Hydrothermal Synthesis of CdSe Quantum Dots and Their

Article pubs.acs.org/IECR

Hydrothermal Synthesis of CdSe Quantum Dots and Their Photocatalytic Activity on Degradation of Cefalexin Xinlin Liu,†,‡ Changchang Ma,‡ Yan Yan,‡ Guanxin Yao,§ Yanfeng Tang,‡ Pengwei Huo,‡ Weidong Shi,‡ and Yongsheng Yan*,‡ †

School of Material Science and Engineering and ‡School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China § Yancheng Institute of Technology, Yancheng 224051, China ABSTRACT: In this paper, CdSe quantum dots (QDs) photocatalysts were successfully synthesized by a hydrothermal method. The factors affecting synthesized CdSe QDs were investigated under different experimental conditions. CdSe QDs were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), ultraviolet−visible diffuse reflectance spectrophotometry (UV−vis DRS), photoluminescence (PL) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The photocatalytic activities of CdSe QDs were evaluated on the decomposition of cefalexin under UV irradiation. The results showed that the degradation ratio of Cefalexin could reach 70.34%. Based on the results, the possible mechanism of the photocatalytic reaction on cefalexin with CdSe QDs was discussed.

1. INTRODUCTION Cefalexin, one kind of cephalosporin antibiotics, has been widely used in the treatment of diseases. However, these antibiotics are very difficult to decompose completely and their residues left in the environment can give rise to the development of antibiotic-resistant bacteria and pose adverse health effects to humans. In order to solve this problem, it is necessary to introduce an effective method. Photocatalytic technology, as a prospective method, exhibits a high efficiency in oxidizing some organic compounds with high degradation ratios and of low cost. Recently, some studies on the photodegradation of antibiotics have been reported,1−3 but there is little report on photocatalysis using quantum dots (QDs) as a photocatalyst. In the past decade, quantum dots (QDs) as a novel fluorescence nanomaterial have attracted great interest. They have two advantages: their band gap can be modified by varying the size to tune the visible response and they can utilize hot electrons or generate multiple charge carriers with a single highenergy photon.4 Due to their unique optical and chemical properties, QDs have been used in sensors,5 disease diagnoses,6,7 light emitting diodes,8 infrared photodetectors,9 and photocatalytic reactions.10,11 The synthetic routes for QDs in the literature can be principally divided into two types: organic-phase12 and aqueous-phase approaches.13,14 Usually, trioctylphosphine (TOP) was used as solvent for the organic synthesis of QDs at high temperatures.15,16 Also, different short-chain thiols as capping groups were widely used in the aqueous synthesis of QDs.17,18 The use of thiols provided stability, solubility, and surface functionality for the QDs.19 As one of the QD nanomaterials, CdSe is an n-type semiconductor. Its band gap energy is reported to be 1.75 eV.20 CdSe QDs have broad absorption spectra, with a first absorption peak and emission profiles that span a wide range of wavelengths in the visible region, both of which shift to longer wavelengths upon increase of the particle size. CdSe QDs’ size-tunable © 2013 American Chemical Society

electronic properties make them a prime candidate for electrooptical applications. Much current research on CdSe is focused on understanding their behavior in solar cells and biological labeling. CdSe QDs are also used in photocatalysis with photoresponse materials to improve the photocatalytic activity.21 Costi22 has reported visible light photocatalysis using highly controlled hybrid gold-tipped CdSe nanorods. Under visible light irradiation, charge separation took place between the semiconductor and metal parts of the hybrid particles. Then they were utilized for direct photodegradation of methylene blue. Chun-won23 has synthesized a graphene−CdSe composite by a facile hydrothermal method. The graphene−CdSe composite exhibited a higher photocatalytic activity for methylene blue (MB) solution. Herein, in this paper, 3-mercaptopropionic acid capped CdSe QDs with UV absorption were used as a basis for the photocatalyst to photodegrade cefalexin. During the synthesis, different experimental conditions for the preparation of CdSe QDs and their photocatalytic activity were investigated. The mechanism of photocatalytic reaction on cefalexin with CdSe QDs photocatalyst was also discussed based on our experimental results.

2. EXPERIMENTAL SECTION 2.1. Materials. Several chemicals, including cadmium chloride (CdCl2·2.5H2O), cadmium carbonate (CdCO3), cadmium nitrate (Cd(NO3)2), cadmium sulfate (CdSO4), sodium hydroxide (NaOH), selenium (Se) powder, sodium borohydride (NaBH4), mercaptosuccinic acid (C4H6O4S), L-cysteine hydrochloride (C3H7NO2S·HCl), mercaptoacetic acid (C2H4O2S), sodium citrate (C6H5Na3O7), 3-mercaptopropionic acid (C3H6O2S), and cefalexin, were used in this study. All were of analytical grade and were used as received without any further purification. All aqueous solutions were prepared with deionized water. The reagents used Received: May 6, 2013 Accepted: October 8, 2013 Published: October 8, 2013 15015

dx.doi.org/10.1021/ie4028395 | Ind. Eng. Chem. Res. 2013, 52, 15015−15023

Industrial & Engineering Chemistry Research

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for the mobile phase of high performance liquid chromatography (HPLC) including Milli-Q ultrapure water and HPLC-grade methanol were from Dikma Chemical (China). 2.2. Sample Preparation. In this study the CdSe QDs were synthesized via aqueous phase based on a previous publication24 with some modifications. 2.2.1. 3-Mercaptopropionic Acid Capped CdSe QDs. Samples of 0.75 mmol of Se powder and 10 mmol of NaBH4 were added to 5 mLof deionized water under nitrogen atmosphere to form a black mixture. The mixture was continuously stirred under nitrogen until the black color disappeared. The Se source solution containing NaHSe was then obtained. Samples of 0.4 mmol of CdCl2·2.5H2O and 0.65 mmol of 3-mercaptopropionic acid were dissolved in 30 mL of deionized water to form a Cd source solution, and its pH value was adjusted to 9 by using a 1.0 mol/L NaOH solution. The solution was then deoxygenated by bubbling nitrogen for at least 30 min. Then the precursors were transferred to an autoclave, sealed, and hydrothermally treated at 160 °C for 40 min. After the container cooled to room temperature, the final products were collected by centrifugation, washed with the deionized water and ethanol several times, and dried in a vacuum at 60 °C for 12 h. Different capping groups (L-cysteine hydrochloride, mercaptosuccinic acid, 3-mercaptopropionic acid, sodium citrate, and mercaptoacetic acid) were employed in these synthesis conditions. 2.2.2. 3-Mercaptopropionic Acid Capped CdSe QDs Prepared with Different Se/Cd Molar Ratios. The Se/Cd molar ratios (5/1, 5/2, 15/8, and 5/4) were altered. The Se ratio was kept constant and the Cd concentration of CdCl2· 2.5H2O was varied. The temperature during these syntheses was at 160 °C. 2.2.3. 3-Mercaptopropionic Acid Capped CdSe QDs Prepared with Different Cadmium Sources. The cadmium source was adjusted by addition of CdCl2·2.5H2O, CdCO3, Cd(NO3)2, and CdSO4. The fixed Se/Cd molar ratio was 15/8. The temperature during these syntheses was at 160 °C. 2.2.4. 3-Mercaptopropionic Acid Capped CdSe QDs Prepared at Different Temperatures. The temperature of CdCl2·2.5H2O was varied from 120 to 200 °C. The fixed Se/ Cd molar ratio was 15/8. 2.3. Characterization. The photoluminescence (PL) spectral analysis of the samples was carried out by adding 1 mg of CdSe QDs prepared with different conditions to 5 mL of ethanol solution at room temperature as a suspension solution and measuring with a QuantaMaster and TimeMaster spectrofluorometer under a 270 nm excitation light source with PMT voltage of 500 V and slit width of 5 nm. The X-ray diffraction (XRD) technique was used to characterize the crystal structure of asprepared photocatalyst. In this work, XRD patterns were obtained with a D/max-RA X-ray diffractometer (Rigaku, Japan) equipped with Ni-filtered Cu Kα radiation (40 kV, 200 mA). The 2θ scanning angle range was 10−90° with a step of 0.02°/0.2 s. X-ray photoelectron spectroscopy (XPS) data were recorded with a PHI5300 spectrometer using an Al Kα (12.5 kV) X-ray source. Transmission electron microscopy (TEM) images were collected on an F20 S-TWIN electron microscope (Tecnai G2, FEI Co.), using a 200 kV accelerating voltage. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet NEXUS 470 FT-IR (Thermo Electric Co.) with 2 cm−1 resolution in the range 400− 4000 cm−1, using KBr pellets. UV−vis absorption spectra of photocatalyst powder were obtained for the dry-pressed disk samples using a Specord 2450 spectrometer (Shimadzu, Japan)

Figure 1. Powder XRD patterns of CdSe QDs prepared with different cadmium sources: (a) CdCO3; (b) Cd(NO3)2; (c) CdSO4; (d) CdCl2· 2.5H2O.

Figure 2. FT-IR spectra of (a) 3-mercaptopropionic acid and (b) CdSe QDs.

Figure 3. UV−vis spectra of CdSe QDs prepared with different cadmium sources: (a) CdCl2·2.5H2O; (b) CdCO3; (c) Cd(NO3)2; (d) CdSO4.

equipped with the integrated sphere accessory for diffuse reflectance spectra, using BaSO4 as the reflectance sample. An Agilent Co. high 15016



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spectrometer (MS). TOC (total organic carbon) was measured with Shimadzu TOC-Vwp equipment. 2.4. Photocatalysis Experiments. The photodegradation reactions of cefalexin antibiotic wastewater were carried out at 303 K in a homemade photocatalytic reactor under UV irradiation. The photochemical reactor contained 0.5 g/L catalyst and 100 mL of 15 mg/L aqueous solution of antibiotic wastewater. After 30 min in the dark, it reached an adsorption balance, and its initial absorbency was determined. The photocatalytic reaction was initiated by irradiating with a 300 W Model GGZ300 UV high pressure mercury lamp (365 nm). The sampling analysis was conducted in 10 min intervals. The photocatalytic degradation ratio (Dr) was calculated by the formula

performance liquid chromatography (HPLC) system containing a quaternary pump and an ultraviolet−visible detector (Agilent, USA) was applied to the determination of cefalexin. Agilent ChemStation software was used for the instrument control and data processing. An Eclipse TC-C18(2) reversed-phase column (150 mm × 4.6 mm, 5 μm) was employed for chromatographic separation at the column temperature of 25 °C. The flow rate of the mobile phase of methanol and water was 1.0 mL min−1 with a ratio of 90:10. The injected volume was 20 μL, and the column effluent was monitored at a wavelength of 262 nm. In this work, the degradation mechanism of cefalexin aqueous solution was detected by a Thermo LXQ mass

Dr = [(1 − A i /A 0)]·100%

(1)

where A0 is the initial absorbency of the antibiotic solution which reached absorbency balance and Ai is the absorbency of the reaction solution. The cefalexin concentration was measured by a UV−vis spectrophotometer with the maximum absorption wavelength at 262 nm.

3. RESULTS AND DISCUSSION 3.1. XRD Analysis. Figure 1 shows the XRD patterns of the 3-mercaptopropionic acid capped CdSe QDs prepared with

Figure 4. TEM and HRTEM images of CdSe QDs.

Figure 5. PL spectra of CdSe QDs prepared (A) with different Se/Cd molar ratios, (B) with different capping groups, (C) with different cadmium sources, and (D) at different temperatures. 15017



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different cadmium sources. The diffraction peaks in Figure 1 can be indexed as the cubic CdSe phase by comparison with JCPDS Card File No. 19-0191. Three typically distinct features can be found. The (111) reflection is attributed to 2θ = 25.3°, and the other two peaks appearing at 42 and 49.7° correspond to the (220) and (311) reflections. Though the cadmium sources among Figure 1a−d were different, the main peaks are the same. The above results imply that CdSe QDs were obtained, which is similar to the phase of monodisperse thiolcapped CdSe synthesized in aqueous solution.25 3.2. FT-IR Spectra Analysis. The FT-IR transmittance spectra of 3-mercaptopropionic acid capped CdSe QDs and 3mercaptopropionic acid are shown in Figure 2. On comparison of the two spectra, the most significant observation is the band at 2550 cm−1, which corresponds to S−H stretching of 3mercaptopropionic acid.26 The peaks at 935 and 1710 cm−1 correspond to CO stretching,27 but they are broad and small. The peaks at 1370 and 2970 cm−1 correspond to −CH2 stretching.28 The band at 1040 cm−1 shows the C−O stretching vibrations.29 It is known that 3-mercaptopropionic acid is water-soluble and would readily bind to the surface of CdSe QDs. It is inferred from the present results that the surface of the CdSe QDs is mainly covered with 3-mercaptopropionic acid ligand. The hydrophilic hydroxyl groups face outward and render CdSe QDs water-soluble. 3.3. UV−Vis Analysis. UV−vis absorption spectroscopy is a very common analytical tool used in the characterization of CdSe QDs as the lowest energy absorption feature (the first exciton) and can yield information on the band gap.30 UV−vis spectra for the 3-mercaptopropionic acid capped CdSe QDs prepared with different cadmium sources are illustrated in Figure 3. The absorption edge of the CdSe QDs prepared with CdCl2·2.5H2O sample was red shifted, but the intensity of absorption was lower. As is well acknowledged, the process of photocatalysis is initiated by the light absorption with energy equal to or larger than the band gap energy of semiconductors, which means that the narrower the band gap is the more photoexcited electrons are. Therefore, the light absorption ability plays an important role in determining the photocatalytic activity of a photocatalyst. From the result, it can be estimated that the band gap of 3-mercaptopropionic acid capped CdSe QDs prepared with the CdCl2·2.5H2O sample is 1.53 eV, which is smaller than that of CdSe in the literature.31 3.4. TEM and HRTEM Analysis. Figure 4 shows the TEM and HRTEM images of 3-mercaptopropionic acid capped CdSe QDs. The QDs are spherically shaped, dense and compact in structure with an increasing tendency of agglomeration. The sizes of the QDs were close to 5 nm. The HRTEM image for the CdSe QDs revealed clearly observed lattice planes of CdSe nanoparticles. The lattice spacing of approximately 0.337 nm corresponds to the (111) phase of CdSe QDs in the XRD patterns. Thus, the as-prepared samples were pure CdSe QDs, and the HRTEM results of the QDs were in good agreement with those of the XRD patterns. 3.5. PL Analysis. It is well-known that the recombination of excited electrons and holes causes PL emission. Lesser PL intensity resulted from the lower recombination of electrons and holes in the photochemical process. Figure 5 shows PL spectra of CdSe QDs prepared with different synthesis conditions under the same solvent and concentration. For the PL spectra of Figure 5A, there was a blue shift with the Se/Cd molar ratio of 15/8 compared with other molar ratios, which was attributed to the dots shrinking in size. Also, the intensity

Figure 6. XPS survey spectra of (a) CdSe, (b) Cd 3d, and (c) Se 3d.

of the emission that came from the 15/8 molar ratio was the lowest. Figure 5B shows the PL spectra of CdSe QDs prepared with different ligands in solution. In comparison with the PL intensity, that of the 3-mercaptopropionic acid ligand is lowest among those of the four ligands. Based on these results, the 15018



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due to the existence of 3-mercaptopropionic acid, which as a cap layer passivated the CdSe QDs.33 Figure 6b gives the highresolution XPS spectra of the Cd 3d5/2 region around 404.65 eV and Cd 3d3/2 at 411.53 eV, which is in accord with the literature and confirms the existence of cadmium.34 The Se 3d5/2 region around 53.66 eV is shown in Figure 6c, which confirms the existence of selenium in the QDs.35 The Cd 3d5/2 peak positioned at 404.65 eV and the Se 3d5/2 peak at 53.66 eV gave 351 eV of the difference between the two peaks, which was in agreement with the CdSe nanocrystals.36 3.7. Photocatalystic Experiments. 3.7.1. Photocatalytic Activity of CdSe QDs. The photocatalytic activity of 3mercaptopropionic acid capped CdSe QDs under UV irradiation is shown in Figure 7. From the result, it can be clearly found that the degradation ratio of cefalexin wastewater without any photocatalyst is almost unchanged in 60 min. However, the degradation ratio of cefalexin wastewater with CdSe QDs could reach 70.34%. The result shows that CdSe QDs as photocatalyst can exhibit a remarkable enhancement of photocatalytic activity.

CdSe QDs with 3-mercaptopropionic acid ligand is the best photocatalyst, better than the others. The PL spectra of CdSe QDs prepared with different cadmium sources are shown in Figure 5C. The CdSe QDs ethanol solution exhibits a strong intrinsic emission band centered at 538 nm. With different cadmium sources, the fluorescence intensity of as-prepared CdSe QDs was different, but the emission wavelength had almost no change. Figure 5D shows the PL intensities of CdSe QDs prepared under different temperatures. The PL intensities of the spectra significantly changed with the reaction temperature. For temperatures from 120 to 160 °C, the PL intensities over this temperature range decreased. At 120 °C, a red shift was observed. When the temperature increased to 180 and 200 °C, the PL intensities were increased, but lower than at 120 °C. 3.6. XPS Analysis. The elemental composition of 3mercaptopropionic acid capped CdSe QDs was analyzed by XPS, which has been proven to be a useful tool to investigate the chemical composition.32 Figure 6a shows the XPS spectra of CdSe QDs. The peaks related to Cd, O, C, and Se elements can be observed. The signals of C 1s and O 1s in the QDs are

Figure 9. Influence of CdSe QDs prepared with different Se/Cd molar ratios ((a) 5/1; (b) 5/2; (c) 15/8; (d) 5/4) on the degradation of cefalexin.

Figure 7. Photodegradation of cefalexin with (a) CdSe QDs and (b) blank.

Figure 8. High-performance liquid chromatography (HPLC) analysis of cefalexin photodegradation over CdSe QDs under UV irradiation. 15019



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The photocatalytic degradation process of cefalexin with 3mercaptopropionic acid capped CdSe QDs under UV irradiation was also investigated by means of HPLC analysis. Figure 8 shows the HPLC analysis of the photodegradation of cefalexin. Figure 8 shows a very symmetrical peak of cefalexin at the start. As time goes on, the peak of cefalexin is decreased gradually, and nearly disappears after 60 min. It can be found that the decomposition of cefalexin with 3-mercaptopropionic acid capped CdSe QDs under UV irradiation is rapid and efficient. 3.7.2. Photocatalytic Activity Effect of CdSe QDs Prepared with Different Se/Cd Molar Ratios. Figure 9 shows the degradation ratio of cefalexin by CdSe QDs photocatalyst prepared with different Se/Cd molar ratios. From the result, it is obviously found that the Se/Cd molar ratio with 15/8 photocatalyst possesses the highest photocatalytic activity. It is well-known that the highest PL intensity indicates higher probability of electron−hole recombination in the surface of the CdSe QDs photocatalyst. In a comparison with the four

different photocatalysts, the Se/Cd molar ratio with 15/8 photocatalyst showed low PL intensity, which prevented the electron−hole recombination. It is obvious that the emitted electrons resulting from the recombination between energized electrons and holes is decreased, the PL intensity is decreased, and, subsequently, the photocatalytic activity is increased.37,38 3.7.3. Photocatalytic Activity Effect of CdSe QDs Prepared with Different Capping Groups. In this work, five different capping groups, L-cysteine hydrochloride, mercaptosuccinic acid, 3-mercaptopropionic acid, sodium citrate, and mercaptoacetic acid, have been chosen to functionalize CdSe QDs. Figure 10 exhibits the photocatalytic activities of CdSe QDs prepared with the five different capping groups. There is no obvious photocatalytic activity change except for 3-mercaptopropionic acid capped CdSe QDs. The 3-mercaptopropionic acid ligand is commonly used to render CdSe QDs water-soluble and is advantageous in photocatalysis applications due to the compact thickness of the coating layer. The 3-mercaptopropionic acid molecule has the carboxyl and thiol ends to facilitate the binding of CdSe QDs to other particles. The system started with the

Figure 10. Effect of CdSe QDs prepared with different capping groups ((a) L-cysteine hydrochloride; (b) mercaptosuccinic acid; (c) 3mercaptopropionic acid; (d) sodium citrate; (e) mercaptoacetic acid) on the degradation of cefalexin.

Figure 12. Effect of CdSe QDs prepared at different temperatures on the degradation of cefalexin.

Figure 11. Effect of CdSe QDs prepared with different cadmium sources ((a) CdCl2·2.5H2O; (b) CdCO3; (c) Cd(NO3)2; (d) CdSO4) on the degradation of cefalexin.

Figure 13. Photocatalytic degradation of cefalexin under UV irradiation with (a) CdSe, (b) CdSe + t-BuOH, and (c) CdSe + EDTA-Na. 15020



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photocatalytic oxidation of the surface thiol ligands to form the disulfides using CdSe QDs as the photocatalysts. This photocatalytic process prevented the photooxidation of QDs. Compared to atoms on the CdSe QDs, the binding abilities of the atoms on the curved surfaces may be affected by their diverse structual environments and size dependent electron configruiations.20

Therefore, the 3-mercaptopropionic acid capped CdSe QDs showed higher photocatalytic activity. 3.7.4. Photocatalytic Activity Effect of CdSe QDs Prepared with Different Cadmium Sources. Figure 11 shows the degradation ratio of CdSe QDs prepared with different cadmium sources. From Figure 11, there is no obvious change among the CdSe QDs for the photocatalytic activity under UV irradiation. This illustrates that there is no obvious effect on photocatalytic activity for the CdSe QDs with different cadmium sources. 3.7.5. Photocatalytic Activity Effect of CdSe QDs Prepared at Different Temperatures. Figure 12 shows the degradation ratio of cefalexin by CdSe QDs prepared at different temperatures. The temperature is an important factor affecting the synthesis of QDs and their optical properties. With the temperature varying, the peak position and intensity of the PL spectra changed directly. Also, the transfer rates of dissolved oxygen to the reaction sites on the surfaces of QDs were also changed. Therefore, the photocatalytic activity differed. From the viewpoint of above, it was found that the photocatalytic activities among 140, 160, and 180 °C were not obviously changed. When the temperature decreased or increased, the photocatalytic activity was lower. 3.7.6. Mechanism of Photocatalysis. It is important to identify the main activity oxidant in the photocatalytic reaction

Figure 14. Total organic carbon (TOC) for the photodegradation reaction in the presence of the CdSe QDs photocatalyst under UV irradiation.

Figure 15. LC chromatograms and m/z of degraded cefalexin: (a) blank; (b) cefalexin; (c) degradation of cefalexin in 30 min; (d) degradation of cefalexin in 60 min. 15021



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4. CONCLUSION In conclusion, the CdSe QDs photocatalysts were successfully synthesized by a hydrothermal method. The XRD and XPS spectra of the photocatalysts were assigned to the existence of CdSe QDs. From the PL spectra, it could be found that CdSe QDs prepared with the Se/Cd molar ratio of 15/8, using 3mercaptopropionic acid as the capping group and cadmium chloride as the cadmium source, had a lower PL intensity. It is well acknowledged that the PL emission intensity of a semiconductor is decided by the recombination of photoinduced electron−hole pairs. This means that the CdSe QDs have a lower recombination of photoinduced electron−hole pairs, which improved the photocatalytic activity. In addition, the photogenerated holes can react with the adsorbed water of CdSe QDs surface to give hydroxyl radicals; the produced OH• groups could further react with cefalexin solution to produce CO2 and H2O or other inorganic molecules.

process for understanding the photocatalytic mechanism. The main oxidative species in photocatalytic process could be detected through the trapping experiments of radicals and holes.39 Figure 13 shows the photodegradation of cefalexin with the addition of disodium ethylenediamine tetraacetate (EDTANa) and tert-butyl alcohol (t-BuOH), respectively. The addition of t-BuOH as a hydroxyl radical scavenger caused a higher photocatalytic degradation of cefalexin, while cefalexin degraded little with the addition of the capture of holes (EDTA-Na). This clearly indicated that holes were the main active species of the cefalexin degradation. The photogenerated holes can react with the adsorbed water to give hydroxyl radicals, and the produced OH• groups can further react with cefalexin solution to produce CO2 and H2O or other inorganic molecules.40,41 Figure 14 shows the degree of mineralization of photodegraded cefalexin wastewaters with CdSe QDs photocatalyst under UV irradiation. The initial TOC concentration of cefalexin wastewaters was 39.331 mg/L. After photodegradation reaction for 60 min, the TOC concentration decreased to 18.731 mg/L, which was a conversion of 52.38%. It can be concluded that the cefalexin molecules can be mineralized by CdSe QDs photocatalyst. From the TOC removal ratio, it also inferred that there were a lot of intermediate products generated in the solution. To investigate the direct detection of reactive intermediates in the photocatalytic system of cefalexin degradation, MS was employed precisely to identify the intermediates. The results are shown in Figure 15. From Figure 15 it can be seen that an intense prominent ion with m/z = 347 was the deprotonated cefalexin molecular ion and the peaks of the major byproducts were found. From the analysis of MS, the OH radical, as the oxidated species, attacked the cefalexin and substituted the H of cefalexin, to obtain m/z = 374. The following sequence was yielded as successive ions attacked and their fragmentation occurred upon collision-induced dissociation: m/z = 347 → m/z = 309 (by loss of OH, H, and CH3) → m/z = 265 (by loss of C, O, and NH2) → m/z = 241 (by loss of C) → m/z = 207 (by loss of O) → m/z = 119 (by loss of CH2 and CH). The process of the degradation is shown in Figure 16. At last, the intermediate products would be degraded to small inorganic molecular material.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 511 8879 0187. Fax.: +86 511 8879 1108. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of Jiangsu Province (No. SBK2011460), the Ph.D. Programs Foundation of Ministry of Education of China (No. 20113227110019), the Ph.D. Innovation Programs Foundation of Jiangsu Province (No. CXLX12_0634), and the Natural Science Foundation of China (No. 21207053).

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Figure 16. Process of degradation of cefalexin (C16H17N3O4S) with CdSe QDs photocatalyst. 15022



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Hydrothermal Synthesis of CdSe Quantum Dots and Their

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