintherangeof0.02–0.1at.%.Themixtureisgrindedinamortarandovendriedat120 ◦ Cfor1h.The process of grinding and heating is repeated for four times andthe powder is finally calcined at 550 ◦ C for 4.5h. 0921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2009.09.008  2  L.G. Devi et al. / Materials Science and Engineering B 166 (2010) 1–6 Fig. 1.  Structure of CR dye.  2.2. Characterization of the catalyst  The phase structure of the catalyst were studied by powder X-raydiffraction(PXRD)patternsusingPhilipsPW/1050/70/76X-raydiffractometerwhichwasoperatedat30kVand20mAwithCuK  radiation with a nickel filter at a scan rate of 2 ◦ min − 1 . The Dif-fuse Reflectance Spectra (DRS) of the samples in the wavelengthrange of 190–600nm were obtained by UV–vis spectrophotome-terShimadzu-UV3101PCUV–vis-NIRusingBaSO 4  asthestandardreference. The band gaps were calculated by the Kubelka–Munkmethod. The Fourier transform infra-red (FTIR) spectral analysiswas carried out using KBr discs in the range of 4000–400cm − 1 using Nicollet IMPACT 400 D FTIR spectrometer. The specificsurface area of the samples were determined by Digisorb 2006surface area Nova Quanta Chrome corporation instrument multi-point BET adsorption system. Surface morphology was analyzedby SEM analysis using JSM 840 microscope operating at 25kVon specimen upon which a thin layer of gold had been evapo-rated and an electron microprobe was used in the EDX mode toobtain quantitative information of the metal ions in the TiO 2  lat-tice.  2.3. Photocatalytic degradation procedure The photocatalysis using solar light was performed between 11a.m. and 2 p.m. during the summer season (May–June) in Banga-lore, India. The solar intensity in this period was maximal. Thelatitude and longitude are 12.58N and 77.38E respectively. Theaverage intensity of sunlight was around 1200Wcm − 2 . The inten-sity of solar light was concentrated using a convex lens and thereaction mixture was exposed to this concentrated solar light.In a typical experiment, 250ml of 10ppm CR solution is takenalong with 100mg of the photocatalyst and stirred in dark for Fig. 2.  PXRD pattern of photocatalysts. (A) TiO 2 , (B) Zn 2+ (0.1at.%)-TiO 2 , (C) V 5+ (0.1at.%)-TiO 2 . 15min to ensure equilibrium adsorption of the CR on catalystsurface. The degradation was followed by UV–vis spectroscopyusing Shimadzu UV-1700 pharmaspec UV–vis spectrophotome-ter. 3. Results and discussion PXRD pattern of TiO 2 , Zn 2+ doped TiO 2  (Zn 2+ -TiO 2 ) and V 5+ doped TiO 2  (V 5+ -TiO 2 ) is shown in Fig. 2. Both the doped cata- lysts showed only anatase phase, which suggests that both thedopants stabilised anatase phase irresepective of their nature, oxi-dationstateandelectronicconfiguration.Thecrystallitesizesofthe  Table 1 Detailed characterization of TiO 2 , Zn 2+ -TiO 2  and V 5+ -TiO 2  samples.Photocatalyst a 2    of crystal plane(101) of anataseLattice parameters (Å) Unit cellvolume (Å) 3 Crystallitesize (nm)Surface area(m 2 /g)Band gapin eV  b (nm)TiO 2  25.32  a = b =3.7828 135.97 26.2 18 3.2 380(0.00%)  c  =9.5023Zn 2+ -TiO 2  25.34  a = b =3.7832 136.14 23.6 24 2.9 420(0.02%)  c  =9.5123Zn 2+ -TiO 2  25.37  a = b =3.7824 136.35 19.8 30 2.7 456(0.06%)  c  =9.5311Zn 2+ -TiO 2  25.39  a = b =3.7820 135.75 15.6 31 2.8 442(0.10%)  c  =9.5009V 5+ -TiO 2  25.34  a = b =3.7822 136.05 26.2 23 2.9 424(0.02%)  c  =9.5113V 5+ -TiO 2  25.36  a = b =3.7824 136.10 26.3 26 2.7 456(0.06%)  c  =9.5136V 5+ -TiO 2  25.33  a = b =3.7825 136.13 28.4 24 2.8 446(0.10%)  c  =9.5152 a The dopant concentration is in at.%. b Band gap absorption of the photocatalyst estimated by UV–vis absorption spectroscopy.  L.G. Devi et al. / Materials Science and Engineering B 166 (2010) 1–6 3 samples were calculated using Scherrer’s equation D = kˇ  cos    (1)where  k  is the constant (shape factor, about 0.9),    is the X-raywavelength (0.15418nm),  ˇ  is the full width at half maximum(FWHM) of the diffraction line and     is the diffraction angle. Thevaluesof  ˇ and   aretakenforcrystalplane(101)ofanatasephase.It is well known that the anatase phase is thermodynamicallystableforsmallercrystallites[12].ThecrystallitesizesforZn 2+ -TiO 2 samplesdecreasedsignificantlywithincreaseindopantconcentra-tion,whileV 5+ -TiO 2  samplesshowedalmostsimilarcrystallitesizecompared to undoped TiO 2  (Table 1). Hence it can be concluded thatZn 2+ dopantstabilisedtheanatasephasemainlybydecreasingthe anatase grain growth. While the higher valent vanadium iondecreases the concentration of oxygen vacancies for charge com-pensation preventing the nucleation necessary for rutile growth[9,13].Itisreportedthatinthecaseofmetaloxidesthereiscriticalvalue of dispersion capacity, at values lower than which the oxidemightbecomehighlydispersedonthesupportwithouttheforma-tion of a separate crystalline phase. Since, no characteristic peakcorresponding to Zn and V species are present, it can be concludedthat the metal ion doping is below the dispersion capacity.The diffraction peaks of crystal planes (101), (200) and (004)inanatasephaseareselectedtodeterminelatticeparametersofthesamples.Thechangesinlatticeparameterondopingwerereflectedin the slight elongation of ‘ c  ’ axis while ‘ a ’ and ‘ b ’ remained almostconstant. Since only ‘ c  ’ parameter was changing marginally, it canbeconcludedthatthedopantsoccupiedbccandfccpositionsintheanatase structure [14].Itiswellknownthatunderthestressorstraineffects,thediffrac-tion peaks get displaced and broadened. In the observed powderpatterns,the d -spacingforthemostintensereflection(100%)of2   value25.32wasobservedtobe3.5096forundopedTiO 2 .Ondoping,this  d -spacing shows a slight shift to 3.5148. Since the displace-mentwasnotquitepronounced,weassumethatstraininducedbythe dopant is too minimal and our calculation of crystallite size byScherrer’s line broadening can be justified.UVabsorptionspectralstudiesindicatedthebandgapextensionofthedopedsamplestovisibleregion(Fig.3).Thisisduetothecre- ationofimpuritylevelsbythedopantwithinthebandgapstatesof TiO 2 .ThemidbandgapstatescalculatedbyKubelka–Munkplotforboth Zn 2+ and V 5+ doped sample was found to be 2.7eV as shownin Fig. 4, due to the similarity in the electronegativity of both the dopants [15]. The extent of red shift increased when dopant con- centrationwasincreasedfrom0.02to0.06at.%andaslightdecreasewas observed for higher dopant concentration (0.1at.%). Energydispersive X-ray analysis confirmed the presence of dopant in thesamples (Table 2) while scanning electron microscopy showed almost similar morphology for all the doped samples (Fig. 5). BET analysis showed increase in the surface area for the doped sam-plescomparedtoundopedsampleduetointroductionofadditionalnucleation sites by the dopant. The surface area for the Zn 2+ dopedsamples was higher compared to V 5+ doped samples due to theirsmaller crystallite size (Table 1).  3.1. FT-IR analysis The following conclusions were drawn from FT-IR analysis:1. TiO 2  showsstrongabsorptionbandsat484and563cm − 1 whichcan be assigned to Ti–O bond in the TiO 2  lattice [16].2. The bands at 3432cm − 1 and 1655cm − 1 can be attributed tothe vibrations of surface adsorbed water molecules and Ti–OHbonding [17]. Fig. 3.  UV–vis absorption spectra of the various photocatalysts. (A) TiO 2 , (B) V 5+ (0.02at.%)-TiO 2 , (C) V 5+ (0.06at.%)-TiO 2 , (D) Zn 2+ (0.06at.%)-TiO 2 . Fig. 4.  Kubelka–Munk plot of   X   versus wavelength for V 5+ /Zn 2+ (0.06at.%)-TiO 2 ;where  X  =(1 − R ∞ ) 2 /2 R ∞ . The inert plot shows the DRS of catalyst.  Table 2 EDX data for undoped TiO 2  and M-TiO 2  (M=V 5+ and Zn 2+ ).Dopant concentration V 5+ -TiO 2  Zn 2+ -TiO 2 Ti V Ti Zn0.02at.% 97.92 2.08 97.94 2.060.06at.% 93.98 6.02 93.94 6.060.10at.% 90.05 9.95 90.04 9.96  4  L.G. Devi et al. / Materials Science and Engineering B 166 (2010) 1–6 Fig. 5.  SEM images of the photocatalysts. 3. The higher charge of V 5+ compared to Ti 4+ additionally attractsone hydroxyl group for charge compensation which results inhigher concentration of surface adsorbed hydroxyl groups andwatermoleculescomparedtoundopedTiO 2  [18,19].Zn 2+ havinglower charge than the host Ti 4+ ion, reduction in the concen-tration of hydroxyl groups were observed (Fig. 6). However the bands were highly broadened for Zn 2+ doped samples due totheir smaller crystallite size compared to V 5+ doped samples[17]. 4. Photocatalytic activity  It is well known that the amount of adsorption of the substrateand the number of active sites on the catalyst surface is crucialfor efficient degradation [20]. The percentage of CR adsorption on Fig. 6.  FT-IR spectra of photocatalysts. (A) V 5+ (0.06at.%)-TiO 2 , (B) TiO 2 , (C) Zn 2+ (0.1at.%)-TiO 2 . the catalyst surface was calculated by comparing its concentrationbefore and after stirring, using the formula C  0 − C C  0 × 100 (2)where  C  0  and  C   are the initial and residual concentration of CR respectively. The decreasing adsorption capacity of the catalystsis of the order: Zn 2+ (0.1at.%)-TiO 2 >Zn 2+ (0.06at.%)-TiO 2 >V 5+ (0.06at.%)-TiO 2 >Zn 2+ (0.02at.%)-TiO 2 >V 5+ (0.02at.%)-TiO 2 >V 5+ (0.1at.%)-TiO 2 >TiO 2 .Allthedopedsamplesshowedstrongadsorp-tion capacities compared to undoped TiO 2 . The photocatalyticdegradation reaction of CR was carried out under natural solarlight. The doped catalysts showed better activity compared toundoped TiO 2  due to its absorption ability in the visible region.Zn 2+ (0.06at.%)-TiO 2  showed enhanced activity compared to allthe other samples and its efficiency was almost five fold higherthantheDegussaP-25TiO 2  (Table3).Thoughboththedopedsam- ples showed maximum red shift in the band gap to an extent of  ∼ 456nm,theenhancedactivityofZn 2+ -TiO 2  comparedtoV 5+ -TiO 2 samples is mainly attributed to its smaller crystallite size and alsoto the stable electronic configuration of Zn 2+ .It is well known that the photogenerated hole reacts with sur-facewatertoproducehydroxylradicalswhichisapowerfuloxidantfor the degradation of organic pollutants. This reaction competeswith the electron–hole recombination reaction.TiO 2 + h →  e − + h + (3)h + + H 2 O ads →  OH ã ads  (4)e − + h + → recombination (5)Thus higher activity in the case of Zn 2+ (0.06at.%)-TiO 2  can beattributed to the generation of excess hydroxyl radicals due to theprolonged separation of charge carriers. At this optimum concen-tration (0.06at.%) of Zn 2+ , the surface barrier becomes higher andthespacechargeregiongetsextended,leadingtotheefficientsep-
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