Fabrication of a Novel SnO2 Photonic Crystal Sensitized by CdS Quantum Dots and Its Enhanced Photocatalysis under Visible Light Irradiation

Fabrication of a Novel SnO2 Photonic Crystal Sensitized by CdS Quantum Dots and Its Enhanced Photocatalysis under Visible Light Irradiation

Electrochimica Acta 121 (2014) 352–360 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/elect...

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Electrochimica Acta 121 (2014) 352–360

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Fabrication of a Novel SnO2 Photonic Crystal Sensitized by CdS Quantum Dots and Its Enhanced Photocatalysis under Visible Light Irradiation Sujin Guo, Deyi Li, Yalei Zhang ∗ , Yonggang Zhang, Xuefei Zhou Key Laboratory of Yangtze Water Environment of Ministry of Education, State Key Laboratory of Pollution Control and Resource Reuse, UNEP-Tongji Institute of Environment for Sustainable Development, Tongji University, 1239 Siping Road, 200092 Shanghai, China

a r t i c l e

i n f o

Article history: Received 12 October 2013 Received in revised form 12 December 2013 Accepted 26 December 2013 Available online 8 January 2014 Keywords: Photonic crystal Quantum dot Sensitization Photocatalysis

a b s t r a c t SnO2 photonic crystal (SnO2 PC) sensitized with CdS quantum dots (SnO2 PC/CdS QDs) was structured on FTO substrate to enhance photocatalysis. Scanning electron microscopy indicated that the CdS QDs with an average of 5 nm to 10 nm were dispersed uniformly into the SnO2 PC. The results of the transmittance spectra indicated that the light absorption was redshifted to 500 nm, and that the optical absorption for SnO2 PC/CdS QDs was sharply more than for the SnO2 film in the 400 nm to 600 nm ranges. Meanwhile, good match between the peak position of photonic band gap (PBG) and the absorption of CdS enhanced the photoconversion efficiency up to 16.8 times and improved the photocurrent up to 9 times than that of SnO2 film. The Bode phase plot of the samples were also conducted to further demonstrates the lower recombination of the photogenerated carriers. After 3 hours’ treatment, the removal rate of carbamazepine (CBZ) on SnO2 PC/CdS QDs was 98.8%, and the rate constant (k) on SnO2 PC/CdS QDs was 12.5 times than that on the SnO2 film/CdS QDs. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction As an efficient pollution control technology, photocatalysis has attracted widespread attention due to its low energy consumption, environmentally friendly property, and excellent photocatalytic degradation activity [1–3]. However, the common photocatalyst has shortcomings such as narrow optical response and low quantum efficiency, which limits the further development of photocatalytic technology. Numerous novel semiconductor photocatalytic materials have been designed to improve light harvesting [4–6]. According to these designs, all of the efforts to enhance light harvesting can be classified into two approaches. The first involves metal ion doping [7,8], nonmetal doping [9,10], dye sensitization [11,12], and narrow band gap semiconductor coupling [13,14], which can increase the production of the photogenerated carriers and the quantity of photons absorbed by photocatalysts by extending the photo-response region. The second is to intensify the interaction of light with the photocatalyst by bringing in periodic optical dielectric structures [15,16] which can increase the light harvesting efficiency of the photocatalyst by affecting the motion of photons and electrons.

∗ Corresponding author. Tel.: +86 21 6598 3803; fax: +86 21 6598 8885. E-mail address: [email protected] (Y. Zhang). 0013-4686/$ – see front matter © 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.12.155

The photonic crystals are made of certain optical materials or structures designed to confine, control, and manipulate photons [17,18]. Photonic crystals with ordered macroporous structure give rise to the photonic band gap (PBG) for certain frequencies of light. The light within the wavelength range of PBG cannot propagate in the PC due to Bragg diffraction and scattering, that is, the PC as a dielectric mirror, which increases the effective path length of light in the wavelength range of the photonic band gap [19,20]. Given this property, photonic crystals can be used to enhance the light absorption of photo-responsive material via multiple scattering. For example, Silvia et al. reported that light absorption of dyesensitized solar cells is amplified by introducing one dimensional photonic crystal as a back reflector [21]. At the frequency edges of these photonic stop bands, photons propagate with strongly reduced group velocity in the periodic photonic structures, giving rise to “slow photons” that have a long effective path length through the photonic crystal [22,23]. Chen et al. fabricated TiO2 photonic crystals that can highly improve the photocatalytic activity when the blue edge of the stop-band was accordant with the absorption maximum of TiO2 [24]. Both the band gap scattering effect and the slow photon effect could enhance the interaction of light with photo-responsive material, thus amplifying the optical absorption and photochemical reaction. XieQuan et al. prepared 3D Au modified plasmonic photocatalyst TiO2 photonic crystals that synergistically enhanced light harvesting and demonstrated excellent photocatalysis performance for 2,4-dichlorophenol [5].

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Tin oxide (SnO2 ) is one of the major wide-gap semiconductors that possess certain inherent advantages such as higher electron mobility and more positive conduction band-edge position than TiO2 [25–27]. In other words, SnO2 has a better ability to accept electrons and is intrinsically more resistive than TiO2 . Specifically, suitable antimony-doped tin oxide has favorable electrical conductivity that benefits the transfer of photogenerated electrons [28], and reduces the recombination of photogenerated carriers. SnO2 is also a promising material as adhesion layer because it resembles FTO (F-doped SnO2 ) in crystal structure [29]. Guo-hua Zhao and other researchers have implanted Sb-SnO2 into TiO2 nanotubes to increase their electrochemical performance on pollutant degradation [30]. However, the wide band gap (3.7 eV) of SnO2 leads to a lower utilization rate of visible light (43% of solar light). In this study, we employ a liquid-phase deposition method to obtain the SnO2 PC on the FTO glass substrate, and then CdS Quantum dots are infiltrated into SnO2 PC using sonication-assisted a sequential chemical bath deposition approach. This novel SnO2 PC/CdS QDs photocatalyst has the following functions: (1) The CdS is supposed to serve as a sensitizer for the efficient use of solar energy to extend the light absorption spectra to the visible region; (2) Because of the scattering effect and the slow photon effect, the photonic band gap of SnO2 PC could strengthens the interaction of light with photocatalysis. The good match between the peak position of (PBG) and the absorption of CdS could improve the light harvesting of photocatalyst; (3) The photocatalyst suppresses the electron-hole recombination by forming the FTO/SnO2 /CdS heterojunction; (4) The intimate combination of SnO2 and FTO with the addition of excellent electrochemical performance of SnO2 can transfer photogenerated electrons from the conduction band of CdS promptly, thus preventing the fast recombination of charge carriers; (5) The PC structure can function as a template to provide highly ordered nanochannels for charge carrier transport, and also to provide a large surface area. To the best of our knowledge, this is the first report about combining a sensitizer with photonic crystal structures to increase the photocatalytic activity of SnO2 .

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solution at room temperature for 5 min, and then dried in air. This procedure was repeated three times to ensure that the sol-gel was fully filled in the voids. The polystyrene latex spheres were removed by calcination in a tube furnace at 450 ◦ C for 2 h with a heating rate of 2 ◦ C/min. After calcination, monodisperse polystyrene sphere films result in a highly ordered SnO2 PC. These samples will be referred to as SnO2 PC. 2.3. Synthesis of SnO2 Nanoparticles SnO2 nanoparticles were synthesized via the sol-gel method. The FTO (15 × 40 mm) substrate was immersed vertically in the aforementioned tin dioxide sol-gel solution at room temperature for 5 min. Then, the samples were dried at 100 ◦ C for 10 min and baked in a 500 ◦ C muffle furnace for 10 min. This procedure was repeated 10 times. Finally, the samples were annealed at 450 ◦ C for an hour. 2.4. Preparation of CdS-Sensitized SnO2 PC CdS QDs were fabricated on the SnO2 PC via a sonication-assisted sequential chemical bath deposition (S-CBD) approach. SnO2 PC was sequentially dipped in four different beakers for 2 min in each beaker during sonication. The first beaker contained 0.025 M cadmium chloride (CdCl2 ), the next contained 0.025 M sodium sulfide (Na2 S) solution, and the other two contained DI water. The excess reagent was removed by washing with DI water. Such an immersion procedure was repeated for 5 cycles, which produced yellow samples. The as-prepared yellow samples were dried in a vacuum oven at 80 ◦ C for 24 h. These samples are referred to as SnO2 PC/CdS QDs. 2.5. Characterization

Monodisperse polystyrene latex spheres (190 nm, 5 wt% in water) were purchased as a suspension from Suzhou Nanomicro Tech. FTO glass with 1.1 mm thickness and 15 /sq sheet resistance was obtained from Shenzhen Display Photoelectric Material Co., Ltd. All of the other reagents (analytical grade purity) were purchased from Sinopharm Chemical Reagent Co., Ltd, and were used without further purification.

The amount of deposited SnO2 was determined according to weight method. The content of CdS QDs deposited on SnO2 was investigated by inductively coupled plasma (ICP, Agilent 720ES, Japan). The scanning electron microscopy (SEM) images were taken by a Hitachi S-4800 operating at 3.0 KV. The crystal structure of the catalyst is characterized by X-ray diffraction (XRD, Model D/max2550VB3+/PC, Rigaku, Japan). The X-ray photoelectron spectroscopy (XPS) date was obtained by using a Perkin Elmer PHI 5000 ESCA System with monochromatic Al KR radiation (40 W, 15 KV). The optical absorption characteristics were determined by UV-visible diffuse reflectance spectroscopy (UV-vis DRS, Model BWS002, BWtek) in the 200 nm to 800 nm range. BaSO4 was used as the reflectance standard. The band-gap energy of a semiconductor can be calculated based on the following formula [31]:

2.2. Fabrication of SnO2 PC

˛h = A (h − Eg )

2. Experimental 2.1. Chemicals

The FTO glass substrate was cleaned in ultrasonic bath with acetonum and ethanol for 10 min, respectively, and then rinsed with deionized water (DI) and dried in air. The polystyrene opal template for the photonic crystal was prepared by using a solventevaporation method. The suspension of monodisperse polystyrene spheres (d = 190 nm) was diluted with DI water to a concentration of 0.05 wt%, and then was sonicated in a glass vial for 60 min. The FTO substrates were immersed vertically in the suspension and the water was evaporated at 45 ◦ C overnight in an oven. To obtain the inverse SnO2 opal, the liquid-phase deposition (LPD) method was used to infiltrate tin dioxide sol-gel into the voids of the template. The sol-gel consisted of a molar ratio of Sn/Sb = 20:1 using a mixture of ethanol and concentrated hydrochloric as solvents. The polystyrene opals were dipped into the aforementioned aged

n/2

(1)

where ␣, ␯, Eg , and A are the absorption coefficient, light frequency, band-gap, and a constant, respectively. The parameter n depends on the characteristics of the transition in the semiconductor, i.e., direct transition (n = 1) or indirect transition (n = 4). Here n refer to 4, A refer to 1. According to the Bragg’s law [32] for normal incidence, the position of the transmission dip (␭) can be expressed as:  = 2(2/3)

1/2

D (fnSnO2 + (1 − f )nair )

(2)

where f is the filling fraction of the SnO2 phase, and n (≈2.04) and n (=1.0) are the refractive indices of SnO2 and air, respectively. For FCC structures, f = 0.74, and D is the diameter of the spheres. The photocurrent density was measured via a CHI electrochemical analyzer (CHI 660D) in the standard three-electrode system with a platinum foil cathode and an SCE as reference electrode. A

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Scheme 1. Schematic procedure for preparing SnO2 PC/CdS QDs.

300-W high-pressure xenon short arc lamp was used as a visiblelight source (center wavelength of 420 nm to 600 nm, light intensity of 2 mW/cm2 ), and 0.1 M Na2 SO4 was the electrolyte. The incident light intensity was measured via optical power meter (I190, PerfectLight, USA). The photoconversion efficiency (␩) of light energy to chemical energy in the presence of an external applied potential Eapp is calculated as [33]: 0 %εeff (photo) = jp [(Ere v − |Eapp |)] × 100/I0

(3)

where jp is the photocurrent density (mA/cm2 ), jp Erev 0 is the total power output, |jp Eapp | is the electrical power input, and I0 is the power density of the incident light (mW/cm2 ). Erev 0 is the standard state-reversible potential, which is 1.23 V (versus NHE); the potential corresponding to the Gibbs free energy change per photon in the water-splitting reaction at pH = 0; and the applied potential is Eapp = Emeas - Eaoc , where Emeas is the electrode potential (versus SCE) of the working electrode at which the photocurrent is measured under illumination, and Eaoc is the electrode potential (versus SCE) of the same working electrode under open circuit conditions, under the same illumination, and in the same electrolyte.

Ammonium oxalates (AO) for the holes, AgNO3 for the electrons and Tert-butyl alcohol (TBA) for the • OH radicals. The content of Cd leached off from the SnO2 PC/CdS QDs and SnO2 films/CdS QDs was determined by inductively coupled plasma (ICP, Agilent 720ES, Japan) analysis. The HPLC analyses were performed on an Agilent 1200 (Agilent Technologies, USA) HPLC system equipped with a G1329A auto sampler, a G1311A quaternary pump, aG1322A degasser, a G1314B VWD-detector, and a G1316A column oven. The UV detection wavelength was 230 nm, and the column temperature was set at 30 ◦ C. A Gemini-NX C18 column (250 mm x 4.6 mm, 5 mm) is used for separation. The mobile phase consists of 60% mixed solution of methanol-acetonitrile (1:1, 0.1% acetic acid) and 40% Millipore water (0.1% phosphoric acid). The flow rate was 1.0 mL/min, and the injection volume was 2 mL. Samples were filtered through a 0.45 ␮m syringe filter (Millipore) before injection, and quantification of CBZ was conducted via an external standard method. The linear range was established between 0.1 mg/L and 1.0 mg/L with a correlation coefficient (R2160) of 0.9992. For CBZ, the limit of detection (LOD) was 0.01 mg/L, and the limit of quantization (LOQ) was 0.1 mg/L [34].

2.6. Photocatalytic (PC) degradation, EPR and radicals control experiments 3. Results and discussion The PC degradation of carbamazepine (CBZ) experiment was performed in a round-bottom quartz reactor with magnetic stirring, in which water was recycled to maintain the reaction temperature at 25 ± 2 ◦ C. The SnO2 PC/CdS QDs, SnO2 films/CdS QDs, SnO2 PC and SnO2 films work as the anode; Pt foil was the cathode; the electrode area was 3 cm2 , and the electrode gap was 1.0 cm. The bias potential applied on the photocatalyst was 0.6 V (vs. SCE). The initial concentration of CBZ was 20 mg/L using 0.01 M Na2 SO4 as the electrolyte. The electron paramagnetic resonance (EPR) measurements were performed by a Bruker EMX spectrometer operating at the Xband frequency. The radical control experiments were performed by adding different radicals scavengers during the photocatalytic CBZ degradation: Benzoquinone (BQ) for the • O2 - radicals,

3.1. Morphology Examination The procedure for preparing SnO2 PC/CdS QDs was illustrated in Scheme 1. Polystyrene (PS) opals template on FTO was prepared by solvent evaporation method. The structure of the SnO2 PC was maintained via replicating the polystyrene opals template. After the polystyrene opals were removed by calcination, SnO2 PC was formed. Subsequently, the CdS quantum dots were covered on the SnO2 PC by the S-CBD method. The amount of deposited SnO2 PC was 49.7 mg. The width of the SnO2 PC film was 15 mm. The content of CdS QDs on SnO2 PC was 10.0% wt. The amount of deposited SnO2 films was controlled in 49.0 mg and the content of CdS QDs was controlled in 10.2 wt% by appropriated impregnation times.

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Fig. 1. (a). SEM images of SnO2 PC. (b). SEM images of SnO2 PC/CdS QDs. (c). TEM images of SnO2 PC. (d). TEM images of SnO2 PC/CdS QDs. (e). HRTEM images of SnO2 PC/CdS QDs.

Fig. 1a presents the SEM images of SnO2 PC. An inverse SnO2 opal was composed of highly uniform spherical voids with a diameter of 170 nm. The shrinkage might be due to the decomposition and vaporization of the colloidal crystal template during the calcination process. Fig. 1b presents the SEM images of SnO2 PC/CdS QDs, which shows that well-ordered pore structure still existed, which suggests that the CdS deposition process does not damage the ordered SnO2 PC structure. In order to depict clearly the CdS nanoparticles, TEM images of SnO2 PC and SnO2 PC/CdS QDs were presented in Fig. 1c and d, respectively. As shown in Fig. 1d, the CdS QDs with an average size of approximately 5 nm to 10 nm were uniformly distributed into the SnO2 PC. Moreover, two sets of lattice fringes with spacings of 0.47 and 0.34 nm were observed in Fig. 1e, respectively, which confirmed the presence of the hexagonal phase of SnO2 and the cubic phase of CdS [35,36]. This homogenous dispersion of CdS QDs infiltrated into the SnO2 PC was desirable for efficient light harvesting, yielding high photocatalytic activity for pollutant degradation. 3.2. XRD, XPS and Raman Fig. 2 shows the XRD patterns of SnO2 PC/CdS QDs. All of the dominant peaks of SnO2 PC were similar to the reflection of the

Fig. 2. XRD patterns of SnO2 PC/CdS QDs on FTO substrate.

FTO substrate, which indicated that SnO2 was close to FTO in crystal structure, which guaranteed the intimate combination of SnO2 and FTO. The diffractive peak at 2␪ = 26.7◦ was assigned to the crystal phase of the cassiterite phase. No diffraction peaks of CdS species

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Fig. 4. Raman spectra of SnO2 PC and SnO2 PC/CdS QDs.

Fig. 5. Transmittance spectra of SnO2 film, SnO2 film/CdS QDs, SnO2 PC, and SnO2 PC/CdS QDs.

In order to further determine the crystal phase of CdS in SnO2 PC/CdS QDs, Raman spectroscopy was investigated. As shown in Fig. 4, three peaks were observed, 1-LO at 300 cm−1 and its overtone 2- as well as 3-LO phonon vibrational modes at 598 and 897 cm−1 , respectively. The observed LO Raman peak positions quite agreed with those reported for cubic phase. This is consistent with those reported that low temperature tends to form cubic phase of CdS [35,36,39–41].

Fig. 3. (A). XPS spectra of SnO2 PC/CdS QDs. (B). Cd 3d spectra of SnO2 PC/CdS QDs. (C). S 2p spectra of SnO2 PC/CdS QDs.

3.3. Transmittance Measurements

were observed, which presumably resulted from the low content incorporation of CdS. These samples were further characterized by XPS, as illustrated in Fig. 3. In the XPS survey spectrum (Fig. 3A), no peaks other than Cd and S were observed, which indicated that high purity CdS were synthesized successfully. The XPS spectra for Cd 3d and S 2s peaks were shown in Fig. 3B, C, respectively. The peaks observed at 404 eV and 410 eV were attributed to Cd 3d5/2 and Cd 3d3/2 , and the peaks that appeared at 161 eV and 163 eV were also assigned to S 2p transitions, respectively. All the bindingenergy values for Cd 3d and S 2p were nearly in accord with the reported data of pure CdS in the literatures [35,37,38]. In addition, to quantitative analysis of Cd and S in SnO2 PC/CdS QDs, the Cd 3d5/2 and S 2p3/2 peak areas were determined. As a result, the Cd: S was approximately 1:1, which further confirmed that the as-prepared photocatalyst approximate CdS bulk stoichiometry.

To confirm the enhancement of light harvesting on this SnO2 PC/CdS QDs photocatalyst, the transmittance spectra were characterized as shown in Fig. 5. Almost no optical absorption of SnO2 film was observed in the visible light region of incident light due to the limitation of the wide band gap energy. After the sensitization of CdS quantum dots, SnO2 film/CdS displayed obvious visible light absorption at 490 nm. To fully use the light energy on this wavelength, the PBG of SnO2 PC must be approximately 490 nm. According to Formula 2, we calculated the diameter of the template sphere at around 190 nm. As shown in Fig. 5, the SnO2 PC and SnO2 PC/CdS QDs indicated more efficient light harvesting at 490 nm, and were highly in accordance with the peak positions of CdS quantum dots, which would take full advantage of both the Bragg diffraction effect and slow photon effect of SnO2 PC to enhance the absorption of CdS in the visible light region.

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Fig. 6. Photocurrent densities of SnO2 PC, and SnO2 PC/CdS QDs under visible light irradiation (␭> 420 nm).

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Fig. 8. Bode phase of the SnO2 PC and SnO2 PC/CdS QDs.

Fig. 9. Process of photocatalytic degradation of CBZ under visible light irradiation (␭> 420 nm). Fig. 7. Photoconversion efficiency as a function of bias potential for the SnO2 film, the SnO2 PC, and SnO2 PC/CdS QDs.

3.4. Photoelectrochemical Measurements Fig. 6 shows the photocurrent densities of SnO2 film, SnO2 PC, and SnO2 PC/CdS QDs under visible light illumination (␭ > 420 nm). The photocurrent of pristine SnO2 was approximately 1.09 ␮A/cm2 under visible light irradiation due to the limitation of the wide band gap. The SnO2 PC exhibited a slightly increased photocurrent under visible light irradiation compared with the SnO2 film due to the existence of PBG. The photocurrent of SnO2 PC/CdS QDs was 9 times as large as that of SnO2 film. Fig. 7 reports the photo conversion efficiency as a function of electrode potential for these three samples under visible light illumination. A maximum photo conversion efficiency of 11.8% for the SnO2 PC/CdS QDs is obtained at 0.6 V versus SCE. Accordingly, the photo response of the SnO2 PC/CdS QDs could be up to 16.8 times and 3.6 times higher than that of the SnO2 film and the SnO2 PC, respectively. Two factors lead to this enhancement. First, the good match between the peak positions of PBG and the absorption of CdS QDs increases the efficiency of visible absorption significantly. Secondly, the excellent conductivity of SnO2 PC/CdS QDs and the heterojunction between FTO, SnO2 , and CdS can reduce the recombination of photogenerated carriers, as shown in Fig. 8. The characteristic frequency in the Bode phase plot of the SnO2 PC/CdS QDs shifts to a lower frequency relative to that of the SnO2 PC, which indicates that the charge-recombination rate is reduced in the SnO2 PC/CdS QDs.

3.5. Photocatalytic Performance To further evaluate the photocatalytic activity of these photocatalysts, several experiments were performed for the degradation of CBZ under visible light illumination. The adsorption of CBZ using SnO2 PC/CdS QDs in the dark served as control. Fig. 9 illustrates the evolution of CBZ concentration with elapsed time on the five different photocatalysts. At 180 min, the concentration of CBZ on the SnO2 film showed extremely low photocatalytic ability due to the limitation of the wide band gap. The concentration of CBZ on the SnO2 film/CdS was 14.094 mg/L, with a removal rate of less than 30%. On the SnO2 PC/CdS QDs, the removal rate of CBZ is improved to 98.8%, which indicates almost complete removal. Fig. 10 shows a linear correlation between the logarithm of CBZ concentration and the reaction time of these photocatalysts, which indicates apparent pseudo-first-order reactions. The apparent rate constants (k) for the SnO2 film, SnO2 film/CdS QDs, and SnO2 PC were 7.41 × 10−6 , 3.24 × 10−5 , and 1.57 × 10−5 /s, respectively. For the SnO2 PC/CdS QDs, k reached 4.04 × 10−4 /s, which was 54.5, 12.5, and 25.7 times more than that of the SnO2 film, SnO2 film/CdS QDs, and SnO2 PC, respectively. 3.6. Stability and Cd2+ leaching Lastly, we also investigated the stability of SnO2 PC/CdS QDs and the possibility of Cd2+ leaching during the photocatalytic test. Fig. 11A showed the time profile of eight repeated CBZ photocatalytic test using SnO2 film/CdS QDs and SnO2 PC/CdS QDs under

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Fig. 10. The kinetics of CBZ degradation under visible light irradiation (␭> 420 nm).

Scheme 2. Schematic illustration of the pollutant degradation mechanism using SnO2 PC/CdS QDs under visible light irradiation (␭> 420 nm).

enhanced photocatalytic stability. The results were consistent with the reported in the literature, which demonstrated that the forming of heterojunction and the strong interaction between CdS QDs and SnO2 PC can effectively protect the CdS from photo-corrosion or oxidation of CdS as well as leaching off of Cd [42–44]. 3.7. Mechanism

Fig. 11. (A). Eight repeated CBZ photocatalytic degradation experiments using SnO2 films/CdS QDs and SnO2 PC/CdS QDs. The red line refers to SnO2 PC/CdS QDs and the blue line refers to SnO2 PC. (B). The ICP analysis of residual Cd2+ in the used photocatalysts.

the same experimental conditions. SnO2 film/CdS QDs losed photocatalytic activity (a degradation rate of 5%) only after three repeated CBZ photocatalytic test due to the photo-corrosion of CdS. According to the ICP analysis (Fig. 11B), the Cd in SnO2 film/CdS QDs was leaching off rapidly. There were only 8% Cd remained in SnO2 film/CdS QDs after being used repetitively for 8 times. However, a degradation rate of 80% was retained even after repeating the experiment eight times. The residual Cd in SnO2 PC/CdS QDs was 85% after being used repetitively for 8 times, leading to the

Base on the above experiments, the mechanism for the excellent photocatalytic performance of SnO2 PC/CdS QDs was illustrated in Scheme 2. On the one hand, compared with that of the SnO2 film/CdS, the influence of surface area on the photocatalytic performance of the SnO2 PC photocatalyst was significant and can be divided into three aspects: (1) the macro-mesoporous surface structure can promote the diffusion of organic pollutant molecules through the pores effectively and provide a large amount of the internal contact surface area; (2) the macro-mesoporous surface structure can prevent the CdS quantum dots from aggregating. Thus, CdS QDs were highly dispersed in SnO2 PC in comparison with that of the SnO2 film/CdS; (3) the macro-mesoporous surface structure of SnO2 PC can take full advantage of multiple radiations scattering, Bragg diffraction effect and slow photon effect of SnO2 PC to enhance the absorption ability of CdS in the visible light region. On the other hand, the efficiency of the photocatalytic reaction was restricted by the following reasons: (1) the efficiency of light

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Fig. 12. EPR spectra of SnO2 PC and SnO2 PC/CdS QDs.

photocatalytic degradation. The relative process was displayed in (eqs 11). In addition, when a scavenger (TBA) for • OH radicals was added, the CBZ removal decreased to 68.8%. However, it is well known that the valance band holes (1.7 eV vs. NHE) are not strong enough to oxidize water toward hydroxyl radicals (• OH/H2 O = 2.27 eV). Therefore, the • OH radicals detected from EPR spectra referred to the subsequent reaction of • O2 − radical. In particular, CBZ removal of values down to 31.1% and 18.8% when scavengers for • O2 − radical (BQ) and electrons (AgNO3 ) were added, indicating that electrons and subsequent generated • O2 − radicals were the dominant active species during the photocatalytic process. It is clearly that the • O2 − radical, the • OH radical and holes participated in the photocatalytic degradation over SnO2 PC/CdS QDs. Therefore, the photocatalytic activity was enhanced significantly. The specific process of generation, transfer, and consumption of the charge carriers over SnO2 PC/CdS QDs could be presented as follows [48,49]: CdS + h␯ → CdS(e + h)

(4)

CdS(e) + SnO2 → CdS + SnO2 (e)

(5)

SnO2 (e) + O2 → SnO2 + • O2 −

(6)

•O − 2

+ H2 O → • HO2 + OH−

(7)

• HO

+ H2 OH2 O2 + • OH

(8)

2

H2 O2 + O2 •− → OH• + OH− +O2 OH• + CBZ → degradedormineralizedproducts Fig. 13. Radicals control experiments using different radical scavengers on SnO2 PC/CdS QDs during the CBZ photocatalytic degradation; Benzoquinone (BQ) as scavenger for superoxide radicals (• O2 − ); AgNO3 as scavenger for electrons; Tert-butyl alcohol (TBA) as scavenger for hydroxyl radicals (• OH); ammonium oxalate (AO) for holes.

absorption; (2) the utilization of absorbed light; and (3) the reduction of the recombination process [45]. Any optimized approach that refers to each factor above may improve photocatalytic performance. As shown in Scheme 2, the sensitization of the CdS quantum dots caused the redshift to 500 nm of the optical absorption of SnO2 . After the formation of PC, the cooperation of the band gap scattering effect and slow photons effect enhanced the visible absorption of the photocatalyst. In addition, SnO2 possessed excellent conductivity and more positive ability to accept electrons than TiO2 , so that Sb-SnO2 could act as the conducting line to transfer photogenerated electrons injected from the conduction band of CdS promptly to the surface of FTO, thus avoiding the recombination of charge carriers. The formation of the FTO/SnO2 /CdS heterojunction also promoted the transfer and separation of photogenerated carriers. Specifically, EPR was employed to monitor the generated reactive oxygen species during the electrons transfer process. As shown in Fig. 12, two different types of signals, a major sextet component and a minor quartet component were observed, indicating the generation of DMPO-superoxide radicals and DMPO-hydroxyl radicals, respectively. No radicals’ signals of SnO2 PC were observed. The results were consistent with those reported of CdS QDs in the literatures [46,47]. To validate the radical reaction mechanism for photocatalytic CBZ degradation over SnO2 PC/CdS QDs, several control experiments were performed, as displayed in Fig. 13. When a radical scavenger, such AO for the holes, was added, the CBZ removal moderately decreased, indicating that the valance band holes of CdS QDs may act as reactive species that participated in the

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(9) (10)

CdS(h) + CBZ(orintermediates) → degradedormineralizedproducts

(11)

4. Conclusions We successfully demonstrated the remarkable enhancement of photocatalytic capability under visible light irradiation by depositing CdS quantum dots into SnO2 PC. The sensitization of the CdS quantum dots makes the absorption of SnO2 PC/CdS QDs redshift to 500 nm, which will greatly improve the efficient use of solar energy. This enhanced photocatalytic capability benefited from the enhanced visible light absorption owing to photonic crystal structure and facilitated separation of photogenerated carriers owing to the heterojunction built between SnO2 and CdS. We believe that the configuration and fabrication method of the photocatalyst in this work can not only provide a valuable knowledge for the development of highly efficient photocatalysts, but also open up new perspectives on cooperation of photos and electrons. Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 20976139, 41070641, 51138009, 51278356), the National Key Technologies R&D Program of China (No. 2012BAJ25B02), New Century Excellent Talents in University (NCET-11-0391), and the Project of Shanghai Science and Technology Commision (No. 11QH1402600).

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