Dielectric and photocatalytic properties of sulfur doped TiO2 nanoparticles prepared by ball milling

Materials Research Bulletin 48 (2013) 3351–3356

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Dielectric and photocatalytic properties of sulfur doped TiO2 nanoparticles prepared by ball milling Mohammed Jalalah a,b, M. Faisal a, Houcine Bouzid a,c, Adel A. Ismail a,d,e,*, Saleh A. Al-Sayari a,e a Promising Centre for Sensors and Electronic Devices (PCSED), Advanced Materials and Nano-Research Centre, Najran University, P.O. Box 1988, Najran, 11001, Saudi Arabia b Electrical Engineering Department, College of Engineering, Najran University, P.O. Box 1988, Najran, 11001, Saudi Arabia c Department of Physics, Faculty of Sciences and Arts, Najran University, P.O. Box 1988, Najran, 11001, Saudi Arabia d Nanostructured & Nanotechnology Materials Division, Central Metallurgical R&D Institute, CMRDI, P.O. Box 87, Helwan, 11421, Cairo, Egypt e Chemistry Department, Science and Art at Sharurah, Najran University, Saudi Arabia

A R T I C L E I N F O

A B S T R A C T

Article history: Received 13 February 2013 Received in revised form 7 May 2013 Accepted 7 May 2013 Available online 15 May 2013

Sulfur (S) doped commercial TiO2 P-25 has been achieved by changing the amount of thiourea using ball milling technique. The results of XRD clearly reveal biphasial anatase and rutile mixtures for all prepared samples and doping of S does not change the morphology of the TiO2. The optical absorption edge of Sdoped TiO2 was red shifted with indirect bandgap energy of 2.8 eV. The dielectric studies confirm that the dielectric constant of TiO2 increases after doping, however it becomes more conductive. Newly designed S-doped TiO2 photocatalysts exhibited excellent photocatalytic performance for the degradation of methylene blue (MB) under visible light. The overall photocatalytic activity of 0.11 at.% S-doped TiO2 was significantly 3-times higher than that of commercial TiO2 P-25 and complete degradation of MB has taken place after 90 min of irradiation under visible light while only 35% dye degraded when the reaction has been carried out in the presence of undoped TiO2. ß 2013 Elsevier Ltd. All rights reserved.

Keywords: A. Inorganic compounds A. Oxides B. Intercalation reactions D. Optical properties

1. Introduction In recent years, semiconductor mediated photocatalytic treatment of organic pollutants has received much attention. These reactions are of particular interest due to their ability to use the solar energy. TiO2 always remain as first choice photocatalyst due its relatively high photocatalytic activity, biological and chemical stability, low cost, non-poisonous and long stable life [1–4]. Utilization of TiO2 semiconductors is promising method for the elimination of hazardous environmental pollutants [5–9], especially for the degradation of biorecalcitrant organic contaminants. It is also known that a mixture of anatase and a small percentage of rutile shows optimal photocatalytic efficiency. Titania has a large band gap (3.20 eV for anatase TiO2) and therefore utilized only a small fraction of visible solar spectrum about 4–5% [10]. Visiblelight-driven photo catalysts have attracted much attention in recent years because visible light is an important clean energy and can be easily utilized. In order to use visible energy efficiently, the

* Corresponding author at: Nanostructured & Nanotechnology Materials Division, Advanced Materials Department, Central Metallurgical R&D Institute, CMRDI, P.O. Box 87, Helwan, 11421, Cairo, Egypt. Tel.: +20 225010641; fax: +20 225010639. E-mail addresses: [email protected], [email protected] (A.A. Ismail). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.05.023

key step is to explore new materials as visible light-driven photocatalysts. Many attempts have been made in order to increase the visible light adsorption capacity of titanium dioxide, such as doping with transition metals [11–23], precious metals [24–32], sensitized with organic materials [33–35] and nonmetal atoms [36–40]. Narrowing of band gap by introducing nonmetal anions (N, C, S and F) into TiO2 was recently found to be more proficient than the conventional technique to capitulate catalyst with high catalytic activity under visible light source. Several works concerning nonmetal cations doping was reported [36–41]. Ohno et al. reported that S cation doped TiO2 powder absorbed visible light more strongly than S anion doped TiO2 powders and showed good photocatalytic activity under visible light [41]. Kangle and coworker studied the synergistic effect of different nonmetal co-doping on the photocatalytic activity of TiO2 hollow microspheres by cysteine modification and also make a comparative study of Bi, C and N co-doped TiO2 nanoparticles under visible and UV light irradiation [42,43]. It is interesting to note that sulfur, unlike other non-metal dopants, may exist in more than one oxidation state (S2, S4+ or S6+) in TiO2 depending on the synthetic conditions or the types of S precursor [44]. The photocatalytic activity still enhances regardless of the S oxidation state. The present study was undertaken to produce S doped TiO2 powder by mechanical ball milling. Ball milling is an effective process for

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mechanical milling. The final product contains nano-scale crystalline structures produced through mechanical ball milling process [45–47]. The main focus is on shifting the absorption edge of TiO2 to visible light region by introducing S into the TiO2 lattice structure. Also, from economic point of view, commercial photocatalysts P25 Degussa were used for preparation S-doped TiO2 at different contents. The developed photocatalysts have been employed for the photocatalytic degradation of methylene blue in order to seek the relationship between various percentages of Sdoped TiO2 and additionally exploit for their dielectric properties. The present study focuses on the designing of ‘‘elegant’’ catalyst material showing potential in photocatalysis, with the capability of destruction of pollutants under visible light and carrying out other selective catalytic processes.

4. Dielectric study To prepare the ceramics using the powders acquired previously, all five powders samples were pressed into a disk shape at 6– 7 MPa. The diameter of the sample was 13 mm and the thickness was 1.5 mm. Specimen disks were plated with silver electrode and connected with two electrodes, then heated at 60 8C for 1 h to dry. The capacitance value and the dielectric loss were measured at room temperature by LCR meter BR2827 and 4285A PRECISION. The following formula was used to obtain dielectric constant e: 

e¼

 Cp  e A

where, Cp is the parallel mode capacitance measured by the LCR meter, A is the area of the overlap of the two silver electrodes and e is the separation between the electrodes.

2. Experimental work 2.1. Materials Titanium dioxide P25 (Degussa, Germany) was obtained from Evonik and thiourea (99.9%) were purchased from Aldrich. The chemicals were used as received and all aqueous solutions were prepared with deionized water (resistivity = 18.2 MV cm). 3. Synthesis of S-doped TiO2 Commercial TiO2-P25 and thiourea SC(NH2)2 were mixed in distilled water. The suspension of TiO2 in thiourea solution was dried at 110 8C to obtain the dried mixture of TiO2 and SC(NH2)2. A planetary ball mill (Fritsch, P-7) was used for milling. Five Ø15 mm hard steel balls with mixture of P25 and thiourea were introduced to a hard steel vessel of 45 cm3 inner volume. The milling was performed at 700 rpm for 30 min. The final powders obtained were filtered and washed with distilled water for several times then calcined at 400 8C for 2 h to remove organic residues. 2 g TiO2-P25 and different amount thiourea 0, 0.02, 0.05, 0.2 and 0.6 g were mixed to obtain sulfur doped TiO2 at different S at.% 0, 0.05, 0.11, 0.16 and 0.22 were denoted as T0, T1, T2, T3 and T4, respectively.

4.1. Photocatalytic activity tests Photocatalytic performance of prepared photocatalysts has been carried out by the decomposition of Methylene Blue utilizing optical absorption spectroscopy. The photocatalytic reaction was carried out in a beaker, which contain 100 ml of Methylene Blue dye solution (0.02 mM) and 100 mg of catalyst. Prior to irradiation, the solution was stirred and bubbled with oxygen for at least 15 min in the dark to allow equilibrium of the system so that loss of compound due to the adsorption can be taken into account. The suspension was continuously purged with oxygen bubbling throughout the experiment. Irradiation was carried out using 250 W visible lamps (Osram, Germany) placed horizontally above beaker. Samples (5.0 ml) were collected at regular intervals during the irradiation and Methylene Blue solution was separated from the photocatalyst by centrifugation before analysis. The degradation was monitored by measuring the absorbance using UV–vis spectrophotometer (Lambda 950 PerkinElmer). The absorbance of Methylene Blue (0.02 mM) was followed at 663 nm wavelength. 5. Results and discussions

R (301)

A (204)

R (211) A (211)

R (111)

R (101) A (004)

T4

A (200)

A (101)

Fig. 1 shows the X-ray diffraction (XRD) patterns of bare TiO2 P25 and prepared S doped TiO2 using ball milling for 30 min and then calcined at 400 8C for 2 h. In order to confirm the bulk composition for each sample, the XRD patterns were compared

R (110)

Field emission-secondary electron microscope (FE-SEM) images were carried out with a FE scanning electron microanalyzer (JEOL-6300F, 5 kV). X-ray diffraction (XRD) data were acquired on a Bruker AXS D4 Endeavour X diffractometer using Cu Ka1/2, la1 = 154.060 pm, la2 = 154.439 pm radiation. Raman spectroscopy was carried out using a WITEC CRM200 Raman system in the range from 500 to 2000 cm1. Infrared spectra were obtained on PerkinElmer Spectrun 100 FTIR-spectrometer in the range 500– 4000 cm1. Reflectance spectrum of all samples as well as TiO2 P25 were taken at room temperature using UV–visible spectrophotometer (lambda 950 PerkinElmer) fitted with universal Reflectance accessory in the range of 200–800 nm. UV–vis spectra were performed in the diffuse reflectance mode (R) and transformed to the Kubelka–Munk function F(R) to separate the extent of light absorption from scattering. Furthermore the band gap values were obtained from the plot of the modified Kubelka–Munk function (F(R)E)1/2) versus the energy of the absorbed light E [48].

Intensity (a.u)

3.1. Characterization

T3 T2

T1

1=2

FðRÞE

¼

ð1  RÞ2  hy 2R

!1=2

The band gap energy was determined from the intersection of tangent through the point of inflexion in the absorption band and the photon energy axis.

T0 20

30

40

2θ /

o

50

60

70

Fig. 1. XRD pattern of undoped TiO2 P-25 and sulfur doped TiO2 samples at different sulfur at.%. Shifted for sake of clarity.

M. Jalalah et al. / Materials Research Bulletin 48 (2013) 3351–3356

Intensity (a.u)

T0 T1 T4

100

200

300

400

500

600

700

800

Raman shift/ nm Fig. 2. Raman spectra of P-25 TiO2 and sulfur doped TiO2 samples at 0.05 and 0.22 sulfur at.%.

50

40

T0 T1 T3 T4

30

%T

with the JCPDS-ICDD standards for anatase (21-1272). The diffractograms for TiO2 are essentially equivalent, exhibiting peaks at 25.48, 36.48, 48.18, 54.28 and 62.88 that are consistent with the (1 0 1), (0 0 4), (2 0 0), (2 1 1) and (2 0 4) planes associated with tetragonal anatase. Also, there are four typical peaks with 2u values of 27.38, 35.98, 41.18 and 54.18, corresponding to the (1 1 0), (1 0 1), (1 1 1) and (2 1 1) crystal planes of the rutile phase (JCPDS No. 21-1276) [49], respectively. No diffraction lines of S or other phases were observed, which means that all S is incorporated into the TiO2 structure. The slow phase transformation from anatase to rutile in the presence of SC(NH2)2 can be thought due to the easing of mechanical stress by SC(NH2)2 during milling. These results are in agreement with pervious published works where they mixed TiO2 and Hexamethylenetetramine (C6H12N4) [50,51]. The structural properties of the S doped TiO2 were further investigated by Raman spectroscopy. Anatase has six Raman active modes: A1g + 2B1g + 3Eg. For a single crystal, Ohsaka et al. [52] determined the following allowed bands: 144 cm1 (Eg), 197 cm1 (Eg), 399 cm1 (B1g), 513 cm1 (A1g), 519 cm1 (B1g) and 639 cm1 (Eg). By contrast, rutile has four Raman active modes: A1g + B1g + B2g + Eg. From a single crystal, the allowed modes were detected [53] at 143 cm1(B1g), 447 cm1 (Eg), 612 cm1 (A1g) and 826 cm1 (B2g). Fig. 2 displays the Raman spectra of TiO2-P25 and sulfur doped TiO2. All samples exhibit the characteristic Ramanactive modes of the anatase TiO2 phase. Increase of the sulfur content, especially at T4, results in significant narrowing and shift of the Raman bands. Multi-peak fitting of the Raman spectra shows that the most intense anatase modes occur at frequencies of 145 cm1 (Eg), 195 cm1 (Eg), 393 cm1 (B1g), 513 cm1 (A1g), and 635 cm1 (Eg), whereas they shift and narrow to 142 cm1 for T4. The latter parameters are very close to those of the reference TiO2, while they approach that of bulk anatase [54]. This variation can be qualitatively explained by the partial release of the optical phonon confinement effect, resulting in both broadening and blue shift of the Raman modes in nanosized materials [54,55]. Fig. 3 shows the FT-IR spectra of TiO2-P25 and S doped TiO2. For samples T3 and T4, the presence of Ti–S bond can be suggested based on the distinct bands at 1130 cm1[56]. On the other hand, composite IR bands are expected in the same frequency range (1000–1200 cm1) due to the presence of SO42 groups bound onto TiO2, while at high 0.22 at.% S doped TiO2, a characteristic S5 5O stretching mode is anticipated at 1400 cm1 [57,58]. However, the latter band is absent in low S at.%-doped TiO2, indicating either a low amount of sulfate groups or most likely the presence of surface sulfates in a

3353

20

10

0 500

1000

1500

2000

2500

Wavenumber/ cm-1

3000

3500

4000

Fig. 3. FTIR spectra for undoped TiO2 P-25 and sulfur doped TiO2 samples at different sulfur at.%.

highly ionic form on the hydrated TiO2 surface. Two peaks at 3460 and 1632 cm1 are assigned to the surface adsorbed water and hydroxyl groups [59]. Fig. 4 shows FE-SEM micrographs of the pure TiO2 and 0.11 at.% S doped TiO2 samples. The grains are found to be uniform, spherical and slightly agglomerated. Further observation indicates that morphology of the samples is very rough and may be beneficial to enhancing the adsorption of reactants due to its great surface roughness and high surface area. It also reveals that the doping of S does not leave any change in the morphology of the TiO2 catalyst surface (Fig. 4a and c). Energy dispersive X-ray (EDX) analysis reveals the presence of S, Ti and O and confirms that the final S and Ti content in the sample is consistent with the S:Ti ratio used in the starting mixtures. And, the quantitative results of EDX of 0.11 at.% S doped TiO2 sample clearly show that the at.% of S, Ti and O are 0.11, 39.1 and 60.79, respectively. Reflectance spectrum of the P-25 (TiO2) and various S-doped TiO2 samples were taken at room temperature using UV–vis spectrophotometer (lambda 950 PerkinElmer) fitted with universal Reflectance accessory in the range of 200–800 nm. Fig. 5 shows diffuse reflectance spectra of pure TiO2 and various S-doped TiO2. All the reflectance spectra show a similar shape. It can be seen from the DRS spectra that S doping resulted in an intense increase in absorption in the visible light region and a red shift in the absorption edge of the TiO2 (Fig. 5). The optical energy gap (Eg) can be determined by the tangent lines of the square root of F(R) against photon energy as shown in Fig. 5 inset for 0.11 at.% S-doped TiO2. The bandgap energy of the S-doped TiO2 nanoparticles (determined from the optical measurements) ranges between 2.67 and 2.98 eV based on the S at.%-doped TiO2, and thus does decrease from that of undoped TiO2 (3.1 eV). These results are in agreement with the previous published work [60]. They confirmed that based on the theoretical analyses by the firstprinciples band calculations using the full potential linearized augmented plane-wave methods within the generalized gradient approximation, the mixing of the S 3p states with the valence band (VB) of TiO2 was found to contribute to the increasing width of the VB. This leads to the band gap narrowing in the S-doped TiO2. Therefore, the photon-to-carrier conversion was induced during irradiation by visible light above 420 nm (>2.9 eV) [60]. Fig. 6 shows the dielectric constants of undoped and S-doped TiO2. The dielectric studies confirm that the dielectric constant k of TiO2 increases after doping, however it becomes more conductive (increase of the dissipation factor), this can also be explained due to decrease in band gap as shown in Fig. 5. In this results are matching the photocatalytic section as you see below.

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Fig. 4. FE-SEM and EDS of TiO2 P-25 (a and b) and FE-SEM and EDS of 0.11 at.% Sulfur doped TiO2 (c and d).

6. Photocatalytic properties Irradiation of an aqueous solution of MB in the presence synthesized photocatalyst leads to decrease in absorption intensity. The decomposition of the dye under visible light irradiation was determined by measuring absorption spectra using a UV/vis spectrophotometer. The data obtained during the illumination of the S doped TiO2 photocatalysts with visible light are shown in Figs. 7–9 and summarized in Table 1. Fig. 7 exhibits the change in absorption spectra for the photocatalytic degradation of MB dye as a function of irradiation time under visible light in the presence of 0.11 at.% S doped TiO2. The strong absorption bands of MB located

at l = 663 nm and l = 291 nm steadily decrease upon increasing irradiation times. This experiment clearly shows that the decoloration of MB can be achieved under visible-light irradiation when the solution is put in contact with photocatalysts. MB can decolorize either by the oxidative degradation of the dye or by the two-electron reduction to its colorless form [61]. We could detect a small peak of the characteristic absorption band of leuco-MB at 256 nm. Hence, the decoloration of MB is attributed to the oxidative degradation of the dye. It can be seen that the maximum absorbance at 663 nm gradually decreases with passage of time and disappear almost completely after 90 min which indicates that the MB dye has almost completely degraded after 90 min in the 60 100

9

T1 T2 T3 T4 P-25

7

1/2

6

5

ε 'r

5

40

40

20

35

0 10

10

10

10

10

10

10

10

Frequency (Hz)

30 25

3 2

20

1

15

Undoped TiO2

10

S-doped TiO2

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

E/ eV

300

ε''r

4

0 2.0

4 250

60

45

7

6

80

50

[F(R)E)] 1/2/ eV

Diffuse releftance

8

55

350

400

450

Wavelenght (nm) Fig. 5. Diffuse reflectance spectra for the samples of commercial P-25 abd S doped TiO2 at different concentrations. Inset, plot of transferred Kubelka–Munk versus energy of the light absorbed of 0.11 at.% S doped TiO2.

1

10

2

10

3

10

4

10

5

10

6

10

7

10

8

10

Frequency (Hz) Fig. 6. Real part of the permittivity vs frequency for undoped (squares) and 0.11 at.% S doped TiO2 (circles). Inset: Imaginary part of the permittivity and the photo of the LCR meter.

M. Jalalah et al. / Materials Research Bulletin 48 (2013) 3351–3356

1.4

Table 1 Optical energy gap of pure P-25 TiO2 and Sulfur doped TiO2 at different sulfur at.% photocatalysts and their photocatalytic properties.

1.2

Sample

Sulfur, at.%

Optical energy gap (Eg) in eV

Photodegradation rate mol L1 min1  107

Photodegradation efficiency (%)

T0 T1 T2 T3 T4

0 0.05 0.11 0.16 0.22

3.1 2.76 2.83 2.95 2.98

0.82 1.66 6.66 2.3 1.2

35 58 99 68 43

Absorbance

1.0

0 min 15 min 30 min 45 min 60 min 75 min 90 min

0.8 0.6 0.4 0.2 0.0 200

300

400

500

600

700

Wavelength/ nm Fig. 7. Typical plot for change in the absorption spectrum in the presence of 0.11 at.% S doped TiO2 (1 g L1) containing Methylene Blue (0.02 mM, V = 100 ml).

1.0

Blank

0.8

T0 0.6

C/Co

T4 T1

0.4

T3

0.2

T2 0.0 0

10

20

30

40

50

60

70

80

90

100

Illumination time/ min Fig. 8. Typical plot for comparision of change in concentration vs irradiation time in presence of P-25 and various % of S doped P-25 (1 g L1) containing Methylene Blue (0.02 mM, V = 100 ml).

-4

7.0x10

-1 -1 -3 Degradation rate [molL min *10 ]

3355

-4

6.0x10

presence of synthesized 0.11 at.% S-doped TiO2 under visible light irradiation. Fig. 8 shows the change in concentration as a function of irradiation time for the dye derivative in the absence and presence of undoped and various percentage of S-doped TiO2. It could be seen that under visible irradiation no observable loss of dye takes place if the reaction has been carried out in the absence of photocatalysts which indicates that MB is stable under visible irradiation. The data obtained during the illumination of the Sdoped TiO2 with visible light are shown in Figs. 8 and 9 and summarized in Table 1. They reveal that the photocatalytic activities and photodegradation rates increase with increasing S content up to 0.11 at.% S-doped TiO2 with the maximum photocatalytic activity being 99%. Subsequently, the photocatalytic activity gradually decreases with increasing S/Ti ratio reaching a value of 43.7% for the sample containing 0.22 at.% S-doped TiO2. In addition, the S-doped TiO2 nanoparticles are more photoactive than the commercially available TiO2 P-25 (Table 1 and Figs. 8 and 9). It has been observed that irradiation of aqueous suspension of MB dye in the presence of synthesized photocatalyst shows beneficial effect and leads to decrease in absorption intensity. It is known that doping S in TiO2 brings visible light absorption photocatalytic activity of TiO2. The band-gap narrowing of TiO2 by S doping was led to enhance photocatalytic activity of the TiO2 under visible light. Because the prepared doped samples can be activated by visible light, thus more electrons and holes can be generated which results in the efficient generation of hydroxyl radicals (OH) and superoxide radical anions (O2) which are the active oxidizers and drive the photodegradation or mineralization of the dye molecule [62]. All S doped samples show much higher photocatalytic activity than undoped TiO2. But we can also see that at higher loadings photocatalytic activity of TiO2 doped S samples has decreased, it might be due to the fact that, the excess dopent acts as recombination centers which facilitates electron–hole recombination thus lowering the activity. So, the photocatalytic activity is de-pressed to a certain extent. To conclude, the higher activity of the 0.11 at.% S-doped TiO2 among all samples may due to its lower band gap i.e. strong adsorption in visible region indicates a possibility of utilizing more visible light for photocatalysis, which can easily generate electron–hole pair and contribute in oxidation of organic pollutants.

-4

5.0x10

7. Conclusions

-4

4.0x10

-4

3.0x10

-4

2.0x10

-4

1.0x10

0.0

T0

T1

T2

T3

T4

Fig. 9. Comparison of degradation rate for the decomposition of Methylene Blue in the presence of P-25 and various at.% of S doped P-25 (1 g L1) containing Methylene Blue (0.02 mM, V = 100 ml).

S-doped Commercial TiO2 P25 at different atomic percentage have been synthesized using ball milling by changing the amount of thiourea and subsequent thermal annealing at 400 8C for 2 h. The addition of S into the TiO2 has been found to enhance the photocatalytic activity of TiO2 and all S-doped samples showing much higher photocatalytic activity than undoped TiO2 as a result of the dielectric constant of TiO2 increases after doping. TiO2 containing 0.11 at.% S has been found to be an efficient photocatalyst for the visible-light decontamination of MB. The 0.11 at.% S doped TiO2 have the highest photocatalytic activity because of the smallest crystallite size and of the important effect on the distortion of anatase lattice even at 400 8C, when its lattice is

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well-formed. S doping of TiO2 increases the photocatalytic activity by a factor of three as compared with undoped TiO2. The visiblelight photocatalytic activity of S-doped TiO2 nanoparticles towards MB is remarkable and it constitutes an environmentally-friendly system operating under ambient light at the green atmosphere without the need of corrosive or toxic chemicals. Acknowledgement Advanced Material and Nano Research Centre, Najran University, Najran has been gratefully acknowledged for all instrumental and financial support. References

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