Journal of Physics and Chemistry of Solids 74 (2013) 1026–1031

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Journal of Physics and Chemistry of Solids

0022-36

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n Corr

E-m

journal homepage: www.elsevier.com/locate/jpcs

Sol–gel synthesis and photocatalytic activity of B and Zr co-doped TiO2

Derya Kapusuz a,n, Jongee Park b, Abdullah Ozturk a

a Department of Metallurgical and Materials Engineering, METU, 06800 Ankara, Turkeyb Department of Metallurgical and Materials Engineering, Atilim University, 06836 Ankara, Turkey

a r t i c l e i n f o

Article history:

Received 10 August 2012

Received in revised form

22 February 2013

Accepted 26 February 2013Available online 6 March 2013

Keywords:

A. Oxides

B. Sol-gel growth

97/$ - see front matter & 2013 Elsevier Ltd. A

x.doi.org/10.1016/j.jpcs.2013.02.022

esponding author. Tel.: þ90 312 210 59 23; f

ail addresses: dkapusuz@metu.edu.tr, dkapus

a b s t r a c t

Effects of boron (B) and/or zirconium (Zr) doping on photocatalytic activity of sol–gel derived titania

(TiO2) powders were investigated. A conventional, non-hydrous sol–gel technique was applied to

synthesize the B, Zr doped/co-doped TiO2 powders. Doping was made at molar ratios of Ti/B¼1 and

Ti/Zr¼10. Sol–gel derived xero-gels were calcined at 500 1C for 3 h. The crystal chemistry and the

morphology of the undoped and B, Zr doped/co-doped TiO2 nanoparticles were investigated using X-ray

diffractometer and scanning electron microscope. Nano-scale (9–46 nm) TiO2 crystallites were obtained

after calcination. Doping and co-doping decreased the crystallite size. Photocatalytic activity was

measured through the degradation of methylene blue (MB) under 1 h UV-irradiation using a UV–vis

spectrophotometer. Results revealed that B doping into anatase caused the formation of oxygen

vacancies, whereas Zr addition caused Ti substitution. Both B and Zr ions had a profound effect on the

particle morphology and photocatalytic activity of TiO2. The photocatalytic activity of B and Zr doped

TiO2 particles increased from 27% to 77% and 57%, respectively. The best activity (88.5%) was achieved

by co-doping.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Recently, TiO2 nanoparticles have attracted great interest forthe degradation of organic and inorganic pollutants and toxics inenvironmental purification owing to their high efficiency, lowcost, and long term stability upon commercial use [1,2]. Manyinvestigations have been performed to improve the photocatalyticproperties of TiO2 since the discovery of photocatalyst TiO2 byFujishima and Honda in 1972 [2]. Anatase phase of TiO2 exhibitsbetter chemical and photon characteristics due to its goodabsorbability and lower electron–hole recombination rate thanthose of rutile [3]. However, its large band gap (3–3.2 eV) limitsthe light interaction only to ultraviolet (UV) light. This accountsfor only 5% of solar energy [4–6]. Thus, many studies have beenperformed to extend the spectral response of anatase to visiblelight and to enhance its photocatalytic activity. Doping and co-doping with metals and non-metals have been shown to beamong the most effective strategies to improve the photocatalyticperformance of TiO2 [7–12]. Asahi et al. [5] studied the effectof N doping into TiO2 and achieved longer wavelength photo-absorption than 400 nm. In this respect, B–N co-doping has beenfound to be one of the most efficient ways of increasing photo-catalytic activity in visible region [13].

ll rights reserved.

ax: þ90 312 210 25 18.

uz@live.com (D. Kapusuz).

B doping is of interest because it shifts the light absorption tovisible range [7]. On the other hand, the doping of metal atomspossibly causes the formation of new phases dispersed into TiO2,temporarily trapping the photogenerated charge carriers andinhibiting the recombination of photoinduced electron–hole pairswhen the electron–hole pairs migrate from the inside of thephotocatalyst to the surface [14]. Since each method has differentadvantages, novel attempts include the investigation of optimumcompositions of co-doping into TiO2 to earn from their synergeticeffects. Unfortunately, especially for B doped TiO2, conflictingresults have been reported on structural evolution of TiO2 inliterature. Geng et al. [15] stated that B atoms can be added intoTiO2 lattice either as interstitial atoms or at the O sites. Thissubstitution at O sites causes a decrease in the band gap.Conversely, Chen et al. [16] stated that B atoms were interstitiallypresent in the lattice forming a Ti–B–O structure.

Non-metal doping like B tends to increase the photocatalyticactivity to visible region. However, the behavior of B atoms inTiO2 lattice is still vague. In addition, metal dopings such asGa3þ , Cr3þ , Sb5þ , and V5þ were reported to reduce the photo-catalytic activity since both trivalent and pentavalent ions act asrecombination centers for photogenerated charge carriers [17].However, Zr doping may enhance the photocatalytic efficiency ascompared to undoped TiO2. TiO2 and ZrO2 both belong to thesame group, 4A elements, and both oxides are n-type semicon-ductors [18]. It is envisaged that Zr doping causes O defects and/or Ti4þ to Zr4þ exchange and hence enhances the photoactivity.

D. Kapusuz et al. / Journal of Physics and Chemistry of Solids 74 (2013) 1026–1031 1027

This work was undertaken to investigate the structural andfunctional evolution with B and/or Zr doping to TiO2, andinvestigate the possible increase in photocatalytic activity. Effectsof B–Zr co-doping on the properties of TiO2 have never been thesubject of an earlier investigation. The central feature of ourapproach is the use of sol–gel technique followed by calcinationfor the doping of Zr ion as a transition metal rather than using anexpensive ion implantation technique. Also, the effects of B, Zrdoping/co-doping on particle morphology and photocatalyticactivity were investigated.

Fig. 1. XRD patterns of (a) undoped, (b) B doped, (c) Zr doped, and (d) B–Zr

co-doped TiO2 powders.

2. Experimental

2.1. Synthesis of undoped and doped TiO2 nanoparticles

Undoped-TiO2 forming solution was prepared via continuousstirring of 0.1 mol of titanium ethoxide (Ti(OC2H5)4, Merck)dissolved in 40 ml EtOH (C2H5OH, 99.9%, Fluka) for 30 min atroom temperature (RT). Aqueous catalyst solutions were avoidedand thus the powders were aged at RT for 8 days for completegelation. After drying at 100 1C for 3 days, the powder wascalcined at 500 1C for 3 h. B and Zr ion doping processes wereperformed upon the formation of TiO2 network by hydrolysis ofthe ethoxide precursor. 0.1 mol of boric acid (H3BO3, Merck) and0.01 mol of zirconium acetylacetonate (Zr(C5H7O2)4, Dong SanChemical Co. Ltd) were used as the B and Zr doping precursors,respectively. In order to prevent lattice distortion by Zr ions dueto higher atomic radii, the amount of doping was kept lowerthan those of Ti and B. As a co-doping strategy, the same molaramounts of B and Zr doping were used. Dopant precursorswere dissolved in 20 ml EtOH. 10 ml of glacial acetic acid (GAA,CH3COOH, Merck) was added for each doping process. GAA wasused to increase crystallization. For the co-doping, Zr doping wasmade for 10 min after B doping under continuous stirring at RT.The aging time was a minimum of 12 h with the increased dopingamount in co-doped powders. After aging, drying and calcinationprocedures were similar to those of undoped-TiO2 particles.

2.2. Characterization of undoped and doped TiO2 nanoparticles

X-ray diffraction (XRD) patterns of dried and calcined powderswere recorded by a Rigaku Model D/MAK/B diffractometer usingCu-Ka radiation with a constant scan rate of 0.021 between 201and 801. XRD patterns were analyzed by using Rigaku 4.2 program.The morphology of the powders was studied using a scanningelectron microscope (SEM, Nova NANOSEM 430) at about 10–15 kVoperating voltage. Chemical compositions were investigated byusing an EDS detector.

2.3. Photo-degradation measurements

The photocatalytic activity of B, Zr doped/co-doped TiO2 wasevaluated by degrading aqueous methylene blue (MB) solutionunder UV light irradiation. The testing procedure was based onthe study of Park [19]. The MB degradation test was performed ina vessel surrounded by a closed container preventing daylight topass through. A 100 W UV lamp (UVP Co., South Korea) was usedas the light source. Testing solutions were prepared by adding0.3 g of undoped, doped, and co-doped TiO2 powders to a 300 mlaqueous MB solution with an initial concentration of 10 mg/ml.Before the test, all powders containing MB solutions were magne-tically stirred in dark for 30 min to establish the absorption–desorption equilibrium. Then, the solutions were irradiated underUV light with constant stirring rate of 500 rpm. After 60 minirradiation, 3 ml of supernatants was taken from the suspension

by a syringe filter unit (Millipore, pore size 0.22 mm) to scan theUV–vis absorption spectrum. The UV–vis absorption spectra ofthe powders was measured between 200 and 800 nm, using aUV–vis spectrophotometer (Scinco S-3100). The changes of absor-bance intensity at 664 nm under UV irradiation were calculatedwith respect to time.

3. Results and discussion

3.1. XRD analyses

Fig. 1 shows the XRD patterns of undoped, B doped, Zr dopedand co-doped TiO2 powders calcined at 500 1C. XRD patternsreveal that anatase phase was the dominant phase and no otherdopant related phases were present at all powder compositions.The formation of a single phase, anatase, points out an effectivedoping and co-doping strategy. Therefore, sol–gel method seemsto be a better and easier alternative for the doping of transitionmetal ions such as Zr, since doping requires no expensive ionimplantation technique [20].

Anatase(A)-to-rutile(R) transformation temperature range isaround 400–600 1C [21]. It is clear that rutile formation wasremarkably suppressed by B, Zr doping/co-doping by using sol–gel synthesis followed by the calcination at 500 1C. XRD patternssuggest that Zr doping is more effective than B doping insuppressing the rutile formation. This cause is attributed to thefact that Zr acts as a stabilizing agent in anatase with a valence of4þ . Furthermore, doping of anatase with cations with valencessmaller than 4þ provides a charge countervailing by formingvacancies. This amplifies the ionic transport in anatase andaccelerates the anatase-to-rutile transformation. On the contrary,the ions with valence higher than 4þ hinder the ionic transportby forming Ti3þ cations and suppress the transformation [22].Specifically, Kingery et al. [23] linked the inhibitory effect of Zr tothe increased strain energy by the Ti4þ substitution in anataselattice. From the free energy perspective, first the high strainenergy must be overcome to initiate the phase transformation.Thus, the rutile transformation was inhibited to higher tempera-tures by doping. In addition, formation of Ti–Zr–O bonds inhibitthe titanium mobility [24,25].

On the other hand, diffraction peaks related to (103) planewere broadened by doping and disappeared by co-doping dueto the change in the number of dangling bonds in the chemicalstructure of anatase. It has been previously shown that the

Table 1Lattice constants of undoped, doped and co-doped TiO2 powders.

Powder a (nm) c (nm)

Undoped TiO2 0.37827 0.95141

B doped TiO2 0.37789 0.94919

Zr doped TiO2 0.37836 0.95643

Co-doped TiO2 0.37956 0.95168

Table 2Crystallite size of B, Zr doped/co-doped TiO2 powders.

Powder Crystallite size (nm)

Undoped TiO2 31715

B doped TiO2 1879

Zr doped TiO2 23711

Co-doped TiO2 26717

D. Kapusuz et al. / Journal of Physics and Chemistry of Solids 74 (2013) 1026–10311028

decrease in the density of dangling bonds stabilizes the surfaceenergy of anatase [26]. In this manner, (101) plane was stated tobe the thermodynamically most stable surface in anatase. It wasalso expressed in terms of electron affinity energy capacity ofobtaining electrons from the adsorbate that (103) plane has thehighest energy [26]. Since significant amounts of any other B or Zrrelated phases were not identified in XRD detection limits, itcould be deduced that B and Zr ions were successively dopedand co-doped in the anatase lattice. The crystal lattice constantsof the powders were calculated in order to investigate theincorporation of the doping ions in anatase lattice. Unit cells

program (Thomas and Redfern, 1995) was used for the calculationof lattice constants using the XRD patterns after Ka-2 eliminationby Rigaku 4.2 program. The most intense three peaks relatedto (101), (200) and (004) planes were used in calculation. Theresultant lattice constants are shown in Table 1.

The calculated lattice constants of the undoped TiO2 wereconsistent with those given in Rigaku database (JCPDS file no: 21-1272, a¼0.37852 nm, c¼0.95139 nm). Thus, the change in ‘‘c’’parameter could be taken as a sign of the effective doping and co-doping processes. Zr doping was reported to prevent electron–hole recombination by trapping them [27]. Zr is a deep energylevel, isoelectric impurity in TiO2 [8]. Since the ionic radii of Zr4þ

(72 pm) and Zr3þ (89 pm) ions are higher than that of Ti4þ

(61 pm) [28,29], the TiO2 lattice is expected to expand and com-pensate the lattice strain by forming defects [30]. The increase in‘‘c’’ parameter by Zr doping has ruled out the expansion of anataselattice by Zr4þ incorporation. Since the rate of condensationbetween Zr species is lower than that of Ti species, ZrO2 networkcould not be formed initially by the sol–gel process. Thus, the Zrspecies remain as dopants in anatase [31].

B doping, on the other hand, influenced the crystal structure ina different way. The ‘‘c’’ parameter of anatase decreased with Bdoping whereas the ‘‘a’’ remained almost unchanged. It has beenreported that B can act as both an interstitial and substitutionaldefect in anatase microstructure [15]. Due to the large differencein ionic radii between B3þ (23 pm) and Ti4þ (61 pm), it is difficultfor B to substitute Ti. On the contrary, the presence of B ions asinterstitial defects would cause an increase in the ‘‘c’’ parameter[32]. However, B doping caused a decrease in ‘‘c’’ parameter of theanatase suggesting that B ions do not substitute Ti but leave thelattice and form diboron trioxide (B2O3). The absence of B2O3

peaks in XRD patterns is caused by its amount being underXRD detection limits [32]. For the co-doped TiO2 powder, thelattice distortion was significantly balanced by the compensationbetween the opposite actions induced by B–Zr co-doping.

In addition to phase and crystallographic analyses, the averagecrystal size of the powders was also calculated by using theDebye–Scherer equation based on the three strongest peaksof anatase. The results are shown in Table 2. It is evidentthat doping of anatase with B and Zr strongly affected thecrystallite size. On one hand, photocatalytic activity is maximizedby the area of catalytic material surface. On the other hand, thesurface charges being closer, the increase in the surface areaincreases the electron–hole recombination during the photocatalytic

reactions [21]. Thus, due to the opposing effects of surface areaand the recombination, there must be an optimum for thecrystallite size. Wang et al. [33] stated the optimum size as11 nm for the decomposition of chloroform in water, whereas itwas found by Maira et al. [34] as 7 nm for the gas phase photo-oxidation of trichloroethylene. As Almquist and Biswas [21]stated, since the photo-irradiated dyes differ from each other inchemistry and mechanism, the effective crystal size of anataseshould be less than 30 nm.

Doping and co-doping of B and Zr ions decreased the crystallitesize of anatase. The influence of B ions on anatase crystallite sizewas much more effective than that of Zr ions. All doped powdersshowed a crystallite size distribution around 20 nm. The onlypowder whose crystallite size was completely lower than 30 nmwas the B doped one.

The broadest distribution was achieved by the co-doping of Band Zr ions. Both B and Zr ions were mentioned to suppress thecrystal growth compared to undoped anatase [31,32]. However,increased amount of Zr ions leads to an expanded anatase lattice[18]. Therefore, from these findings it is deduced that the effect ofZr ions on decreasing the crystallite size was lower than that of Bions. This caused a broad size distribution of anatase crystals.

3.2. SEM analysis

Fig. 2 shows the typical field emission SEM images of undopedand B, Zr doped/co-doped TiO2 particles. In general, the morphol-ogies of the powders are quite similar. All of the TiO2 powderscalcined at 500 1C exhibited spherical particles with quiterounded edges. However, a detail look into the images can revealthe aggregation of crystallites. As Penn et al. [35] stated, theaggregation is a considerable mode of anatase crystal growth inall types of powders. The least crystal aggregation was observedin the B doped TiO2. Oxygen vacancies leading to the smallamount of B2O3 formation stabilize the surface charges of the Bdoped anatase particles. This provides a better dispersion and lessaggregation of the crystallites.

In addition to SEM analysis, EDS analysis was performed onpowders in order to investigate the chemical structure. Theanalyses revealed the existence of Ti and O as the main elements.The atomic ratio of Ti:O was calculated by taking the meanaverage of 5 measurements. For the undoped TiO2, the Ti:O ratiowas 0.33/0.67 giving a value of 0.49. The other values were 2.94,0.47 and 0.37 for B doped, Zr doped, and co-doped TiO2,respectively. B and Zr peaks were also observed in the EDSspectrum, however, it is more convenient to talk about Ti:Oatomic ratio rather than giving the atomic percentage of lightelements like B. Deviations in Ti:O ratio from 0.49 suggest thedomination of oxygen vacancy formation in case of B doping andthe domination of Ti substitution in case of Zr doping. For the co-doped powder, both effects were observed. The substitution of Tiby Zr dominated upon the formation of oxygen vacancies whichwere formed by B doping. Therefore, the Ti:O ratio decreasedfurther. The morphological and chemical analyses results areconsistent with the lattice analysis results obtained by XRD.

Fig. 2. FE-SEM images of (a) undoped, (b) B doped, (c) Zr doped, and (d) B–Zr co-doped TiO2 powders.

Fig. 3. MB degradation of undoped, doped and B–Zr co-doped TiO2 in 90 min.

D. Kapusuz et al. / Journal of Physics and Chemistry of Solids 74 (2013) 1026–1031 1029

3.3. Photo-activity

Fig. 3 displays the methylene blue (MB) degradation behaviorof doped and co-doped TiO2 powders in comparison to undopedTiO2 after 30 min of dark condition and 60 min of UV irradiationwith a total testing time of 90 min. It is worth mentioning that thecritical fact was to compare the intensity and rate of photo-activity in a fixed reaction time that would be short enoughfor the possible commercial use of the final products rather thancomparing the time needed for total degradation. Thus, the60 min irradiation was chosen in analyses.

The characteristic UV–vis absorption bands of MB solutions at664 nm decreased in intensity after doping/co-doping. MB degra-dation is expressed in terms of C/C0% where ‘‘C’’ corresponds to

the difference in concentration (Ct�C0) at reaction time ‘‘t’’ and‘‘C0’’ shows the concentration at t¼0. It is well known that MB is ahighly photosensitive molecule that can gradually degrade itselfunder light irradiation. However, this value is as small as 7% in100 min that it was neglected for the current 60 min analyses[36]. It was apparent that doping of TiO2 was highly effective ondegradation of MB dye since the undoped TiO2 had very lowphotocatalytic activity (27%). B–Zr co-doped powder representedthe highest activity with 88.5% and it was followed by the Bdoped TiO2 with 77%. Zr doped TiO2 exhibited an intermediatevalue of 57% between undoped and B doped powders.

Many factors were reported to influence photocatalytic activ-ity. All of them are closely related to each other. Kao et al. [37]reported that the most remarkable ones are crystallite size andthe phase assembly. Since the anatase phase formation is domi-nant for all powders, the activity is mostly related to the crystal-lite size and the microstructure in a chemical aspect that wouldlead to different rates of electron–hole recombination on the TiO2

surfaces.The increased photocatalytic performance of doped TiO2 pow-

ders can be explained in terms of two main aspects which are (i)the formation of discrete energy levels above valence band (VB) orbelow conduction band (CB) by doping and (ii) the formation ofoxygen vacancies. Zr ions with unoccupied 4d states above thelowest level of CB of TiO2 improved photocatalytic activity [38].In addition, formation of oxygen vacancies as found in B dopedTiO2 powders, namely ‘‘electron traps’’, avoided recombination ofphotogenerated charge carriers by leaving the holes free to movethrough the surface. Besides, as mentioned earlier, the smaller thecrystallite size, the higher the photocatalytic activity owing to thedecreased distance for the charge carriers to reach the surfacewithout recombination. Additionally, B doping in TiO2 may cause

Fig. 4. Apparent rate constant (kt) of MB degradation reaction in 90 min.

D. Kapusuz et al. / Journal of Physics and Chemistry of Solids 74 (2013) 1026–10311030

formation of some Ti3þ ions facilitating the separation of recom-bined electron–hole pairs [36]. Thus, B addition was much moreeffective than Zr doping on photocatalytic activity. For the co-doping, it can be suggested that the apparently higher photo-catalytic activity was caused mainly by the synergetic effects ofthe decreased crystallite size by B doping and the increased latticedistortion by Zr doping.

In addition, the apparent rates of photocatalytic degradationin 90 min were also calculated according to the Langmuir–Hinselwood (LH) model [39]

� ln(C/C0)¼ktt (1)

where kt stands for the apparent rate of degradation (min�1 ) at reaction

time ‘‘t’’ according to the first order kinetics. The apparent rateconstant of MB degradation reaction for the powders are shown inFig. 4.

The photo-activity results, shown in Figs. 3 and 4, clarify thedifference between undoped and doped/co-doped powders interms of the MB degradation rate. As is obvious, the reaction rateis low for all powders except for the B doped TiO2 without UVirradiation. The small crystallite size of B doped powders, 18 nm,results in a high surface area and interconnected pores betweenthe particles. The agglomeration level is relatively low and theinteraction between the particles is easier. By the irradiationof UV light for 60 min, the rate constant increases instantaneouslyin the co-doped powders but lower slopes were observed forundoped, B doped and Zr doped TiO2. This result accounts forthe synergetic effects of B and Zr ions on the electron–holerecombination rate. The lowest crystallite size and oxygen vacan-cies formed by B doping decrease the recombination rate by theformation of electron traps and by leading the holes to the surfacefreely. Additionally, the defects formed by Zr doping prevent therecombination of the electron–hole pairs which increases reactionrate (kt) instantaneously for the co-doped powder.

4. Conclusions

B and Zr ions could be doped and co-doped successively inanatase TiO2 lattice by using sol–gel synthesis method followedby calcination. The co-doping of B and Zr ions increased thephotocatalytic performance to a higher level, from each dopanteffect on crystal structure.

Photocatalytic activity is affected mostly by the crystallite sizeand the formation of oxygen vacancies in anatase lattice. B dopingfacilitated the formation of oxygen vacancies and decreased thecrystallite size to the lowest value of 18 nm which caused anincrease in the surface area and improvement in photocatalytic

activity. When doped solely, the photocatalytic activity washigher than that of Zr doping. Zr doping also increased thephotocatalytic activity by acting as a stabilizing agent in TiO2

lattice. It increased the lattice strain to a harmless level avoidingunbalanced charge states and created Ti4þ defects. This providedZr ions to create additional energy states in the band gap, causingan increase in photocatalytic activity. For the co-doped powderthe activity was 88.5% with an average crystallite size of 26 nmand a broader size distribution.

Acknowledgments

Authors gratefully acknowledge the partial financial supportprovided by National Institute of Boron Research (BOREN—

Project no. 2010.C- 0275.) of Turkey.

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