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Enhanced photocatalytic properties of CdS nanoparticles decorated α-Fe2O3 nanopillar arrays under visible light
Accepted Manuscript Enhanced photocatalytic properties of CdS nanoparticles decorated α-Fe2O3 nanopillar arrays under visible light ShuangShuang, Zheng Xie, Zhengjun Zhang PII: DOI: Reference:
S0021-9797(17)30074-7 http://dx.doi.org/10.1016/j.jcis.2017.01.086 YJCIS 21992
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
22 November 2016 14 January 2017 19 January 2017
Please cite this article as: ShuangShuang, Z. Xie, Z. Zhang, Enhanced photocatalytic properties of CdS nanoparticles decorated α-Fe2O3 nanopillar arrays under visible light, Journal of Colloid and Interface Science (2017), doi: http:// dx.doi.org/10.1016/j.jcis.2017.01.086
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Enhanced photocatalytic properties of CdS nanoparticles decorated α-Fe2O3 nanopillar arrays under visible light Shuang Shuang,a Zheng Xie,a, c and Zhengjun Zhang b*
a. State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China b. Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China c. High-Tech Institute of Xi’an, Xi’an 710025, China *Corresponding author at: School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China. E-mail: [email protected]
Abstract: CdS nanoparticles (NPs) decorated α-Fe2O3 nanopillar arrays (NPAs) were fabricated through several steps. Fe NPAs were firstly fabricated by glancing angle deposition technique and oxidized in air to gain α-Fe2O3 NPAs. Then these NPAs were decorated by CdS NPs through successive ion layer adsorption and reaction (SILAR). Here we have tested photodegredation of methylene blue (MB) and photoelectrochemical properties under visible light. Especially, when SILAR cycle number reaches to 10, it shows the highest degradation efficiency (94 % in 75 min on MB) which improves 72% comparing with pure one and the highest photocurrent density (2.0 mA cm-2 at 0.4 V vs Ag/AgCl electrode). α-Fe2O3/CdS hetero-junctions could greatly enhance photocatalytic performance, which can help to accomplish sufficient usage of solar energy and be exploited on pollution abatement in future.
Key words: α-Fe2O3 NPAs, CdS NPs, decoration, glancing angle deposition, photoelectrochemical, SILAR, MB, degradation efficiency, photocatalytic performance, visible light.
Graphical abstracts:
Introduction Fujishima and Honda first observed photoelectrochemical (PEC) splitting of water over TiO2 electrodes in 1972 [1], which made semiconductors as good photocatalyst candidates. Due to its property of conversion solar energy into electrical and chemical energy efficiently, which can be applied into many fields, especially decomposing organic pollutants, hydrogen and oxygen generation, etc [2]. Because of ~3.0 eV wide bang gap, TiO2 makes it be active just under UV region which accounts only ~4% in solar energy [3]. Thus broadening lights activation range has been highly demanded to a great photocatalyst. However, since researches began, there still remain various big challenges on optimizing physical and chemical properties of catalysts. Thus it's worth noting that besides exploiting new type materials, traditional materials can be skillfully modified through different methods as well to make them be more active under visible light wavelength range eventually. Hematite (α-Fe2O3) is a kind of typical n-type traditional semiconductor, which has been found usable in many application as catalysts [4], pigment [5], and gas sensors [6]. Due to its narrow band gap (~2.1 eV), highly stability against photocorrosion and high theoretical solar-to-hydrogen efficiency (~17%), α-Fe2O3 could work effectively under visible light range [7]. Therefore, it can be found that many research work have been focus on α-Fe2O3 during these twenty years [8-19]. However, there is also some drawback hindering the wide usage of α-Fe2O3 such as poor minority charge carrier mobility (0.2 cm2•V-1 •s-1) and short hole diffusion length (2~4 nm). These special material properties eventually cause a high electrons and
holes recombination rate, a quite short excited state lifetime (~10 ps), and poor electrical conductivity [20, 21]. To enhance advantage and avoid disadvantage, a large amount of efforts and tries have been made, like constrcting α-Fe2O3 as nanotubes, nanoparticles, nanocubes, nanowires, nanofibers, nanorods, and hierarchical structures [8-19, 22]. Or fabricating heterojunctions, doping and so on, have been taken into optimization of photocatalytic performance [9, 23-26]. Good electron-hole separation efficiency can greatly help to improve the photocatalytic property [27]. PbS [28], CdSe [29], ZnS [30], and CdS [31, 32] all belong to narrow band gap semiconductors, which have been already regarded as nanostructured TiO2 sensitizers showing greatly enhanced visible light response. Among them CdS (band gap ~2.4 eV), with high absorption coefficient, is a good candidate in photocatalytic material. After irradiation, electrons generated from CdS nanoparticles (NPs) transfer into α-Fe2O3 quickly to reach efficiently separation of induced electrons and holes. Thus it is suitable to combine CdS with α-Fe2O3 and can be greatly developed into photoelectrochemisty and photocatalytic applications [33]. CdS NPs have superiority such as high extinction coefficient, spectral tunability by size, and good stability. The fabrication methods of CdS NPs had been exploited through electrodeposition [30], chemical bath deposition (CBD) [35], and successive ion layer adsorption and reaction (SILAR) [36, 37]. And among them, SILAR methods could be easily controlled by just changing the cycle number. Tranditional photocatalysts are mostly powders, and they are hard to be collected and recycled. Here the fabrication of vertically aligned α-Fe2O3 nanopillar arrays
(NPAs) on different substrates are introduced. These kind of materials are more convenient to recycle. When CdS NPs are decorated on α-Fe2O3 NPAs with the incensement of SILAR cycle numbers, the photocatalytic performance gets better first and then worse. Thus the decoration of CdS is an efficient method to enhance degradation efficiency only when appropriate loading quantity.
Methods Fabrication of α-Fe2O3 NPAs. Fe NPAs were firstly deposited vertically by the e-beam glancing angle deposition technique respectively on three substrates: 1) quartz substrates for degradation dye test, 2) F-doped SnO2 (FTO) substrates (20 Ω per square) for PEC curve characteristic, and 3) (001) planar silicon substrates for sample characterization. Substrates before deposition were ultrasonically cleaned in acetone, ethanol and deionized water baths each for 7 min, consequently. Deposition chamber was firstly evacuated to a vacuum level above 1×10−8 Torr. After deposition process, the samples were oxidized in a quartz tube furnace at temperatures of 400 °C for 1 hour at a ramp of 2 °C min-1 in air to obtain α-Fe2 O3 NPAs. CdS NPs deposition on α-Fe2O3 NPAs. SILAR method of CdS NPs decoration with slight modification is adopted, as previously reported [32]. α-Fe2O3 NPAs substrates were dipped in Cd(Ac)2 and Na2S solutions alternatively each for several seconds and then got the CdS nanocrystallites eventually. The substrates were immersed into Na 2S (0.05M) for 30 seconds, then cleaned in deionized water for 30 seconds and then turned to Cd(Ac)2 (0.05M) for another 30 seconds. Later substrates were cleaned by deionized water again. This whole cycle was regarded as one single SILAR cycle and
it was repeated until desired CdS NPs decoration quality. Materials characterization. The structure and morphology of the α-Fe2O3 NPAs/CdS NPs were examined by field-emission scanning electron microscope (FESEM, JEOL-7001F), Raman spectroscopy (LABRAM HR800, excitation wavelength set at 633 nm) and high-resolution transmission electron microscope (HRTEM, JEOL-2011), respectively. The special element analysis of the samples was analyzed by x-ray photon electron spectrometer (XPS, Perkin Elmer PHI 5300), and the binding energy was calibrated with the reference to the C 1s peak centered at 284.6 eV. Property Measurement. The steady sate current densities (j–V) test was characterized by an electrochemistry workstation (CHI 660D, Chenhua instrument). The products were regarded as the working electrode. And a Pt and Ag/AgCl electrode (saturated KCl) sheet were used as the counter electrode and reference electrode, respectively. The working electrode was illuminated by a 300 W Xe lamp. And between the light source and the quartz cell there is an ultraviolet filter placed to cut off the UV light in wavelength <420 nm. Photocurrent densities were tested in the light on-off process with a pulse of 30 s at 0.4 V bias vs Ag/AgCl electrode under visible light (200 mW cm-2). The photocatalytic performance was studied by the photodegradation of methylene blue (MB) dye with Xe lamp exposure. The sample deposited on quartz substrate was immersed into beaker containing 5 mL of MB (5 μM) under fixed time. At different time point during whole photodegradation, the UV-vis absorbance spectra of the MB solution were also measured.
Results and Discussions Characterization of Photocatalysts. Top-view SEM images of samples fabricated at different steps are shown in Figure 1. And from Figure 1(a), we can find that vertically aligned Fe NPAs with the diameter of ~40 nm and length of ~200 nm are quite uniform. And it can be seen that after annealing process, the diameter of Fe NPAs grows to ~60 nm. Besides, due to the swelling, such nanopillars touch with each other in some typical part according to Figure 1(b). Then Figures 1(c-f) present the morphology of CdS NPs decorated α-Fe2O3 NPAs with 5, 10, 15, and 20 SILAR cycle numbers. This method has been reported previously and regarded as one of most effective way of chemical reaction deposition [38, 39]. When SILAR cycle number increases, there will be new generated small CdS nanocrystalines and bigger crystallites grown from small one produced before [32]. It can be seen that after 15 cycles, the quantity of large CdS crystallites becomes more and more and when cycle number reaches to 20, there has been cohesion of CdS crystallites with each other. X-ray diffraction (XRD) patterns of the α-Fe2 O3 NPAs and decorated with different cycle number samples are showed in Figure 2(a). The pure α-Fe2O3 NPAs sample’s XRD pattern is matched with standard spectrum (JCPDS 33-0664) greatly which confirms the substance we have synthesized. And with the increase of SILAR cycle number, the peaks at 2θ = 28.1o arise and become more and more obvious. It
means that a preferred orientation along the (111) plane of the cubic phase CdS (JCPDS No.89-0440) have been created. Furthermore, it also demonstrates that CdS NPs have been successfully deposited on α-Fe2O3 nanopillars. Figure 2(b-d) show XPS spectra of the sample, where the binding energies were corrected by referencing the C 1s line to 284.6 eV. The peaks located at ~710.5 and ~723.3 eV are in good agreement with the Fe 2p3/2 and Fe 2p1/2 binding energy of Fe3+. The little shakeup satellite structure between two main peaks is the fingerprint of the electronic structure of Fe3+. The two peaks at 405.5 eV and 412.2 eV in Fig. 4b are corresponding to the 3d5/2 and Cd3d3/2 spin states of the Cd2+. And there are S2p2/3 peak at 161.5 eV and S2p1/2 peak at 169.5 eV. Raman results are as well characterized in order to study the crystal phase of sample. Figure 3 shows the Raman spectrum signal of pure α- Fe2O3 NPAs in which 224, 287, 408, 496 and 610 cm-1 peaks are in accordance with A1g(1), Eg(2), Eg(4), A1g(2) and Eg(5) of α-Fe2O3 [40]. While with the increase of SILAR cycle number, an additional peak at 295 cm‑1 which can be ascribed to CdS appears from insert figure and becomes stronger and stronger, which confirms the material composition again. Furthermore, from the spectrum we can see that the intensity of this peak becomes significantly higher after enough deposition cycle number. What’s more, the peak also becomes broader because of the LO (longitudinal optical) type confined vibrations of CdS NPs [41]. The structure and morphology of single α-Fe2O3 nanopillar and α-Fe2O3 NPAs/CdS NPs with 10 cycles sample are also studied by TEM analysis. As shown in
Figure 4(a-b), typical low and high resolution TEM images of α-Fe2O3 nanopillars and decorated with 10 cycles are all observed. It is clear that 1D pillar-like morphology presents the diameter of ~50 nm and length of ~200 nm. The lattice fringes of 0.256 nm is corresponding with interplanar spacing of α-Fe2O3 (110) planes, which confirms the substance composition. From Figures 3(c-d) one can see that pillars are coated with lots of CdS NPs and lattice fringes of 0.331 nm can be checked to (420) plane of CdS. Photodegradation of MB We have also tested the photocatalytic performance of nanocomposites and pure α-Fe2O3 NPAs by MB degradation under visible light irradiation. As we all known, the intensity of the λ=664 nm absorbance peak of MB is proportional to the solution concentration, as long as below 0.8 from Beer’s law [42]. According to , the degradation rate can be calculated, where A0 and A(t) are absorbance values at wavelength of 664 nm, at the irradiation time of 0 and t, respectively. The variation of MB concentration with time goes by are showed in Figure 5(a-b). After 75 mins’ visible light irradiation, over 94% of total MB quantity was degraded by α-Fe2O3/CdS nanocomposites with cycle number of 10. It shows superiority compared with bare α-Fe2O3 nanopillars of which just 78% of dye under the same condition. With the incensement of SILAR cycle number, photocatalytic property becomes better comparing with pure one. While the degradation efficiency of 5 SILAR cycles CdS decorated α-Fe2O3 NPAs reaches to 82 %. However with more than 10 SILAR cycle number, efficiency reduces gradually in which 15 SILAR cycles sample declines to
90 %. And when cycle number turns to 20, its efficiency result goes down to 81 % after 75 mins’ irradiation. Figure 5(b) shows that all degradation reaction rate all fitted with first-order kinetics. And the reaction rate constant of sample with 10 cycle number is 1.72 times comparing with that of bare one. In detail, as shown in Figure 5(c), optimal sample’s evolution of the MB solution’s absorption spectra are recorded during the whole reaction process. The intensity of peak gradually weakens with irradiation time going by which accordingly means that concentrate of dye molecule reduced. PEC performances were measured for all samples in a three-electrode cell. We used samples as working electrode with an exposed area of 1.5 cm2. And 0.01 M Na2SO4 aqueous solution was selected as the electrolyte. The photocurrent density on-off curves of all CdS-decorated α-Fe2O3 NPAs and pure α-Fe2O3 NPAs versus the incident light irradiation time at a bias potential of +0.4 V are shown in Figure 5(d). In the figure we can see that the photocurrent density values under irradiation of decorated samples is superior than that of pure one obviously [43]. Besides, we can see that the variation trend of photocurrent density is in accordance with degradation efficiency of all samples. When SILAR cycle number increases to 10, the photocurrent density reaches to the largest of 2.0 mA cm-2. This result suggests that the best sample shows a higher efficiency on the separation of electrons and holes and a better photocatalytic performance [44]. Photocatalytic Mechanism. After irradiated by visible light, electrons can be excited from semiconductor’s
valence band into the unoccupied conduction band, and holes generated stay in valence band 41. Figure 6 shows the schematic of light absorption, electrons separation, transfer and degradation of MB process for α-Fe2O3 NPAs/CdS NPs. When light reaches material surface, CdS NPs will instantly excite electrons and holes. Because the conduction band of CdS is more positive than that of Fe2O3, electrons generated from CdS tend to transfer into the conduction band of α-Fe2O3. Similarly, holes generated from α-Fe2O3 are inclined to move to the valence band of CdS for the more positive valence band of CdS. While holes excited from CdS still stay in its conduction band. Thus electron/hole pairs can be efficiently separated. This would eventually promote photocatalytic and photoelectrochemical performance of composite
materials
compared with pure
α-Fe2O3.
Additionally,
electrons
accumulated after a period of time will react with oxygen and turn to active oxygen species (e.g.•O2-). Furthermore, they could even be reduced to highly reactive hydroxyl radicals (•OH) [45]. Such kind of radicals are able to decompose organic molecular into intermediates or mineralized products. Besides, holes in CdS valence band can react with dye molecules and result into decomposed products [46]. As seen in Figure 3(a-b), though degradation process is actually a part of photobleaching, the reaction rate of it is exactly depended on the density of the electrons and holes. Furthermore, larger photocurrent density means higher potocatalytic efficiency. And here the density of the photocurrent is positive correlation with photoactivity property and in accordance with experiment results.
Conclusions
In summary, we oxidized Fe NPAs obtained by glancing angle deposition technique into α-Fe2O3 NPAs, and synthesized α-Fe2O3 NPAs/CdS NPs through SILAR method. Outstanding visible light photoelectrochemical and photocatalytic properties could be observed from α-Fe2O3 NPAs/CdS NPs composites because of high electrons and holes separation efficiency. Especially the results show that SILAR cycle number of 10 showing both the highest photocurrent density (2.0 mA cm-2 at 0.4 V vs Ag/AgCl electrode) and the best degradation efficiency (94 % in 75 min) on MB. And such kind of self-standing structures are much easier to recycle. They do prevent the occurrence of secondary pollution which means more than recycles. Thus heterojunctions of CdS/α-Fe2O3 provides a potential way on solar energy conversion and environmental governance material preparation.
Acknowledgments The authors are grateful to the financial support by the Research Project by Chinese Ministry of Science and Technology (grant no. 2016YFE0104000) and the financial support by the National Natural Science Foundation of China (grant No. 51372135).
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Figure Captions:
Figure 1. SEM images of the samples: (a) Fe NPAs; (b) Fe2O3 NPAs obtained from oxidation of Fe NPAs; after (c) 5, (d) 10, (e) 15, and (f) 20 SILAR cycles of CdS deposition.
Figure 2. (a) XRD patterns of the α-Fe2O3 NPAs/CdS NPs, (b) Fe2p, (c) Cd3d and (d) S2p XPS spectra of 10 SILAR cycles α-Fe2O3 NPAs/CdS NPs.
Figure 3. Raman patterns of the α-Fe2O3 NPAs/CdS NPs
Figure 4. TEM images and HRTEM images: (a, b) α-Fe2O3 sample; (c, d) 10 SILAR cycles α-Fe2O3 NPAs/CdS NPs
Figure 5. (a) Time course of the decrease in the concentration; (b) ln(C 0/C) for the degradation of MB with visible irradiation under different conditions; (c) Absorption spectral changes of a MB aqueous solution (5 μM) degraded by 10 cycles CdS/α-Fe2O3; (d) Current versus time measurements of α-Fe2O3 with different cycle numbers during visible light illuminations under 0.4 V versus Ag/AgCl electrode bias
Figure 6. Schematic of light absorption, electrons separation, transfer and degradation of MB process for α-Fe2O3 NPAs/CdS NPs
S0021-9797(17)30074-7 http://dx.doi.org/10.1016/j.jcis.2017.01.086 YJCIS 21992
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
22 November 2016 14 January 2017 19 January 2017
Please cite this article as: ShuangShuang, Z. Xie, Z. Zhang, Enhanced photocatalytic properties of CdS nanoparticles decorated α-Fe2O3 nanopillar arrays under visible light, Journal of Colloid and Interface Science (2017), doi: http:// dx.doi.org/10.1016/j.jcis.2017.01.086
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Enhanced photocatalytic properties of CdS nanoparticles decorated α-Fe2O3 nanopillar arrays under visible light Shuang Shuang,a Zheng Xie,a, c and Zhengjun Zhang b*
a. State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China b. Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China c. High-Tech Institute of Xi’an, Xi’an 710025, China *Corresponding author at: School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China. E-mail: [email protected]
Abstract: CdS nanoparticles (NPs) decorated α-Fe2O3 nanopillar arrays (NPAs) were fabricated through several steps. Fe NPAs were firstly fabricated by glancing angle deposition technique and oxidized in air to gain α-Fe2O3 NPAs. Then these NPAs were decorated by CdS NPs through successive ion layer adsorption and reaction (SILAR). Here we have tested photodegredation of methylene blue (MB) and photoelectrochemical properties under visible light. Especially, when SILAR cycle number reaches to 10, it shows the highest degradation efficiency (94 % in 75 min on MB) which improves 72% comparing with pure one and the highest photocurrent density (2.0 mA cm-2 at 0.4 V vs Ag/AgCl electrode). α-Fe2O3/CdS hetero-junctions could greatly enhance photocatalytic performance, which can help to accomplish sufficient usage of solar energy and be exploited on pollution abatement in future.
Key words: α-Fe2O3 NPAs, CdS NPs, decoration, glancing angle deposition, photoelectrochemical, SILAR, MB, degradation efficiency, photocatalytic performance, visible light.
Graphical abstracts:
Introduction Fujishima and Honda first observed photoelectrochemical (PEC) splitting of water over TiO2 electrodes in 1972 [1], which made semiconductors as good photocatalyst candidates. Due to its property of conversion solar energy into electrical and chemical energy efficiently, which can be applied into many fields, especially decomposing organic pollutants, hydrogen and oxygen generation, etc [2]. Because of ~3.0 eV wide bang gap, TiO2 makes it be active just under UV region which accounts only ~4% in solar energy [3]. Thus broadening lights activation range has been highly demanded to a great photocatalyst. However, since researches began, there still remain various big challenges on optimizing physical and chemical properties of catalysts. Thus it's worth noting that besides exploiting new type materials, traditional materials can be skillfully modified through different methods as well to make them be more active under visible light wavelength range eventually. Hematite (α-Fe2O3) is a kind of typical n-type traditional semiconductor, which has been found usable in many application as catalysts [4], pigment [5], and gas sensors [6]. Due to its narrow band gap (~2.1 eV), highly stability against photocorrosion and high theoretical solar-to-hydrogen efficiency (~17%), α-Fe2O3 could work effectively under visible light range [7]. Therefore, it can be found that many research work have been focus on α-Fe2O3 during these twenty years [8-19]. However, there is also some drawback hindering the wide usage of α-Fe2O3 such as poor minority charge carrier mobility (0.2 cm2•V-1 •s-1) and short hole diffusion length (2~4 nm). These special material properties eventually cause a high electrons and
holes recombination rate, a quite short excited state lifetime (~10 ps), and poor electrical conductivity [20, 21]. To enhance advantage and avoid disadvantage, a large amount of efforts and tries have been made, like constrcting α-Fe2O3 as nanotubes, nanoparticles, nanocubes, nanowires, nanofibers, nanorods, and hierarchical structures [8-19, 22]. Or fabricating heterojunctions, doping and so on, have been taken into optimization of photocatalytic performance [9, 23-26]. Good electron-hole separation efficiency can greatly help to improve the photocatalytic property [27]. PbS [28], CdSe [29], ZnS [30], and CdS [31, 32] all belong to narrow band gap semiconductors, which have been already regarded as nanostructured TiO2 sensitizers showing greatly enhanced visible light response. Among them CdS (band gap ~2.4 eV), with high absorption coefficient, is a good candidate in photocatalytic material. After irradiation, electrons generated from CdS nanoparticles (NPs) transfer into α-Fe2O3 quickly to reach efficiently separation of induced electrons and holes. Thus it is suitable to combine CdS with α-Fe2O3 and can be greatly developed into photoelectrochemisty and photocatalytic applications [33]. CdS NPs have superiority such as high extinction coefficient, spectral tunability by size, and good stability. The fabrication methods of CdS NPs had been exploited through electrodeposition [30], chemical bath deposition (CBD) [35], and successive ion layer adsorption and reaction (SILAR) [36, 37]. And among them, SILAR methods could be easily controlled by just changing the cycle number. Tranditional photocatalysts are mostly powders, and they are hard to be collected and recycled. Here the fabrication of vertically aligned α-Fe2O3 nanopillar arrays
(NPAs) on different substrates are introduced. These kind of materials are more convenient to recycle. When CdS NPs are decorated on α-Fe2O3 NPAs with the incensement of SILAR cycle numbers, the photocatalytic performance gets better first and then worse. Thus the decoration of CdS is an efficient method to enhance degradation efficiency only when appropriate loading quantity.
Methods Fabrication of α-Fe2O3 NPAs. Fe NPAs were firstly deposited vertically by the e-beam glancing angle deposition technique respectively on three substrates: 1) quartz substrates for degradation dye test, 2) F-doped SnO2 (FTO) substrates (20 Ω per square) for PEC curve characteristic, and 3) (001) planar silicon substrates for sample characterization. Substrates before deposition were ultrasonically cleaned in acetone, ethanol and deionized water baths each for 7 min, consequently. Deposition chamber was firstly evacuated to a vacuum level above 1×10−8 Torr. After deposition process, the samples were oxidized in a quartz tube furnace at temperatures of 400 °C for 1 hour at a ramp of 2 °C min-1 in air to obtain α-Fe2 O3 NPAs. CdS NPs deposition on α-Fe2O3 NPAs. SILAR method of CdS NPs decoration with slight modification is adopted, as previously reported [32]. α-Fe2O3 NPAs substrates were dipped in Cd(Ac)2 and Na2S solutions alternatively each for several seconds and then got the CdS nanocrystallites eventually. The substrates were immersed into Na 2S (0.05M) for 30 seconds, then cleaned in deionized water for 30 seconds and then turned to Cd(Ac)2 (0.05M) for another 30 seconds. Later substrates were cleaned by deionized water again. This whole cycle was regarded as one single SILAR cycle and
it was repeated until desired CdS NPs decoration quality. Materials characterization. The structure and morphology of the α-Fe2O3 NPAs/CdS NPs were examined by field-emission scanning electron microscope (FESEM, JEOL-7001F), Raman spectroscopy (LABRAM HR800, excitation wavelength set at 633 nm) and high-resolution transmission electron microscope (HRTEM, JEOL-2011), respectively. The special element analysis of the samples was analyzed by x-ray photon electron spectrometer (XPS, Perkin Elmer PHI 5300), and the binding energy was calibrated with the reference to the C 1s peak centered at 284.6 eV. Property Measurement. The steady sate current densities (j–V) test was characterized by an electrochemistry workstation (CHI 660D, Chenhua instrument). The products were regarded as the working electrode. And a Pt and Ag/AgCl electrode (saturated KCl) sheet were used as the counter electrode and reference electrode, respectively. The working electrode was illuminated by a 300 W Xe lamp. And between the light source and the quartz cell there is an ultraviolet filter placed to cut off the UV light in wavelength <420 nm. Photocurrent densities were tested in the light on-off process with a pulse of 30 s at 0.4 V bias vs Ag/AgCl electrode under visible light (200 mW cm-2). The photocatalytic performance was studied by the photodegradation of methylene blue (MB) dye with Xe lamp exposure. The sample deposited on quartz substrate was immersed into beaker containing 5 mL of MB (5 μM) under fixed time. At different time point during whole photodegradation, the UV-vis absorbance spectra of the MB solution were also measured.
Results and Discussions Characterization of Photocatalysts. Top-view SEM images of samples fabricated at different steps are shown in Figure 1. And from Figure 1(a), we can find that vertically aligned Fe NPAs with the diameter of ~40 nm and length of ~200 nm are quite uniform. And it can be seen that after annealing process, the diameter of Fe NPAs grows to ~60 nm. Besides, due to the swelling, such nanopillars touch with each other in some typical part according to Figure 1(b). Then Figures 1(c-f) present the morphology of CdS NPs decorated α-Fe2O3 NPAs with 5, 10, 15, and 20 SILAR cycle numbers. This method has been reported previously and regarded as one of most effective way of chemical reaction deposition [38, 39]. When SILAR cycle number increases, there will be new generated small CdS nanocrystalines and bigger crystallites grown from small one produced before [32]. It can be seen that after 15 cycles, the quantity of large CdS crystallites becomes more and more and when cycle number reaches to 20, there has been cohesion of CdS crystallites with each other. X-ray diffraction (XRD) patterns of the α-Fe2 O3 NPAs and decorated with different cycle number samples are showed in Figure 2(a). The pure α-Fe2O3 NPAs sample’s XRD pattern is matched with standard spectrum (JCPDS 33-0664) greatly which confirms the substance we have synthesized. And with the increase of SILAR cycle number, the peaks at 2θ = 28.1o arise and become more and more obvious. It
means that a preferred orientation along the (111) plane of the cubic phase CdS (JCPDS No.89-0440) have been created. Furthermore, it also demonstrates that CdS NPs have been successfully deposited on α-Fe2O3 nanopillars. Figure 2(b-d) show XPS spectra of the sample, where the binding energies were corrected by referencing the C 1s line to 284.6 eV. The peaks located at ~710.5 and ~723.3 eV are in good agreement with the Fe 2p3/2 and Fe 2p1/2 binding energy of Fe3+. The little shakeup satellite structure between two main peaks is the fingerprint of the electronic structure of Fe3+. The two peaks at 405.5 eV and 412.2 eV in Fig. 4b are corresponding to the 3d5/2 and Cd3d3/2 spin states of the Cd2+. And there are S2p2/3 peak at 161.5 eV and S2p1/2 peak at 169.5 eV. Raman results are as well characterized in order to study the crystal phase of sample. Figure 3 shows the Raman spectrum signal of pure α- Fe2O3 NPAs in which 224, 287, 408, 496 and 610 cm-1 peaks are in accordance with A1g(1), Eg(2), Eg(4), A1g(2) and Eg(5) of α-Fe2O3 [40]. While with the increase of SILAR cycle number, an additional peak at 295 cm‑1 which can be ascribed to CdS appears from insert figure and becomes stronger and stronger, which confirms the material composition again. Furthermore, from the spectrum we can see that the intensity of this peak becomes significantly higher after enough deposition cycle number. What’s more, the peak also becomes broader because of the LO (longitudinal optical) type confined vibrations of CdS NPs [41]. The structure and morphology of single α-Fe2O3 nanopillar and α-Fe2O3 NPAs/CdS NPs with 10 cycles sample are also studied by TEM analysis. As shown in
Figure 4(a-b), typical low and high resolution TEM images of α-Fe2O3 nanopillars and decorated with 10 cycles are all observed. It is clear that 1D pillar-like morphology presents the diameter of ~50 nm and length of ~200 nm. The lattice fringes of 0.256 nm is corresponding with interplanar spacing of α-Fe2O3 (110) planes, which confirms the substance composition. From Figures 3(c-d) one can see that pillars are coated with lots of CdS NPs and lattice fringes of 0.331 nm can be checked to (420) plane of CdS. Photodegradation of MB We have also tested the photocatalytic performance of nanocomposites and pure α-Fe2O3 NPAs by MB degradation under visible light irradiation. As we all known, the intensity of the λ=664 nm absorbance peak of MB is proportional to the solution concentration, as long as below 0.8 from Beer’s law [42]. According to , the degradation rate can be calculated, where A0 and A(t) are absorbance values at wavelength of 664 nm, at the irradiation time of 0 and t, respectively. The variation of MB concentration with time goes by are showed in Figure 5(a-b). After 75 mins’ visible light irradiation, over 94% of total MB quantity was degraded by α-Fe2O3/CdS nanocomposites with cycle number of 10. It shows superiority compared with bare α-Fe2O3 nanopillars of which just 78% of dye under the same condition. With the incensement of SILAR cycle number, photocatalytic property becomes better comparing with pure one. While the degradation efficiency of 5 SILAR cycles CdS decorated α-Fe2O3 NPAs reaches to 82 %. However with more than 10 SILAR cycle number, efficiency reduces gradually in which 15 SILAR cycles sample declines to
90 %. And when cycle number turns to 20, its efficiency result goes down to 81 % after 75 mins’ irradiation. Figure 5(b) shows that all degradation reaction rate all fitted with first-order kinetics. And the reaction rate constant of sample with 10 cycle number is 1.72 times comparing with that of bare one. In detail, as shown in Figure 5(c), optimal sample’s evolution of the MB solution’s absorption spectra are recorded during the whole reaction process. The intensity of peak gradually weakens with irradiation time going by which accordingly means that concentrate of dye molecule reduced. PEC performances were measured for all samples in a three-electrode cell. We used samples as working electrode with an exposed area of 1.5 cm2. And 0.01 M Na2SO4 aqueous solution was selected as the electrolyte. The photocurrent density on-off curves of all CdS-decorated α-Fe2O3 NPAs and pure α-Fe2O3 NPAs versus the incident light irradiation time at a bias potential of +0.4 V are shown in Figure 5(d). In the figure we can see that the photocurrent density values under irradiation of decorated samples is superior than that of pure one obviously [43]. Besides, we can see that the variation trend of photocurrent density is in accordance with degradation efficiency of all samples. When SILAR cycle number increases to 10, the photocurrent density reaches to the largest of 2.0 mA cm-2. This result suggests that the best sample shows a higher efficiency on the separation of electrons and holes and a better photocatalytic performance [44]. Photocatalytic Mechanism. After irradiated by visible light, electrons can be excited from semiconductor’s
valence band into the unoccupied conduction band, and holes generated stay in valence band 41. Figure 6 shows the schematic of light absorption, electrons separation, transfer and degradation of MB process for α-Fe2O3 NPAs/CdS NPs. When light reaches material surface, CdS NPs will instantly excite electrons and holes. Because the conduction band of CdS is more positive than that of Fe2O3, electrons generated from CdS tend to transfer into the conduction band of α-Fe2O3. Similarly, holes generated from α-Fe2O3 are inclined to move to the valence band of CdS for the more positive valence band of CdS. While holes excited from CdS still stay in its conduction band. Thus electron/hole pairs can be efficiently separated. This would eventually promote photocatalytic and photoelectrochemical performance of composite
materials
compared with pure
α-Fe2O3.
Additionally,
electrons
accumulated after a period of time will react with oxygen and turn to active oxygen species (e.g.•O2-). Furthermore, they could even be reduced to highly reactive hydroxyl radicals (•OH) [45]. Such kind of radicals are able to decompose organic molecular into intermediates or mineralized products. Besides, holes in CdS valence band can react with dye molecules and result into decomposed products [46]. As seen in Figure 3(a-b), though degradation process is actually a part of photobleaching, the reaction rate of it is exactly depended on the density of the electrons and holes. Furthermore, larger photocurrent density means higher potocatalytic efficiency. And here the density of the photocurrent is positive correlation with photoactivity property and in accordance with experiment results.
Conclusions
In summary, we oxidized Fe NPAs obtained by glancing angle deposition technique into α-Fe2O3 NPAs, and synthesized α-Fe2O3 NPAs/CdS NPs through SILAR method. Outstanding visible light photoelectrochemical and photocatalytic properties could be observed from α-Fe2O3 NPAs/CdS NPs composites because of high electrons and holes separation efficiency. Especially the results show that SILAR cycle number of 10 showing both the highest photocurrent density (2.0 mA cm-2 at 0.4 V vs Ag/AgCl electrode) and the best degradation efficiency (94 % in 75 min) on MB. And such kind of self-standing structures are much easier to recycle. They do prevent the occurrence of secondary pollution which means more than recycles. Thus heterojunctions of CdS/α-Fe2O3 provides a potential way on solar energy conversion and environmental governance material preparation.
Acknowledgments The authors are grateful to the financial support by the Research Project by Chinese Ministry of Science and Technology (grant no. 2016YFE0104000) and the financial support by the National Natural Science Foundation of China (grant No. 51372135).
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Figure Captions:
Figure 1. SEM images of the samples: (a) Fe NPAs; (b) Fe2O3 NPAs obtained from oxidation of Fe NPAs; after (c) 5, (d) 10, (e) 15, and (f) 20 SILAR cycles of CdS deposition.
Figure 2. (a) XRD patterns of the α-Fe2O3 NPAs/CdS NPs, (b) Fe2p, (c) Cd3d and (d) S2p XPS spectra of 10 SILAR cycles α-Fe2O3 NPAs/CdS NPs.
Figure 3. Raman patterns of the α-Fe2O3 NPAs/CdS NPs
Figure 4. TEM images and HRTEM images: (a, b) α-Fe2O3 sample; (c, d) 10 SILAR cycles α-Fe2O3 NPAs/CdS NPs
Figure 5. (a) Time course of the decrease in the concentration; (b) ln(C 0/C) for the degradation of MB with visible irradiation under different conditions; (c) Absorption spectral changes of a MB aqueous solution (5 μM) degraded by 10 cycles CdS/α-Fe2O3; (d) Current versus time measurements of α-Fe2O3 with different cycle numbers during visible light illuminations under 0.4 V versus Ag/AgCl electrode bias
Figure 6. Schematic of light absorption, electrons separation, transfer and degradation of MB process for α-Fe2O3 NPAs/CdS NPs