TiO2 nanocomposites under visible light irradiation

Materials Research Bulletin 48 (2013) 3025–3031

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Enhanced photosensitized degradation of rhodamine B on CdS/TiO2 nanocomposites under visible light irradiation Wenjuan Li a,b,c, Xiaoli Cui c, Peixian Wang b, Yu Shao b, Danzhen Li b,*, Fei Teng a a

Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of Environmental Sciences and Engineering, Nanjing University of Information Sciences and Engineering, Nanjing 210044, PR China b Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou University, Fuzhou 350002, PR China c Shandong Province Key Laboratory of Life-Organic Analysis, Qufu Normal University, Qufu 273165, PR China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 22 December 2012 Received in revised form 14 April 2013 Accepted 17 April 2013 Available online 30 April 2013

Visible-light-driven photocatalysts, CdS/TiO2 nanocomposites were synthesized by a simple hydrothermal method. Their formation and structures were characterized by X-ray diffractometer, transmission electron microscopy, diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy. Taking rhodamine B (RhB) as a model, their photocatalytic activities in aqueous phase under visible light irradiation (420 < l < 800 nm) were tested. The results showed that the composite of CdS and TiO2 with appropriate oxidation reduction energy levels enhanced the charge separation and extended the absorption response into visible light region. Thus, the photosensitized degradation of RhB was largely enhanced. The degradation mechanism was explored concretely. ß 2013 Elsevier Ltd. All rights reserved.

Keywords: A. Composite A. Semiconductor B. Chemical synthesis D. Catalytic properties

1. Introduction Azo dyes constitute the largest class of dyes used in industry and are resistant to aerobic degradation [1]. The release of the azo dyes and their products in the environment causes serious pollution problems. Popular treatment methods for eliminating dyes from the waste stream include flocculation with lime, activated charcoal adsorption, and biotreatment [2]. However, these methods face many defects such as the generation of solid wastes and the restriction on indigenous soil microorganisms to degrade dye compounds. And they are likely to be inefficient. In recent years, great interest has been focused on the use of semiconductor materials as photocatalysts for the removal of organic and inorganic pollutants from aqueous phase. As an efficient photocatalyst with good stability and nontoxicity, TiO2 has been extensively investigated in the environmental field [3]. However, the photocatalysis on TiO2 would be carried out only by the irradiation of UV light which is not the optimal candidate to bring about the photodegradation of dyes. To exploit effective visible light-active photocatalysts with high quantum efficiency and high stability, some researchers have focused on the synthesis of titania materials doped with S, C, N, etc. atoms [4–6]. Some have exploited a new type of nontitania photocatalysts at the expense of

* Corresponding author. Tel.: +86 591 83779256; fax: +86 591 83779256. E-mail addresses: [email protected], [email protected] (D. Li). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.04.057

a lot of manpower and material resources [7,8]. However, few of them got satisfactory results. Another solution is using the catalytic oxidation with hydrogen peroxide (H2O2). Homogeneous Fenton oxidation is the most widely used catalytic process for dye degradation [9]. But this process requires low pH and forms a significant amount of ferric hydroxide sludge in the course of homogeneous Fenton treatment. These disadvantages inhibit its wide application [10]. Besides these approaches, it is worthy to pay more attention to the photosensitization reactions. The light absorption can be expanded by the photosensitization process through excitation of the sensitizer followed by charge transfer to the semiconductors. Some of these photosensitization reactions have achieved certain quantum efficiency [11,12]. But problems of the low quantum efficiency and low conversion ratio of dyes are still existent. In the photosensitization reactions, the transfer of electrons between the dyes and semiconductors is of importance in the degradation of dyes. Therefore, enhancing the transfer of electrons would increase the activity. Exploring efficient sensitizers with appropriate electronic states to enhance the transfer of electrons is the key challenge in sensitization-type photocatalysis. It is expected to be an effective appropriate solution to the problem of decomposing organic compounds by this method. The composite of two inorganic semiconductors with appropriate oxidation reduction energy levels could enhance the charge separation and extend the absorption response into visible region. For CdS/TiO2 photocatalysts, their preparation not only helps for charge separation by isolating electrons and holes in two distinct

W. Li et al. / Materials Research Bulletin 48 (2013) 3025–3031

particles, but also extends the photo-response of the photocatalyst into the visible region [13]. Although the drawback of CdS, such as its anodic decomposition and the release of toxic cadmium, limits its use as a photocatalyst [14,15], it is worthy to be examined in the mechanistic aspects especially when it deals with dye degradation from a fundamental view. The preparation and photocatalytic activities of the CdS/TiO2 photocatalysts have been reported in many researches and the mechanism of the degradation process has been also deduced and proposed [13,16–18]. However, there were few techniques to detect the active species involved in the degradation process in situ and the evidence for the mechanism proposed was not enough. Furthermore, in our experiments, we found the synergistic effect of the dyes and the photocatalysts largely enhanced the photosensitized degradation of rhodamine B (RhB). Herein, in order to investigate the synergistic effect, we systematically investigated the preparation of the composite CdS/ TiO2 and explored the degradation mechanism of RhB by several techniques in detail. Hopefully, the synergistic effect could be applied in other photocatalysts after our investigation. 2. Experimental 2.1. One step preparation of CdS/TiO2 nanocomposites First, 0.001 mol of analytical grade cadmium acetate (Cd(Ac)22H2O) was dissolved in ethanol to form solution (A). 0.001 mol thiourea and 0.001 mol polyethylene glycol 2000 (PEG) were dissolved in deionized water to form solution (B). Then solution (B) was added dropwise to solution (A) with continuous stirring. After the mixture was stirred for 0.5 h, a certain amount of titania sol (provided by Research Institute of Photocatalysis, Fuzhou University) was added in. The procedure was operated in a Teflon liner with 100 ml capacity. After 1 h of stirring at room temperature, the Teflon liner was then sealed in a stainless steel autoclave and maintained at 200 8C for 12 h. After cooled naturally to ambient temperature, the resultant products were washed several times with water/ethanol and then dried in vacuum at 60 8C for 3 h. For comparison, the CdS was also prepared by the similar method, without the addition of the titania sol. Furthermore, the detailed preparation of TiO2 was shown in the Supplementary materials. 2.2. Characterization of photocatalysts Transmission electron microscopy (TEM) images were collected using a JEOL JEM 2010F microscope working at 200 kV and equipped with an energy-dispersive X-ray analyzer (Phoenix). Xray diffraction (XRD) patterns were recorded on a Bruker D8 Advance X-ray diffractometer with Cu Ka radiation. The UV–vis spectra of various liquid samples and diffuse reflectance spectra (DRS) of the prepared samples were performed on Varian Cary 50 UV–vis spectrophotometer and Varian Cary 500 UV–vis spectrophotometer with an integrating sphere attachment ranging from 200 to 800 nm, respectively. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a ESCALAB 250 photoelectron spectroscopy (Thermo Fisher Scientific Inc.) at 3.0  1010 mbar with monochromatic AlKa Radiation (E = 1486.2 eV). Electron spin resonance (ESR) spectra were obtained using a Bruker model A300 spectrometer with a 500 W Xe-arc lamp equipped with an IRcutoff filter (l < 800 nm) and a UV-cutoff (l > 420 nm) as a visible light source. The settings were center field, 3512 G; microwave frequency, 9.86 GHz; power, 20 mW. The generation of hydroxyl radicals was investigated by the method of photoluminescence technique with terephthalic acid (PL-TA). The photoluminescence spectra were surveyed by Edinburgh FL/FS900 spectrometer. The behavior of pH values in the suspension was monitored by Ross

Ultra Combination pH electrode (ORION 8102BNUWP) with an automatic temperature compensation probe (ORION 927005MD Star ATC Probe). The conductivity test was monitored by conductivity cell (ORION 013605MD). They were both detected on Thermo Fisher Scientific Inc. 5-star meter, which was connected to a computer. 2.3. Tests of photocatalytic activity The visible-light source was a 500 W halogen lamp (Philips Electronics) positioned beside a cylindrical reaction vessel with a plane side. Two cutoff filters (420 and 800 nm) were placed before the vessel to ensure that irradiation of the RhB/CdS/TiO2 system occurred only by visible-light wavelengths. The photo energy density was 230 mW/cm2. The overall system was cooled by wind and water to maintain the room temperature. A 0.04 g portion of catalysts was added to 80 ml of RhB solution (105 mol l1) in a 100 ml Pyrex glass vessel. Prior to irradiation, the suspensions were magnetically stirred for 1 h to ensure the equilibrium of the working solution. After irradiation, at given time intervals, 3 ml aliquots were sampled and centrifuged to remove the catalysts. The degraded solution was analyzed using a Varian Cary 50 Scan UV–vis spectrophotometer and the absorption peak at 552 nm was monitored.

3. Results and discussion 3.1. Physicochemical properties of CdS/TiO2 composites Fig. 1 shows XRD patterns of CdS/TiO2, CdS and TiO2 samples. The molar ratio of CdS/TiO2 in the composite was 12%. It obviously indicated that the CdS/TiO2 was the composite of CdS (#) and TiO2 (*). The phase of TiO2 existed in the composite was the anatase. At the same time, the patterns of CdS/TiO2 composites with different composite ratios are shown in Fig. S1 (see Supplementary data). From this figure, we can find that with the increase of the molar ratio of CdS in the composite, the intensity of the peaks assigned to CdS (#) became stronger and stronger. It verified the composition of the composites again. In the following experiments, we would focus on this CdS/TiO2 composite with CdS/TiO2 = 12%. Fig. 2 shows the DRS of CdS/TiO2 (12%), CdS, and TiO2. The results indicated that the absorption of the nanocomposite in visible light region (l > 400 nm) was clearly more than that of TiO2, which was due to the contribution of CdS. To further obtain information about the structure of the sample, CdS/TiO2 nanocomposite was characterized by TEM. From Fig. S2a, #

* TiO2

(002)

(110)

#

#

CdS

# CdS

#

Intensity

3026

* # #

*

*# # *

CdS/TiO2(12%)

*

(101)

* 10

20

30

40

*

TiO2

*

50

60

70

80

2 Theta / degree Fig. 1. XRD patterns of (a) CdS/TiO2 (12%), CdS, and TiO2 prepared at 200 8C for 12 h.

W. Li et al. / Materials Research Bulletin 48 (2013) 3025–3031

1.0

10

CdS

0.8

C / C0

8

F(R)

3027

6

CdS/TiO2

4

CdS/TiO2-420/800 nm CdS/TiO2-550/800 nm

0.6

0.4 TiO2

2

0.2

0

0.0 300

400

500

600

700

0

Wavelength / nm

20

40

60

80

100

120

Time / min

Fig. 2. DRS of CdS/TiO2 (12%), CdS, and TiO2.

Fig. 4. Comparison of photodegradation of RhB on CdS/TiO2 after light was filtered by a 420/800 or 550/800 nm combined filter.

it indicates that the composite is composed of a large quantity of nanoparticles. As shown in Fig. S2b, two different lattice fringes can be found, allowing for identification of the crystallographic spacings of TiO2 and CdS. The fringes of d = 0.35 and 0.34 nm matched the (1 0 1) crystallographic planes of anatase TiO2 and the (0 0 2) crystallographic planes of hexagonal CdS nanoparticles, respectively. Furthermore, XPS was applied to detect the valence state of elements in the composite. It indicated that the peaks for Cd3d and S2p were found in CdS and CdS/TiO2 samples which were shown in Fig. S3. The chemical valences for Cd and S in the two samples remained the same.

photocatalytic activity of the composite CdS/TiO2 (12%). Fig. 3 shows the photocatalytic activities of CdS/TiO2 (12%), TiO2, CdS, and TiO2xNx (prepared according to the ref [6]). From the comparison, it can be clearly seen that the CdS/TiO2 (12%) sample showed greater activity than that of the other catalysts under identical conditions. We also tested the stability of CdS/TiO2 sample through the detection of the degradation rate of cycling runs for the photodegradation of RhB. In Fig. S5, although the conversion ratios of five cycling runs for the photodegradation are decreased, the sample still remains a certain activity after the fifth run. Therefore, as for the wide application, there are indeed many problems to solve. But as a basic research, the CdS/TiO2 sample with good photocatalytic activity under visible light irradiation in aqueous phase is a good model for the study of the photocatalytic mechanism. In order to distinguish whether the degradation of RhB is driven by the initial excitation of RhB or the CdS/TiO2, two groups of combined filters were used in the degradation system to compare the activities (550/800 and 420/800 nm, Fig. S6). Based on the fact that the light absorption edge of CdS/TiO2 was at l = 540 nm and the maximum absorption peak of RhB was at 552 nm [19], the light was filtered by a 550/800 nm combined filter to avoid the light absorbed by CdS. If the photocatalytic reaction was driven by the excitation of CdS/TiO2, as light was filtered by the 550/800 nm combined filter, there would be no degradation of RhB. Instead, if the degradation was driven by the excitation of RhB, the photocatalytic activity should be the same under the two conditions. From the results shown in Fig. 4, after light is filtered by the 550/800 nm combined filter, although the photodegradation rate of the main absorption peak in CdS/TiO2/RhB system is decreased comparing with that under 420/800 nm combined filter, the RhB is still largely degraded. Therefore, from the comparison, it can be seen that the degradation process was driven mainly by the excitation of RhB and to a lesser extent by the excitation of CdS. The detailed mechanism would be discussed in the following part.

3.2. Photocatalytic activity of CdS/TiO2 nanocomposites Based on the above analysis, it can be found that the composite was exactly composed of CdS and TiO2. The photocatalytic activities of CdS/TiO2 samples with different composite ratios in the degradation of RhB are shown in Fig. S4. After 120 min of irradiation under visible light (420 < l < 800 nm), the photodegradation conversion ratio (PCR) of RhB in the presence of CdS/TiO2 sample (12%) was up to 94%. It was superior to that of the other two samples with different composite ratios. Other comparative experiments were also made to investigate the liquid-phase

1.0 0.8

C / C0

0.6 0.4 0.2

CdS/TiO2 CdS TiO2

0.0

TiO2-xNx

0

20

3.3. Mechanism

40

60

80

100

120

Time / min Fig. 3. Comparison of the photocatalytic activity in the degradation of RhB on CdS/ TiO2, TiO2, CdS, and TiO2xNx under visible light irradiation (420 < l < 800 nm).

Fig. 5 is the changes of the conductivity in the degradation process of RhB on CdS/TiO2, TiO2, and CdS under visible light irradiation (420 < l < 800 nm). Before irradiation, the conductivity of RhB without catalysts was about 4 mS cm1. In the presence of TiO2, the conductivity of RhB was about 16 mS cm1. After light irradiation, the conductivity of RhB in the absence or presence of TiO2 still kept the values. However, in the presence of CdS/TiO2 or

W. Li et al. / Materials Research Bulletin 48 (2013) 3025–3031

80

7.2

RhB blank CdS/RhB TiO2/RhB

light on

dark

CdS

6.6

(CdS/TiO2)/RhB C1=(CdS/TiO2/AO/RhB)-(AO/RhB)

40

C2=(CdS/TiO2/AgNO3/RhB)-(AgNO3/RhB)

6.0

(CdS/TiO2)/RhB/N2

pH

Conductivity / uS cm -1

3028

CdS/TiO2

5.4 RhB blank

0 4.8 light on

dark

4.2

-40 0

1

2

TiO2

3

Time / h Fig. 5. Changes of the conductivity in the degradation process of RhB in the presence of CdS/TiO2, TiO2, CdS, CdS/TiO2/AO, and CdS/TiO2/AgNO3 under visible light irradiation (420 < l < 800 nm).

CdS sample, the conductivity of RhB in each system was increased with the increasing light irradiation time. The conductivity of RhB in the presence of CdS/TiO2 was higher than that of CdS. After 120 min of irradiation, the order of the conductivity values of RhB in the presence of CdS/TiO2, TiO2, and CdS was in accord with the photocatalytic activity (Fig. 3). The transfer of electrons in the photocatalytic process is very important, which would inhibit the recombination of electrons and holes, and thus increase the activity. The separation of electrons and holes was always recognized to be the initial step in the photodegradation mechanism. Therefore, to investigate the mechanism of the process, scavengers for holes and electrons were employed to test their effect on the conductivity. When ammonium oxalate (AO), which is often used as a hole-capturer, is added to the reaction system, the conductivity (C1, shown in Fig. 5) is nearly decreased to zero. In theory, its addition would increase the separation of holes and electrons. However, the increase of the amount of electrons did not increase the conductivity of the system. This indicated that the decrease of the amount of holes played an important role in the degradation process. AgNO3 is an electron capturer. When it is added, the conductivity (C2, shown in Fig. 5) is decreased to minus. The large decrease indicated that the electrons were indeed captured by AgNO3. The decrease of the amount of electrons largely decreased the degradation process. O2 is also an electron-capturer to produce O2 which is an important intermediate species. The absence of O2 would increase the recombination of the electrons and holes and then induce the decrease of the conductivity. Therefore, after N2 was bubbled through the suspension, the conductivity was exactly decreased comparing with that in the original system. This verified the role of electrons again. Therefore, from the three experiments, it indicated that the transfer of electrons in the system was very important. In addition, the holes also played an important role in the process. The temporal evolution of pH values in the degradation process of RhB on CdS, TiO2, and CdS/TiO2 is shown in Fig. 6. Before irradiation, the initial pH values in the three systems were 6.4, 4.4 and 5.8, respectively. After the light was on, the pH values in CdS/ RhB system were increased with the increasing irradiation time, whereas that in the TiO2/RhB system did not change. On the opposite, the pH values in CdS/TiO2/RhB system were decreased. The changes of the pH values can be associated with the changes of the surface charge of the photocatalyst, hydrophobicity, net charge of pollutant, changes in its adsorption modes, and amount of produced OH. This can lead to a modification of the overall rate

0

1

Time / h

2

3

Fig. 6. Changes of pH values in the degradation process of RhB on CdS, TiO2, and CdS/ TiO2.

[20]. Therefore, combined the results of Figs. 5 and 6, the better photocatalytic activity in the presence of CdS/TiO2 can be explained from the following two steps. First, the changes of the surface charge of the photocatalysts affected the pH values in the degradation system. There is a net charge of zero on the catalysts surface at a definite pH of the surrounding solution; this is the zero point of charge (z.p.c.). In the case of TiO2, the z.p.c. of TiO2 is pH < 7 [21]. Water is considered as a neutral solution with pH = 7. According to Pauling’s electrostatic valence rule, when TiO2 was dispersed in water, in order to maintain the electroneutrality, the surface of TiO2 would release some acidic groups. This is the reason that why the TiO2 suspension shows acidic property. In the TiO2/ RhB system under visible light irradiation, as reported in the Ref. [22], the degradation of RhB on TiO2 was actually a photosensitized reaction driven by the excitation of RhB. The TiO2 was just the transporter of electrons and the activity was low. Therefore, in Figs. 5 and 6, the surface properties of TiO2 did not change, so did the conductivity and pH values in the presence of TiO2. In the CdS/ RhB system, the z.p.c. of CdS is close to pH = 7. With visible light irradiation, the surface of CdS would release some groups to maintain the electroneutrality. This led to the increase of the pH values and the conductivity in this system. As for CdS/TiO2/RhB system, the pH values were decreased while the conductivity of the solution was increased. It indicated that the surface properties of CdS/TiO2 changed. Some acidic species on the surface of CdS/TiO2 released to the solution and then induced the changes of the conductivity and the pH values. Second, the adsorption of RhB is affected by the pH values [23,24]. From Fig. 6, it can be seen that the composite of CdS and TiO2 changes the pHzpc of the surrounding solution which would be in favor of the adsorption of RhB. The appropriate pH values in the presence of CdS/TiO2 would enhance the adsorption of RhB and increase the generation of the active species, and then induce the better photocatalytic activity. Furthermore, the conductivity is also associated with the concentration of the species in the solution. The more species are in the solution, the more the conductivity of the solution presents. In the presence of CdS/TiO2, the photocatalytic activity showed the best. And the concentration of the byproducts was also higher than that of the other systems. Therefore, the larger value of the conductivity in the presence of CdS/TiO2 further verified its better photocatalytic activity. To testify the active species existent in the process, ESR spintrap method with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was applied with water or methanol for DMPO-OH and DMPO-O2

W. Li et al. / Materials Research Bulletin 48 (2013) 3025–3031

light irradiation

3029

0.20

Intensity

Absorbance

0.15

b

0.10

0.05

dark

a 0.00 3480

3495

3510

3525

400

3540

Fig. 7. DMPO spin-trapping ESR spectra in CdS/TiO2 methanol dispersion under visible light irradiation (420 < l < 800 nm).

test [25]. As shown in Fig. S7, the intensity ratio of main peaks engendered in CdS/TiO2 water system is about 1:1:1:1 under visible light irradiation. The weak signals are different from the ones reported previously and there is no report about these signals of CdS/TiO2 yet. They may be assigned to other active species generated in the process. For O2 radicals, another important intermediate species, as shown in Fig. 7, six strong and obvious peaks of DMPO-O2 species can be observed under visible light irradiation in methanolic dispersions. This indicated that O2 species were generated under irradiation and would participate in the degradation process. To further investigate the generation of OH, the PL-TA technique was used in the detection [26]. Fig. 8 shows the fluorescence spectra of suspensions containing CdS/TiO2 and TA under visible light irradiation. It can be seen that the fluorescence intensity increases steadily with the irradiation time. Therefore,  OH radicals were indeed generated on CdS/TiO2. In ESR spectra, the reason that OH was not detected may be due to the strong signal intensity of other species. The signal intensity of other species was so strong that the signal of OH may be covered.

50000

50 min 40 min

Intensity

30 min 30000

20 min 10 min 0 min

20000

10000

0 343.0

393.0

443.0

493.0

543.0

593.0

Wavelength / nm 

500

550

600

Wavelength / nm

Magnetic Field / G

40000

450

Fig. 8. OH-trapping PL spectra of suspensions containing CdS/TiO2 and TA under visible light irradiation (420 < l < 800 nm).

Fig. 9. Detection of H2O2 in CdS/TiO2 water dispersions (CdS/TiO2 0.04 g/80 ml). Curve a was obtained by water without catalysts; curve b was obtained by addition of DPD and POD to the dispersions after 1 h of irradiation under visible light (420 < l < 800 nm).

Another important intermediate species H2O2 in the photocatalytic process would be generated by O2 species. The DPD method [27] employed for peroxide measurements was used for the detection of H2O2. In the absence of catalysts, no H2O2 is detected in water (curve a, Fig. 9). In the presence of CdS/TiO2 under visible light irradiation (420 < l < 800 nm), when N,N-diethyl-p-phenylenediamine (DPD) and horseradish peroxidase (POD) were added to the system, the oxidized DPD showed two absorption maxima (at 510 and 551 nm). It showed that in the degradation of RhB, the intermediate H2O2 species were generated. They were inevitable sources of OH species. When the condition that the satisfactory overlap between the conduction band of CdS/TiO2 and the energy level of the excitated dye RhB* (E(RhB*)) was fulfilled, the photocatalytic degradation of RhB can be carried out. Considered the relative positions of the standard oxidation redox potential of the RhB u u ( U(RhB*jRhB+) = 1.09 V, U(RhBjRhB+) = 1.46 V vs. NHE) [28,29]), the transfer of electrons between RhB and CdS/TiO2 was allowed. Therefore, under visible light irradiation, electrons transferred from the adsorbed dye in its singlet excited state to the conduction band of CdS/TiO2. Furthermore, electrons generated in the CdS can transfer to the conduction band of TiO2. Then the transferred electrons can be trapped by surface oxygen to give O2. And active species such as H2O2 and OH would be generated in the following reactions. At the same time, holes generated in CdS can oxide adsorbed RhB dyes directly. The detailed mechanism was shown in Fig. 10. In CdS/TiO2/RhB system, the advantages to increase the photocatalytic activity of the degradation of RhB were as follows: (1) first, the composite increased the visible light absorption of the catalyst. The absorption edge shifted to long wavelength region. In this region, both RhB molecules and CdS were excitated. Therefore, the CdS/TiO2 semiconductors can transfer electrons, injected by the excitated dye and CdS, to a suitable oxidizing agent (O2) at a separate position on the surface of CdS and TiO2. Thus, the recombination of the excitated electrons and holes was effectively inhibited, and RhB+ could undergo N-dealkylation with high efficiency [29]. (2) Second, considered the relative positions of the oxidation redox potential of CdS and TiO2, the heterojunction structure may be formed in the composite, causing the generation of an internal electrical field. Under visible light irradiation, the photogenerated electrons and holes were separated in the build-in

W. Li et al. / Materials Research Bulletin 48 (2013) 3025–3031

3030

Fig. 10. Scheme diagram of the photocatalytic reaction of RhB on CdS/TiO2.

field of CdS and TiO2. There may be more electrons gathered on the surface. As a consequence, the transfer rate of electrons was increased at the same time. (3) Third, as for TiO2/RhB system, only TiO2 was the transporter of electrons and it cannot be excitated under visible light irradiation. But for our CdS/TiO2/RhB system, both CdS and TiO2 can serve as the role. In addition to this, there were photogenerated holes on the composite which may play some roles in the degradation process. (4) Fourth, the appropriate pH values affected the adsorption of RhB and the generation of the active species. For example, pH values may influence the amount of  OH formed. The OH radicals can be generated by the reaction of light-excited holes with H2O/OH. Besides this approach, H2O2 might be another source of OH radicals through the following chain reactions (reactions (6)–(8)). A pH higher than the pKa (reactions (6) and (7)) leads to an inversion of the reaction. And a lack of HO2 inhibits the formation of H2O2. Therefore, at an appropriate pH value, OH can be supplied by both H2O2 and positive holes. In a word, the synergistic effect of RhB and CdS/TiO2 composite largely enhanced the photosensitized degradation of RhB. The following schemes to account for the mechanism were proposed. visible light irradiation

RhB þ hn

!

RhB

(1)

CdS=TiO2 þ RhB  ! RhBþ þ CdS=TiO2 ðe Þ visible light irradiation

CdS þ hn

!

þ

CdS ðhVB þ e CB Þ

(2) (3)

CdSðeCB  Þ ! TiO2 ðeCB  Þ

(4)

eCB  ðe Þ þ O2 ! O2 

(5)

O2  þ Hþ ! HO2  ðpKa1 Þ

(6)

O2  ; HO2  ; H2 O2 ;  OH or hVB þ þ RhBþ ! peroxyorhydroxylated::: intermediates ! degraded products

(9)

4. Conclusions CdS/TiO2 nanocomposite photocatalysts were synthesized by a hydrothermal method. All the characterizations of XRD, TEM and XPS showed the composites were exactly formed by CdS and TiO2. The prepared sample at the conditions 200 8C, 12 h, CdS/TiO2 = 12% possessed the best photocatalytic activity to the degradation of RhB under visible light irradiation. The photosensitized degradation activity of RhB on CdS/TiO2 nanocomposite was obviously superior to that on CdS and TiO2 under identical conditions. The better activity was due to the synergistic effect of the RhB and CdS/ TiO2 composite. Active oxygen free radicals have been tested and were mostly responsible for the degradation. The mechanism was discussed at last. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (21173047 and 21073036), Open Project from Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control of Nanjing University of Information Science and Technology (KHK1118) and the Innovation Platform for Superiority Subject of Environmental Science and Engineering of Jiangsu Province, and Scientific Research Foundation of Qufu Normal University. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.materresbull.2013. 04.057. References

eCB  ðe Þ þ HO2  þ Hþ ! H2 O2 ðpKa2 Þ

(7)

H2 O2 þ eCB  ðe Þ !  OH þ OH

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