Photocatalytic Decomposition of RhB by Newly Designed and Highly

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Photocatalytic decomposition of RhB by newly designed and highly effective CF@ZnO/CdS hierarchical heterostructures Zhenjiang Yu, M.Rajesh Kumar, Yang Chu, Haixia Hao, Qingyao Wu, and Hongde Xie ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02005 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 15, 2017

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ACS Sustainable Chemistry & Engineering

Photocatalytic decomposition of RhB by newly designed and highly effective CF@ZnO/CdS hierarchical heterostructures Z.J. Yua, M. Rajesh Kumara,b, Y. Chua, H.X. Haoa, Q.Y. Wua, H.D. Xiea* a

College of Chemistry, Chemical Engineering and Material Science, Soochow University, Suzhou 215123, China b

Institute of Natural Science and Mathematics, Ural Federal University, Mira 19, Ekaterinburg, Russia

ABSTRACT A two-step hydrothermal method was developed to produce CdS nanoparticle– sensitized ZnO nanoneedle heterostructures, which array on copper fiber (CF), and its applications in the field of photocatalyst were also explored. In this novel heterostructures, the CF plays a role in supporting for the carriers of CdS nanoparticles–sensitized ZnO nanoneedles. Furthermore, CF is capable of accelerating the exportation of charge carriers. Therefore, CF@ZnO/CdS hierarchical heterostructures present excellent photocatalytic performance under visible light, enabling the decomposition of organic dyes such as RhB within 60 min, with desirable cycling ability. Due to the sustainability and engineering potential of CF in chemistry, it is easy to be recycled. This method we developed also conformed to the development of green chemistry without using organic solvents in the whole fabrication process of CF@ZnO/CdS hierarchical heterostructures. This work may pave the way for the useful system to realize efficient charge separation and transmission, which could exert significant influence on the large-scale synthesis of photocatalytic materials with low cost and enhanced performance. KEYWORDS:

Copper fiber; ZnO nanoneedles; CdS nanoparticles; Hierarchical

heterostructures; Photocatalytic activities.

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INTRODUCTION Recently, many efforts have been devoted to the environmental applications of semiconductor photocatalysts to meet the emerging demand in environment fields such as the reduction of heavy metals and the degradation of organic compounds.1-5 Metallic-oxides such as TiO2 (3.30 eV), ZnO (3.27 eV) and WO3 (2.80 eV) have been explored extensively as facile photocatalytic materials because of their high photocatalytic performance and economical synthetic routes6-9 ENREF_6_ENREF_6_ENREF_6_ENREF_6. However, these single photocatalytic nanocrystals have wide band gap, which is adverse to absorption and ENREF_8 transformation of the visible light. To enhance photocatalytic performance,

photogenerated charge carriers should be effectively transmitted inside semiconductor composite heterostructures in the light of different band gap structures of their constituents due to the following important aspects. First, nanocrystals (ZnO) with wide band-gap are expected to harvest effectively visible light by coupling narrow band-gap (CdS) nanoparticles photosensitizers. Second, charge carriers transmission from one nanocrystals to another can cause efficient charge separation. Third, CF possesses excellent conductivity, which can help to minimize the electron-hole pair recombination. Different morphologies of ZnO nanostructures, such as nanoparticles, nanowires, nanobelts and nanoflakes, have been achieved and studied for various application fields from photocatalytic powders to semiconductor devices.5,10–14 It is well-known that different morphologies of ZnO can be produced by a variety of methods such as sol-gel, spray pyrolysis, chemical deposition and co-precipitation. Till now, chemical gas phase deposition method has been widely employed for the growth of one-dimensional ZnO. Although it helps to synthesize uniform ZnO nanostructures, high cost and harsh reaction conditions limit the utilization of this method. For example, metal-organic chemical vapor depositions (MOCVD) and thermal evaporation method have to be conducted at about 500 ºC and 900 ºC, 2


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respectively.15-17 Recently, with the development of semiconductor materials, scientists placed much emphasis on one-dimensional ZnO nanostructures owing to their high photocatalytic performance. However, in order to take fully advantage of visible light and generate more charge carriers, narrow band gap photosensitizers are expected to composite with ZnO nanoneedles. Here, we developed a facile synthesis of ZnO nanoneedles using citric acid as complex agent and PVP as surfactant. For the sake of better environment protection, it is critical to gather and recycle the powder-type photocatalysts after used in treatment. Coupling ZnO with narrow band-gap nanocrystals, such as ZnS, CdSe, CdTe, and CdS, has been believed to be an efficient way to enhance the visible light absorption of ZnO.18-22 Among the above, CdS is believed to be the most applicable visible photosensitizers for ZnO nanocrystals because of the similar lattice structure, which can lead to a close combination between the two types of nanocrystals.23-28 Although, CdS nanocrystals possess excellent photocatalytic performance, the toxicity of CdS limits its application. Meanwhile, there is trouble in gathering and recycling the powder-type CdS. In previous study, much emphasis has been placed on the preparation of CdS nanoparticles/ZnO nanoneedles core/shell heterostructures for photocatalysis and the results indicate that the heterostructures present excellent photocatalytic performance.29-35 CdS nanoparticles/ZnO nanoneedles synthesized by high-temperature methods, such as spray pyrolysis and chemical deposition methods, have been reported,36-39 while hydrothermal route is another critical method due to its low cost and probability for large-scale fabrication.40-44 In this work, hydrothermal method was adopted to produce CdS nanoparticles on a large scale at 140 oC. Meanwhile, to gather powder-type photocatalysts and find out a suitable substrate for powder-type nanoparticles is of great concern. In the past years, carbon fiber was a good carrier applied in photocatalytic fields owing to its low cost and excellent conductivity.45-48 Well known, the conductivity of carriers 3



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can exert a great influence on photocatalytic properties. Copper fiber, as a novel metal nanomaterial, possesses much excellent material performance, such as superior conductivity, large specific area, and outstanding chemical resistance. Therefore, growth of ZnO nanoneedle/CdS nanoparticles heterostructures on CF would be a promising strategy to improve the properties of one-dimensional semiconductor materials. However, the above designing concept for the novel structure hasn’t been investigated by now. Herein, we developed a facile, low-cost and environmental fabrication method for the growth of highquality ZnO/CdS hierarchical heterostructures on CF, as shown in Scheme 1.

Scheme 1. Schematic diagram of the preparation of the CF@ZnO/CdS hierarchical heterostuctures and its application in the decomposition of RhB. CF has a large specific area, which renders the nucleation and growth of a large quantity of ZnO nanoneedles with strong mechanical interactions. Furthermore, copper fiber can tremendously promote the separation and transmission of photo-generated charge carriers 4


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in CdS/ZnO heterostructures due to its excellent conductivity. To measure the photocatalytic performance, rhodamine B (RhB) dye was selected as the photodecomposition resources. To our knowledge, CF@ZnO/CdS hierarchical heterostructure arrays have not been investigated until now. This is the first time reporting on preparing a novel CF@ZnO/CdS hierarchical heterostructures by a facile two-step hydrothermal method. EXPERIMENTAL SECTION Materials Zn(CH3COO)2·2H2O, NaOH, citric acid monohydrate, Na2S·9H2O (flakes) and CF were purchased from Sinopharm Chemical Reagent Co., Ltd. PVP (MW=30K) was purchased from Shanghai Lingfeng Chemical Reagent. CdCl2·2.5H2O was purchased from Shanghai Titan Scientific Co., Ltd. All the chemicals were of analytical grade without further purification. RhB was used to test the photocatalytic performance of CF@ZnO/CdS hierarchical heterostructures. Preparation of CF@ZnO/CdS heterostructures CF@ZnO/CdS hierarchical heterostructure arrays were prepared via a two-step process. Synthesis of homogeneous ZnO nanoneedle arrays on copper fiber: CF@ZnO nanoneedle arrays with ZnO loadings were initially synthesized by chemical bath deposition (CBD). In brief, a certain amount of Zn(CH3COO)2·2H2O (0.25, 0.5, 0.75, 1.00 g) was dissolved in 50 ml de-ionized water, respectively. The solution was stirred magnetically at room temperature. Subsequently, citric acid monohydrates (0.25, 0.5, 0.75, 1.00 g) were added in the above solution, respectively. Then, 0.28 g of PVP (MW=30K) was dissolved in 20 ml de-ionized water and added dropwise into the above solution, respectively. NaOH (0.8, 1.6, 2.4, 3.2 g) was dissolved in 10 ml de-ionized water and added into the above solution drop by drop, respectively. Finally, 32 ml of the above solution was transferred to a Teflonlined autoclave (50 ml capacity) and 0.446 g of CF was immersed in the solution at 160 oC 5



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for 8 h in a homogeneous reactor. The above product is as CF@ZnO (a), CF@ZnO (b), CF@ZnO (c) and CF@ZnO (d), respectively. The photocatalytic activities of CF@ZnO (a), CF@ZnO (b) CF@ZnO (c) and CF@ZnO (d) were studied by the degradation of RhBcontaining wastewater and suspended into 100 ml RhB solution with a concentration of 10 mg L-1 in the presence of a little H2O2, respectively. We notice that CF@ZnO (d) possess excellent photocatalytic activities. Next, CF@ZnO (d) are as the research object which will be used in photocatalytic activities. We also investigate the effect of CA and PVP on the morphologies of CF@ZnO (d). First, we synthesize CF@ZnO in the absence of PVP, and the product is as CF@ZnO (a). Subsequently, CF@ZnO are fabricated in the absence of CA, and the product are as CF@ZnO (b). Then, CF@ZnO synthesized in the presence of both CA and PVP are as CF@ZnO (c). Preparation of CF@ZnO/CdS heterostructure arrays: The uniform CdS nanoparticles grew on the surface of ZnO nanoneedles by hydrothermal method. Briefly, 0.145 g of CdCl2·2.5H2O, 0.153 g of Na2S and 0.05 g of PVP (MW=30K) were added to a certain amount of 32 ml de-ionized water, respectively. The above mixture was transferred to a Teflon-lined autoclave (50 ml capacity) and the CF@ZnO nanoneedle heterostructures were subsequently immersed in the solution at 140 oC for 12 h in a homogeneous reactor. Characterization X-ray diffraction analysis (XRD) equipped with Cu Kα radiation (λ=1.54060 Å) and scanning electron microscopy (SEM; Hitachi S-4800) were used to determine the structural, morphological, elemental composition and size of the CF@ZnO/CdS heterostructures. X-ray photoelectron spectra (XPS; Axis Ultra HAS) were employed to study the effect of the CF, ZnO and CdS constituents of CF@ZnO/CdS on its structure. The photocatalytic performance was carried out by UV-vis diffuse reflectance spectroscopy (UV-vis DRS, Shimadzu UV3600). 6


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Photocatalytic activities analysis The photocatalytic activities of CF@ZnO/CdS hierarchical heterostructures were studied by the degradation of RhB-containing wastewater and suspended into 100 ml RhB solution with a concentration of 10 mg L-1. Before the reaction, the mixtures were stirred in the dark for 30 min to gain adsorption-deposition equilibrium. Then, the solution was irradiated using a Xe lamp (500 W, λ > 420 nm) for 60 min and add a little H2O2. The results of photocatalytic and recycle performance of CF@ZnO/CdS hierarchical heterostructures were also explored. The electrochemical impedance spectroscopy (EIS) EIS measurements of the as-obtained CF, CF@ZnO and CF@ZnO/CdS were performed using electrochemical workstation (CHI-660E) using three-electrode system. Ag/AgCl and Pt were as reference electrode and counter electrode, respectively. CF, CF@ZnO and CF@ZnO/CdS were as working electrode and they are tested in the solution of 0.01 mol L-1 Na2SO4. The frequency range was investigated from 0.1 Hz to 100000 Hz and the initial voltage was set at V=0. Figure S1 shows the EIS of CF, CF@ZnO and CF@ZnO/CdS. RESULTS AND DISCUSSION Fig. 1(a-e) shows the XRD spectra of CF@ZnO/CdS hierarchical heterostructures. To make it clear that CF is initially pure copper metallic, X-ray diffraction was carried out before preparing heterostructures, as is shown in Fig. 1a. All the observed peaks of copper reveals that the polycrystalline nature of the copper fiber and the peaks 43.6o, 50.6o, 73.9o indexed to (1 1 1), (2 0 0) and (2 2 0) planes of fcc structure of copper (JCPDS NO. 85-1326).49 For pure ZnO nanoneedles (shown in Fig. 1b), the diffraction peaks at the value of 31.9o, 34.7o, 36.6o, 47.7o, 56.2o, 63.1o and 68.1o represents (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) planes, respectively, of wurtzite structure of ZnO (JCPDS NO. 89-7102), and the relative 7



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intensity of (0 0 2) planes with the (1 0 0) planes demonstrates the special anisotropic growth of the nanoneedled ZnO along the (0 0 1) planes,50 as shown in Figure S2. XRD patterns of CdS nanoparticles shown in Fig. 1c at the peak values of 26.4o, 43.8o and 51.9o indexed to (1 1 1), (2 2 0) and (3 1 1) planes, respectively, and the peaks are consistent with Bragg reflections of the standard cubic structure (JCPDS NO. 89-0440).51 Fig. 1d indicates that CF@ZnO nanostructures obtained by the first-step hydrothermal route. Moreover, CF was not involved in the reaction and just acted as a substrate.

Figure 1. XRD patterns of (a) copper fiber, (b) ZnO nanoneedles, (c) CdS nanoparticles, (d) CF@ZnO nanostructures and (e) CF@ZnO/CdS hierarchical heterostures. 8


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Subsequently, CF@ZnO/CdS hierarchical heterostuctures were finally synthesized by the second-step hydrothermal method as is shown in Fig. 1e, and CF@ZnO nanostructures did not vary with the hydrothermal reaction. All the diffraction peaks are in good agreement with the fcc structure of copper, wurtzite ZnO and cubic CdS. TEM and HRTEM were characterized to explore more detailed information about the morphology and crystal structure of the heterostructures shown in Fig. 2. Moreover, no impurities diffraction peaks were observed, which indicate

that the high-quality CF@ZnO/CdS hierarchical

heterostructures were successfully prepared.

Figure 2. TEM image of the sample ZnO/CdS; HRTEM image of the interface between core ZnO and shell CdS. Scanning electron microscope (SEM) images of the as-obtained products at different reaction conditions are shown in Fig. (3, 5 & 6). Fig. 3 shows low and high magnification SEM images of the CF@ZnO (a), CF@ZnO (b), CF@ZnO (c) and CF@ZnO (d), which are obtained by using 0.25 g, 0.5 g, 0.75 g and 1.00 g Zn(CH3COO)2, respectively. And the dimension of copper fiber substrate is 5 x 10 cm. From the Fig. 3, we clearly notice that the quality and quantity of CF@ZnO (d) are optimal when the mass of Zn(CH3COO)2 are 1.00 g. 9



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The photocatalytic activities of CF@ZnO (a), CF@ZnO (b) CF@ZnO (c) and CF@ZnO (d) were studied by the degradation of RhB-containing wastewater and suspended into 100 ml RhB solution with a concentration of 10 mgL-1, as shown in Fig. 4. Fig. 4a shows that CF@ZnO (d) possesses excellent degradation rate and efficiency, so CF@ZnO (d) are as the next research objection. The rate constant of CF@ZnO (d) photocatalysis was derived from the liner fitting curve of equation ln (C0/C) vs light time t, as shown in Fig. 4b. The liner fitting curve of equation is ln (C0/C) = 0.013 t(min) + 0.162, and the liner fitting degree R2 = 0.99049 is high, demonstrating that the photocatalytic reaction is similar to the pseudo-first order reaction. Fig. 5 shows SEM images of the CF@ZnO (CA), CF@ZnO (PVP) and CF@ZnO. CF@ZnO (CA) is obtained in the absence of PVP, and CF@ZnO (PVP) are synthesized in the absence of (CA). CF@ZnO is fabricated in the presence of both CA and PVP. It is both CA and PVP that play a crucial role in the formation of excellent CF@ZnO.

Figure 3. Low and high magnification SEM images of the CF@ZnO (a), CF@ZnO (b), CF@ZnO (c) and CF@ZnO (d). 10


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Figure 4. (a) The degradation percentage of CF@ZnO (a), CF@ZnO (b) CF@ZnO (c) and CF@ZnO (d). (b) The fitting curves of sample CF@ZnO (d).

Figure 5. SEM images of the CF@ZnO in the presence of CA (a), CF@ZnO in the presence of PVP (b) and CF@ZnO in the presence of CA and PVP (c). SEM images of as-obtained copper fiber show uniform diameter of approximate 50 µm and smooth surface, as is depicted in Fig. 6(a-c). It was worth noting that copper fiber with 3D microstructure possesses large specific surface area, which is highly beneficial for further modification of photocatalytic materials. Herein, low-temperature and efficient chemical bath deposition (CBD) was developed to grow crystalline ZnO/CdS hierarchical heterostructures, as is shown in Fig. 6(d-i). It is clear from Fig. 6(d-f) that the quasi-vertical aligned ZnO nanoneedles grew on a large surface area of the copper fiber substrate with a diameter of around 150-200 nm and a length of around 1.5-2 µm. The large specific surface area of nanoneedled ZnO provided excellent surroundings to deposit CdS nanoparticles, as 11



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shown in Fig. 6(g-i). From the Fig. 6i, CdS nanoparticles evenly grew on the surface of ZnO in situ with a diameter of approximately 10-15 nm. It is clear from Fig. 6(d-f) that the surface of ZnO nanoneedles is smooth, which is contrary to the rough surface of modified ZnO nanoneedles in Fig. 6(g-i). This kind of high quality and uniform CF@ZnO/CdS hierarchical heterostructures could provide an effective pathway for the transportation of the photogenerated carriers. That is to say, the combined effects of CF, wide band-gap ZnO and narrow band-gap CdS gave rise to excellent photocatalytic properties. Fig. 7 shows a SEM image of CF@ZnO/CdS hierarchical heterostructures and corresponding elemental mapping of Cu, O, Zn, S and Cd, respectively. Notice that Zn and O elements are evenly distributed along the ZnO nanoneedles, while Cd and S are discovered on the same sites in agreement with the location of nanoparticles.

Figure 6. Low and high magnification SEM images of the (a, b, c) copper fiber, (d, e, f) CF@ZnO, and (g, h, i) CF@ZnO/CdS hierarchical heterostructures. 12


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Figure 7. (a) SEM image of CF@ZnO/CdS and corresponding elemental mapping of (b) Cu, (c) O, (d) Zn, (e) S, (f) Cd, respectively. XPS spectra were adapted to determine the surface composition and chemical valence state of the CF@ZnO/CdS hierarchical heterostructures, as is shown in Fig. 8. The spectra provide further evidence for the formation of ZnO nanoneedles and CdS nanoparticles. Fig. 8 shows the XPS of (a) wide scan spectra, (b) Cu 2p, (c) O 1s, (d) Zn 2p, (e) S 2p, (f) Cd 3d positions of CF@ZnO/CdS hierarchical heterostructures. The wide scan spectra for CF@ZnO/CdS hierarchical heterostructures are shown in Fig. 8a and all the peaks on the spectra can be attributed to Cu, O, Zn, S and Cd elements. The presence of C1s peak at 284.5 eV plays a role of reference for binding energy measurement. The two strong peaks are 13



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located at 931.3 and 951.1 eV in Fig. 8b, respectively, which indicates that Cu element still exists steadily in the form of Cu (0) chemical valence state52 after preparing CF@ZnO/CdS hierarchical heterostructures. Therefore, it laid the foundation of photocatalytic application. The peaks at around 531.0 (Fig. 8c), 1044.1 and 1021.1 eV (Fig. 8d), mean that O and Zn element exist in the form of O2- and Zn2+ chemical valence state.53

Figure 8. The XPS of (a) wide scan spectra, (b) Cu 2p, (c) O 1s, (d) Zn 2p, (e) S 2p, (f) Cd 3d positions of CF@ZnO/CdS hierarchical heterostructures. 14


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Fermi level of Cu is lower than that of ZnO. When they come into contact, electrons in ZnO will transfer to Cu, and it will generate an electric field from ZnO towards Cu, which is in good agreement with the other results. The position of S 2p peak (Fig. 8e) is at around 161.3 eV in Fig. 8e, which shows that S element exists in the form of S2- chemical valence state. From Fig. 8f, the two peaks in Cd 3d core level results from the spin-orbit separation between Cd 3d3/2 (411.3 eV) and Cd 3d5/2 (404.6 eV).28 The above results demonstrate that CF@ZnO/CdS hierarchical heterostuctures have been achieved, which is in accordance with XRD and SEM results. UV-vis diffuse reflection spectra (DRS) were adapted to study optical properties of CF@ZnO, CdS nanoparticles and CF@ZnO/CdS, as shown in Fig. 9. It is clear from Fig. 9a that the absorption spectrum of CF@ZnO heterostructure is in the UV region. However, the corresponding spectrum of CdS is in the visible region. To obtain excellent photocatalytic properties, combined the wide band-gap semiconductor of ZnO with narrow band-gap semiconductor of CdS. CF@ZnO/CdS hierarchical heterostructures absorb light across a wider wavelength region, which means that the heterostructures can efficiently harvest visible light. The corresponding band-gap energy is shown in Fig. 9b. According to the wave energy equation (E=hc/λ), the band gap energy of CF@ZnO is 3.30 eV, which is similar with the band gap of pure ZnO.54,55 For cubic phase CdS, the band gap energy is also calculated simply, which is equal to 2.63 eV.56 However, the band gap of ZnO and CdS on the copper fiber changes a little. The absorption peaks of ZnO on the CF@ZnO/CdS heterostructures are blue shifted in comparison with ZnO on the CF@ZnO, which is due to the presence of CdS nanoparticles. The absorption peaks of CdS on the CF@ZnO/CdS heterostructures are a little red shifted due to the presence of CF@ZnO. Thus, the above results indicate that the hierarchical heterostructures have coupled with the optical properties of the two semiconductors. 15



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Figure 9. (a) UV-vis diffuse reflection spectra (DRS) of CF@ZnO, CdS and CF@ZnO/CdS hierarchical heterostructures. (b) The band-gap energy of CF@ZnO, CdS and CF@ZnO/CdS hierarchical heterostructures. Photocatalytic activities investigation: The photocatalytic performance of the ZnO, CdS and CF@ZnO/CdS hierarchical heterostructures was investigated by degrading RhB solution (10 mg/L), using Xe lamp (PLSSXE300-300UV). The dimension of copper fiber substrate is 5 x 10 cm. After the first hydrothermal method, the mass of CF@ZnO is 0.477 g. The mass of ZnO on the copper fiber is 0.031 g. The above CF@ZnO was utilized to the second hydrothermal method, and the mass of CF@ZnO/CdS is 0.517 g, so the mass of CdS on the CF@ZnO is 0.040 g. Herein, 0.031 g ZnO, 0.040 g CdS, 0.071 g ZnO/CdS and 0.517 g (0.071 g ZnO/CdS) CF@ZnO/CdS were adapted to degrading RhB solution. Fig. 10a shows the characteristic UV-vis adsorption spectra of RhB solution (10 mg/L) in the presence of CF@ZnO/CdS hierarchical heterostructures. Although there is 0.071 g of ZnO/CdS on the CF, the enhancement photocatalytic performance of CF@ZnO/CdS is ascribed to efficient charge separation and transmission, as well as the coupling effect of ZnO 16


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nanoneedles and CdS nanoparticles. Fig. 10b shows the decomposition of RhB in the presence of ZnO, CdS, ZnO/CdS and CF@ZnO/CdS, respectively. The rate constant of CF@ZnO/CdS photocatalysis was derived from the liner fitting curve of equation ln (C0/C) vs light time t, as shown in Fig. 10c. The liner fitting curve of equation is ln (C0/C) = 0.03306 t(min) + 0.21, and the liner fitting degree R2 = 0.9842 is high, demonstrating that the photocatalytic reaction is similar to the pseudo-first order reaction. CF@ZnO/CdS hierarchical heterostructures exhibits the best photocatalytic activity and 90% of RhB was decomposed within 60 min. Moreover, it also possesses desirable cycling ability as is shown in Fig. 10d.

Figure 10 (a) UV-vis adsorption spectra change of RhB solution in the presence of CF@ZnO/CdS hierarchical heterostructures. (b) The degradation percentage of ZnO, CdS, ZnO/CdS and CF@ZnO/CdS. (c) The fitting curves of sample CF@ZnO/CdS. (d) The recyclable ability of CF@ZnO/CdS for the first, second and third times.

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The schematic illustration of the photocatalytic degradation process of RhB solution is shown in Scheme 2. When under visible light, although both of the two components can be photexcited, the photogenerated charge carriers could only be transferred from CdS to ZnO through their well-coupled phase interface on the basis of their band gap structures.57 That is to say, electrons were photoexcited on the valence band (VB) of the CdS nanoparticles due to its narrow band energy, and then they transferred to the conduction band (CB) of ZnO nanoneedles. The basic three steps of photocatalytic reactions are 1) photoexcitation of charge carriers; 2) charge carrier separation and diffusion to the photocatalyst surface; and 3) oxidation and reduction reaction on the catalyst surface. During step (2), recombination can occur via different mechanisms. The major pathway is the already mentioned relaxation of photoexcited electrons back into the VB, which can directly happen from the CB. After the photoexcitation, electrons can also be trapped in electron traps both at and below the surface of a semiconductor, from where the recombination can proceed. The same principle is applicable for the holes, which can also be trapped in interband or surface states.58 Photoexcited electrons are transferred upon light irradiation from the CdS CB to the CB of ZnO. This charge injection process is very fast, it was first reported to occur in

Photocatalytic Decomposition of RhB by Newly Designed and Highly

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