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TiO2 composite under magnetic field
Solar Energy 196 (2020) 505–512
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Investigation of photocatalytic activity through photo-thermal heating enabled by Fe3O4/TiO2 composite under magnetic field Lei Shi, Xinzhi Wang, Yanwei Hu, Yurong He
T
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Heilongjiang Key Laboratory of New Energy Storage Materials and Processes, Harbin 150001, China School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Photo-thermal conversion Nanofluids Magnetic-enhanced photocatalytic
The significant improvement in photocatalytic activity achieved in recent years has generated widespread attention due to its utilization in solar energy. In this study, two shapes of magnetic nanoparticles were synthesized using hydrothermal method. The magnetic nanoparticles exhibited superior photocatalytic activity for water treatment, which takes into consideration the photo-thermal conversion of magnetic nanoparticles. The effect of morphology on the photo-degradation was analyzed by the specific surface area measurement and photoluminescence test. Furthermore, the suspension degradation rate with magnetic nanoparticles was enhanced by ~200% compared to a commercial photocatalyst. Moreover, the photo-degradation efficiency further increased from 96.4 to 99.6% with the magnetic field intensity increased because of the absorbance ability enhancement. This result indicates that the photocatalytic activity of a magnetic photocatalyst can be enhanced through photothermal conversion and magnetic field regulation, which expands the application of solar power technology.
1. Introduction Wastewater pollution, particularly in the form of organic dyes, is a nonnegligible environmental problem that can cause great harm to human health. Thus, there has been much interest in the development of a cost-effective and efficient wastewater treatment for removing organic dyes [Chaudhary and Rizwan et al., 2017; Gupta and Pandey, 2019]. It is noteworthy that photocatalytic processes, as one way to achieve photochemical conversion, have been recognized as an ideal treatment of environmental pollutants [Li et al., 2016; Sharma et al., 2019]. With recent advances in nanotechnology, various photocatalytic nanoparticles with different structures, and consisting of various materials, have been prepared to eliminate organic pollutants by enhancing their photo absorption properties [Nafey et al., 2001; Soltani et al., 2013]. Due to its chemical inertness and long-term stability in solution, nanostructure TiO2 has been demonstrated to have perfect catalytic effects for water treatment under solar illumination [Borges et al., 2016]. This process of solar energy to chemical energy transformation is accompanied by solar energy to heat transformation, which in return influences the photo-chemical transformation [Li et al., 2017; Rajrana et al., 2019]. The development of sustainable energy technologies, especially solar energy, is critical to ensuring future energy sources [Abdelrahman et al., 2019; Kundu and Lee, 2012; Vig et al., 2019]. The acquisition of ⁎
renewable solar energy and the transition from solar energy to other energy forms, such as photoelectric, photochemical, and photo-thermal energy has attracted significant attention [Liu et al., 2015; Zhou et al., 2017]. Both photo-thermal and photochemical conversion studies have mainly considered the expansion of radiation absorption into the visible solar spectrum, then its production of more photo carriers to enhance the efficiency [Cuevas et al., 2015; Chen et al., 2015]. For this reason, a number of researchers have focused on nanofluids that exhibit outstanding photo-thermal conversion properties. Carbon-based and metallic-based micro-nano structure have been widely applied to manufacture solar conversion absorbers and enhance photo-thermal conversion efficiency [Ge et al., 2014; Chehadi et al., 2016]. For example, Jin et al. [Jin et al., 2016] experimentally and numerically discussed the photo-thermal transformation mechanism of Au nanoparticle direct absorption photo-collectors. Wang et al. [Wang et al. (2018)] researched the effects of photo-thermal conversion on the treatment of dye-contaminated wastewater and explained the relationship between photo-thermal conversion and photochemical conversion. Gupta et al. investigated the effect of heterojunction on photodegradation efficiency of methylene blue under visible irradiation [Gupta and Pandey, 2019; Kaur et al., 2018]. Magnetic photocatalysts combine catalytic properties and magnetism to facilitate recyclable purification and separation for the treatment of organic pollutants [Bahnemann, 2004; Abdelrahman et al.,
Corresponding author at: Heilongjiang Key Laboratory of New Energy Storage Materials and Processes, Harbin 150001, China. E-mail address: [email protected] (Y. He).
https://doi.org/10.1016/j.solener.2019.12.053 Received 2 June 2019; Received in revised form 14 December 2019; Accepted 17 December 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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hexahydrate (4 mmol) was stirred vigorously in 80 ml DI water for 10 min. Then, the suspension was mixed with 2.35 g sodium citrate (3 mmol) and 0.72 urea (4 mmol). Next, 0.8 g polyacrylamide was added into the suspension within stirred up condition for 20 min. Then the reaction suspensions were transferred into an autoclave sealed with polytetrafluoroethylene. And it was maintained at 200 °C for 12 h and cooled with air natural convection. Then, the black precipitate (Fe3O4 nanoparticles) was washed with ethanol solution by magnetic separation before oven drying at 50 °C for 12 h. The Fe3O4 nanoparticles were mixed with 5 ml titanium tetrafluoride solution of 40 mmol and stirred vigorously for 15 min, which was in order to ensure Fe3O4 can be covered with enough TiO2 nanoparticles. The suspensions were then placed in an autoclave sealed with polytetrafluoroethylene. Then, the autoclave was maintained at two different temperatures (180 and 200 °C) for 48 h to prepare two kinds of nanoparticles and cooled with air natural convection. Finally, the magnetic nanoparticles coated with TiO2 were obtained from the reaction suspensions by magnetic attraction.
2019]. There was a gap between conduction band and valence band in the energy band structure of the magnetic photocatalyst [Abdelrahman et al., 2019]. The magnetic photocatalyst maintained an unstable excited state when the energy was greater than the forbidden bandwidth of the illumination. The electron excitation of the valence band jumped to the conduction belt and formed holes, which resulted in the magnetic photocatalyst releasing external energy for water purification Wang et al., 2018. Previous research has shown that, the separation cost of catalyst particles is even higher than the energy saved in the solar purification process because the catalyst particles are small [Pardeshi and Patil, 2008; Abdelrahman et al., 2019]. Magnetic separation is an efficient and low-cost method to recover magnetic nanoparticles applying a suitable magnetic field [Zhang et al., 2009; Zhuang et al., 2010]. For example, Li et al. (2018) reported an approach to synthesize magnetic photocatalyst and confirmed that these nanoparticles have excellent photocatalytic properties in mixture solutions. Tan et al. [Tan et al., 2015] synthesized nanostructure Fe3O4/TiO2 core-shell nanoparticles, and the degradation performance of the nanoparticles with pH and temperature was researched. However, most studies in this area have focused on the separation efficiency of a magnetic photocatalyst [Liu et al., 2014; Soltani et al., 2017]. Despite the great successes achieved in recent years, the catalytic performance is still lower than expected. Hence, there is a need to investigate improved alternative strategies of catalytic efficiency [He et al., 2013; Ding et al., 2012]. Some studies have attempted to improve catalytic efficiency using other physical field forms, such as electric and magnetic fields [Fan et al., 2012; Barrett et al., 1951]. New researches suggest that both the degradation rate constant and efficiency can be enhanced with the utilization of magnetic fields [Ghasemi et al., 2014; Kaloudis et al., 2016]. In this work, we prepared two shapes of magnetic photo-catalyst (MPC) using hydrothermal method with different temperatures, which combined catalytic and magnetic properties to provide a composite material. Initially, the spinous and petaloid morphology of MPCs were characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The structural characterization of MPCs were characterized by X-ray diffraction spectra (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform-infrared spectroscopy (FT-IR) and magnetic property measurement system (MPMS). Then, an analysis of photocatalytic experiments was conducted to study the degradation performance of two shapes of MPC compared to P25, a standard commercial photocatalyst. The reason of different catalytic properties between three photocatalyst was discussed through specific surface area analysis and photoluminescence measurement. Furthermore, the relationship between photo-thermal conversion and photocatalytic performance was investigated in photocatalytic experiments with cooling. Finally, we observed the photocatalytic performance of a MPC under different magnetic fields, and the mechanism of magnetic-enhanced photocatalytic degradation was explained.
2.3. Characterization The morphology of the magnetic photocatalyst materials was characterized by SEM (ZEISS SUPRA 55; Zeiss, Germany) and recorded with a bias voltage of 30 kV. Energy dispersive spectroscopy (EDS) was performed using an X-MaxN 80 Silicon Drift Detector. The TEM patterns of the magnetic nanoparticles were characterized by TEM (JEM-2010; Jeol, Japan). The crystallographic structures of the magnetic nanoparticles were determined by X-ray diffraction (D8-Advance, Bruker, USA), with a monochromatic radiation source at room temperature operating in step-scan mode. The chemical composition of the magnetic nanoparticles was obtained through Fourier transform-infrared spectroscopy (Nicolet-560, Nicolet, US). The nanoparticle magnetism was investigated using a vibrating sample magnetometer (VSM; Quantum Design, USA). X-ray photoelectron spectroscopy (AXIS Ultra, Kratos Analytical, UK) was used with the exciting source (Mg-KR radiation) provided by the Electron Spectroscopy for Chemical Analysis (ESCA) Laboratory MKII instrument. And the absorption spectrum was recorded through the ultraviolet-visible spectrophotometer (UV-Vis; Persee Co., Ltd., Beijing, China) at wavelengths of 300–800 nm. The photoluminescence (PL) analysis was carried out by fluorescence spectrophotometer (Hitachi F-7000, Japan). The specific surface area analysis was obtained by Brunauer-Emmett-Teller (BET) methods (Quantachrome Autosorb-1C-VP, US). 2.4. Photo-catalytic efficiency test set-up Rhodamine B (RhB), one of the organic dyes, was used as the degraded pollutant in experiments. The experimental facility was showed in Fig. 2, in which photocatalytic activity occurs through the photothermal conversion of a magnetic photocatalyst under a magnetic field Li et al., 2018; Huang et al., 2017. As shown in Fig. 2, 30 mg magnetic nanoparticles were added into 50 ml Rhodamine B aqueous suspension (1 × 10−4 mol/L) in a double-walled acrylic beaker, with the temperature maintained by recirculated cooling water. The RhB aqueous solution containing magnetic nanoparticles was illuminated with a solar simulator (NP2000; CeauLight, Beijing, China) for 35 min to determine the photocatalytic performance. RhB was decomposed into inorganic matter and changes in the aqueous solution concentration were measured every 5 min by the UV-Vis spectrophotometer. The temperature at different heights in the test beaker was determined by three T-type thermocouples (SLE-40; Omega, Norwalk, USA) and read by a data-acquisition receiver (CA34972; Agilent, Santa Clara, USA). A constant magnetic field generated by the Helmholtz coil was applied to the test beakers and the strength of the magnetic field could be regulated by electric current I. And the strength of magnetic field was measured by digital gauss meter (Model 931; Honor Top Magnetic
2. Experimental section 2.1. Chemical reagents Iron(III) chloride hexahydrate (FeCl3·6H2O, AR, 99%), ascorbic acid (C6H8O6, AR, 99%), polyacrylamide ((C3H5NO)n, AR, 99%), urea (CN2H4O, AR, 99%), titanium tetrafluoride (TiF4, AR, 99%), and sodium citrate (Na3C6H5O7·2H2O, AR, 99%) were commercially purchased (Aladdin Reagents; Shanghai, China). Deionized water (DI) purified in a laboratory ultrapure water system (Arium-mini Plus; Sartorius, Göttingen, Germany) was applied in all process of nanoparticles preparation. 2.2. Synthesis of magnetic nanoparticles The preparation process of magnetic nano-composite using hydrothermal method is shown as follows (Fig. 1): 1.08 g Iron (III) chloride 506
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Fig. 1. Preparation schematic of magnetic nano-composite using hydrothermal method.
calculated from 12 to 19 nm, which is consistent with the TEM images (Fig. 3h) [Hou et al., 2018]. This indicated that each magnetic Fe3O4/ TiO2 photo-catalyst is composed of small primary crystals that retain their excellent paramagnetism and catalytic property, which also can be demonstrated by TEM images (Fig. 3d–f). The XPS spectra of SMPC were recorded and three different peaks (Fe 2p, O 1s, and Ti 2p) were apparent, as shown in Fig. 4b, which further confirmed the elemental components of magnetic nanoparticles. From the FT-IR spectrum (Fig. 4c), the peaks at 596, 3401, and 1633 cm−1 were ascribed to FeeO, eOH stretching, and bending vibrations, which indicated that the surface of the nanoparticles contained hydrophilic groups, endowing the nanoparticles with well-dispersed properties in solution [Zhang et al., 2011]. The peaks at 1241 and 669 cm−1 were the characterized vibrations of the TieO groups, which further justify the XRD and XPS results. The peak at 1400 cm−1 was the characterization of the COOeFe vibrations, both the peak at 1633 and 1400 cm−1 show the hydrophilic groups on the surface of Fe3O4 nanoparticles. The hysteresis loops (Fig. 4d) showed that the Fe3O4 nanoparticles saturation magnetization was 59.3 emu/g. The SMPC and PMPC had the same saturation magnetization of 32.4 emu/g, while the PMPC was more sensitive to the magnetic field. This was probably due to the differences in morphology and size between SMPC and PMPC [Qi et al., 2017]. The saturation magnetization of the SMPC and PMPC can be converted into weight percent of the Fe3O4 magnetization [Guardia et al., 2007]. It means that adding TiO2 into nano-composites weaken the magnetic property of the magnetic bulk material, which led to the decrease in saturation magnetization.
Fig. 2. Experimental schematic for the enhancement of photo-catalytic activity by a magnetic field.
Technology, China). The catalytic performance of both kinds of morphological particles was compared to P25 in the experimental procedure. The influence of applied magnetic field on the catalytic performance was studied at five different intensities of magnetic field (0, 100, 200, 400, and 800 Oe).
3. Results and discussion 3.1. Structure of magnetic photo-catalyst
3.2. Photocatalytic activity of magnetic nano-composite
The shape of the prepared Fe3O4 magnetic nanoparticles, spinous magnetic Fe3O4/TiO2 photo-catalyst (SMPC), and petaloid magnetic Fe3O4/TiO2 photo-catalyst (PMPC) were studied by SEM, as shown in Fig. 3a–c. The SEM images show that Fe3O4 nanoparticles, SMPC, and PMPC had uniform morphologies and well-dispersed properties, with average diameters of about 160, 280, and 300 nm, respectively. The reaction temperature during material preparation was the main reason for the two different morphologies, and the higher temperature (200 °C) promoted sharpening of the surface of SMPC comparing to the PMPC prepared with lower temperature (180 °C) [Wang et al., 2010]. From the TEM images (Fig. 3d–f), it was obvious that the darkness was heterogeneously distributed, which indicated a porous structure in the nanoparticles Lenert and Wang, 2012. As shown in Fig. 3h, the high magnification TEM images further demonstrated that Fe3O4/TiO2 are composed of small primary loose nanoparticles with the diameter below 20 nm, and the lattice fringe of 0.21 nm and 0.36 nm belong to Fe3O4 and TiO2 nanoparticle which are visible. Fig. 4a shows the XRD pattern, and the phase of the sample was determined by XRD analysis. The diffraction peaks at 2θ angles of 30.8°, 35.8°, 43.4°, 53.6°, 57.5°, and 62.9° match well with the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) reflections of the standard pattern of crystalline Fe3O4 (CPDS No. 89–0691). The diffraction peaks at 2θ angles of 24.9°, 37.7°, 48.1°, 54.8°, and 62.6° were detected that corresponded to the (1 1 0), (1 1 2), (1 0 1), (0 0 4), and (2 0 0) reflections of TiO2 (PDF No. 21-1272) [Chalasani and Vasudevan, 2013]. Based on the Scherrer equation, the crystal sizes of Fe3O4 and TiO2 in pristine are
To evaluate the photocatalytic performance of the prepared photocatalyst nanoparticles, SMPC and PMPC were used to catalyze the degradation of RhB in comparison to the P25 photocatalyst under simulated solar light. The intensity of the UV–Vis original absorption spectra decreased gradually with the duration of solar irradiation, and the concentration of RhB was directly proportional to the absorption peak intensity of absorption spectra based on the Beer-Lambert law [Hogan et al., 2014]. The intensity changes of the characteristic peak intensity were therefore used to demonstrate catalytic performance. The concentration of the degraded suspension liquid after solar-light illumination was recorded, and the degradation efficiency (Ed) could be calculated by Eq. (1) [Shi et al., 2018]:
Ed =
RCi - RCf × 100\% RCi
(1)
where RCi was the initial RhB concentration and RCf was the final concentration of the degraded RhB solution. The RhB degradation dynamics were expressed as a function of time. In the absence of photocatalyst solar radiation, the decrease in RhB concentration was negligible. The photocatalysis kinetics based on semiconductor were defined by the first-order-reaction of Langmuir-Hinshelwood model [Abdelrahman et al., 2019]:
ln(RCi/RCf )= kt
(2)
where t was the solar illumination time and k was the photo-degradation rate constant. Eq. (2) provides the photo-degradation rate constant 507
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Fig. 3. Morphology characterization of Fe3O4, spinous magnetic Fe3O4/TiO2 photo-catalyst (SMPC), and petaloid magnetic Fe3O4/TiO2 photo-catalyst (PMPC): scanning electron microscopy (SEM) images of (a) Fe3O4, (b) SMPC, and (c) PMPC nanoparticles; transmission electron microscopy (TEM) images of (d) Fe3O4 (e) SMPC, and (f) PMPC nanoparticles; (g) EDS images and (h) high magnification TEM images of magnetic Fe3O4/TiO2 photo-catalysts.
Fig. 4. Characterization of Fe3O4, SMPC, and PMPC nanoparticles: (a) X-ray diffraction spectra for confirming composition and structure; (b) XPS spectra of SMPC for measuring the chemical state of the elements; (c) Fourier transform transform-infrared spectroscopy spectra for analyzing functional group and (d) vibrating sample magnetometer for confirming their saturation magnetization.
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Fig. 5. (a) Ultraviolet-visible spectrum of P25, SMPC, and PMPC at the same concentration; (b) UV-visible absorption spectra of Rhodamine B (RhB) suspension decrease over time during 35 min; (c) temperature increase in bulk fluid with P25, SMPC, and PMPC; and (d) RhB suspensions degradation efficiency with watercooling and without water-cooling.
photocatalytic performance, photocatalytic experiments were conducted with water-cooling and the temperature was maintained at 20°, as shown in Fig. 5c, while the degradation efficiency, as shown in Fig. 5d. The degradation efficiency of the RhB suspension with added P25 was 83.1% (with cooling) and 87.2% (without cooling), while the degradation efficiency of the RhB suspension with added SMPC was 93.1% (with cooling) and 94.9% (without cooling), representing an increase of 12.0% and 8.8%, respectively. Furthermore, the degradation efficiency of the RhB suspension with SMPC without water-cooling was higher than that with water-cooling. The most likely reason for this was that the photo-thermal conversion accelerated the photocatalytic degradation resulting from the temperature increase of the RhB suspension. In addition, the higher absorption of SMPC in the visible spectrum further enhanced the effect of photo-thermal conversion in suspension. The morphology is considered to be the significant factor affecting the photocatalytic activity. The specific surface area of magnetic photocatalysts (SMPC and PMPC) was larger than P25 nanoparticles resulting from their nanoporous structure, which could be seen in the N2 adsorption-desorption isotherms and the pore distributions (Fig. 6a). The BET surface area was measured to be as high as 98.6 m2 g−1 (SMPC), respectively, compared to 79.4 m2 g−1 for PMPC nanoparticles, which may due to the porous structure of the magnetic Fe3O4/ TiO2 nanoparticles. The high specific surface area not only promote the transmission of electrons but also accelerate the absorption of photons. Meanwhile, the nanoporous structure in magnetic photocatalysts also promotes the adsorption of rhodamine B, which also contributes to water treatment. From the photoluminescence (PL) spectra (Fig. 6b), the emission peak intensity of the SMPC heterojunction is lower than that of the PMPC, which both lower than that of P25. The lower the emission intensity of PL, the lower the recombination efficiency of photo-generated carriers, and thus the higher the photocatalytic activity [Li et al. (2018)]. It may due to the P25 has lower recombination rates of photo-generated electron-hole pairs, and the heterogeneous spinous structure between Fe3O4 and TiO2 has a positive correlation with active reaction sites on its surface. The concentration changes of the RhB suspension could be determined from the ratio of the final concentration to the initial concentration of the RhB suspension, as shown in Fig. 6c. Compared to the RhB solution with added P25, the
equivalent to ln(RCi/RCt) versus the solar irradiation time through the slope of the best fit line.
3.2.1. Effect of photo-thermal conversion on photocatalytic activity It can be speculated that photocatalytic process is accompanied by the conversion process from solar energy to thermal form, by increasing the temperature of the reaction system [Wang et al., 2016]. One effective method to improve the efficiency of solar energy harvesting is to improve the absorption ability of solar spectrum [Zhang et al., 2014]. The optical properties of P25, SMPC, and PMPC are displayed in Fig. 5a through UV-Vis spectrum measurements. From Fig. 5a, it can be seen that SMPC and PMPC had a stronger broadband absorption than P25 at the same concentration in the visible spectrum (380–780 nm). The enhanced absorption of visible photons observed in SMPC and PMPC may result from the addition of Fe3O4 nanoparticles. And the absorption of SMPC was clearly higher than that of PMPC. The energy of solar radiation is mainly concentrated in the visible spectrum, so that the magnetic catalyst can absorb more energy to drive electron transfer and heat up fluid. Because of the SMPC has a smaller particle size compared with PMPC, there are more SPMC nanoparticles in suspension to absorb solar energy with the same mass concentration of the suspension, and more TiO2 in SPMC indicated by EDS images can further enhance the ability to absorb solar energy. On the other hand, the absorption spectrum is related to the structure and morphologies of nanoparticles, which can change the light path of liquid. Spinous Fe3O4/TiO2 suspensions realize higher absorbance visible wavelengths due to its superior scattering and absorption characteristics which results from its spinous structure. During the photocatalytic process, the UV-Vis absorption spectrum of the RhB suspension were recorded at every 5 min without water-cooling. From Fig. 5b, it can be indicated that the spectrum peak intensity decreased rapidly to low absorption values at a solar illumination intensity of 1000 W/m2. The selection of photocatalyst had a significant effect on the photo-thermal conversion, leading to the temperature rise in the suspension (Fig. 5c) that may in turn affect the photocatalytic performance. The temperature of the RhB suspension with SMPC increased more quickly than the other suspensions, indicating that SMPC had a superior ability to absorb the solar intensity. To determine the effect of photo-thermal conversion on 509
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Fig. 6. (a) N2 adsorption-desorption isotherms for comparing specific surface area and pore distributions (b) Photoluminescence (PL) spectra of Fe3O4, SMPC, and PMPC nanoparticles; (c) time dependence of RhB suspensions concentration changes; (d) plot of ln(RCi/RCf) with irradiation time and the photo-degradation rate constant of Fe3O4, SMPC, and PMPC suspensions.
3.2.2. Effect of a magnetic field on photocatalytic activity To determine the effect of a magnetic field on photocatalytic performance, photocatalytic experiments using magnetic nano-composite were conducted under different magnetic fields (0, 100, 200, 400, and 800 Oe). The photocatalytic performance of magnetic nano-composite under a magnetic field was clearly better than that without a magnetic field, as shown in Fig. 7a. It was found that the concentration of RhB suspension decreased more quickly with an intensity increase of magnetic field under solar illumination. The final concentration of the RhB suspension was lowered when increasing the magnetic field strength, indicating a limitation in the degradation effect with an increase in magnetic field strength. The minimum final RhB concentration when
concentration of the RhB suspension with added SMPC or PMPC decreased faster and the final concentrations were clearly reduced. It can also be seen that the RhB solution with added SMPC had a superior photocatalytic performance to that with added PMPC. A stronger absorption in the visible spectrum may be the main reason for the enhancement of the degradation process. As shown in Fig. 6d, the photodegradation rate constant of photocatalytic performances with magnetic nano-composite was calculated by Equation (2). The photo-degradation rate constant of the RhB suspension with different photocatalysts increased from 0.054 (P25) to 0.080 (PMPC) and 0.111 (SMPC), representing the photo-degradation rate has an increase of almost 200%.
Fig. 7. (a) Concentration decrease in of the RhB suspension during 35 min under different magnetic fields (0, 100, 200, 400, and 800 Oe); (b) plot of ln(RCi/RCf) with irradiation time and the photo-degradation rate constant; (c) Comparison of degradation efficiency with different photo-catalyst and (d) 7 cycles degradation performance of SMPC suspension under different magnetic fields; (e) UV-Vis spectrum of the same concentration of SMPC where the increasing spectral absorption peaks with the magnetic field intensify increasing and (f) the temperature rise in bulk fluid with the increase of magnetic fields (0, 100, 200, 400, and 800 Oe). 510
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using a magnetic field was 0.41% of the initial concentration, which means that the effect of magnetic field on degradation efficiency decreased in higher field strength (800 Oe). The photo-degradation rate constant increased from 0.138 to 0.214 when increasing the magnetic field strength from 0 to 800 Oe, i.e. the photo-degradation rate increased by 55% resulting from the increase of magnetic field strength, and a stable value was maintained under the stronger magnetic fields (Fig. 7b). From Fig. 7c, it was observed that the degradation efficiency of the RhB suspension with magnetic nano-composite was clearly higher than that of P25. When increasing the magnetic field strength from 100 to 800 Oe, the degradation efficiency of the RhB suspension with PMPC increased from 95.4 to 98.6% and the degradation efficiency of the RhB suspension with SMPC increased from 96.4 to 99.6%. It was also found that the degradation efficiency of the RhB suspension with SMPC was higher than that of the RhB suspension with PMPC under the same magnetic field, which contributed to greater absorption of the RhB suspension with SMPC in the visible spectrum. The SMPC nanoparticles were redispersed in RhB suspension for catalytic activity measurements over 7 cycles under different magnetic field strengths (Fig. 7d). It can be seen that the degradation efficiency increased with the magnetic field strength from 0 to 800 Oe and maintain stable. This results indicate that the SMPC can exhibit excellent reusability under different magnetic field strengths. The most likely reason for magnetically enhanced photocatalytic degradation is that the increasing magnetic field strength enhances the solar absorption of the nanofluid (Fig. 7e). This may be due to the directional arrangement of magnetic particles under magnetic volume force, which changes the optical structure of the magnetic photocatalyst nanofluid [Shi et al., 2017]. The optical structure within orientation of magnetic particles in a solution under a magnetic field promote the absorption and scattering of light between the magnetic photocatalyst and based fluid. It can enhance the ability of magnetic photocatalyst to generate and transport photogenerated charge carriers. The magnetic photocatalyst nanofluid can enhance the temperature of the bulk liquid as the magnetic field intensity increases under solar irradiation, as shown in Fig. 7f. The temperature of the RhB suspension increased from 53.5 to 55.5 °C, indicating that the magnetic field accelerated magnetic nano-composite to absorb the solar intensity, and that the energy conversion from photo to thermal form enhanced the photocatalytic performance. On the other hand, the magnetic field can effectively promote photo-thermal conversion by enhancing the adsorption performance of solar energy due to the enlarged reaction area and Lorentz force [Wang et al. (2018)]. All above indicated that the photocatalytic activity of a magnetic photocatalyst can be enhanced by magnetic field regulation.
Declaration of Competing Interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled. Acknowledgments This project is funded by the National Natural Science Foundation of China (Grant No. 51676060), the Natural Science Founds of Heilongjiang Province for Distinguished Young Scholars (Grant No. JC2016009), and Young Scientist Studio of Harbin Institute of Technology. References Abdelrahman, E.A., Tolan, D.A., Nassar, M.Y., 2019. A tunable template-assisted hydrothermal synthesis of hydroxysodalite zeolite nanoparticles using various aliphatic organic acids for the removal of zinc (II) ions from aqueous media. J. Inorg. Organomet. P. 29 (1), 229–247. Bahnemann, D., 2004. 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4. Conclusions In conclusion, a convenient and efficient approach was developed to synthesize magnetic nano-composite by controlling the temperature to improve absorbance ability in the visible solar spectrum. SMPC had excellent optical and magnetic properties compared to PMPC and P25 resulting from high specific surface area and spinous morphology. Comparing to photocatalytic activity of P25, the photo-degradation rate constant of the RhB suspension with SMPC increased by almost 200%. The degradation efficiencies of the RhB suspension with PMPC and SMPC were 92.1% and 94.3%. The excellent reusability of SMPC was demonstrated under different magnetic field strengths. The mechanism of excellent photocatalytic performance of magnetic nano-composite under a magnetic field was clearly identified. The photo-degradation rate constant increased with the increase of magnetic field intensity. The degradation efficiency of RhB suspension with SMPC increased from 96.4 to 99.6%, with the magnetic field intensity increased from 100 to 800 Oe. This resulted in an enlarged reaction area and enhancement of photo-thermal conversion due to the improved absorbance of the magnetic nanofluid under a magnetic field. 511
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Li, J., Pei, Q., Wang, R., Zhou, Y., Zhang, Z., Cao, Q., et al., 2018a. Enhanced photocatalytic performance through magnetic field boosting carrier transport. ACS Nano. 12, 3351–3359. Li, J., Zhang, Y., Wang, Y., Xue, C., Liang, J., Jiang, G., Liu, W., Zhu, C., 2016. Formation of Cu2ZnSnS4 thin film solar cell by CBD-annealing route: comparison of Cu and CuS in stacked layers SnS/Cu(S)/ZnS. Sol. Energy 129, 1–9. Liu, C., Rao, Z., Zhao, J., Huo, Y., Li, Y., 2015. Review on nanoencapsulated phase change materials: preparation, characterization and heat transfer enhancement. Nano Energy 12, 814–826. Liu, J., Chen, H., Xu, Y., Wang, L., Tan, C., 2014. A solar energy storage and power generation system based on supercritical carbon dioxide. Renew. Energy 64, 43–51. Li, Y., Ruan, Z., He, Y., Li, J., Li, K., Jiang, Y., Xu, X., Yuan, Y., Lin, K., 2018b. In situ fabrication of hierarchically porous g-C3N4 and understanding on its enhanced photocatalytic activity based on energy absorption. Appl. Catal. B-Environ. 236, 64–75. Nafey, A.S., Abdelkader, M., Abdelmotalip, A., Mabrouk, A.A., 2001. Solar still productivity enhancement. Energy Convers. Manage. 42, 1401–1408. Pardeshi, S.K., Patil, A.B., 2008. A simple route for photocatalytic degradation of phenol in aqueous zinc oxide suspension using solar energy. Sol. Energy. 82, 700–705. Qi, C., Wang, G., Yang, L., Wan, Y., Rao, Z., 2017. Two-phase lattice Boltzmann simulation of the effects of base fluid and nanoparticle size on natural convection heat transfer of nanofluid. Int. J. Heat. Mass. Tran. 105, 664–672. Rajrana, K., Gupta, A., Mir, R.A., et al., 2019. Facile sono-chemical synthesis of nanocrystalline MnO2 for catalytic and capacitive applications. Phys. B: Condensed Matter. 564, 179–185. Sharma, J., Gupta, A., Pandey, O.P., 2019. Effect of Zr doping and aging on optical and photocatalytic properties of ZnS nanopowder. Ceramics Int. 45 (11), 13671–13678. Shi, L., He, Y., Huang, Y., Jiang, B., 2017. Recyclable Fe3O4@CNT nanoparticles for highefficiency solar vapor generation. Energy Convers. Manage. 149, 401–408. Shi, L., He, Y., Wang, X., Hu, Y., 2018. Recyclable photo-thermal conversion and purification systems via Fe3O4@TiO2 nanoparticles. Energy Convers. Manage. 171, 272–278. Soltani, N., Saion, E., Mahmood, M.Y.W., Navasery, M., Bahmanrokh, G., Erfani, M., Zare, M.R., Gharibshahi, E., 2013. Photocatalytic degradation of methylene blue under visible light using PVP-capped ZnS and CdS nanoparticles. Sol. Energy. 97, 147–154.
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Solar Energy journal homepage: www.elsevier.com/locate/solener
Investigation of photocatalytic activity through photo-thermal heating enabled by Fe3O4/TiO2 composite under magnetic field Lei Shi, Xinzhi Wang, Yanwei Hu, Yurong He
T
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Heilongjiang Key Laboratory of New Energy Storage Materials and Processes, Harbin 150001, China School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Photo-thermal conversion Nanofluids Magnetic-enhanced photocatalytic
The significant improvement in photocatalytic activity achieved in recent years has generated widespread attention due to its utilization in solar energy. In this study, two shapes of magnetic nanoparticles were synthesized using hydrothermal method. The magnetic nanoparticles exhibited superior photocatalytic activity for water treatment, which takes into consideration the photo-thermal conversion of magnetic nanoparticles. The effect of morphology on the photo-degradation was analyzed by the specific surface area measurement and photoluminescence test. Furthermore, the suspension degradation rate with magnetic nanoparticles was enhanced by ~200% compared to a commercial photocatalyst. Moreover, the photo-degradation efficiency further increased from 96.4 to 99.6% with the magnetic field intensity increased because of the absorbance ability enhancement. This result indicates that the photocatalytic activity of a magnetic photocatalyst can be enhanced through photothermal conversion and magnetic field regulation, which expands the application of solar power technology.
1. Introduction Wastewater pollution, particularly in the form of organic dyes, is a nonnegligible environmental problem that can cause great harm to human health. Thus, there has been much interest in the development of a cost-effective and efficient wastewater treatment for removing organic dyes [Chaudhary and Rizwan et al., 2017; Gupta and Pandey, 2019]. It is noteworthy that photocatalytic processes, as one way to achieve photochemical conversion, have been recognized as an ideal treatment of environmental pollutants [Li et al., 2016; Sharma et al., 2019]. With recent advances in nanotechnology, various photocatalytic nanoparticles with different structures, and consisting of various materials, have been prepared to eliminate organic pollutants by enhancing their photo absorption properties [Nafey et al., 2001; Soltani et al., 2013]. Due to its chemical inertness and long-term stability in solution, nanostructure TiO2 has been demonstrated to have perfect catalytic effects for water treatment under solar illumination [Borges et al., 2016]. This process of solar energy to chemical energy transformation is accompanied by solar energy to heat transformation, which in return influences the photo-chemical transformation [Li et al., 2017; Rajrana et al., 2019]. The development of sustainable energy technologies, especially solar energy, is critical to ensuring future energy sources [Abdelrahman et al., 2019; Kundu and Lee, 2012; Vig et al., 2019]. The acquisition of ⁎
renewable solar energy and the transition from solar energy to other energy forms, such as photoelectric, photochemical, and photo-thermal energy has attracted significant attention [Liu et al., 2015; Zhou et al., 2017]. Both photo-thermal and photochemical conversion studies have mainly considered the expansion of radiation absorption into the visible solar spectrum, then its production of more photo carriers to enhance the efficiency [Cuevas et al., 2015; Chen et al., 2015]. For this reason, a number of researchers have focused on nanofluids that exhibit outstanding photo-thermal conversion properties. Carbon-based and metallic-based micro-nano structure have been widely applied to manufacture solar conversion absorbers and enhance photo-thermal conversion efficiency [Ge et al., 2014; Chehadi et al., 2016]. For example, Jin et al. [Jin et al., 2016] experimentally and numerically discussed the photo-thermal transformation mechanism of Au nanoparticle direct absorption photo-collectors. Wang et al. [Wang et al. (2018)] researched the effects of photo-thermal conversion on the treatment of dye-contaminated wastewater and explained the relationship between photo-thermal conversion and photochemical conversion. Gupta et al. investigated the effect of heterojunction on photodegradation efficiency of methylene blue under visible irradiation [Gupta and Pandey, 2019; Kaur et al., 2018]. Magnetic photocatalysts combine catalytic properties and magnetism to facilitate recyclable purification and separation for the treatment of organic pollutants [Bahnemann, 2004; Abdelrahman et al.,
Corresponding author at: Heilongjiang Key Laboratory of New Energy Storage Materials and Processes, Harbin 150001, China. E-mail address: [email protected] (Y. He).
https://doi.org/10.1016/j.solener.2019.12.053 Received 2 June 2019; Received in revised form 14 December 2019; Accepted 17 December 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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hexahydrate (4 mmol) was stirred vigorously in 80 ml DI water for 10 min. Then, the suspension was mixed with 2.35 g sodium citrate (3 mmol) and 0.72 urea (4 mmol). Next, 0.8 g polyacrylamide was added into the suspension within stirred up condition for 20 min. Then the reaction suspensions were transferred into an autoclave sealed with polytetrafluoroethylene. And it was maintained at 200 °C for 12 h and cooled with air natural convection. Then, the black precipitate (Fe3O4 nanoparticles) was washed with ethanol solution by magnetic separation before oven drying at 50 °C for 12 h. The Fe3O4 nanoparticles were mixed with 5 ml titanium tetrafluoride solution of 40 mmol and stirred vigorously for 15 min, which was in order to ensure Fe3O4 can be covered with enough TiO2 nanoparticles. The suspensions were then placed in an autoclave sealed with polytetrafluoroethylene. Then, the autoclave was maintained at two different temperatures (180 and 200 °C) for 48 h to prepare two kinds of nanoparticles and cooled with air natural convection. Finally, the magnetic nanoparticles coated with TiO2 were obtained from the reaction suspensions by magnetic attraction.
2019]. There was a gap between conduction band and valence band in the energy band structure of the magnetic photocatalyst [Abdelrahman et al., 2019]. The magnetic photocatalyst maintained an unstable excited state when the energy was greater than the forbidden bandwidth of the illumination. The electron excitation of the valence band jumped to the conduction belt and formed holes, which resulted in the magnetic photocatalyst releasing external energy for water purification Wang et al., 2018. Previous research has shown that, the separation cost of catalyst particles is even higher than the energy saved in the solar purification process because the catalyst particles are small [Pardeshi and Patil, 2008; Abdelrahman et al., 2019]. Magnetic separation is an efficient and low-cost method to recover magnetic nanoparticles applying a suitable magnetic field [Zhang et al., 2009; Zhuang et al., 2010]. For example, Li et al. (2018) reported an approach to synthesize magnetic photocatalyst and confirmed that these nanoparticles have excellent photocatalytic properties in mixture solutions. Tan et al. [Tan et al., 2015] synthesized nanostructure Fe3O4/TiO2 core-shell nanoparticles, and the degradation performance of the nanoparticles with pH and temperature was researched. However, most studies in this area have focused on the separation efficiency of a magnetic photocatalyst [Liu et al., 2014; Soltani et al., 2017]. Despite the great successes achieved in recent years, the catalytic performance is still lower than expected. Hence, there is a need to investigate improved alternative strategies of catalytic efficiency [He et al., 2013; Ding et al., 2012]. Some studies have attempted to improve catalytic efficiency using other physical field forms, such as electric and magnetic fields [Fan et al., 2012; Barrett et al., 1951]. New researches suggest that both the degradation rate constant and efficiency can be enhanced with the utilization of magnetic fields [Ghasemi et al., 2014; Kaloudis et al., 2016]. In this work, we prepared two shapes of magnetic photo-catalyst (MPC) using hydrothermal method with different temperatures, which combined catalytic and magnetic properties to provide a composite material. Initially, the spinous and petaloid morphology of MPCs were characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The structural characterization of MPCs were characterized by X-ray diffraction spectra (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform-infrared spectroscopy (FT-IR) and magnetic property measurement system (MPMS). Then, an analysis of photocatalytic experiments was conducted to study the degradation performance of two shapes of MPC compared to P25, a standard commercial photocatalyst. The reason of different catalytic properties between three photocatalyst was discussed through specific surface area analysis and photoluminescence measurement. Furthermore, the relationship between photo-thermal conversion and photocatalytic performance was investigated in photocatalytic experiments with cooling. Finally, we observed the photocatalytic performance of a MPC under different magnetic fields, and the mechanism of magnetic-enhanced photocatalytic degradation was explained.
2.3. Characterization The morphology of the magnetic photocatalyst materials was characterized by SEM (ZEISS SUPRA 55; Zeiss, Germany) and recorded with a bias voltage of 30 kV. Energy dispersive spectroscopy (EDS) was performed using an X-MaxN 80 Silicon Drift Detector. The TEM patterns of the magnetic nanoparticles were characterized by TEM (JEM-2010; Jeol, Japan). The crystallographic structures of the magnetic nanoparticles were determined by X-ray diffraction (D8-Advance, Bruker, USA), with a monochromatic radiation source at room temperature operating in step-scan mode. The chemical composition of the magnetic nanoparticles was obtained through Fourier transform-infrared spectroscopy (Nicolet-560, Nicolet, US). The nanoparticle magnetism was investigated using a vibrating sample magnetometer (VSM; Quantum Design, USA). X-ray photoelectron spectroscopy (AXIS Ultra, Kratos Analytical, UK) was used with the exciting source (Mg-KR radiation) provided by the Electron Spectroscopy for Chemical Analysis (ESCA) Laboratory MKII instrument. And the absorption spectrum was recorded through the ultraviolet-visible spectrophotometer (UV-Vis; Persee Co., Ltd., Beijing, China) at wavelengths of 300–800 nm. The photoluminescence (PL) analysis was carried out by fluorescence spectrophotometer (Hitachi F-7000, Japan). The specific surface area analysis was obtained by Brunauer-Emmett-Teller (BET) methods (Quantachrome Autosorb-1C-VP, US). 2.4. Photo-catalytic efficiency test set-up Rhodamine B (RhB), one of the organic dyes, was used as the degraded pollutant in experiments. The experimental facility was showed in Fig. 2, in which photocatalytic activity occurs through the photothermal conversion of a magnetic photocatalyst under a magnetic field Li et al., 2018; Huang et al., 2017. As shown in Fig. 2, 30 mg magnetic nanoparticles were added into 50 ml Rhodamine B aqueous suspension (1 × 10−4 mol/L) in a double-walled acrylic beaker, with the temperature maintained by recirculated cooling water. The RhB aqueous solution containing magnetic nanoparticles was illuminated with a solar simulator (NP2000; CeauLight, Beijing, China) for 35 min to determine the photocatalytic performance. RhB was decomposed into inorganic matter and changes in the aqueous solution concentration were measured every 5 min by the UV-Vis spectrophotometer. The temperature at different heights in the test beaker was determined by three T-type thermocouples (SLE-40; Omega, Norwalk, USA) and read by a data-acquisition receiver (CA34972; Agilent, Santa Clara, USA). A constant magnetic field generated by the Helmholtz coil was applied to the test beakers and the strength of the magnetic field could be regulated by electric current I. And the strength of magnetic field was measured by digital gauss meter (Model 931; Honor Top Magnetic
2. Experimental section 2.1. Chemical reagents Iron(III) chloride hexahydrate (FeCl3·6H2O, AR, 99%), ascorbic acid (C6H8O6, AR, 99%), polyacrylamide ((C3H5NO)n, AR, 99%), urea (CN2H4O, AR, 99%), titanium tetrafluoride (TiF4, AR, 99%), and sodium citrate (Na3C6H5O7·2H2O, AR, 99%) were commercially purchased (Aladdin Reagents; Shanghai, China). Deionized water (DI) purified in a laboratory ultrapure water system (Arium-mini Plus; Sartorius, Göttingen, Germany) was applied in all process of nanoparticles preparation. 2.2. Synthesis of magnetic nanoparticles The preparation process of magnetic nano-composite using hydrothermal method is shown as follows (Fig. 1): 1.08 g Iron (III) chloride 506
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Fig. 1. Preparation schematic of magnetic nano-composite using hydrothermal method.
calculated from 12 to 19 nm, which is consistent with the TEM images (Fig. 3h) [Hou et al., 2018]. This indicated that each magnetic Fe3O4/ TiO2 photo-catalyst is composed of small primary crystals that retain their excellent paramagnetism and catalytic property, which also can be demonstrated by TEM images (Fig. 3d–f). The XPS spectra of SMPC were recorded and three different peaks (Fe 2p, O 1s, and Ti 2p) were apparent, as shown in Fig. 4b, which further confirmed the elemental components of magnetic nanoparticles. From the FT-IR spectrum (Fig. 4c), the peaks at 596, 3401, and 1633 cm−1 were ascribed to FeeO, eOH stretching, and bending vibrations, which indicated that the surface of the nanoparticles contained hydrophilic groups, endowing the nanoparticles with well-dispersed properties in solution [Zhang et al., 2011]. The peaks at 1241 and 669 cm−1 were the characterized vibrations of the TieO groups, which further justify the XRD and XPS results. The peak at 1400 cm−1 was the characterization of the COOeFe vibrations, both the peak at 1633 and 1400 cm−1 show the hydrophilic groups on the surface of Fe3O4 nanoparticles. The hysteresis loops (Fig. 4d) showed that the Fe3O4 nanoparticles saturation magnetization was 59.3 emu/g. The SMPC and PMPC had the same saturation magnetization of 32.4 emu/g, while the PMPC was more sensitive to the magnetic field. This was probably due to the differences in morphology and size between SMPC and PMPC [Qi et al., 2017]. The saturation magnetization of the SMPC and PMPC can be converted into weight percent of the Fe3O4 magnetization [Guardia et al., 2007]. It means that adding TiO2 into nano-composites weaken the magnetic property of the magnetic bulk material, which led to the decrease in saturation magnetization.
Fig. 2. Experimental schematic for the enhancement of photo-catalytic activity by a magnetic field.
Technology, China). The catalytic performance of both kinds of morphological particles was compared to P25 in the experimental procedure. The influence of applied magnetic field on the catalytic performance was studied at five different intensities of magnetic field (0, 100, 200, 400, and 800 Oe).
3. Results and discussion 3.1. Structure of magnetic photo-catalyst
3.2. Photocatalytic activity of magnetic nano-composite
The shape of the prepared Fe3O4 magnetic nanoparticles, spinous magnetic Fe3O4/TiO2 photo-catalyst (SMPC), and petaloid magnetic Fe3O4/TiO2 photo-catalyst (PMPC) were studied by SEM, as shown in Fig. 3a–c. The SEM images show that Fe3O4 nanoparticles, SMPC, and PMPC had uniform morphologies and well-dispersed properties, with average diameters of about 160, 280, and 300 nm, respectively. The reaction temperature during material preparation was the main reason for the two different morphologies, and the higher temperature (200 °C) promoted sharpening of the surface of SMPC comparing to the PMPC prepared with lower temperature (180 °C) [Wang et al., 2010]. From the TEM images (Fig. 3d–f), it was obvious that the darkness was heterogeneously distributed, which indicated a porous structure in the nanoparticles Lenert and Wang, 2012. As shown in Fig. 3h, the high magnification TEM images further demonstrated that Fe3O4/TiO2 are composed of small primary loose nanoparticles with the diameter below 20 nm, and the lattice fringe of 0.21 nm and 0.36 nm belong to Fe3O4 and TiO2 nanoparticle which are visible. Fig. 4a shows the XRD pattern, and the phase of the sample was determined by XRD analysis. The diffraction peaks at 2θ angles of 30.8°, 35.8°, 43.4°, 53.6°, 57.5°, and 62.9° match well with the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) reflections of the standard pattern of crystalline Fe3O4 (CPDS No. 89–0691). The diffraction peaks at 2θ angles of 24.9°, 37.7°, 48.1°, 54.8°, and 62.6° were detected that corresponded to the (1 1 0), (1 1 2), (1 0 1), (0 0 4), and (2 0 0) reflections of TiO2 (PDF No. 21-1272) [Chalasani and Vasudevan, 2013]. Based on the Scherrer equation, the crystal sizes of Fe3O4 and TiO2 in pristine are
To evaluate the photocatalytic performance of the prepared photocatalyst nanoparticles, SMPC and PMPC were used to catalyze the degradation of RhB in comparison to the P25 photocatalyst under simulated solar light. The intensity of the UV–Vis original absorption spectra decreased gradually with the duration of solar irradiation, and the concentration of RhB was directly proportional to the absorption peak intensity of absorption spectra based on the Beer-Lambert law [Hogan et al., 2014]. The intensity changes of the characteristic peak intensity were therefore used to demonstrate catalytic performance. The concentration of the degraded suspension liquid after solar-light illumination was recorded, and the degradation efficiency (Ed) could be calculated by Eq. (1) [Shi et al., 2018]:
Ed =
RCi - RCf × 100\% RCi
(1)
where RCi was the initial RhB concentration and RCf was the final concentration of the degraded RhB solution. The RhB degradation dynamics were expressed as a function of time. In the absence of photocatalyst solar radiation, the decrease in RhB concentration was negligible. The photocatalysis kinetics based on semiconductor were defined by the first-order-reaction of Langmuir-Hinshelwood model [Abdelrahman et al., 2019]:
ln(RCi/RCf )= kt
(2)
where t was the solar illumination time and k was the photo-degradation rate constant. Eq. (2) provides the photo-degradation rate constant 507
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Fig. 3. Morphology characterization of Fe3O4, spinous magnetic Fe3O4/TiO2 photo-catalyst (SMPC), and petaloid magnetic Fe3O4/TiO2 photo-catalyst (PMPC): scanning electron microscopy (SEM) images of (a) Fe3O4, (b) SMPC, and (c) PMPC nanoparticles; transmission electron microscopy (TEM) images of (d) Fe3O4 (e) SMPC, and (f) PMPC nanoparticles; (g) EDS images and (h) high magnification TEM images of magnetic Fe3O4/TiO2 photo-catalysts.
Fig. 4. Characterization of Fe3O4, SMPC, and PMPC nanoparticles: (a) X-ray diffraction spectra for confirming composition and structure; (b) XPS spectra of SMPC for measuring the chemical state of the elements; (c) Fourier transform transform-infrared spectroscopy spectra for analyzing functional group and (d) vibrating sample magnetometer for confirming their saturation magnetization.
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Fig. 5. (a) Ultraviolet-visible spectrum of P25, SMPC, and PMPC at the same concentration; (b) UV-visible absorption spectra of Rhodamine B (RhB) suspension decrease over time during 35 min; (c) temperature increase in bulk fluid with P25, SMPC, and PMPC; and (d) RhB suspensions degradation efficiency with watercooling and without water-cooling.
photocatalytic performance, photocatalytic experiments were conducted with water-cooling and the temperature was maintained at 20°, as shown in Fig. 5c, while the degradation efficiency, as shown in Fig. 5d. The degradation efficiency of the RhB suspension with added P25 was 83.1% (with cooling) and 87.2% (without cooling), while the degradation efficiency of the RhB suspension with added SMPC was 93.1% (with cooling) and 94.9% (without cooling), representing an increase of 12.0% and 8.8%, respectively. Furthermore, the degradation efficiency of the RhB suspension with SMPC without water-cooling was higher than that with water-cooling. The most likely reason for this was that the photo-thermal conversion accelerated the photocatalytic degradation resulting from the temperature increase of the RhB suspension. In addition, the higher absorption of SMPC in the visible spectrum further enhanced the effect of photo-thermal conversion in suspension. The morphology is considered to be the significant factor affecting the photocatalytic activity. The specific surface area of magnetic photocatalysts (SMPC and PMPC) was larger than P25 nanoparticles resulting from their nanoporous structure, which could be seen in the N2 adsorption-desorption isotherms and the pore distributions (Fig. 6a). The BET surface area was measured to be as high as 98.6 m2 g−1 (SMPC), respectively, compared to 79.4 m2 g−1 for PMPC nanoparticles, which may due to the porous structure of the magnetic Fe3O4/ TiO2 nanoparticles. The high specific surface area not only promote the transmission of electrons but also accelerate the absorption of photons. Meanwhile, the nanoporous structure in magnetic photocatalysts also promotes the adsorption of rhodamine B, which also contributes to water treatment. From the photoluminescence (PL) spectra (Fig. 6b), the emission peak intensity of the SMPC heterojunction is lower than that of the PMPC, which both lower than that of P25. The lower the emission intensity of PL, the lower the recombination efficiency of photo-generated carriers, and thus the higher the photocatalytic activity [Li et al. (2018)]. It may due to the P25 has lower recombination rates of photo-generated electron-hole pairs, and the heterogeneous spinous structure between Fe3O4 and TiO2 has a positive correlation with active reaction sites on its surface. The concentration changes of the RhB suspension could be determined from the ratio of the final concentration to the initial concentration of the RhB suspension, as shown in Fig. 6c. Compared to the RhB solution with added P25, the
equivalent to ln(RCi/RCt) versus the solar irradiation time through the slope of the best fit line.
3.2.1. Effect of photo-thermal conversion on photocatalytic activity It can be speculated that photocatalytic process is accompanied by the conversion process from solar energy to thermal form, by increasing the temperature of the reaction system [Wang et al., 2016]. One effective method to improve the efficiency of solar energy harvesting is to improve the absorption ability of solar spectrum [Zhang et al., 2014]. The optical properties of P25, SMPC, and PMPC are displayed in Fig. 5a through UV-Vis spectrum measurements. From Fig. 5a, it can be seen that SMPC and PMPC had a stronger broadband absorption than P25 at the same concentration in the visible spectrum (380–780 nm). The enhanced absorption of visible photons observed in SMPC and PMPC may result from the addition of Fe3O4 nanoparticles. And the absorption of SMPC was clearly higher than that of PMPC. The energy of solar radiation is mainly concentrated in the visible spectrum, so that the magnetic catalyst can absorb more energy to drive electron transfer and heat up fluid. Because of the SMPC has a smaller particle size compared with PMPC, there are more SPMC nanoparticles in suspension to absorb solar energy with the same mass concentration of the suspension, and more TiO2 in SPMC indicated by EDS images can further enhance the ability to absorb solar energy. On the other hand, the absorption spectrum is related to the structure and morphologies of nanoparticles, which can change the light path of liquid. Spinous Fe3O4/TiO2 suspensions realize higher absorbance visible wavelengths due to its superior scattering and absorption characteristics which results from its spinous structure. During the photocatalytic process, the UV-Vis absorption spectrum of the RhB suspension were recorded at every 5 min without water-cooling. From Fig. 5b, it can be indicated that the spectrum peak intensity decreased rapidly to low absorption values at a solar illumination intensity of 1000 W/m2. The selection of photocatalyst had a significant effect on the photo-thermal conversion, leading to the temperature rise in the suspension (Fig. 5c) that may in turn affect the photocatalytic performance. The temperature of the RhB suspension with SMPC increased more quickly than the other suspensions, indicating that SMPC had a superior ability to absorb the solar intensity. To determine the effect of photo-thermal conversion on 509
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Fig. 6. (a) N2 adsorption-desorption isotherms for comparing specific surface area and pore distributions (b) Photoluminescence (PL) spectra of Fe3O4, SMPC, and PMPC nanoparticles; (c) time dependence of RhB suspensions concentration changes; (d) plot of ln(RCi/RCf) with irradiation time and the photo-degradation rate constant of Fe3O4, SMPC, and PMPC suspensions.
3.2.2. Effect of a magnetic field on photocatalytic activity To determine the effect of a magnetic field on photocatalytic performance, photocatalytic experiments using magnetic nano-composite were conducted under different magnetic fields (0, 100, 200, 400, and 800 Oe). The photocatalytic performance of magnetic nano-composite under a magnetic field was clearly better than that without a magnetic field, as shown in Fig. 7a. It was found that the concentration of RhB suspension decreased more quickly with an intensity increase of magnetic field under solar illumination. The final concentration of the RhB suspension was lowered when increasing the magnetic field strength, indicating a limitation in the degradation effect with an increase in magnetic field strength. The minimum final RhB concentration when
concentration of the RhB suspension with added SMPC or PMPC decreased faster and the final concentrations were clearly reduced. It can also be seen that the RhB solution with added SMPC had a superior photocatalytic performance to that with added PMPC. A stronger absorption in the visible spectrum may be the main reason for the enhancement of the degradation process. As shown in Fig. 6d, the photodegradation rate constant of photocatalytic performances with magnetic nano-composite was calculated by Equation (2). The photo-degradation rate constant of the RhB suspension with different photocatalysts increased from 0.054 (P25) to 0.080 (PMPC) and 0.111 (SMPC), representing the photo-degradation rate has an increase of almost 200%.
Fig. 7. (a) Concentration decrease in of the RhB suspension during 35 min under different magnetic fields (0, 100, 200, 400, and 800 Oe); (b) plot of ln(RCi/RCf) with irradiation time and the photo-degradation rate constant; (c) Comparison of degradation efficiency with different photo-catalyst and (d) 7 cycles degradation performance of SMPC suspension under different magnetic fields; (e) UV-Vis spectrum of the same concentration of SMPC where the increasing spectral absorption peaks with the magnetic field intensify increasing and (f) the temperature rise in bulk fluid with the increase of magnetic fields (0, 100, 200, 400, and 800 Oe). 510
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using a magnetic field was 0.41% of the initial concentration, which means that the effect of magnetic field on degradation efficiency decreased in higher field strength (800 Oe). The photo-degradation rate constant increased from 0.138 to 0.214 when increasing the magnetic field strength from 0 to 800 Oe, i.e. the photo-degradation rate increased by 55% resulting from the increase of magnetic field strength, and a stable value was maintained under the stronger magnetic fields (Fig. 7b). From Fig. 7c, it was observed that the degradation efficiency of the RhB suspension with magnetic nano-composite was clearly higher than that of P25. When increasing the magnetic field strength from 100 to 800 Oe, the degradation efficiency of the RhB suspension with PMPC increased from 95.4 to 98.6% and the degradation efficiency of the RhB suspension with SMPC increased from 96.4 to 99.6%. It was also found that the degradation efficiency of the RhB suspension with SMPC was higher than that of the RhB suspension with PMPC under the same magnetic field, which contributed to greater absorption of the RhB suspension with SMPC in the visible spectrum. The SMPC nanoparticles were redispersed in RhB suspension for catalytic activity measurements over 7 cycles under different magnetic field strengths (Fig. 7d). It can be seen that the degradation efficiency increased with the magnetic field strength from 0 to 800 Oe and maintain stable. This results indicate that the SMPC can exhibit excellent reusability under different magnetic field strengths. The most likely reason for magnetically enhanced photocatalytic degradation is that the increasing magnetic field strength enhances the solar absorption of the nanofluid (Fig. 7e). This may be due to the directional arrangement of magnetic particles under magnetic volume force, which changes the optical structure of the magnetic photocatalyst nanofluid [Shi et al., 2017]. The optical structure within orientation of magnetic particles in a solution under a magnetic field promote the absorption and scattering of light between the magnetic photocatalyst and based fluid. It can enhance the ability of magnetic photocatalyst to generate and transport photogenerated charge carriers. The magnetic photocatalyst nanofluid can enhance the temperature of the bulk liquid as the magnetic field intensity increases under solar irradiation, as shown in Fig. 7f. The temperature of the RhB suspension increased from 53.5 to 55.5 °C, indicating that the magnetic field accelerated magnetic nano-composite to absorb the solar intensity, and that the energy conversion from photo to thermal form enhanced the photocatalytic performance. On the other hand, the magnetic field can effectively promote photo-thermal conversion by enhancing the adsorption performance of solar energy due to the enlarged reaction area and Lorentz force [Wang et al. (2018)]. All above indicated that the photocatalytic activity of a magnetic photocatalyst can be enhanced by magnetic field regulation.
Declaration of Competing Interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled. Acknowledgments This project is funded by the National Natural Science Foundation of China (Grant No. 51676060), the Natural Science Founds of Heilongjiang Province for Distinguished Young Scholars (Grant No. JC2016009), and Young Scientist Studio of Harbin Institute of Technology. References Abdelrahman, E.A., Tolan, D.A., Nassar, M.Y., 2019. A tunable template-assisted hydrothermal synthesis of hydroxysodalite zeolite nanoparticles using various aliphatic organic acids for the removal of zinc (II) ions from aqueous media. J. Inorg. Organomet. P. 29 (1), 229–247. Bahnemann, D., 2004. 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4. Conclusions In conclusion, a convenient and efficient approach was developed to synthesize magnetic nano-composite by controlling the temperature to improve absorbance ability in the visible solar spectrum. SMPC had excellent optical and magnetic properties compared to PMPC and P25 resulting from high specific surface area and spinous morphology. Comparing to photocatalytic activity of P25, the photo-degradation rate constant of the RhB suspension with SMPC increased by almost 200%. The degradation efficiencies of the RhB suspension with PMPC and SMPC were 92.1% and 94.3%. The excellent reusability of SMPC was demonstrated under different magnetic field strengths. The mechanism of excellent photocatalytic performance of magnetic nano-composite under a magnetic field was clearly identified. The photo-degradation rate constant increased with the increase of magnetic field intensity. The degradation efficiency of RhB suspension with SMPC increased from 96.4 to 99.6%, with the magnetic field intensity increased from 100 to 800 Oe. This resulted in an enlarged reaction area and enhancement of photo-thermal conversion due to the improved absorbance of the magnetic nanofluid under a magnetic field. 511
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