Materials Research Bulletin 51 (2014) 326–331

Tuning the morphology, stability and photocatalytic activity of TiO2

nanocrystal colloids by tungsten doping

Haiping Xu a, Jianhua Liao a,b, Shuai Yuan a,*, Yin Zhao a, Meihong Zhang a, Zhuyi Wang a,Liyi Shi a,*a Research Center of Nanoscience and Nanotechnology, Shanghai University, 99 Shangda Road, Shanghai 200444, Chinab School of Pharmaceutical Sciences, Gannan Medical University, Ganzhou, Jiangxi 341000, China

A R T I C L E I N F O

Article history:

Received 4 August 2013

Received in revised form 3 November 2013

Accepted 24 December 2013

Available online 29 December 2013

Keywords:

Semiconductors

TiO2 nanocrystal

Catalytic properties

A B S T R A C T

The effects of tungsten doping on the morphology, stability and photocatalytic activity of TiO2

nanocrystal colloids were investigated. The nanostructure, chemical state of Ti, W, O, and the properties

of tungsten doped TiO2 samples were investigated carefully by TEM, XRD, XPS, UV–vis, PL and

photocatalytic degradation experiments. And the structure–activity relationship was discussed

according to the analysis and measurement results. The analysis results reveal that the morphology,

zeta potential and photocatalytic activity of TiO2 nanocrystals can be easily tuned by changing the

tungsten doping concentration. The tungsten doped TiO2 colloid combines the characters of high

dispersity and high photocatalytic activity.

� 2013 Elsevier Ltd. All rights reserved.

Contents lists available at ScienceDirect

Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

In the past several decades, the efforts of scientists andtechnicians all over the world have realized the application ofphotocatalytic technology in air and water purification. It has beenconfirmed that almost all of organic pollutants dissolved in watercan be completely mineralized into carbon dioxide, water andmineral acids by photocatalysts [1,2]. TiO2 nanomaterials withwide bandgap are considered to be the most potential photo-catalysts due to the unique size effect, surface effect, quantumeffect, chemical stability and excellent photocatalytic activity.

During practical applications, both the photocatalytic efficiencyand reusability should be considered. According to these require-ments, the immobilized photocatalysts, such as TiO2 filmssupported on other substrates, are conveniently for reuse [3,4].However, the efficiency is usually restricted significantly by thelimited catalyst surface area and mass transfer rate. In contrast,TiO2 nanoparticles usually show high photocatalytic efficiency dueto the large surface area. However, it is very difficult for thepowder-type photocatalysts to be separated from the purifiedwater and reused. Although magnetic photocatalysts can berecovered easily from the suspension after treatment [5,6], thecomplicated producing process makes the cost high.

The photocatalysis-membrane processes based on the hybrid-ization of photocatalysis with membrane process can eliminate the

* Corresponding authors. Tel.: +86 21 66134852; fax: +86 21 66134852.

E-mail addresses: s.yuan@shu.edu.cn (S. Yuan), shiliyi@shu.edu.cn (L. Shi).

0025-5408/$ – see front matter � 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.materresbull.2013.12.052

conflict between the dispersion and recovery for the powder-typephotocatalysts [7,8]. To make better performance, the photo-catalyst for the photocatalysis-membrane processes shouldcombine the features of high dispersity, good stability and highphotocatalytic activity. The colloidal photocatalyst shows a highpotential for the application in photocatalysis-membrane processdue to the high dispersity and good stability.

In the past decades, a lot of efforts have been devoted toimprove the photocatalytic activity in UV range or visible range bymetal ion doping, such as Fe3+, Cu2+, W6+, Cr6+, etc. [9–15].However, the effects of metal ion doping on the dispersity andstability in solvent is still very interesting and worth to investigate.

Based on our previous research on the colloidal TiO2 nanocrystalsuspension [16–18], we investigated herein the preparation ofcolloidal TiO2 nanocrystals with high dispersity and stability bytungsten ion doping. The effects of tungsten doping concentrationon the dispersity and stability were discussed. And the photo-catalytic activities were evaluated by the photocatalytic degradationof phenol. The results are encouraging that the tungsten doped TiO2

nanocrystal can possess the advantages of high dispersity in waterand high UV-sensitive photocatalytic activity.

2. Experimental

2.1. Materials

Titanium (IV) sulfate (Ti(SO4)2, CR) was provided by ShanghaiChemical Reagent Co. China. Aqueous ammonia solution (NH4OH,25%, AR), aqueous hydrogen peroxide solution (H2O2, 30%, AR) and

H. Xu et al. / Materials Research Bulletin 51 (2014) 326–331 327

tungsten powder (AR) were purchased from Sinopharm ChemicalReagent Co. Ltd. All chemicals were used as received withoutfurther purification. Deionized water was used for solutionpreparation.

2.2. Preparation of tungsten doped TiO2 nanocrystal

Similar to the previous method synthesizing TiO2 nanocrystal[16], in a typical synthesis, 0.75 g of titanium (IV) sulfate wasdiluted in 15.0 ml deionized water to form a 0.2 M Ti(SO4)2

solution. Under vigorous stirring, 3.0 M of ammonia solution wasslowly added, adjusting the pH value to 7.5, which produced awhite precipitate in the solution immediately. The precipitate wasfiltered and washed until the electric conductivity of the filtratewas lower than 100 ms/cm. The filter cake was redispersed in62.5 mL deionized water to get a suspension by ultrasonictreatment for 0.5 h.

Under continuous magnetic stirring, tungsten powder and1.25 ml of H2O2 (30 wt%) was added to the suspension in an ice-water bath. The mixture was refluxed at 100 8C for 4 h to gain thesolvothermal precursor. Finally, the resulting solvothermal pre-cursor was transferred to a Teflon autoclave lined with Teflon 70%filled and tightly closed, and then held at 180 8C for 15 h. After thesolvothermal crystallization, the prepared tungsten-doped TiO2

nanocrystal was filtered again and was redispersed in 125 mldeionized water. The tungsten doped TiO2 nanocrystal colloid wasthus obtained. According to the calculated molar ratio of W:Ti = 0:100, 1:100, 2:100, 3:100, the products are noted as TW-0,TW-1, TW-2, and TW-3, respectively.

2.3. Characterization

The TiO2 powders were prepared through rotatory evaporationof the TiO2 colloids at 45 8C. The morphologies of the products wereviewed by transmission electron microscopy (TEM). The TEMimages were recorded on a JEOL JEM-200 CX microscope at anacceleration voltage of 200 kV. The specific surface area of theprepared sample was measured by N2 adsorption–desorption on aMicromeritics ASAP 2000 system. The XRD analysis was performedusing a Rigaku D/MAX-2000 X-ray diffractometer at roomtemperature, operating at 30 kV and 30 mA, using Cu ka radiation(l = 0.15418 nm). X-ray photoelectron spectroscopy (XPS) spectrawere recorded by a PHI 5000C ESCA spectrometer using Mg Karadiation (hn = 1253.6 eV). The zeta potential of the TiO2 nano-particles was measured on a Zetasizer 3000HS (Malvern Instru-ments Ltd. UK). The surface area of TiO2 nanoparticles wasdetermined using a nitrogen adsorption apparatus (model 3H-2000III, China). Diffuse reflectance spectra (DRS) were obtained forthe dry-pressed disk samples using a Scan UV–vis spectrophotom-eter (Varian, Cary 500) equipped with an integrating sphereassembly, using BaSO4 as the reflectance sample. The photo-luminescence (PL) spectra were measured through the fluores-cence spectrophotometer (Hitachi, F-7000) using 300 nm line of Xelamp as excitation source at room temperature.

2.4. Evaluation of photocatalytic activity by the degradation of phenol

The photocatalysts obtained were diluted by deionized water to1.0 g/L. The photocatalytic degradation was performed in an 80 mLtest tube with 75 mL of reaction solution. The initial concentrationof phenol was 10 mg/L. The suspension was stirred in dark for30 min to obtain the saturated adsorption of phenol before UV-light irradiation. During the photoreaction process, the mixedsolution was irradiated by an UV lamp (16 W, 365 nm) withconstant magnetic stirring. The phenol concentration of thesolution was measured every 1 h after removing powders by

centrifugation at 10,000 rpm for 30 min. The remaining phenol inthe solution system was detected by an UV-2501 PC spectrometerat wavelength of 200–400 nm. The efficiency of degradation wascalculated from Eq. (1) as follows:

C

C0¼ A

A0(1)

where A0 and A are the initial and final absorbance at 269 nm forphenol.

3. Results and discussion

Fig. 1 shows the TEM images of products with different W:Ticalculation molar ratio. For the pure TiO2 nanocrystals obtained byhydrothermal at 180 8C for 15 h, the length is about 34.2 nm, andthe diameter is about 13.2 nm. With the increase of tungstendoping concentration, the shape of nanocrystals changed from rod-like to rectangular, and the aspect ratio of TW-0, TW-1, TW-2, TW-3 are 2.59, 1.76, 1.56 and 1.24, respectively, revealing the tungstendoping really affects the surface energy of the nanocrystal facetsand resulting different shapes. The specific surface area of theprepared samples were measured by N2 adsorption–desorption,and the results are listed in Table 1. Compared with pure TiO2

nanocrystal, the tungsten-doped samples have larger specificsurface area. And the sample TW-2 shows the highest specificsurface area 100.2 m2/g.

The colloidal suspension of tungsten doped TiO2 in water(Fig. 1c inset picture) shows high transparency, which confirmsthat the nanocrystals can be dispersed well in water. The stabilityof water-based colloid depends on the zeta potential greatly.Generally, the colloid with absolute zeta potential value higherthan 30.0 mV is a stable dispersion, while higher than 50.0 mV is agood stable dispersion [19]. Table 1 shows the zeta potential ofpure TiO2 colloid and tungsten doped TiO2 hydrosols at pH value ofapproximately 7.5. As shown in Table 1, compared with pure TiO2

hydrosol (�43.7 mV), the absolute zeta potential value of everytungsten doped TiO2 colloid is a little higher, indicating higherstability of the tungsten doped TiO2 colloid. The high dispersity ofprepared TiO2 samples should be attributed to the high zetapotential value of nanocrystals, which is favorable for the exposureof large surface area of TiO2 nanocrystals to the reactants.

The XRD patterns of TiO2 with different doping concentrationare shown in Fig. 2. All the samples are of anatase crystal phase.However, WO3 phase does not appear in all XRD patterns even ifthe content of doped W6+ is improved to 3%. It is probably that W6+

is incorporated into the titania lattice to form W–O–Ti bonds orhighly dispersed on the TiO2 surface [20]. The crystal sizes of TW-0,TW-1, TW-2 and TW-3 calculated by Scherrer formula from theFWHM of the (1 0 1) peak of anatase are 15.3, 16.0, 20.0 and24.5 nm, respectively [21]. With the increase of tungsten dopingconcentration, the diffraction angles are shifted to lower degrees.The lattice parameters can be calculated from the diffractionangles according to the Bragg equation (Eqs. (2) and (3)). The latticeparameters are summarized in Table 1. Where d(hkl), u are theBragg lattice spacing and angle, respectively. a, b and c are thelattice parameters [22,23].

dðhklÞ ¼ l2sinu

(2)

d2ðhklÞ ¼ h2a�2 þ k2b�2 þ l2c�2 (3)

The effective ionic radii of W6+ and Ti4+ with 6-fold coordinationare 60.0 pm and 60.5 pm, respectively [24]. Compared with thepure TiO2, the W6+ doped TiO2 shows higher volume of unit cell.The lattice distortion of TiO2 should be attributed to thesubstitution of Ti atom by W atom.

Fig. 1. TEM images of tungsten doped TiO2 nanocrystals: (a) TW-0, (b) TW-1, (c) TW-2 and (d) TW-3.

H. Xu et al. / Materials Research Bulletin 51 (2014) 326–331328

Fig. 3 shows the UV–visible absorption of TiO2 powders dopingby different amount of tungsten through rotatory evaporation ofTiO2 colloid at 45 8C. Tungsten-doped TiO2 samples are gray. Thecolor of TiO2 samples becomes darker with the increasing tungstendoping concentration. Typically, the color of semiconductornanoparticles is determined by the absorption band edge position.The absorption band edge extending to longer wavelength can leadto enhanced absorption in the visible light range. The absorptionspectra of tungsten-doped TiO2 exhibit a red shift to visible lightrange. The optical band gap energy can be estimated by using thefollowing equation for a semiconductor [25,26]:

a ¼ Kðhy-EgÞn=2

hy(4)

where a is the absorption coefficient, K is a constant, Eg is the bandgap, and n is equal to 1 for the direct transition. The band gap can beestimated from the plot of (ahv)2 versus photon energy (hy). The

Table 1Characteristics of the W6+ doped TiO2 samples.

Samplesa D/nm a/nm c/nm V/nm3 SBETb (m2/g) Zc (mV) Eg

d

TW-0 15.3 3.78444 9.52764 0.13645 86.7 �43.7 3.17

TW-1 16.0 3.78984 9.50574 0.13653 88.9 �53.3 3.16

TW-2 20.0 3.80038 9.50124 0.13723 100.2 �48.2 3.09

TW-3 24.5 3.79703 9.50314 0.13701 95.1 �44.0 3.16

a TW-0, TW-1, TW-2 and TW-3 correspond to the W6+ doped TiO2 nanocrystals

prepared with 0, 1, 2 and 3 at% of tungsten dopant, respectively.b SBET is the surface area examined using a nitrogen adsorption apparatus.c z is the zeta potential of as-prepared sample.d Eg is the bandgap of as-prepared sample.

intercept of the tangent to the plot will give a good approximationof the band gap energy for this direct band gap material (shown inFig. 3 and Table 1). The band gap energy of the tungsten doped TiO2

is close to that of pure TiO2, while the tungsten doped TiO2 exhibitstronger absorption in the visible light range. The proper reason isthat there are two kinds of tungsten species whose relativeproportions change with the tungsten content, that is, the W6+

doped in the lattice and the partial aggregation tungsten oxidesspecies on the TiO2 surface [27].

To study the chemical states of the as-prepared samples, XPSspectra were measured and analyzed. Fig. 4 shows the XPS spectra

Fig. 2. XRD patterns of tungsten doped TiO2 nanocrystals: (a) TW-0, (b) TW-1, (c)

TW-2 and (d) TW-3.

Fig. 3. UV–vis absorption of TiO2 nanocrystals doping by the different amount of

tungsten: (a) TW-0, (b) TW-1, (c) TW-2 and (d) TW-3.

Table 2Surface analysis by XPS spectra for TiO2 nanocrystals doping by the different

amount of tungsten.

SamplesW/Ti molar ratioBinding energy (eV) OL (%)OH (%)

Ti 2p1/2W 4f5/2OL 1s (Ti–O)OH 1s (O–H)

TW-0 0 463.8 – 529.4 531.0 49.6 50.4

TW-1 0.012 463.9 37.2 529.6 531.2 47.3 52.7

TW-2 0.021 464.0 37.2 529.8 531.4 40.6 59.4

TW-3 0.034 463.9 37.3 529.7 531.3 44.1 56.9

H. Xu et al. / Materials Research Bulletin 51 (2014) 326–331 329

of Ti 2p, W 4f and O 1s of TiO2 nanocrystals doped by the differentamount of tungsten. The XPS peaks of Ti 2p3/2 and Ti 2p1/2 locate atapproximately 458 and 464 eV, which are assigned to the latticetitanium, indicating that Ti element mainly exists as the chemicalstate of Ti4+ [28,29]. The molar ratio of W/Ti is a little higher thanthe theoretical value, which confirms the similar result that thetungsten species is a little richer on the surface than in the bulk(Table 2). As expected in TiO2, the oxidation state of titanium is +4.And the TiO2 phase has not been changed by the tungsten doping.The binding energy peaks located at approximately 36.0 and37.0 eV are attributed to the spin–orbit splitting of the W 4fcomponents (W 4f7/2 and W 4f5/2) [30], which shift to a lowerbinding energy compared to those of tungsten (VI) trioxide power[31]. It should be ascribed to the interaction of Ti and W atom [32].W 4f7/2 corresponds to tungsten in the +6 oxidation state (W6+).The results indicate that tungsten atoms replace titanium atomsand assume the same oxidation state of +4 as titanium atoms. FromXPS spectra of O 1s core level of TiO2 nanocrystals doping by thedifferent amount of tungsten, they are asymmetric, indicatingthere are at least two kinds of chemical forms. After curve fitting,one peak is at the binding energy of about 529 eV corresponding tooxygen in TiO2 lattice (OL), and the other is about 531 eVcorresponding to oxygen in TiO2 surface adsorption of H2O (OH)(Fig. 5 and Table 2) [17,33]. As shown in Table 2, the amount ofsurface hydroxyl group is enhanced by doping tungsten, which is

Fig. 4. XPS spectra of Ti 2p (A), W 4f (B) and O 1s(C) for the TiO2 nanocrystals doping

favorable to the high �OH generation, thus to the high photo-catalytic efficiency [34].

The photoluminescence spectra were analyzed to understandthe recombination of photogenerated electrons and holes [35].Fig. 5 shows the broad PL spectra of the samples TW-0, TW-1, TW-2and TW-3, excited by a laser with a wavelength of 300 nm. All thesamples exhibit a broad peak at about 398 nm, which arose fromthe luminescence of self-trapped excitons [36]. Some other peakswith central wavelength beyond 400 nm in the PL spectra resultedpossibly from the surface states and oxygen vacancies. Serponeet al. reported that the PL bands of anatase nanocrystals at the longwavelength range (442, 455, 465, and 502 nm) are attributed to theoxygen vacancies [37]. The PL emission peaks of tungsten-dopedsamples are red shifted compared with TW-0, which is consistentwith the evaluated band gap according to the UV–vis spectra. Theemission intensity of the PL spectra has been related closely to theluminescence of the recombination of photoinduced electrons andholes, the free excitons, and self-trapped excitons. In this study, thePL emission intensity of as-prepared samples is TW-0 > TW-1 > TW-3 > TW-2 (Fig. 5), which indicates that TiO2 incorporatingwith an appropriate amount of tungsten may slow the radiativerecombination process [38,39].

As shown in Fig. 6 and Table 3, the photocatalytic degradationcorresponds to a pseudo-first-order reaction. Pseudo-first-orderkinetics was assumed to calculate the corresponding degradationrate constant (k) [40].

LnC

C0

� �¼ �kt (5)

where C0 is the original phenol concentration (mg/L) after theadsorption/desorption reached equilibrium, C is the concentration(mg/L) at a given time t (h) and k is the first-order degradation rate

by the different amount of tungsten: (a) TW-0, (b) TW-1, (c) TW-2 and (d) TW-3.

Fig. 5. PL patterns of TiO2 nanocrystals doping by the different amount of tungsten

(a) TW-0, (b) TW-1, (c) TW-2 and (d) TW-3.

Table 3The effects of the different amount of tungsten on degradation ratio and

degradation rate K.

Samples K/h�1 t1/2/(h) Dphenol (%)

TW-0 0.1092 6.3 48.9

TW-1 0.1896 3.7 68.1

TW-2 0.2694 2.6 80.0

TW-3 0.1734 4.0 64.8

Dphenol is the degradation ratio of phenol under the irradiation of UV light for 6 h.

H. Xu et al. / Materials Research Bulletin 51 (2014) 326–331330

constant (h�1). Half-life, t1/2 (h), can be calculated from k by usingthe following equation [18]:

t1=2 ¼ ln2

k

� �(6)

The sample TW-2 shows the highest degradation rate constant(0.2694 h�1) (Table 3). Correspondingly, the degradation ratio after6 h under UV light irradiation of TW-2 is about 80.0%. Thephotocatalytic degradation experiment results confirm that thetungsten doping concentration has great effects on the photo-catalytic activity. Larger specific surface area and higher dispersityis better for the photocatalytic degradation of phenol. However, itshould be noticed that the photocatalytic activity not only dependson the specific surface area, but also depends on the tungstendoping concentration.

The replacement of Ti atom by W atom in the lattice will createimpurity energy level below the conduction band of TiO2. Theimpurity level of W6+/W5+ is the shallow trap for the photo-generated electrons, which benefits the separation of photogen-erated charges and will results in higher photocatalytic activity[41]. However, higher doping concentration will create morerecombination center for the photogenerated electrons and holes,which will decrease the photocatalytic activity of TiO2.

Fig. 6. The reaction kinetics fitting curve of the tungsten ion modified TiO2

nanoparticles on photocatalytic degradation phenol (a) TW-0, (b) TW-1, (c) TW-2

and (d) TW-3.

On another aspect, the valence state of tungsten dopant ions ishigher than that of Ti4+. The replacement of Ti by W will enhancethe free electron density of as-prepared samples in order tobalance charge. Consequently, the density of electron distributionin the surface of TiO2 nanocrystals will be increased, which willgenerate more hydroxyl groups (OH�) on the particle surface. Thisresult agrees well with the XPS analysis of the samples. It is wellknown that surface hydroxyl groups can react with photoinducedholes and generate surface hydroxyl radicals with high oxidationcapability [42], which leads to higher photocatalytic degradationactivity. Meanwhile, more negative charges on the particle surfacewill enhance the stability of colloids. Briefly, the optimized amountof tungsten incorporation for samples TW-2 with highestphotocatalytic activity for degradation of phenol can be attributedto the modification of photoelectric properties and surface state.

4. Conclusions

Colloidal tungsten-doped TiO2 nanocrystals with high disper-sity, stability and photocatalytic activity were synthesis byhydrothermal method. The analysis results reveal that thetungsten doping concentration has great effects on the morpholo-gy, zeta potential and photocatalytic activity of TiO2. The tungstendoped TiO2 combines the characters of high dispersity and highphotocatalytic activity, which will be a promising candidate for thewater purification in photocatalysis-membrane processes.

Acknowledgements

The authors acknowledge the supports of Shanghai LeadingAcademic Discipline Project (S30107), National Natural ScienceFoundation of China (51202138), Natural Science Foundation ofShanghai (12ZR1410500).

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