Photocatalytic Degradation of Tetracycline Hydrochloride via a ... - MDPI

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Photocatalytic Degradation of Tetracycline Hydrochloride via a CdS-TiO2 Heterostructure Composite under Visible Light Irradiation Wei Li 1,2 , Hao Ding 1, *, Hua Ji 3 , Wenbin Dai 2 , Jianping Guo 2 and Gaoxiang Du 1 1

2 3

*

Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences (Beijing), 100083 Beijing, China; [email protected] (W.L.); [email protected] (G.D.) State Key Laboratory of Solid Waste Reuse for Building Materials, Beijing Building Materials Academy of Sciences Research, 100041 Beijing, China; [email protected] (W.D.); [email protected] (J.G.) Suez Water Treatment Company Limited, 100026 Beijing, China; [email protected] Correspondence: [email protected]; Tel.: +86-010-8232-2982

Received: 9 May 2018; Accepted: 4 June 2018; Published: 8 June 2018

Abstract: A photocatalytic active CdS-TiO2 heterostructure composite was prepared by hydrothermal method and its morphology and properties were characterized. Results indicate that the CdS nanoparticles deposited on the surface of a TiO2 nanoparticles, which was in anatase phase. The particle scale of both of the components reached approximately 15 nm. In comparison to pure TiO2 (410 nm), the light absorption edge of the heterostructure composite was 550 nm, which could extend the light response from UV to the visible region. Under visible light irradiation, the degradation efficiency of tetracycline hydrochloride by the CdS-TiO2 composite achieved 87.06%, significantly enhancing photocatalytic activity than the as-prepared pure TiO2 and commercial TiO2 (Degussa P25). This character is attributed to the synergetic effect of these two components in the absorption of visible light. Keywords: CdS-TiO2 heterostructure composite; photocatalyst; tetracycline hydrochloride

1. Introduction As one kind of N-type semiconductor, titanium dioxide (TiO2 ) has been extensively studied for its excellent properties, such as its low cost, chemical stability, non-toxicity, and high photocatalytic activity [1–5]. Of these merits, photocatalytic application has attracted a great attention due to its potential for efficiently exploiting solar energy to solve the global energy crisis [6,7] and environmental pollution [8]. Nevertheless, it can only absorb ultraviolet (UV) light and cannot be excited by visible light irradiation for its wide band gap energy (≥3.2 eV) [9] and fast recombination of photogenerated electron-hole pairs [10,11]. To take full advantage of visible light, the light response of the semiconductor must be extended from UV to the visible region. To accomplish this, efforts have been made, such as metal doping [12,13], nonmetal doping [14–17], reducing its band gaps by hydrogenation [18,19] and sensitizing with a low band gap semiconductor material [20–26]. Among previous methods, cadmium sulfide (CdS) nanocrystal was widely used to sensitize TiO2 for its high activity and quantum efficiency in the visible light region as a result of its reasonable band-gap energy (about 2.3 eV) [27–31]. Another advantage for this combination of CdS and TiO2 is that the photogenerated electrons in the conduct band of CdS can be transferred to the conduct band of TiO2 while leaving holes in CdS, which effectively prolong the lifetime of the photogenerated charge carriers [32–34].

Nanomaterials 2018, 8, 415; doi:10.3390/nano8060415

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TiO2 while leaving holes in CdS, which effectively prolong the lifetime of the photogenerated charge carriers [32–34]. There are numerous reports reports about about CdS-sensitized CdS-sensitized TiO TiO22 of binary binary semiconductor semiconductor composites, composites, most of 2 and micro-CdS as well as how CdS plays a role in quantum dots of which whichare areabout aboutbulk-TiO bulk-TiO and micro-CdS as well as how CdS plays a role in quantum 2 [27,32,35,36]. Although photogenerated electrons could be excited from valence band to conduction dots [27,32,35,36]. Although photogenerated electrons could be excited from valence band to band of CdS,band it is difficult electrons to transfer conduction of TiO2 band as a result of2 conduction of CdS, itfor is these difficult for these electronstotothe transfer to theband conduction of TiO the area between CdSbetween and TiOCdS 2, leading to 2low photocatalytic efficiency. Until now, as alimited result ofcontact the limited contact area and TiO , leading to low photocatalytic efficiency. little has been paid the attempt of two components in a nano-scale Herein, we Untilattention now, little attention hastobeen paid to the attempt of two components in aparticle. nano-scale particle. present preparation of binary semiconductor composites, composites, using Titanium oxysulfate titanium Herein, awe present a preparation of binary semiconductor using Titaniumasoxysulfate precursor. In this composite, CdS nanoparticles are uniformly decorated on the surface of TiO 2 and as titanium precursor. In this composite, CdS nanoparticles are uniformly decorated on the surface both particles are similar in size. CdS and 2 contacted in nano-scale rather than aggregating of TiO particles are similar inTiO size. CdS and closely TiO2 contacted closely in nano-scale rather 2 and both respectively. This respectively. nanostructureThis provides a higher degree of acontact between CdS and 2 than than aggregating nanostructure provides higherarea degree of contact areaTiO between traditional binary CdS-TiO 2 nanostructures and demonstrates high photocatalytic activity in CdS and TiO2 than traditional binary CdS-TiO2 nanostructures and demonstrates high photocatalytic degradation within a within tetracycline hydrochloride solution. Our research provides an insight in activity in degradation a tetracycline hydrochloride solution. Our research provides an insight designing highly efficient visible-light photocatalysts which areare based on on a heterostructure as well as in designing highly efficient visible-light photocatalysts which based a heterostructure as well better understanding of the photocatalytic reaction mechanism. as better understanding of the photocatalytic reaction mechanism. 2. Experimental Procedure 2.1. Raw Raw Materials Materials and and Reagents Reagents 2.1. In this this study, study, thioglycolic from Tianjin In thioglycolic acid acid was was purchased purchased from Tianjin Guangfu Guangfu Fine Fine Chemical Chemical Research Research Institute (Tianjin, reagents, including titanium precursor Titanium oxysulfate—sulfuric Institute (Tianjin,China). China).Other Other reagents, including titanium precursor Titanium oxysulfate— acid hydrate xH22O), cadmiumcadmium acetate, and sodium were purchased 2 SO44··xH sulfuric acid (TiOSO hydrate4 ·xH (TiOSO SO4ethanol, ·xH2O), ethanol, acetate, andsulfide, sodium sulfide, were from Aladdin Chemical Co., Ltd. (Shanghai, China). All reagents were of analytical grade, without purchased from Aladdin Chemical Co., Ltd. (Shanghai, China). All reagents were of analytical grade, further purification. Figure 1 shows scanning electronic microscope (SEM) images of titanium precursor without further purification. Figure 1 shows scanning electronic microscope (SEM) images of · xH SO · xH O). (TiOSO 4 precursor 2 4 (TiOSO 2 titanium 4·xH2SO4·xH2O).

Figure 1. Micrographs of titanium precursor-Titanium oxysulfate. Figure 1. Micrographs of titanium precursor-Titanium oxysulfate.

2.2. Preparation Method 2.2. Preparation Method 2.2.1. Synthesis of TiO2 Nanoparticles 2.2.1. Synthesis of TiO2 Nanoparticles 8.00 g of Titanyl sulfate (TiOSO4) was added to 145 mL absolute ethanol (molar ratio = 1:50) and 8.00 g of Titanyl sulfate (TiOSO4 ) was added to 145 mL absolute ethanol (molar ratio = 1:50) and kept magnetic stirring for 24 h. After mixing evenly, 40 mL suspension was extracted and added to a kept magnetic stirring for 24 h. After mixing evenly, 40 mL suspension was extracted and added 50 mL autoclave. The solvothermal treatment in the autoclave was processed at 110 °C for◦ 24 h. to a 50 mL autoclave. The solvothermal treatment in the autoclave was processed at 110 C for Afterwards, the white precipitate was filtered using a vacuum filter holder (Tianjin Jinteng 24 h. Afterwards, the white precipitate was filtered using a vacuum filter holder (Tianjin Jinteng Experimental Equipment Co., Ltd., Tianjin, China), washed thoroughly with absolute ethanol, dried Experimental Equipment Co., Ltd., Tianjin, China), washed thoroughly with absolute ethanol, dried in

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vacuum oven (Gongyi Yuhua Instrument Co., Ltd., Gongyi, China) at 100 ◦ C, and finally calcined in a furnace (Beijing Zhongke Aobo Technology Co., Ltd., Beijing, China) at 550 ◦ C for 3 h [37]. 2.2.2. Preparation of CdS-TiO2 Heterostructure Composites To load the CdS onto the TiO2 surface, a reported procedure in the literature was introduced [38]. 0.16 g of TiO2 nanoparticles was put into 50 mL deionized water and stirred for 0.5 h. Next, 4 mL of Cd(CH3 COO)2 solution (0.1 M), 300 µL of analytical grade thioglycolic acid and 4 mL of Na2 S solution (0.1 M) were added into the suspension, respectively. After magnetic stirring for 1 h, 40 mL of the suspension was added to a 50 mL Teflon-lined stainless steel autoclave (Shanghai Kesheng Instrument Co., Ltd., Shanghai, China) and heated at 160 ◦ C in an oven (Gongyi Yuhua Instrument Co., Ltd., Gongyi, China) for 14 h. The autoclave was then cooled at room temperature. Afterwards, the product was centrifuged (Shanghai Anting Scientific Instrument Factory, Shanghai, Chian) and then washed with deionized water. Subsequently, the yellow powder was dried at 60 ◦ C in a vacuum oven for 10 h. For comparison, a pure CdS nanoparticles was also synthesized following the same protocol described above, without the addition of TiO2 . 2.3. Characterization 2.3.1. Characterization of Structure and Morphology The products were characterized by X-ray diffraction (XRD) in reflection mode (Cu Kα radiation) on an UltimaIV X-ray diffractometer (Rigaku, Japan) at a scanning rate of 4◦ /min in 2θ ranging from 15◦ to 85◦ (λ = 0.15418 nm). The particle size and morphology was visualized using a field-emission scanning electronic microscope (FESEM) (Gemini SEM 500, Carl Zeiss, Jena, Germany) with the energy dispersive X-ray (EDX) (X-Max Extreme, Oxford Instruments, Oxford, UK) spectrum analysis capability, operating at accelerating voltages of 20 kV. Transmission electron microscopy and high-resolution transmission electron microscopy (HRTEM) (FEI Tecnai G2 F30 TEM, Hillsboro, OR, USA) The electron accelerating voltage was 300 kV. A small amount of the sample was first dispersed in alcohol by sonication. One drop of the suspension was then dropped onto a thin, hole-filled carbon film. The girds were then dried under an infrared lamp (Shanghai Kesheng Instrument Co., Ltd., Shanghai, China) for 10 min before TEM measurement. The optical properties of CdS-TiO2 heterostructure composite as well as pure TiO2 and pure CdS nanoparticles were investigated using an Ultraviolet-visible Lambda 365 diffuse reflectance spectrophotometer (PerkinElmer, Waltham, Massachusetts, USA), which was equipped with a Labsphere diffuse reflectance accessory using a standard white board as a reference. In addition, the adsorption values of the tetracycline hydrochloride concentration were also measured by Lambda 365 UV-vis spectrometer. 2.3.2. Photocatalytic Degradation of Tetracycline Hydrochloride under Visible Light Irradiation The photocatalytic degradation of tetracycline hydrochloride was carried out at room temperature in an 80 mL self-designed quartz photochemical reactor containing 50 mL of aqueous solution (50 mg/L). 50 mg of sample was dispersed in the solution and then the suspension was stirred for 1 h to reach the adsorption-desorption equilibrium. All reactors were irradiated using a 500 W Xenon lamp (Beijing NBeT Technology Co., Ltd., Beijing, China) with an ultraviolet filter (λ > 400 nm) (Nbet) to cut off UV light [11,39]. 5 mL of the reacted solution was extracted from the quartz reactor at a given irradiation time interval and then measured using a UV-vis spectrometer at the maximal absorption wavelength of 356 nm to calculate the degradation efficiency (C/C0 ).


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3. Results and Discussion Nanomaterials 2018, 8, x FOR PEER REVIEW

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3.1. Structure and Morphology of CdS-TiO2 3. Results and Discussion

3.1.1. Phase and Chemical Constitution of CdS-TiO2 Heterostructure Composite 3.1. Structure and Morphology of CdS-TiO2

Figure 2 presents the diffraction patterns of the pure TiO2 nanoparticles, the pure CdS 3.1.1. Phaseand and the Chemical Constitution of CdS-TiO2 Heterostructure Composite nanoparticles, CdS-TiO 2 heterostructure composite with 50% (w/w) of CdS. For the TiO2 ◦ , 37.80◦ , 48.0◦ , 53.9◦ , 55.1◦ , 62.7◦ , and 75.1◦ in the XRD nanoparticles, at 25.3patterns Figurethe 2 diffraction presents thepeaks diffraction of the pure TiO2 nanoparticles, the pure CdS nanoparticles, and the CdS-TiO 2 heterostructure composite with 50% CdS. planes For theofTiO 2 pattern can be attributed to the (101), (004), (200), (105), (211), (204), and (w/w) (215) of crystal anatase nanoparticles, the diffraction peaks at 25.3°, 37.80°, 48.0°, 53.9°, 55.1°, 62.7°, and 75.1° in the XRD TiO2 (JCPDS no. 21-1272) [20], respectively. From the XRD patterns of CdS, it can be seen that the ◦ , 28.2 ◦ , 43.7 ◦ , 47.9 ◦ are planes patternpeaks can be at attributed to the (105), (211), (204),◦ ,and (215) of anatase diffraction 2θ values of(101), 24.8◦(004), , 26.5(200), and 51.9crystal in good agreement TiO2 (JCPDS no. 21-1272)[20], respectively. From the XRD patterns of CdS, it can be seen that the with the (100), (002), (101), (110), (103), and (112) crystal planes of the hexagonal structure of the CdS diffraction peaks at 2θ values of 24.8, 26.5, 28.2, 43.7, 47.9, and 51.9° are in good agreement with the (JCPDS no. 75-1545), respectively [40]. All the XRD patterns of the CdS-TiO2 are consistent with both (100), (002), (101), (110), (103), and (112) crystal planes of the hexagonal structure of the CdS (JCPDS the anatase TiO and greenockite CdS, indicating that the heterostructure composite is composed no. 75-1545),2 respectively [40]. All the XRD patterns of the CdS-TiO2 are consistent with both the of theanatase two phases. also calculated the average crystal sizes of greenockite CdS, anatase TiO2 andWe greenockite CdS, indicating that the heterostructure composite is composed of theTiO2 , and CdS-TiO composite nanoparticles via the peak width of the (002) plane of greenockite CdS and 2 two phases. We also calculated the average crystal sizes of greenockite CdS, anatase TiO2, and CdSthe (101) of anatase TiO2 by formula as follows). The CdS results TiO2plane composite nanoparticles viaScherrer’s the peak width of the(shown (002) plane of greenockite andare the shown (101) in Table plane 1 [41].of anatase TiO2 by Scherrer’s formula (shown as follows). The results are shown in Table 1 [41]. Kλ D = Kλ = β cos θ cos

wherewhere K is aK constant (shape factor, about X-raywavelength, wavelength, is the FWHM of the is a constant (shape factor, about0.89), 0.89), λλ is is the the X-ray β isβ the FWHM of the diffraction line, line, and and θ is θthe diffraction angle. diffraction is the diffraction angle. The results indicate thatthat thethe average of pure pureCdS CdSand and TiO close without The results indicate averageparticle particle sizes sizes of TiO 2 are close andand without 2 are any change afterafter forming the the composite, which was preparationrequirements requirements of any change forming composite, which wasalso alsoable ableto tomeet meet the preparation of CdS-TiO 2 heterostructure. CdS-TiO 2 heterostructure.

Figure 2. X-ray diffraction (XRD)patterns patternsof of CdS, CdS, TiO 2 composite. Figure 2. X-ray diffraction (XRD) TiO22, ,and andCdS-TiO CdS-TiO 2 composite.


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Table 1. Table 1. Crystal Crystal size size and and band band gap gap energy energy of of the the samples. samples.

Crystal Size of Nanoparticles Calculated by the Band Gap Energy Crystal Size of Nanoparticles Band Gap Energy (eV) CalculatedPeak by the Peak Width Width (nm) (nm) (eV) CdS CdS 2.30 1919 2.30 1111 3.24 TiO2 TiO2 3.24 in CdS-TiO2 22 CdS inCdS CdS-TiO 2 22 TiO2 in CdS-TiO2 12 TiO2 in CdS-TiO2 12 Sample Sample

3.1.2. Microstructure of 3.1.2. Microstructure of CdS-TiO CdS-TiO22 The The morphology morphology of of the the pure pure TiO TiO22 nanoparticles, nanoparticles, CdS CdS nanoparticles, nanoparticles, and and CdS-TiO CdS-TiO22 composites composites have been analyzed by FESEM. Spherical morphology, as depicted in Figure 3a, shows uniformity with have been analyzed by FESEM. Spherical morphology, as depicted in Figure 3a, shows uniformity a diameter of 15 nm. As nm. shown Figurein 3b,Figure the CdS show an oval-liked morphology with a diameter of 15 Asinshown 3b,nanoparticles the CdS nanoparticles show an oval-liked in the SEM image and their diameter are about 20 nm. Figure 3c shows that, although were morphology in the SEM image and their diameter are about 20 nm. Figure 3c shows that,CdS although deposited, the two components are similarare in similar size, making difficultitto distinguish them via them SEM CdS were deposited, the two components in size,itmaking difficult to distinguish image. Figure 3d shows the distribution of the four elements (Ti, O, Cd, and S) in the composite. via SEM image. Figure 3d shows the distribution of the four elements (Ti, O, Cd, and S) in the Distribution overlappingoverlapping indicates a uniform of CdS and TiO signals Cdsignals and S 2 . The composite. Distribution indicatescombination a uniform combination of CdS and TiO2. of The are greater Ti and O, indicating that the TiO2that particles were decorated by CdS. of Cd and Sthan are greater than Ti and O, indicating the TiO 2 particles were decorated by CdS.

Figure 3. Scanning electronic microscope (SEM) micrographs of TiO2 (a); CdS (b); and CdS-TiO2 Figure 3. Scanning electronic microscope (SEM) micrographs of TiO2 (a); CdS (b); and CdS-TiO2 composite (c) and energy dispersive X-ray (EDX) mapping results of the composite (d). composite (c) and energy dispersive X-ray (EDX) mapping results of the composite (d).

Transmission electron microscopy (TEM) was also applied to verify the fabrication of the CdSelectron microscopy applied to verify the fabrication of the CdS-TiO2 TiO2 Transmission heterostructure. As seen in Figure(TEM) 4a, thewas TiOalso 2 nanoparticles are about 15nm in diameter, which heterostructure. As seen in Figure 4a, the TiO nanoparticles are about 15nm in diameter, which 2 agrees well with the SEM observations. In Figure 4b, the CdS nanoparticles are olive-like in agrees shape well with the SEM observations. In Figure 4b, the CdS nanoparticles are olive-like in shape with with a diameter of about 20 nm. However, in Figure 4c, the CdS nanoparticles aggregates with TiO 2. a diameter of about 20 nm. However, in Figure 4c, the CdS nanoparticles aggregates with TiO 2. The representative high resolution TEM (HRTEM) images revealed that the lattice spacing of the CdSThe representative high resolution TEM (HRTEM) images revealed that the lattice spacing of the TiO2 composite was 0.317 nm and 0.352 nm, which corresponded well to the (101) plane of CdS-TiO was(101) 0.317 nm of and 0.352 TiO nm,2,which corresponded to the (101) plane of 2 composite greenockite CdS and the plane anatase respectively. It furtherwell confirmed the interfacial greenockite CdS and (101) of anatase TiO , respectively. It further confirmed the interfacial junction between CdSthe and TiOplane 2, in which CdS was 2closely attached to that of TiO2. junction between CdS and TiO2 , in which CdS was closely attached to that of TiO2 .

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Figure 4. Transmission electron microscopy (TEM) micrographs of pure TiO2 (a); pure CdS (b) and Figure 4. Transmission electron microscopy (TEM) micrographs of pure TiO2 (a); pure CdS (b) and CdS-TiO2 composite (c). Figure 4. Transmission electron microscopy (TEM) micrographs of pure TiO2 (a); pure CdS (b) and CdS-TiO 2 composite (c). CdS-TiO2 composite (c).

3.1.3. Formation Mechanism of the TiO2 Nanoparticles and CdS-TiO2 Heterostructure Composite 3.1.3. Formation Mechanism of the TiO2 Nanoparticles and CdS-TiO2 Heterostructure Composite 3.1.3. Formation of the TiO2 Nanoparticles CdS-TiO2 Heterostructure The principleMechanism of the morphology evolution isand summarized in Figure 5. Composite A dissolutionThe principle of the morphology evolution is summarized in Figure 5. AFigure dissolution-recrystallization recrystallization process explains the reaction: Although the longin rod-like precursor TiOSO4 The principle of the morphology evolution is summarized 5. A dissolutionprocess explains the reaction: Although the long rod-like precursor TiOSO dispersed in the ethanol dispersed in the ethanol solution, there was still a small amount of water, which was released from 4 recrystallization process explains the reaction: Although the long rod-like precursor TiOSO 4 the crystal water (TiOSO 4 ·xH 2 O). Subsequently, hydrolysis and alcoholysis were triggered under solution, there was still a small amount of water, which was released from the crystal water (TiOSO 4 ·xH2 O). dispersed in the ethanol solution, there was still a small amount of water, which was released from condition of hydrolysis high temperature in the autoclave [42]. It is of assumed that some of OSubsequently, and alcoholysis were triggered under high triggered pressure and high the crystal waterpressure (TiOSO 4and ·xH 2high O). Subsequently, hydrolysis andcondition alcoholysis were under Ti-O bondsin inthe TiOSO 4 were and broken the solvothermal TiO(OH) condition of high pressure in the autoclave [42].and It isthe assumed that2 4precipitated some Otemperature autoclave [42].high It during istemperature assumed that some ofreaction O-Ti-O bonds in TiOSO wereofbroken via hydrolysis Because limited hydroxide radical was provided from the crystal water,limited the Ti-O inreaction. TiOSOreaction 4 were broken during the solvothermal reaction and the TiO(OH) 2 Because precipitated during thebonds solvothermal and the TiO(OH) via hydrolysis reaction. 2 precipitated product could not develop but became a nanoparticle in situ. As the reaction progressed, the threevia hydrolysis reaction. Because limited hydroxide radical was provided from the crystal water, the hydroxide radical was provided from the crystal water, the product could not develop but became a product could notAs develop a nanoparticle in situ. and As the progressed, the TiOSO threedimensional framework of but the became raw material decomposed thereaction long rod precursor 4 nanoparticle in situ. the reaction progressed, the three-dimensional framework of the raw material dimensionalreplaced framework of the composed raw material decomposed and the long rod precursor TiOSO4 disappeared, by blocks of numerous nanoparticles. Anatase TiO 2 was obtained decomposed and the long rod precursor TiOSO4 disappeared, replaced by blocks composed of numerous disappeared, by blocks Anatase TiO2 was obtained after calcinationreplaced at 550 °C h. composed of numerous nanoparticles. nanoparticles. Anatase TiO2 for was3 obtained after calcination at 550 ◦ C for 3 h. after calcination at 550 for 3 h. radicals can be absorbed onto a TiO2 surface in aqueous solutions. It is well known that°Chydroxyl It is well known that hydroxyl radicals can be absorbed onto a TiO2 surface in aqueous solutions. It is well knownion thatcan hydroxyl radicals absorbed a TiO2 surface in aqueous solutions. Moreover, cadmium be drawn to a can TiObe 2 surface in onto the presence of hydroxyl. When S2−2was − Moreover, cadmium ion can be drawn to a TiO surface in the presence of hydroxyl. When 2 Moreover,into cadmium ion canCdS be drawn to a TiO in the to presence hydroxyl. S2−Swaswas introduced the solution, precipitated out2 surface and attached the TiOof 2 particles. InWhen the meantime, introduced intointo thethe solution, CdS precipitated out tothe theTiO TiO In the meantime, 2 particles. introduced solution, CdS precipitated outand and attached attached to 2 particles. the meantime, a CdS-TiO 2 heterostructure was generated, composed of two components of similarInsize. a CdS-TiO heterostructure was generated, composed of two components of similar size. 2 a CdS-TiO 2 heterostructure was generated, composed of two components of similar size.

Figure compositethrough throughaatwo-step two-step Figure5.5.Schematic Schematicofofpreparation preparationroute route of of CdS-TiO CdS-TiO22 heterostructure heterostructure composite Figure 5. Schematic of preparation route of CdS-TiO heterostructure composite through a two-step process. process. 2 process.

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3.2. Properties of CdS-TiO CdS-TiO22 Composite 3.2.1. 3.2.1. Light Light Absorption Absorption Ability Ability of of the the CdS-TiO CdS-TiO22 Composite Composite The The diffuse diffuse reflection reflection UV-vis UV-vis absorption absorption spectra spectra of of CdS-TiO CdS-TiO22 heterostructure heterostructure composite composite is is shown shown in Figure 6, together with those of pure TiO and CdS as comparison. It is evident that pure 2 in Figure 6, together with those of pure TiO2 and CdS as comparison. It is evident that pure TiO TiO22 nanoparticles could only absorb up to 410 nm, which mostly belonged to the UV region because nanoparticles could only absorb up to 410 nm, which mostly belonged to the UV region because of of its its wide wide energy energy band band gap gap (3.2 (3.2 eV) eV) [43] [43] and and was was unlikely unlikely to to respond respond to to visible visible light. light. In In comparison, comparison, the could reach 530530 nmnm arising from the band absorption [38]. the absorption absorptionfeatures featuresofofCdS CdSnanoparticles nanoparticles could reach arising from the band absorption After sensitization with with CdS, CdS, the absorption edgeedge of CdS-TiO processed an obvious red 2 composite [38]. After sensitization the absorption of CdS-TiO 2 composite processed an obvious shift that broadened to about 550 nm, which showed strong absorption capability in the visible light red shift that broadened to about 550 nm, which showed strong absorption capability in the visible region. The existing difference in absorption edge wavelength for pure CdS-TiO 2 and 2 clearly2 light region. The existing difference in absorption edge wavelength forTiO pure TiO 2 and CdS-TiO indicates that thethat light process of TiOof and that of the 2 was clearly indicates theabsorption light absorption process TiO2altered was altered and the thatphoto-response the photo-response of CdS-TiO as greatly improved through sensitization with the CdS nanoparticles [31,44]. The band gap 2 the CdS-TiO2 as greatly improved through sensitization with the CdS nanoparticles [31,44]. The band energy of pure CdS CdS and pure TiO2 TiO was2 estimated by the formula [45–47]. gap energy of pure and pure was estimated byfollowing the following formula [45–47].

= Eg =

1240 1240 λonset

is the the band band gap gap energy energy and and λλonset onset is where EEgg is where isthe theabsorption absorption onset onset determined determined by by linear linear extrapolation extrapolation from the inflection point of the curve to the baseline [41,48,49]. The band gap energy results are are also also from the inflection point of the curve to the baseline [41,48,49]. The band gap energy results shown in in Table Table 1. 1. shown

Figure 6. 6 UV-vis samples. Figure UV-vis spectra spectra of of the the samples.

3.2.2. Photocatalytic Properties of CdS-TiO2 Composite 3.2.2. Photocatalytic Properties of CdS-TiO2 Composite The photocatalytic activities of the CdS-TiO2 heterostructure as well as the sole TiO2 and CdS The photocatalytic activities of the CdS-TiO2 heterostructure as well as the sole TiO2 and CdS nanoparticles were evaluated through degradation of 50 mg/L tetracycline hydrochloride (TH) nanoparticles were evaluated through degradation of 50 mg/L tetracycline hydrochloride (TH) solution solution under visible light irradiation. Although the degradation effect of TH was affected by many under visible light irradiation. Although the degradation effect of TH was affected by many conditions, conditions, such as pH, temperature, type of water matrix (ultrapure water, surface water, or such as pH, temperature, type of water matrix (ultrapure water, surface water, or groundwater), and so groundwater), and so on, in this study, we only examined the TH degradation in ultrapure water. on, in this study, we only examined the TH degradation in ultrapure water. Figure 7 shows the Figure 7 shows the concentration evolution of targeted raw material by the irradiation of visible light concentration evolution of targeted raw material by the irradiation of visible light in the presence of in the presence of different mass ratios of CdS-TiO2 samples (CdS: 0~100 wt %). Because TH is also sensitive to light, for comparison, we performed a blank experiment without any photocatalyst. As expected, the TH almost could not be degraded under the visible light. Additionally, the


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different mass ratios of CdS-TiO2 samples (CdS: 0~100 wt %). Because TH is also sensitive to light, for comparison, performed a blank experiment without any photocatalyst. As expected, the Nanomaterials 2018, 8,we x FOR PEER REVIEW 8 ofTH 12 almost could not be degraded under the visible light. Additionally, the photocatalytic activity of pure TiO2 nanoparticles was of also veryTiO low (7.68%). The degradation efficiency of TH-self pure TiO2 photocatalytic activity pure 2 nanoparticles was also very low (7.68%). Theand degradation was almost negligible. Obviously, the CdS-TiO composite, which had a 50% of CdS, showed the efficiency of TH-self and pure TiO2 was almost negligible. Obviously, the CdS-TiO2 composite, which 2 highest activity and showed the degradation of TH reached 87.06%. This phenomenon was explained the had a 50% of CdS, the highest activity and the degradation of TH reached 87.06%.byThis fact that a higher percentage of CdS could absorb more visible light and yield an efficient transfer phenomenon was explained by the fact that a higher percentage of CdS could absorb more visible of excited electrons from the CdS nanoparticles to thefrom conduction of TiO2 nanoparticles [27]. light and yield an efficient transfer of excited electrons the CdSband nanoparticles to the conduction However, when the mass percentage of CdS decreased 25%, the degradation also decreased to band of TiO 2 nanoparticles [27]. However, when the masstopercentage of CdS decreased to 25%, the about 51.64%. This was a result of fewer electrons able to be generated by CdS under visible light degradation also decreased to about 51.64%. This was a result of fewer electrons able to be generated irradiation. However, higher mass ratio of CdS the (75%) also could activity by CdS under visible the light irradiation. However, higher mass not ratioincrease of CdSphotocatalytic (75%) also could not (61.04%).photocatalytic This may have been (61.04%). a result of a smaller TiObeen proportion, which both diminished the increase activity This may have a result of a smaller TiO 2 proportion, 2 chance of photogenerated electrons moving from CdS to TiO and increased the charge recombination. which both diminished the chance of photogenerated electrons moving from CdS to TiO2 and 2 This explanation also can be applied toThis the pure CdS nanoparticles in which thetodegradation was increased the charge recombination. explanation also can be applied the pure CdS lower (42.26%). nanoparticles in which the degradation was lower (42.26%).

Figure of tetracycline tetracycline hydrochloride hydrochloridesolution solutionininthe thepresence presenceofofdifferent different mass ration Figure 7. 7. Degradation Degradation of mass ration of 2 samples. of CdS-TiO CdS-TiO samples. 2

For further comparison as a reference material, the photocatalytic activity of the commercial TiO2 For further comparison as a reference material, the photocatalytic activity of the commercial (Degussa P25) (Merck, Darmstadt, Germany) was also examined. As shown in Figure 8, the TiO2 (Degussa P25) (Merck, Darmstadt, Germany) was also examined. As shown in Figure 8, degradation efficiency could reach to 50% for P25 TiO2 after 8 h of photoreaction, which showed the degradation efficiency could reach to 50% for P25 TiO2 after 8 h of photoreaction, which showed higher photocatalytic activity than the pure TiO2 synthesized in this study. Although both TiO2 are higher photocatalytic activity than the pure TiO2 synthesized in this study. Although both TiO2 are similar in morphology and structure, the commercial P25 TiO2 particles were composed of two kinds similar in morphology and structure, the commercial P25 TiO2 particles were composed of two kinds of of crystal shape: anatase and rutile TiO2, which is easy to utilize the visible light when compared with crystal shape: anatase and rutile TiO2 , which is easy to utilize the visible light when compared with the the single-phase TiO2 [29]. However, the composite catalyst displayed much greater photocatalytic single-phase TiO [29]. However, the composite catalyst displayed much greater photocatalytic activity activity after the2CdS was introduced. The great improvement of photocatalytic activity of the CdSafter the CdS was introduced. The great improvement of photocatalytic activity of the CdS-TiO TiO2 heterostructure composite can be understood as follow: because the properly aligned2 heterostructure composite can be understood as follow: because the properly aligned conduction conduction bands (CB) existed in the CdS-TiO2 composite, CdS nanoparticles could harness the bands (CB) existed in the CdS-TiO2 composite, CdS nanoparticles could harness the visible light and visible light and effectively transfer photogenerated electrons to the CB of TiO2 [20]. In so doing, TH effectively transfer photogenerated electrons to the CB of TiO2 [20]. In so doing, TH could be degraded could be degraded much more easily than the P25 TiO2 degradation method. much more easily than the P25 TiO2 degradation method.


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Figure 8. Degradation of tetracycline hydrochloride solution in the presence of CdS-TiO2 composites

Figure 8. Degradation of tetracycline hydrochloride solution in the presence of CdS-TiO2 composites and P25 TiO2. and P25 TiO2 . 3.2.3. Mechanism of the Photodegradation by CdS-TiO2 Composite Figure 8.of Degradation of tetracycline hydrochloride solution in in theCdS-TiO presence2 of CdS-TiO2 composites 3.2.3. Mechanism the Photodegradation CdS-TiO The photodegradation mechanism ofby organic molecules heterostructure composite 2 Composite 2. andhas P25 TiO system been discussed in previous studies [26,27,29,50]. As shown in Figure 9, the CdS-TiO2

The photodegradation mechanism of organic molecules in CdS-TiO2 heterostructure composite heterostructure composite featured the direct Z-scheme charge carrier transfer process [51]. Because the(about Photodegradation by CdS-TiO system3.2.3. has been in previous studies [26,27,29,50]. As shown in Figure 9, the(VB) CdS-TiO2 of theMechanism lowerdiscussed bandof gap 2.3 eV), the carrier of CdS2 Composite could be excited from the valence band heterostructure composite featured the direct Z-scheme charge carrier transfer process [51]. Because of to the conduction band (CB)mechanism by the visible light irradiation. combined with the more positive The photodegradation of organic moleculesWhen in CdS-TiO 2 heterostructure composite CB band of TiO 2, the photogenerated electrons could transferred from CB of CdS to the that9,valence ofthe TiOCdS-TiO 2, leaving the lower gap (about 2.3 eV), the carrier ofbeCdS could be from band2 (VB) to system has been discussed in previous studies [26,27,29,50]. Asexcited shown in Figure high oxidation capability vacancies in the VB of charge CdS which could degrade TH. heterostructure featured thelight direct Z-scheme carrier transferdirectly process [51]. Because the conduction band composite (CB) by the visible irradiation. When combined with the more positive CB Simultaneously, these vacancies could be trapped on the surface of the photocatalyst, promoting the of the lower band gap (about 2.3 eV), the carrier of CdS could be excited from the valence band (VB) of TiO2 , the photogenerated electrons could be transferred from CB of CdS to that of TiO2 , leaving high splitting of adsorbed water or OHhydroxyl (·OH). These radicals have to the conduction band (CB)molecules by the visible lightforming irradiation. Whenradicals combined with the more positive oxidation capability vacancies in the VB oxidizing of CdS which could directly degrade TH. Simultaneously, these been as a type of strong agents in which THCB can oxidized during the CB of considered TiO2, the photogenerated electrons could be transferred from ofalso CdSbe to that of TiO 2, leaving vacancies could be trapped on the surface of the photocatalyst, promoting the splitting of adsorbed photocatalytic [52].vacancies Similar-sized CdS VB nanoparticles were located on the surface of TiO high oxidationreaction capability in the of CdS which could directly degrade TH.2 water molecules or OHforming hydroxyl radicals (the ·OH). These radicals have considered as a nanoparticles, enlarging the contact area.on The photogenerated electrons in been the CB of CdS Simultaneously, these vacancies couldsurface be trapped surface of the photocatalyst, promoting the could efficiently transfer toin the CB ofTH TiO 2 at the easilyradicals generating energy holesreaction and of adsorbed water molecules or OHforming hydroxyl (·OH). These radicals have type of splitting strong oxidizing agents which can alsointerface, be oxidized during the high photocatalytic [52]. electrons, which allowed good utilization visible [43]. been considered as a typefor ofwere strong oxidizing agents inlight which TH can also be oxidized during the Similar-sized CdS nanoparticles located onofthe surface of TiO 2 nanoparticles, enlarging the contact photocatalytic reaction [52]. Similar-sized CdS nanoparticles were located on the surface of TiO2 surface area. The photogenerated electrons in the CB of CdS could efficiently transfer to the CB of TiO2 nanoparticles, enlarging the contact surface area. The photogenerated electrons in the CB of CdS at the interface, easily generating high energy holes and electrons, which allowed for good utilization could efficiently transfer to the CB of TiO2 at the interface, easily generating high energy holes and of visible light [43]. electrons, which allowed for good utilization of visible light [43].

Figure 9. Schematic of the direct Z-scheme charge-carrier transfer process in the CdS-TiO2 heterostructure composite.


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4. Conclusions In summary, a CdS-TiO2 heterostructure composite was produced through a simple hydrothermal method by using TiOSO4 as a titanium precursor. In this composite, the CdS nanoparticles were uniformly loaded onto the surface of an anatase TiO2 nanoparticles. Both kinds of particles were similar in size. In comparison with the as-prepared pure TiO2 and Degussa P25, the CdS-TiO2 composite exhibited higher photocatalytic activity for tetracycline hydrochloride degradation, which reached 87.06% under visible light irradiation. The enhanced activity for the CdS-TiO2 composite was attributed to the more effective transfer of the photogenerated electrons due to the larger contact area between the two semiconductors. Author Contributions: W.L. and H.D. conceived and designed the experiments; W.L. performed the experiments; H.J. provided a professional English editing service; W.D. drew the schematic; J.G. and G.D. contributed reagents/materials/analysis tools; W.L. wrote the paper. Conflicts of Interest: The authors declare no competing financial interest.

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