Photo-thermal characteristics of water-based Fe3O4@SiO2 nanofluid for solar-thermal applications

This work proposes and demonstrates the novel idea of using Fe3O4@SiO2 core/shell structure nanoparticles (NPs) to improve the solar thermal conversion efficiency. Magnetite (Fe3O4) NPs are synthesized by controlled co-precipitation method. Fe3O4@SiO2 NPs are prepared based on sol–gel approach, then characterized. Water-based Fe3O4@SiO2 nanofluid is prepared and usedto illustrate the photo-thermal conversion characteristics of a solar collector under solar simulator. The temperature rise characteristics of the nanofluids are investigated at different heights of the solar collector, for duration of 300 min, under a solar intensity of 1000 W m−2. The experimental results show that Fe3O4@SiO2 NPs have a core/shell structure with spherical morphology and size of about 400 nm. Fe3O4@SiO2/H2O nanofluid enhances the photo-thermal conversion efficiency compared with base fluid and Fe3O4/H2O nanofluid, since the silica coating improves both the thermodynamic stability of the nanofluid and the light absorption effectiveness of the NPs. At a concentration of 1 mg/1 ml of Fe3O4@SiO2/H2O, and with the utilization of kerosene into the solar collector, and exposure for radiation for 5 min, the photo-thermal conversion efficiency has shown an enhancement at the bottom of the collector of about 32.9% compared to the base fluid.


Introduction
In the last decade, magnetite (Fe 3 O 4 ) nanoparticles (NPs) have attracted the interest of many researchers because of their broad range of applications in the fields of biology and energy, such as biocatalysis, bio-labeling, immunoassay, biosensors, separation and purification of biomolecules, magnetic resonance imaging (MRI), and molecular interactions in live cells [1,2], drug delivery [3], CO 2 capture [4] and thermal absorption systems [5]. In photo-thermal applications, Fe 3 O 4 NPs have attracted researchers' attention because they have an extensive photon absorption cross-section, and strong intermolecular bonds, economical, and environmental friendly [5]. Fe 3 O 4 NPs are used in photo-thermal therapy, where Fe 3 O 4 NPs possess the ability to absorb near infrared to kill cancer cells under light irradiation, as the photon energy converts effectively into heat, subsequently the local temperature of tumor raises and scorches the cancer cells [6][7][8]. Furthermore, in solar energy field, the Fe 3 O 4 NPs are used in thermal energy conversion systems, where NPs are mixed in transparent base fluids to form nanofluids. The nanofluids are utilized in varieties of collectors for direct absorption of the solar radiation [5].
In spite of the above benefits, there are some challenges in using Fe 3 O 4 for photo-thermal energy conversion, as concerns stabilizing the Fe 3 O 4 nanofluids and suppressing the reflection of light. Fe 3 O 4 NPs show poor dispersion and high tendency to agglomerate in water and organic solvents. That is due to the large surface to volume ratio and existence of strong dipole-dipole interaction between the NPs. Furthermore, the thermal effect of light can also induce the oxidation of magnetite (Fe 3 O 4 ) to be transformed to hematite (Fe 2 O 3 ) which has different properties [10].
To protect magnetite NPs from agglomeration and oxidation, they are typically coated with a more inert mat erial, such as silica (SiO 2 ) [11]. Silica coating shields the magnetic dipole interaction, and enhances the coulomb repulsion due to being negatively charged. Usually, SiO 2 coating on the surface of Fe 3 O 4 NPs prevents their aggregation in solution, improves their chemical stability, provides better protection against toxicity [12], and provides a good surface for subsequent functionalization for bioimaging, diagnosis, therapeutic [13][14][15][16][17] and bioelectronic [18] applications. Fe 3 O 4 @SiO 2 NPs have strong affinity with water which helps realize an efficient and stable nanofluid. Light absorption is enhanced in Fe 3 O 4 @SiO 2 NPs because silica coating minimizes the light reflection allowing its use in solar thermal applications. To the best of our knowledge, so far, no study has been reported on solar photo-thermal conversion of water-based Fe 3 O 4 @SiO 2 nanofluids.
Several researchers have investigated the photo-thermal effect of iron oxide and other NPs (e.g. TiO 2 , Al 2 O 3 , Ag, Cu, SiO 2 , graphite NPs, and carbon nanotubes) dispersed in fluids for heat transfer enhancement [5,[19][20][21][22][23]. Their studies reported remarkable outlet temperature and efficiency improvement by using nanofluids as compared to conventional fluids in energy conversion. Fe 3 O 4 nanofluid is used in solar thermal systems to enhance efficiency through radiative properties (i.e. absorption and scattering) and thermo-physical properties such as thermal conductivity and heat capacity enhancement [22]. Hybrid Graphene/Fe 3 O 4 has been synthesized and prepared as a kerosene-based nanofluid aiming for the heat transfer enhancement [24].
A novel approach in order to combine the advantages of both SiO 2 and Fe 3 O 4 nanomaterials is to utilize Fe 3 O 4 NPs coated with silica for solar photo-thermal conversion application. According to the literature, the photothermal effect induced by solar radiation has never been investigated for Fe 3 O 4 @SiO 2 nanofluid. In the present work, the synthesis procedure and characterization of Fe 3 O 4 @SiO 2 NPs will be elaborated in section 2, where nanofluid preparation and experiment setup will be further discussed. Finally photo-thermal characteristics of the water-based Fe 3 O 4 @SiO 2 nanofluid and the solar collector performance are investigated in section 3. Section 4 summarizes our main experimental findings.

Fe 3 O 4 @SiO 2 NPs synthesis
The Fe 3 O 4 NPs were prepared by a chemical co-precipitation method [25]. The procedure is described as follows: 2.36 g of FeCl 3 · 6H 2 O and 0.86 g of FeCl 2 · 4H 2 O (2:1 M ratio) were dissolved in 40 ml deionized water under atmospheric nitrogen. This aqueous solution containing both Fe 2+ and Fe 3+ ions, it was then heated and kept exposed to nitrogen while maintaining a rigorous magnetic stirring. Once the temperature of the solution reached 80 °C, 5 ml of ammonium hydroxide was added quickly after which the magnetic NPs were formed and started to agglomerate as indicated by a black precipitate. Then 5 ml of PEG aqueous solution was added drop wise, while the suspension was kept at 80 °C for 1 h. PEG which is a hydrophilic and biocompatible polymer, used as an additive coating polymer during the co-precipitation process, in order to increase the content of surface hydroxyl in the composite, and to get very stable and highly water soluble magnetic NPs. The Fe 3 O 4 NPs were precipitated due to both the large surface to volume ratio of the NPs and the magnetic dipole-dipole attraction between them. The resulting black products were washed several times with distilled water and ethanol, and dried in vacuum at 60 °C for 6 h. Fe 3 O 4 NP formation can be described according to the following reaction [26]: The core/shell structured Fe 3 O 4 @SiO 2 NPs were synthesized using a modified Stöber method [27,28].

Photo-thermal conversion experiment
The Fe 3 O 4 @SiO 2 NPs were ultrasonically dispersed in water with concentrations of 1 mg/1 ml and 2 mg/1 ml. The collector is a glass cylinder with diameter of 3.6 cm and height of 5 cm. The cylinder walls were insulated by a thick layer of polystyrene except from the top, where it was covered with a glass plate to minimize the heat loss and allow the light passage through the fluid. The fluid was exposed to illumination using a sunlight simulator. The distance between the light source and the collector was adjusted to measure average heat flux of 1000 W m −2 incident on the collector's surface. Several K-type thermocouples were fixed on the wall of the cylinder through the insulation material. The thermocouples were installed at different heights of the outer side of the cylinder to measure temperatures. The other ends of the thermocouples were connected to a data acquisition unit. Finally, the measurement data was transferred to a computer for analysis. The experimental setup is shown in figure 1. The photo-thermal conversion experiments were conducted by preparing mainly two collectors. The first collector was filled completely with 30 ml of the nanofluid. The second collector was the same except for its bottom was black color, filled with 10 ml of the nanofluid and 20 ml kerosene, in order to trap the heat into the bottom of the collector and reduce the heat dissipation from the top surface.

Characterization methods
The morphology and the size of the prepared NPs were visualized by a transmission electron microscope (TEM, FEI, TecnaiG2 Spirit Bio Twin, Netherland). The x-ray powder diffraction (XRD) patterns of the samples were obtained by x-ray diffractometer (Shimadzu, XRD-6100, Jaban) using Cu Kα radiation (λ = 1.5405 A°). The chemical structure was determined by a Nicolet Fourier transform infrared spectroscopy (FT-IR, Nexus TM 670) spectrophotometer for which the samples were prepared by mixing the product with KBr and pressing into a compact pellet. The size distribution measurements were done by dynamic light scattering (DLS) technique using Zeta Sizer Nano ZS (Malvern, Model ZEN360, England) at 25 °C. Photo-thermal conversion performance was investigated using solar simulator (Newport, Model 71445, US) under illumination of air mass 1.5 global (AM 1.5G) (Arc Lamp Source, Model 66906) that corresponds to 1 kW m −2 . Temperature versus time graphs were recorded in a computer using the Lab VIEW software, with K-type thermocouples (Omega 5TC-TT-K-36-36), which were connected to a data acquisition unit (thermocouple input devices, NI, USB-9211, 4-Channel, 24-bit).

Characteristics of Fe 3 O 4 @SiO 2 NPs
The first step of this work involved the synthesis of the magnetite cores and their coating with silica shells through the hydrolysis and condensation of TEOS in the presence of the magnetic NPs. Figure 2 shows the TEM images of the magnetic NPs before and after the SiO 2 coating. The results indicated that     [30,31]. These peaks are sharp and intense, demonstrating the well crystallized structure of Fe 3 O 4 with typical cubic inverse spinel structure. The XRD pattern of Fe 3 O 4 @SiO 2 core/shell NPs shows a broad peak between 10° and 30° ascribed to the amorphous SiO 2 (JCPDS No. 29-0085) [30,31], in addition to peaks from Fe 3 O 4 cores which exist with less intensity, compared to the resulted XRD patterns of uncoated Fe 3 O 4 . The decrease of diffraction peaks intensity of Fe 3 O 4 indicates that Fe 3 O 4 NPs are coated with a thin layer of SiO 2 . Due to the absorption of x-ray through the SiO 2 shell. Figure 5 shows the FTIR spectra of Fe 3 O 4 and Fe 3 O 4 @SiO 2 NPs in the range of 400-4000 cm −1 . Fe 3 O 4 spectrum shows a broad band at 3400 cm −1 , which is attributed to the asymmetric and symmetric stretching vibrational modes of O-H bond. The peak at 1600 cm −1 is due to the hydroxyl groups of molecular water; the H-O-H bending of the coordinated water [32]. The peak at 590 cm −1 corresponds to the stretching vibration of the Fe-O bond functional group of Fe 3 O 4 [33]. Fe 3 O 4 @SiO 2 spectrum shows a sharp peak at around 1080 cm −1 that corresponding to the strong asymmetric stretching vibration of Si-O-Si. A peak at 797 cm −1 should be related to symmetric vibrational modes of Si-O-Si cm −1 , a peak at 960 cm −1 is assigned to the symmetric stretching vibration  of Si-OH bond, and a peak at 464 cm −1 due to Si-O-Si bending [34]. The broad band at 3400 cm −1 is attributed to the surface hydroxyl group that was adsorbed on the surface of Fe 3 O 4 @SiO 2 NPs. The band around 570 cm −1 corresponds to Si-O-Fe stretching vibration [35] proving the attachment of SiO 2 on the surface of the Fe 3 O 4 as well as demonstrates the successful synthesis of Fe 3 O 4 @SiO 2 .

Photo-thermal conversion properties of Fe 3 O 4 @SiO 2 nanofluid
In order to investigate the photo-thermal conversion characteristic of the nanofluids compared to the base fluid, the fluids were irradiated by a simulated solar light under a solar intensity of 1000 W m −2 . Figures 6(a)-(c) shows the temperature distributions with time for 280 min at three different heights of the solar collector. At the beginning the nanofluid temperature was 17 °C.    nanofluid temperature. The surface temperature of the nanofluid was higher than that of the base fluid (H 2 O) due to the enhanced solar light absorption of Fe 3 O 4 @SiO 2 NPs dispersed in the base fluid. At concentrations as low as 1 mg/ml (0.001 weight), the temperature of H 2 O increases from 21 °C to 30.2 °C within 65 min, with a temperature increase of 8.2 °C. The temperature of the nanofluid increases from 21 °C to 33.7 °C within 65 min, with a temperature increase of 12.7 °C, indicating a significant photo-thermal effect compared to H 2 O. It is concluded that the temperature rise of Fe 3 O 4 @SiO 2 nanofluid is higher than the water base fluid. This should be attributed to the strong absorption and the capturing of solar radiation as well as the scattering effect of the NPs dispersed in the water. Hence, under the same radiation conditions, the temperature of nanofluids is higher than that of pure water.
It is observed from the graphs that the temperature increases steeply with the radiation time at the initial heating stage. Afterward, for the irradiation time above 25 min the heating rate is decreased noticeably until it reaches almost a steady state at the late stage, due to the heat dissipation to the surrounding. Additionally, at the initial heating stage (i.e. about 20 min) the temperature distribution shows decrease with depth, where the temperature at location T1 (the bottom of the container) is the least while the temperature at location T3 (top of container) is the highest. Due to the optical absorption of liquids, the intensity of irradiated solar light weakens with the increase of liquid depth. Hence, the bottom temperature increased slightly and was clearly different from the surface temperature during the first 60 min of the test time. After this initial time period it is noticed that the surface temperature (T3) decreases at a faster rate than the other two locations. This is attributed to the heat dissipation or exchange to the surroundings which is more pronounced in the surface region.
To evaluate the distribution homogeneity of different fluids based on temperature, A coefficient D can be calculated by taking the difference between the maximum temperature of the nanofluid at all temperature measurement positions and the minimum temperature of the nanofluid at all temperature measurement positions after the exposed time Δt [36]. D for the nanofluids are calculated based on the final temperature to evaluate the distribution homogeneity of the temperatures. After 280 min the values for D are 3.94 °C and 2.97 °C for Fe 3 O 4 and Fe 3 O 4 @SiO 2 nanofluids, respectively. The temperature distribution homogeneity coefficient reduced with SiO 2 coating of Fe 3 O 4 NPs. It can be concluded that Fe 3 O 4 @SiO 2 nanofluid has a higher thermal conductivity than Fe 3 O 4 nanofluid. As the thermal conductivity improves the heat transfer become better and result in a more homogenous temperature distribution [36].
The collector thermal efficiency is defined as the ratio of the usable thermal energy to the incident radiation energy on the collector surface [37][38][39]. In the current experimental setup, the solar radiation is uniform and the fluid depth is small so it can be assumed that the temperature of the NP is the same as the surrounding fluid, measured by the thermocouple. The photo-thermal conversion efficiency (η) can then be calculated by the following equation [40,41]: Where, c w and, m w are the specific heat and the mass of the water, respectively; c n and m n are the specific heat and the mass of the NPs, respectively; ΔT is the temperature difference of the nanofluid after an exposed time Δt; A is the irradiation area; G (W m −2 ) is the irradiation intensity. The photo-thermal conversion efficiency is proportional to the temperature gradient of the nanofluid. As the particle concentration is very small, i.e. m n c n /m w c w → 0 the collector efficiency can be simplified to become [42]: Figures 7(a)-(c) shows the photo-thermal conversion efficiency variation with time for the water and the nanofluids, at positions T1 (bottom), T2 and T3 (surface), respectively. Figure 7(d) shows the overall photothermal conversion efficiency, which is calculated by taking the average temperature for the three temperature measurements at different heights of the collector (T1, T2 and T3). The calculated efficiencies after 10 min of irradiation are represented in table 1. It is obvious that, the efficiency increase in the first 10 min due to the high initial temperature rise and the negligible heat losses to the surrounding at the beginning. The calculated efficiency at the surface of the collector (i.e. T3) after 10 min of irradiation for Fe 3 O 4 @SiO 2 nanofluid is higher than that for Fe 3 O 4 nanofluid (figure 7(c)) by 6%. This is attributed to the light absorption improvement, consequently the temperature enhancement in the case of Fe 3 O 4 @SiO 2 nanofluid. The solar collectors exhibit exponential decay of efficiency after 15 min of irradiation due to the decrease the of heat rate of the nanofluid with time as shown in figure 6, which resulted in lowering the collector efficiency. Possible reasons can be analyzed from several aspects. When the temperature rises, the Brownian motion of the dispersed NPs is intensified. This motion will enhance the heat transfer and move the peak temperature away from the surfaces to reduce heat loss to ambient. As a contradictory effect; as the temperature gradient within the nanofluid decreases the heat transfer to the environment will be enhanced. On the other hand, the stability of NPs decline with the increase of temperature as agglomeration is increased due to Brownian motion, consequently the concentration of the NPs decreases with time, leads to an obvious reduction in photo-thermal conversion efficiency. These aspects interact in the photo-thermal conversion process, which has been shown to decrease the photo-thermal conversion with time beyond 15 min (figure 7).

Photo-thermal conversion properties of the solar collector
In order to enhance the temperature of the interior fluid mainly at the bottom of the collector, the bottom of the collector was covered with a black colored paper and a test was conducted with 10 ml of Fe 3 O 4 @SiO 2 / H 2 O (1 mg/1 ml) nanofluid and 20 ml of kerosene in the collector. The nanofluid settled beneath the kerosene as the density of water (0.997 g cm −3 ) [43] is more than the density of kerosene (0.783 g cm −3 ) [44]. Waterbased nanofluid and kerosene have different optical and thermal properties utilizing both fluids in one collector improves the photo-thermal conversion performance of the collector. Figure 8 displays the temperature rise characteristics of the solar collector, which is shown in the inserted sketch in the figure. After 300 min of irradiation the temperature difference between bottom and top surface of the collector reached 9.45 °C. The temperatures in the nanofluid part at locations; T1and T2 were 57.3 °C and 55.5 °C with temperature increments of 34.3 °C and 32.5 °C, respectively. The temperatures in the kerosene part at locations; T3 and T4 were 50.7 °C and 47.8 °C with temperature increments of 27.7 °C and 24.8 °C, respectively. The temper ature distribution homogeneity enlarged with kerosene. The solar energy absorption of Fe 3 O 4 @SiO 2 nanofluid (i.e. T1 and T2 locations) is stronger than in the case of only pure kerosene (i.e. T3 and T4 locations) which increases the nanofluid temperature and leads to a more uneven temperature distribution. In order to evaluate the temperature distribution homogeneity for the 10 ml of Fe 3 O 4 @SiO 2 nanofluid of the collector in the presence of kerosene, the coefficient D is calculated after 300 min of irradiation to be1.8 °C. It is clear that the temperature were trapped at the nanofluid part of the collector which has higher temperature than the upper layer of kerosene.  The advantages of using kerosene are: (i) kerosene has a higher refractive index (1.4) [45] than water (1.33) [46], so the optical path of photons entering the system increases, which is beneficial to the capturing and absorption of light; (ii) specific heat capacity of kerosene (2.01 J/g °C) is less than water (4.186 J/g °C) [43] so the temperature of kerosene can be raised faster than water; (iii) the thermal conductivity of kerosene (0.145 W mK −1 ) is less than water (0.613 W mK −1 ) which reduces the heat loss and transfer of the nanofluid through the collector top surface. Figure 9 displays the temperature rise characteristics at the bottom of the collector at T1 location, the collector is composed of 10 ml of the water-based nanofluid and 20 ml kerosene. The temperatures were recorded for duration of irradiation time of 250 min of pure water and Fe 3 O 4 @SiO 2 /H 2 O nanofluids at different concentrations. It is clear that the temperature rise of the nanofluid is much higher than pure water, and the temperature rise of the Fe 3 O 4 @SiO 2 /H 2 O nanofluid with concentration of 1 mg/1 ml is more than that with concentration of 2 mg/1 ml. This might be attributed to the tendency of the NPs to agglomerate by attractive forces (Van der Waal forces) as their concentration in the nanofluid increases. Another reason might be the decrease of the transparency of the nanofluid which reduce the light intensity and so the optical absorption of the liquid when the particle's concentration in the nanofluids becomes large. The photo-thermal conversion efficiencies of the collectors were calculated for the temperature rise in the first 5 min of the irradiation. It is found that the collectors which contain kerosene and the nanofluid with concentration of 1 mg/1 ml and 2 mg ml −1 Fe 3 O 4 @SiO 2 /H 2 O exhibit efficiencies of 98.5% and 85.4%, respectively. This is a high photo-thermal conversion efficiency compared to the collector which contains kerosene and pure H 2 O (65.6%).
Further research is proposed to investigate the photo-thermal characteristics of kerosene-based Fe 3 O 4 @SiO 2 nanofluid. However, one of the key parameters in nanofluids is the stability of NPs. In order to form a stable kerosene-based Fe 3 O 4 @SiO 2 nanofluid a suitable hydrophobic modification procedure is needed [47,48].

Conclusion
This work experimentally investigates the photo-thermal conversion characteristics of Fe 3 O 4 @SiO 2 NPs dispersed in H 2 O under a solar simulator with intensity of 1000 W m −2 . The synthesized Fe 3 O 4 NPs have average size of about 7.8 nm and the Fe 3 O 4 @SiO 2 NPs have core/shell structure with average size of about 475 nm. Fe 3 O 4 and Fe 3 O 4 @SiO 2 NPs have been applied in water-based nanofluids. They have remarkable performance in improving the photothermal conversion efficiency of the water-base fluid. The silica coating enhances the raise of the fluid's temperature. The temperature distribution homogeneity coefficient reduces with the coating of Fe 3 O 4 NPs with SiO 2 . It can be concluded that Fe 3 O 4 @SiO 2 nanofluid has a higher thermal conductivity than Fe 3 O 4 nanofluid. The improvement of thermal conductivity results in the enhancement of the heat transfer and homogeneity of temperature distribution. Regarding the solar collector with 10 ml of the nanofluid and 20 ml kerosene, we deduced the following. At the bottom of the collector, after 5 min of radiation the efficiency of Fe 3 O 4 @SiO 2 / H 2 O (1 mg/1 ml) nanofluid is about 98.5%, whereas, the efficiency of Fe 3 O 4 @SiO 2 /H 2 O (2 mg/1 ml) nanofluid is about 85.4%; both are greater than that of H 2 O (65.6%). The usage of kerosene in the solar collector enhances the temperature rise, when used with the optimum concentration as 1 mg/1 ml corresponding to the NPs/H 2 O. Further research is proposed to prepare a stable kerosene-based Fe 3 O 4 @SiO 2 nanofluid, and to investigate its photo-thermal characteristics.