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Propylene oxide assisted sol–gel synthesis of zinc ferrite nanoparticles for solar fuel production
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CERAMICS INTERNATIONAL
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Propylene oxide assisted sol–gel synthesis of zinc ferrite nanoparticles for solar fuel production Rahul R. Bhosalea,n, Anand Kumara, Fares AlMomania, Ivo Alxneitb a
Department of Chemical Engineering, College of Engineering, Qatar University, Doha, Qatar b Solar Technology Laboratory, Paul Scherrer Institute, 5232, Villigen PSI, Switzerland
Received 23 August 2015; received in revised form 9 October 2015; accepted 9 October 2015
Abstract This paper reports the synthesis of phase-pure Zn-ferrite nanoparticles with high specific surface area (SSA) via the propylene oxide (PO) assisted sol–gel method. For the synthesis of Zn-ferrite, metal precursors (ZnCl2 and FeCl2 4H2O) were first dissolved in ethanol, and then PO was added dropwise for the gel formation. The effects of a variety of synthesis parameters, such as the concentration of PO, the gel aging time, the calcination temperature, and the calcination dwell time, on the phase/chemical composition, SSA, porosity, crystallite size, and morphology of the Zn-ferrite were studied in detail. Different analytical techniques, such as powder X-ray diffraction (PXRD), BET surface area analyzer (BET), electron dispersive spectroscopy (EDS), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HR-TEM), were used for analyzing the Zn-ferrite samples. The acquired results indicate that the phase/chemical composition of the Zn-ferrite remains unchanged, irrespective of the variation in the experimental conditions. BET analysis further confirms that the SSA of Zn-ferrite increased due to the increase in the concentration of PO and decreased with the upsurge in the calcination temperature and dwell time. The crystallite size of Znferrite was also observed to be higher when the calcination temperature and dwell time were increased. SEM and HR-TEM assessment verify the formation of Zn-ferrite nanoparticles via the sol–gel method employed during the study. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Zn-ferrite; Sol–gel method; Nanoparticles; Solar fuel; Propylene oxide
1. Introduction Conversion of solar energy into chemical fuels, such as solar H2 or solar syngas (a mixture of H2 and CO for the production of liquid transportation fuels), via thermochemical H2O and CO2 splitting reaction is one of the favorable possibilities for the fulfillment of the future energy demand. In recent years, researchers have been working towards metal oxide (MO) based solar thermochemical cycles for the production of solar fuels. A variety of MO-based thermochemical cycles, such as zinc oxide cycle, tin oxide cycle, iron oxide cycle, doped iron oxide (ferrite) cycle, and ceria/doped ceria cycle, have been
n
Corresponding author. Tel.: þ974 4403 4168; fax: þ 974 4403 4131. E-mail addresses: [email protected], [email protected] (R.R. Bhosale).
tested for their performances in the solar H2O and CO2 splitting reaction [1–55]. The results of a literature survey indicate that, in recent years, researchers have primarily been focused on the nonvolatile ferrite cycle. A range of ferrite materials, e.g., Niferrite, Co-ferrite, Zn-ferrite, Mn-ferrite, Ni–Zn-ferrite, Ni– Mn-ferrite, and Ni–Sn-ferrite, were studied regarding their performance in solar thermochemical fuel production [36–53]. Most of these ferrite materials were synthesized using various approaches, including self-propagation high-temperature synthesis (SHS), solid state synthesis (SSS), oxidation of aqueous metal hydroxide suspension (OAMHS), aerosol spray pyrolysis (ASP), and the sol–gel method [36–54]. Recently, Bhosale et al. [37–39,43,45,47,48] reported sol–gel derived ferrites as being quite capable to produce H2 via the solar H2O splitting reaction. The quantities of H2 reported in the case of sol–gel synthesized ferrites were observed to be higher
http://dx.doi.org/10.1016/j.ceramint.2015.10.043 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: R.R. Bhosale, et al., Propylene oxide assisted sol–gel synthesis of zinc ferrite nanoparticles for solar fuel production, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.043
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Fig. 1. Sol–gel route for the synthesis of Zn-ferrite.
compared to previously investigated ferrites prepared using SHS, SSS, etc. Previous investigations reported utilization of Zn-ferrite prepared by ASP [51], SSS, and commercially purchased [55] for solar thermochemical cycles. However, the performance of sol–gel derived Zn-ferrite in the production of solar fuels has not been reported. This study reports the propyleneoxide assisted sol–gel synthesis of Zn-ferrite for solar thermal applications. Physico-chemical characterization of the sol–gel derived Zn-ferrite was performed using different characterization methods. The influence of a variety of synthesis parameters, such as concentration of PO, gel aging time, calcination temperature, and calcination dwell time, on the phase/chemical composition, specific surface area (SSA), porosity, crystallite size, and morphology of the synthesized Zn-ferrite was studied in detail. 2. Materials and methods 2.1. Materials For the synthesis of Zn-ferrite via the sol–gel method, ZnCl2 and FeCl2 4H2O were used as the metal precursors, ethanol (C2H5OH, 95%) was used as the solvent, and propylene oxide (CH3CHCH2O, 99%) was used as the gelation agent. All of these chemicals were purchased from Alfa Aesar and Sigma Aldrich and were used without any pre-treatment. 2.2. Sol–gel synthesis of Zn-ferrite To synthesize Zn-ferrite, ZnCl2 and FeCl2 4H2O were added in ethanol in the appropriate weight ratio (Zn: Fe ¼ 1:2). The mixture of metal salts and ethanol was sonicated until a visibly clear solution was obtained. To this mixed solution, propylene oxide (PO) was added drop-wise as the gelation agent, and then the formation of gel was achieved. Assynthesized Zn-ferrite gel was aged for quite a few hours at
room temperature. After aging, the gel was dried at 100 1C for 2 h using a temperature controlled hot plate. The dried gel powder was further calcined at different temperatures (with different dwell times) in air by using a muffle furnace. The obtained calcined powder was stored in a dry atmosphere for further analysis. The sol–gel route employed in this study for the synthesis of the Zn-ferrite is shown in Fig. 1. 2.3. Zn-ferrite characterization Calcined Zn-ferrite powder was characterized using different analytical methods to determine the phase/chemical composition, crystallite size, specific surface area (SSA), porosity, and particle morphology. A Panalytical XPert MPD/DY636 powder X-ray diffractometer with Cu-Kα radiation (λ ¼ 0.15418 nm, voltage¼ 45 kV, current ¼ 20 mA, angular range ¼ 20–801 2θ, steps ¼ 0.051 2θ, and recording time ¼ 5 s) was used to identify the phase composition and crystallite size of the derived Zn-ferrite. The crystallite sizes were calculated by using the Scherrer equation as follows: crystallite size ¼
Kλ βcosθ
ð1Þ
K ¼ dimensionless shape factor (value close to unity, typical value of 0.9) λ ¼ X-ray wavelength β ¼ line broadening at half the maximum intensity (FWHM), after subtracting the instrumental line broadening (rad) θ ¼ Bragg's angle A BET surface area analyzer, ASAP 2420 from Micromeritics, was used to determine the specific surface area (SSA) and cumulative pore volume (i.e., porosity) (PV) of the Znferrite material via the adsorption/desorption isotherms obtained after degassing the powders at 200 1C for 12 h. The material morphology and chemical composition of the derived Zn-ferrite were analyzed using a Zeiss Supra 55VP field-emission scanning electron microscope (SEM) equipped
Please cite this article as: R.R. Bhosale, et al., Propylene oxide assisted sol–gel synthesis of zinc ferrite nanoparticles for solar fuel production, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.043
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Zn-ferrite powder was further analyzed using a high resolution transmission electron microscope [HRTEM, TECNAI G2 F20, FEI, and JEM-2010]. The samples for the TEM analysis were prepared by dispersing the Zn-ferrite particles in Isopropyl Alcohol (IPA) using a sonic bath. A drop of this solution containing Zn-ferrite was loaded on a TEM grid and then dried before placing the grid on the TEM holder for analysis. 3. Results and discussion 3.1. Effect of the concentration of propylene oxide (PO)
Fig. 2. Effect of concentration of PO on time required for Zn-ferrite gel formation (temperature¼25 1C).
Fig. 3. Effect of the PO concentration on the phase composition of Zn-ferrite (PXRD patterns).
Table 1 Effect of the PO concentration on the chemical composition of Zn-ferrite (EDS analysis). PO (ml) Zn:Fe molar ratio (asprepared)
Zn:Fe molar ratio (after calcination)
Chemical composition (EDS analysis)
5.0 7.5 10.0 12.5 15.0 17.5 20.0
1.01:2.01 1.02:2.00 0.99:2.00 1.00:1.99 0.99:1.99 1.00:1.99 1.01:2.01
Zn1.01Fe2.01O4.025 Zn1.02Fe2.00O4.020 Zn0.99Fe2.00O3.990 Zn1.00Fe1.99O3.985 Zn0.99Fe1.99O3.975 Zn1.00Fe1.99O3.985 Zn1.01Fe2.01O4.025
1:2 1:2 1:2 1:2 1:2 1:2 1:2
with a field emission gun and an X-ray energy dispersive spectrometer (EDS) from Oxford Instruments. Zn-ferrite powder was directly used for the SEM and EDS analyses without coating with a conducting material. A secondary electron detector was used to probe the oxide layer topography with electron high tension (EHT) ¼ 3 kV.
First, the effectiveness of concentration of PO on the time required for gel formation, the phase/chemical composition, the crystallite size, the SSA, and the porosity of the sol–gel derived Zn-ferrite was explored. For the synthesis of Zn-ferrite, 10 g of the metal precursors of Zn and Fe (molar ratio of Zn: Fe ¼ 1:2) was dissolved in 20 ml of ethanol. Different amounts of PO were added to this metal precursor–ethanol mixture to achieve the gel formation. Fig. 2 represents the effect of concentration of PO on the time required for the Zn-ferrite gel formation. The reported results indicate that as the concentration of PO increases the time required for Zn-ferrite gel formation decreases. For example, at PO ¼ 5 ml, the time required for Zn-ferrite gel formation was 584 s. As the PO concentration was increased up to 10 ml, the gel time was decreased to 488 s. Further increase in the PO concentration up to 20 ml significantly decreases the gel time to 288 s. However, if more than 20 ml of PO was added, then the gel time was not further decreased. Sol–gel synthesis involves hydrolysis and poly-condensation reactions of the precursor molecules. The addition of a proton scavenger, such as PO, helps in freezing the molecular chains and reducing the time required for gel formation. Zn-ferrite gels prepared by adding different amounts of PO were further dried, aged, and calcined in air at 600 1C (dwell time ¼ 2 h). The calcined powders thus obtained were analyzed via PXRD and EDS to study the effect of PO concentration on the phase/chemical composition of Zn-ferrite. PXRD and EDS results are reported in Fig. 3 and Table 1; the results indicate no change in the phase/chemical composition of the Zn-ferrite due to the addition of different amounts of PO. For example, the EDS analysis confirms the chemical compositions of Zn1.01Fe2.01O4.025, Zn0.99Fe2.00O3.990, Zn0.99Fe1.99O3.975, and Zn1.01Fe2.01O4.025 of Zn-ferrite synthesized with PO concentrations equal to 5, 10, 15, and 20 ml, respectively. In addition to the gel time and phase/chemical composition, the influence of PO concentration on SSA, porosity, and crystallite size of the Zn-ferrite was also studied. Fig. 4a and b reports the effect of the concentration of PO on the SSA and porosity of Zn-ferrite. According to Fig. 4a, as the concentration of PO increases, the SSA of Zn-ferrite also increases. For example, as the PO concentration increases from 5 to 20 ml, the SSA increases by 72.84%. In addition, the enhancement in the SSA was found to increase when the PO concentration increases from 5 to 15 ml (9.88–34.23 m2/g). However, the
Please cite this article as: R.R. Bhosale, et al., Propylene oxide assisted sol–gel synthesis of zinc ferrite nanoparticles for solar fuel production, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.043
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R.R. Bhosale et al. / Ceramics International ] (]]]]) ]]]–]]] Table 2 Effect of gel aging time on the phase/chemical composition, SSA, porosity, and crystallite size of sol–gel derived Zn-ferrite. Aging time (h)
Chemical composition (EDS analysis)
SSA (m2/g)
Porosity (cm3/g)
Crystallite size (nm)
24 48 72 96 120
Zn1.00Fe1.99O3.985 Zn0.99Fe2.00O3.990 Zn1.00Fe2.00O4.000 Zn0.99Fe1.99O3.975 Zn0.98Fe2.00O3.980
36.38 36.14 35.99 36.65 36.61
0.061 0.059 0.057 0.064 0.062
21.08 22.02 21.56 20.66 20.98
Table 3 Effect of the calcination temperature on the chemical composition of Zn-ferrite. Calcination temperature (1C)
Zn:Fe molar ratio (asprepared)
Zn:Fe molar ratio (after calcination)
Chemical composition (EDS analysis)
600 700 800 900 1000
1:2 1:2 1:2 1:2 1:2
0.99:2.00 0.99:1.99 1.00:2.00 0.98:1.99 1.02:1.99
Zn0.99Fe2.00O3.990 Zn0.99Fe1.99O3.975 Zn1.00Fe2.00O4.000 Zn0.98Fe1.99O3.965 Zn1.02Fe1.99O4.005
the concentration of PO, the crystallite size remained unaffected. Fig. 4b shows that the crystallite size of the Zn-ferrite remained constant (in the range of 20.66–21.08 nm), even though the PO concentration was increased from 5 to 20 ml. Fig. 4. Effect of the PO concentration on (a) SSA and (b) porosity of Znferrite.
3.2. Effect of gel aging time In sol–gel synthesis, to improve the mechanical strength, connectivity of the molecular network, and homogeneity of the product, aging of the as-prepared gels is required. To study the effect of aging time, Zn-ferrite gels prepared with 20 ml of PO were aged for 24–120 h at room temperature. After aging, the gels were dried and calcined in air at 600 1C (dwell time ¼ 2 h). Obtained Zn-ferrite powders were further analyzed using PXRD, BET, and EDS to study the effect of aging time on the physico-chemical properties. The results listed in Table 2 confirm that, with the aging of the as-prepared Zn-ferrite gels for 24–120 h, the phase/chemical composition, SSA, porosity, and crystallite size of the Zn-ferrite were not affected.
Fig. 5. PXRD patterns of the Zn-ferrite gel calcined at different temperatures in air.
increase in the PO concentration from 15 to 20 ml was not found to induce a significant elevation in the SSA. Similar to the SSA, the porosity of the Zn-ferrite was also increased with the increase in the concentration of PO (Fig. 4b). When the PO concentration was equal to 5 ml, the porosity was 0.022 cm3/g. As the concentration of PO increases from 5 to 20 ml, the porosity was also increased by 63.94%. Although, the SSA and porosity of the Zn-ferrite were observed to be significantly affected by the increase in
3.3. Effect of the calcination temperature A set of experiments were performed to investigate the effect of calcination temperature on phase/chemical composition, SSA, porosity, and crystallite size of the Zn-ferrite gels. Zn-ferrite gel prepared with 20 ml of PO was aged for 24 h, dried at 100 1C, and then calcined at different calcination temperatures in air (dwell time ¼ 2 h) using a muffle furnace. Zn-ferrite powders obtained after calcination were analyzed using PXRD and EDS to determine the phase/chemical composition of the derived Zn-ferrite. According to the PXRD and EDS results shown in Fig. 5 and Table 3, the phase and
Please cite this article as: R.R. Bhosale, et al., Propylene oxide assisted sol–gel synthesis of zinc ferrite nanoparticles for solar fuel production, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.043
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Table 4 Effect of the calcination dwell time on the chemical composition of Zn-ferrite.
Fig. 6. Effect of the calcination temperature on the crystallite size of sol–gel derived Zn-ferrite.
Calcination dwell time (h)
Zn:Fe molar ratio (asprepared)
Zn:Fe molar ratio (after calcination)
Chemical composition (EDS analysis)
1 2 3 4 5
1:2 1:2 1:2 1:2 1:2
0.98:1.99 0.99:1.98 1.01:2.01 1.00:2.02 1.01:1.99
Zn0.98Fe1.99O3.965 Zn0.99Fe1.98O3.960 Zn1.01Fe2.01O4.025 Zn1.00Fe2.02O4.030 Zn1.01Fe1.99O3.995
calcination temperatures. In contrast to the phase/chemical composition, the crystallite size of Zn-ferrite was significantly increased (57.17%) with the increase in the calcination temperature from 600 to 1000 1C (Fig. 6). The effect of calcination temperature on the SSA and porosity of the Zn-ferrite was also explored. Fig. 7 shows the results obtained via BET analysis. Both SSA and porosity of the Zn-ferrite was considerably decreased with the increase in the calcination temperature. As the calcination temperature increases from 600 to 1000 1C, the SSA was drastically reduced from 36.38 to 7.65 m2/g. The % reduction in the SSA with the increase in the calcination temperatures is as follows: 600–700 1C (38.29%), 700–800 1C (33.72%), 800– 900 1C (31.25%), and 900–1000 1C (25.22%). The porosity was also decreased from 0.0612 to 0.0099 cm3/g with the increase in the calcination temperature from 600 to 1000 1C. The % decrease in the porosity with the increase in the calcination temperatures is as follows: 600–700 1C (50.65%), 700–800 1C (36.09%), 800–900 1C (31.08%), and 900– 1000 1C (25.56%). Note that the % reduction in both the SSA and the porosity due to the increase in the calcination temperature is of decreasing order. 3.4. Effect of the calcination dwell time
Fig. 7. Effect of the calcination temperature on (a) SSA and (b) porosity of sol–gel derived Zn-ferrite.
chemical composition of the derived Zn-ferrite remained unchanged (nominal phase pure ZnFe2O4), irrespective of the different calcination temperatures (600–1000 1C). The sharpness of the Zn-ferrite PXRD peaks was observed to increase with the increase in the calcination temperature. This result indicates the higher crystallinity of Zn-ferrite at elevated
Similar to the calcination temperature, the influences of the calcination dwell time (t) on the phase/chemical composition, SSA, porosity, and crystallite size of the sol–gel derived Znferrite were also studied in detail. The as-prepared Zn-ferrite gel (prepared with 20 ml of PO) was aged for 24 h, dried at 100 1C, and then calcined at 600 1C for different dwell times in the range of 1–5 h. The calcined powders obtained were further characterized in terms of the phase/chemical composition via PXRD and EDS analyses. The PXRD (not shown here) and EDS results (Table 4) confirm that the calcination dwell time does not affect the phase/chemical composition of the sol–gel derived Zn-ferrite. Fig. 8 shows that the crystallite size of the Zn-ferrite increases with the increase in the calcination dwell time. As the calcination dwell time increases from 1 to 3 h, the crystallite size increased by 5.77 nm. Similarly, the crystallite size was further increased by 7.11 nm when the calcination dwell time was increased from 3 to 5 h. Variations in the SSA and porosity of the Zn-ferrite due to the increase in the calcination dwell time from 1 to 5 h are
Please cite this article as: R.R. Bhosale, et al., Propylene oxide assisted sol–gel synthesis of zinc ferrite nanoparticles for solar fuel production, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.043
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R.R. Bhosale et al. / Ceramics International ] (]]]]) ]]]–]]]
Fig. 8. Effect of the calcination dwell time on the crystallite size of Zn-ferrite.
Fig. 10. (a) SEM and (b) HR-TEM images of the Zn-ferrite derived via the sol–gel method.
increases, the porous nature of the Zn-ferrite crystal structure collapsed, and hence, both the SSA and the porosity were decreased. 3.5. Microstructural analysis
Fig. 9. Effect of the calcination dwell time on (a) SSA and (b) porosity of Znferrite.
reported in Fig. 9a and b. Due to the increase in the calcination dwell time, both the SSA and the porosity of the Zn-ferrite were decreased. For example, as the calcination dwell time increases from 1 to 3 h, the SSA and the porosity were decreased by 11.09% and 20.09%, respectively. Similarly, with the increase in the calcination dwell time from 3 to 5 h, the SSA and the porosity were decreased by factors of 1.124 and 1.312. It is believed that as the calcination dwell time
Effects of the concentration of PO, aging time, calcination temperature, and calcination dwell time on the phase/chemical composition, SSA, porosity, and crystallite size of sol–gel derived Zn-ferrite indicate that the phase pure ZnFe2O4 with high SSA and porosity and lower crystallite size can be prepared as per the conditions explored in this investigation. Furthermore, the nanoparticle morphology was confirmed by performing the SEM and HR-TEM analysis of the Zn-ferrite powder prepared with 20 ml of PO, aged for 24 h, dried at 100 1C for 2 h, and further calcined in air at 600 1C for 2 h (Fig. 10a and b). According to the SEM and HR-TEM images reported, the average particle size of the Zn-ferrite was in the range of 10–50 nm. These results confirm the formation of ZnFe2O4 nanoparticles via the sol–gel method. The results obtained in this investigation reports successful synthesis of ZnFe2O4 nanoparticles via the sol–gel method. The effect of various synthesis parameters was studied, and an optimized experimental synthesis approach was developed. Furthermore, the experimental investigation of the effectiveness of the sol–gel derived ZnFe2O4 nanoparticles towards
Please cite this article as: R.R. Bhosale, et al., Propylene oxide assisted sol–gel synthesis of zinc ferrite nanoparticles for solar fuel production, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.043
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solar thermochemical H2O and CO2 splitting cycles is under progress. 4. Summary and conclusion In this paper, (Zn-ferrite) ZnFe2O4 nanoparticles were successfully synthesized via the sol–gel method using propylene oxide (PO) as the gelation agent. The effects of various synthesis parameters on physico-chemical properties of the sol–gel derived Zn-ferrite were studied in detail. The results obtained indicated that as the concentration of PO was increased from 5 to 20 ml, the time required for the gel formation was decreased by 296 s. In addition, the SSA and porosity of the Zn-ferrite was enhanced by 72.84% and 63.93% with the increase in the PO concentration from 5 to 20 ml. The phase/chemical composition and crystallite size remained unaffected due to the increase in the PO concentration. The study of the effect of gel aging time indicated no change in the physico-chemical properties of the sol–gel derived Zn-ferrite. The phase and chemical composition of the Zn-ferrite was not affected due to the increase in the calcination temperature (600–1000 1C) and dwell time (1–5 h); however, the SSA and porosity were significantly decreased. For example, the SSA was decreased by 28.73 and 8 m2/g when the calcination temperature and dwell time was increased from 600 to 1000 1C and 1 to 5 h, respectively. Similarly, due to the identical increase in the calcination temperature and dwell time, the porosity was decreased by 0.0513 and 0.0261 cm3/g. In contrast, the crystallite size was increased by 28.14 and 12.88 nm due to the similar increases in the calcination temperature and the dwell time. The SEM and HRTEM analyses indicated the formation of Zn-ferrite nanoparticles in the range of 10–50 nm. Acknowledgments The authors gratefully acknowledge the financial support provided by the Qatar University Internal Grants (QUUGCENG-CHE-13/14-4 and QUUG-CENG-CHE-14\15-10), the Indo-Swiss Joint Research Program (ISJRP, Grant 138852), and the Swiss Federal Office of Energy (SFOE). References [1] P. Loutzenhiser, A. Steinfeld, Solar syngas production from CO2 and H2O in a two-step thermochemical cycle via Zn/ZnO redox reactions: thermodynamic cycle analysis, Int. J. Hydrog. Energy 36 (2011) 12141–12147. [2] A. Stamatiou, P. Loutzenhiser, A. Steinfeld, Syngas production from H2O and CO2 over Zn particles in a packed-bed reactor, AIChE J. 58 (2012) 625–631. [3] A. Steinfeld, Solar hydrogen production via a two-step water-splitting thermochemical cycle based on Zn/ZnO redox reactions, Int. J. Hydrog. Energy 27 (2002) 611–619. [4] A. Steinfeld, Solar thermochemical production of hydrogen—a review, Sol. Energy 78 (2005) 603–615. [5] J. Scheffe, A. Steinfeld, Oxygen exchange materials for solar thermochemical splitting of H2O and CO2: a review, Mater. Today 17 (2014) 341–348.
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Please cite this article as: R.R. Bhosale, et al., Propylene oxide assisted sol–gel synthesis of zinc ferrite nanoparticles for solar fuel production, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.043
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CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Propylene oxide assisted sol–gel synthesis of zinc ferrite nanoparticles for solar fuel production Rahul R. Bhosalea,n, Anand Kumara, Fares AlMomania, Ivo Alxneitb a
Department of Chemical Engineering, College of Engineering, Qatar University, Doha, Qatar b Solar Technology Laboratory, Paul Scherrer Institute, 5232, Villigen PSI, Switzerland
Received 23 August 2015; received in revised form 9 October 2015; accepted 9 October 2015
Abstract This paper reports the synthesis of phase-pure Zn-ferrite nanoparticles with high specific surface area (SSA) via the propylene oxide (PO) assisted sol–gel method. For the synthesis of Zn-ferrite, metal precursors (ZnCl2 and FeCl2 4H2O) were first dissolved in ethanol, and then PO was added dropwise for the gel formation. The effects of a variety of synthesis parameters, such as the concentration of PO, the gel aging time, the calcination temperature, and the calcination dwell time, on the phase/chemical composition, SSA, porosity, crystallite size, and morphology of the Zn-ferrite were studied in detail. Different analytical techniques, such as powder X-ray diffraction (PXRD), BET surface area analyzer (BET), electron dispersive spectroscopy (EDS), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HR-TEM), were used for analyzing the Zn-ferrite samples. The acquired results indicate that the phase/chemical composition of the Zn-ferrite remains unchanged, irrespective of the variation in the experimental conditions. BET analysis further confirms that the SSA of Zn-ferrite increased due to the increase in the concentration of PO and decreased with the upsurge in the calcination temperature and dwell time. The crystallite size of Znferrite was also observed to be higher when the calcination temperature and dwell time were increased. SEM and HR-TEM assessment verify the formation of Zn-ferrite nanoparticles via the sol–gel method employed during the study. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Zn-ferrite; Sol–gel method; Nanoparticles; Solar fuel; Propylene oxide
1. Introduction Conversion of solar energy into chemical fuels, such as solar H2 or solar syngas (a mixture of H2 and CO for the production of liquid transportation fuels), via thermochemical H2O and CO2 splitting reaction is one of the favorable possibilities for the fulfillment of the future energy demand. In recent years, researchers have been working towards metal oxide (MO) based solar thermochemical cycles for the production of solar fuels. A variety of MO-based thermochemical cycles, such as zinc oxide cycle, tin oxide cycle, iron oxide cycle, doped iron oxide (ferrite) cycle, and ceria/doped ceria cycle, have been
n
Corresponding author. Tel.: þ974 4403 4168; fax: þ 974 4403 4131. E-mail addresses: [email protected], [email protected] (R.R. Bhosale).
tested for their performances in the solar H2O and CO2 splitting reaction [1–55]. The results of a literature survey indicate that, in recent years, researchers have primarily been focused on the nonvolatile ferrite cycle. A range of ferrite materials, e.g., Niferrite, Co-ferrite, Zn-ferrite, Mn-ferrite, Ni–Zn-ferrite, Ni– Mn-ferrite, and Ni–Sn-ferrite, were studied regarding their performance in solar thermochemical fuel production [36–53]. Most of these ferrite materials were synthesized using various approaches, including self-propagation high-temperature synthesis (SHS), solid state synthesis (SSS), oxidation of aqueous metal hydroxide suspension (OAMHS), aerosol spray pyrolysis (ASP), and the sol–gel method [36–54]. Recently, Bhosale et al. [37–39,43,45,47,48] reported sol–gel derived ferrites as being quite capable to produce H2 via the solar H2O splitting reaction. The quantities of H2 reported in the case of sol–gel synthesized ferrites were observed to be higher
http://dx.doi.org/10.1016/j.ceramint.2015.10.043 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: R.R. Bhosale, et al., Propylene oxide assisted sol–gel synthesis of zinc ferrite nanoparticles for solar fuel production, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.043
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Fig. 1. Sol–gel route for the synthesis of Zn-ferrite.
compared to previously investigated ferrites prepared using SHS, SSS, etc. Previous investigations reported utilization of Zn-ferrite prepared by ASP [51], SSS, and commercially purchased [55] for solar thermochemical cycles. However, the performance of sol–gel derived Zn-ferrite in the production of solar fuels has not been reported. This study reports the propyleneoxide assisted sol–gel synthesis of Zn-ferrite for solar thermal applications. Physico-chemical characterization of the sol–gel derived Zn-ferrite was performed using different characterization methods. The influence of a variety of synthesis parameters, such as concentration of PO, gel aging time, calcination temperature, and calcination dwell time, on the phase/chemical composition, specific surface area (SSA), porosity, crystallite size, and morphology of the synthesized Zn-ferrite was studied in detail. 2. Materials and methods 2.1. Materials For the synthesis of Zn-ferrite via the sol–gel method, ZnCl2 and FeCl2 4H2O were used as the metal precursors, ethanol (C2H5OH, 95%) was used as the solvent, and propylene oxide (CH3CHCH2O, 99%) was used as the gelation agent. All of these chemicals were purchased from Alfa Aesar and Sigma Aldrich and were used without any pre-treatment. 2.2. Sol–gel synthesis of Zn-ferrite To synthesize Zn-ferrite, ZnCl2 and FeCl2 4H2O were added in ethanol in the appropriate weight ratio (Zn: Fe ¼ 1:2). The mixture of metal salts and ethanol was sonicated until a visibly clear solution was obtained. To this mixed solution, propylene oxide (PO) was added drop-wise as the gelation agent, and then the formation of gel was achieved. Assynthesized Zn-ferrite gel was aged for quite a few hours at
room temperature. After aging, the gel was dried at 100 1C for 2 h using a temperature controlled hot plate. The dried gel powder was further calcined at different temperatures (with different dwell times) in air by using a muffle furnace. The obtained calcined powder was stored in a dry atmosphere for further analysis. The sol–gel route employed in this study for the synthesis of the Zn-ferrite is shown in Fig. 1. 2.3. Zn-ferrite characterization Calcined Zn-ferrite powder was characterized using different analytical methods to determine the phase/chemical composition, crystallite size, specific surface area (SSA), porosity, and particle morphology. A Panalytical XPert MPD/DY636 powder X-ray diffractometer with Cu-Kα radiation (λ ¼ 0.15418 nm, voltage¼ 45 kV, current ¼ 20 mA, angular range ¼ 20–801 2θ, steps ¼ 0.051 2θ, and recording time ¼ 5 s) was used to identify the phase composition and crystallite size of the derived Zn-ferrite. The crystallite sizes were calculated by using the Scherrer equation as follows: crystallite size ¼
Kλ βcosθ
ð1Þ
K ¼ dimensionless shape factor (value close to unity, typical value of 0.9) λ ¼ X-ray wavelength β ¼ line broadening at half the maximum intensity (FWHM), after subtracting the instrumental line broadening (rad) θ ¼ Bragg's angle A BET surface area analyzer, ASAP 2420 from Micromeritics, was used to determine the specific surface area (SSA) and cumulative pore volume (i.e., porosity) (PV) of the Znferrite material via the adsorption/desorption isotherms obtained after degassing the powders at 200 1C for 12 h. The material morphology and chemical composition of the derived Zn-ferrite were analyzed using a Zeiss Supra 55VP field-emission scanning electron microscope (SEM) equipped
Please cite this article as: R.R. Bhosale, et al., Propylene oxide assisted sol–gel synthesis of zinc ferrite nanoparticles for solar fuel production, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.043
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Zn-ferrite powder was further analyzed using a high resolution transmission electron microscope [HRTEM, TECNAI G2 F20, FEI, and JEM-2010]. The samples for the TEM analysis were prepared by dispersing the Zn-ferrite particles in Isopropyl Alcohol (IPA) using a sonic bath. A drop of this solution containing Zn-ferrite was loaded on a TEM grid and then dried before placing the grid on the TEM holder for analysis. 3. Results and discussion 3.1. Effect of the concentration of propylene oxide (PO)
Fig. 2. Effect of concentration of PO on time required for Zn-ferrite gel formation (temperature¼25 1C).
Fig. 3. Effect of the PO concentration on the phase composition of Zn-ferrite (PXRD patterns).
Table 1 Effect of the PO concentration on the chemical composition of Zn-ferrite (EDS analysis). PO (ml) Zn:Fe molar ratio (asprepared)
Zn:Fe molar ratio (after calcination)
Chemical composition (EDS analysis)
5.0 7.5 10.0 12.5 15.0 17.5 20.0
1.01:2.01 1.02:2.00 0.99:2.00 1.00:1.99 0.99:1.99 1.00:1.99 1.01:2.01
Zn1.01Fe2.01O4.025 Zn1.02Fe2.00O4.020 Zn0.99Fe2.00O3.990 Zn1.00Fe1.99O3.985 Zn0.99Fe1.99O3.975 Zn1.00Fe1.99O3.985 Zn1.01Fe2.01O4.025
1:2 1:2 1:2 1:2 1:2 1:2 1:2
with a field emission gun and an X-ray energy dispersive spectrometer (EDS) from Oxford Instruments. Zn-ferrite powder was directly used for the SEM and EDS analyses without coating with a conducting material. A secondary electron detector was used to probe the oxide layer topography with electron high tension (EHT) ¼ 3 kV.
First, the effectiveness of concentration of PO on the time required for gel formation, the phase/chemical composition, the crystallite size, the SSA, and the porosity of the sol–gel derived Zn-ferrite was explored. For the synthesis of Zn-ferrite, 10 g of the metal precursors of Zn and Fe (molar ratio of Zn: Fe ¼ 1:2) was dissolved in 20 ml of ethanol. Different amounts of PO were added to this metal precursor–ethanol mixture to achieve the gel formation. Fig. 2 represents the effect of concentration of PO on the time required for the Zn-ferrite gel formation. The reported results indicate that as the concentration of PO increases the time required for Zn-ferrite gel formation decreases. For example, at PO ¼ 5 ml, the time required for Zn-ferrite gel formation was 584 s. As the PO concentration was increased up to 10 ml, the gel time was decreased to 488 s. Further increase in the PO concentration up to 20 ml significantly decreases the gel time to 288 s. However, if more than 20 ml of PO was added, then the gel time was not further decreased. Sol–gel synthesis involves hydrolysis and poly-condensation reactions of the precursor molecules. The addition of a proton scavenger, such as PO, helps in freezing the molecular chains and reducing the time required for gel formation. Zn-ferrite gels prepared by adding different amounts of PO were further dried, aged, and calcined in air at 600 1C (dwell time ¼ 2 h). The calcined powders thus obtained were analyzed via PXRD and EDS to study the effect of PO concentration on the phase/chemical composition of Zn-ferrite. PXRD and EDS results are reported in Fig. 3 and Table 1; the results indicate no change in the phase/chemical composition of the Zn-ferrite due to the addition of different amounts of PO. For example, the EDS analysis confirms the chemical compositions of Zn1.01Fe2.01O4.025, Zn0.99Fe2.00O3.990, Zn0.99Fe1.99O3.975, and Zn1.01Fe2.01O4.025 of Zn-ferrite synthesized with PO concentrations equal to 5, 10, 15, and 20 ml, respectively. In addition to the gel time and phase/chemical composition, the influence of PO concentration on SSA, porosity, and crystallite size of the Zn-ferrite was also studied. Fig. 4a and b reports the effect of the concentration of PO on the SSA and porosity of Zn-ferrite. According to Fig. 4a, as the concentration of PO increases, the SSA of Zn-ferrite also increases. For example, as the PO concentration increases from 5 to 20 ml, the SSA increases by 72.84%. In addition, the enhancement in the SSA was found to increase when the PO concentration increases from 5 to 15 ml (9.88–34.23 m2/g). However, the
Please cite this article as: R.R. Bhosale, et al., Propylene oxide assisted sol–gel synthesis of zinc ferrite nanoparticles for solar fuel production, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.043
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R.R. Bhosale et al. / Ceramics International ] (]]]]) ]]]–]]] Table 2 Effect of gel aging time on the phase/chemical composition, SSA, porosity, and crystallite size of sol–gel derived Zn-ferrite. Aging time (h)
Chemical composition (EDS analysis)
SSA (m2/g)
Porosity (cm3/g)
Crystallite size (nm)
24 48 72 96 120
Zn1.00Fe1.99O3.985 Zn0.99Fe2.00O3.990 Zn1.00Fe2.00O4.000 Zn0.99Fe1.99O3.975 Zn0.98Fe2.00O3.980
36.38 36.14 35.99 36.65 36.61
0.061 0.059 0.057 0.064 0.062
21.08 22.02 21.56 20.66 20.98
Table 3 Effect of the calcination temperature on the chemical composition of Zn-ferrite. Calcination temperature (1C)
Zn:Fe molar ratio (asprepared)
Zn:Fe molar ratio (after calcination)
Chemical composition (EDS analysis)
600 700 800 900 1000
1:2 1:2 1:2 1:2 1:2
0.99:2.00 0.99:1.99 1.00:2.00 0.98:1.99 1.02:1.99
Zn0.99Fe2.00O3.990 Zn0.99Fe1.99O3.975 Zn1.00Fe2.00O4.000 Zn0.98Fe1.99O3.965 Zn1.02Fe1.99O4.005
the concentration of PO, the crystallite size remained unaffected. Fig. 4b shows that the crystallite size of the Zn-ferrite remained constant (in the range of 20.66–21.08 nm), even though the PO concentration was increased from 5 to 20 ml. Fig. 4. Effect of the PO concentration on (a) SSA and (b) porosity of Znferrite.
3.2. Effect of gel aging time In sol–gel synthesis, to improve the mechanical strength, connectivity of the molecular network, and homogeneity of the product, aging of the as-prepared gels is required. To study the effect of aging time, Zn-ferrite gels prepared with 20 ml of PO were aged for 24–120 h at room temperature. After aging, the gels were dried and calcined in air at 600 1C (dwell time ¼ 2 h). Obtained Zn-ferrite powders were further analyzed using PXRD, BET, and EDS to study the effect of aging time on the physico-chemical properties. The results listed in Table 2 confirm that, with the aging of the as-prepared Zn-ferrite gels for 24–120 h, the phase/chemical composition, SSA, porosity, and crystallite size of the Zn-ferrite were not affected.
Fig. 5. PXRD patterns of the Zn-ferrite gel calcined at different temperatures in air.
increase in the PO concentration from 15 to 20 ml was not found to induce a significant elevation in the SSA. Similar to the SSA, the porosity of the Zn-ferrite was also increased with the increase in the concentration of PO (Fig. 4b). When the PO concentration was equal to 5 ml, the porosity was 0.022 cm3/g. As the concentration of PO increases from 5 to 20 ml, the porosity was also increased by 63.94%. Although, the SSA and porosity of the Zn-ferrite were observed to be significantly affected by the increase in
3.3. Effect of the calcination temperature A set of experiments were performed to investigate the effect of calcination temperature on phase/chemical composition, SSA, porosity, and crystallite size of the Zn-ferrite gels. Zn-ferrite gel prepared with 20 ml of PO was aged for 24 h, dried at 100 1C, and then calcined at different calcination temperatures in air (dwell time ¼ 2 h) using a muffle furnace. Zn-ferrite powders obtained after calcination were analyzed using PXRD and EDS to determine the phase/chemical composition of the derived Zn-ferrite. According to the PXRD and EDS results shown in Fig. 5 and Table 3, the phase and
Please cite this article as: R.R. Bhosale, et al., Propylene oxide assisted sol–gel synthesis of zinc ferrite nanoparticles for solar fuel production, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.043
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Table 4 Effect of the calcination dwell time on the chemical composition of Zn-ferrite.
Fig. 6. Effect of the calcination temperature on the crystallite size of sol–gel derived Zn-ferrite.
Calcination dwell time (h)
Zn:Fe molar ratio (asprepared)
Zn:Fe molar ratio (after calcination)
Chemical composition (EDS analysis)
1 2 3 4 5
1:2 1:2 1:2 1:2 1:2
0.98:1.99 0.99:1.98 1.01:2.01 1.00:2.02 1.01:1.99
Zn0.98Fe1.99O3.965 Zn0.99Fe1.98O3.960 Zn1.01Fe2.01O4.025 Zn1.00Fe2.02O4.030 Zn1.01Fe1.99O3.995
calcination temperatures. In contrast to the phase/chemical composition, the crystallite size of Zn-ferrite was significantly increased (57.17%) with the increase in the calcination temperature from 600 to 1000 1C (Fig. 6). The effect of calcination temperature on the SSA and porosity of the Zn-ferrite was also explored. Fig. 7 shows the results obtained via BET analysis. Both SSA and porosity of the Zn-ferrite was considerably decreased with the increase in the calcination temperature. As the calcination temperature increases from 600 to 1000 1C, the SSA was drastically reduced from 36.38 to 7.65 m2/g. The % reduction in the SSA with the increase in the calcination temperatures is as follows: 600–700 1C (38.29%), 700–800 1C (33.72%), 800– 900 1C (31.25%), and 900–1000 1C (25.22%). The porosity was also decreased from 0.0612 to 0.0099 cm3/g with the increase in the calcination temperature from 600 to 1000 1C. The % decrease in the porosity with the increase in the calcination temperatures is as follows: 600–700 1C (50.65%), 700–800 1C (36.09%), 800–900 1C (31.08%), and 900– 1000 1C (25.56%). Note that the % reduction in both the SSA and the porosity due to the increase in the calcination temperature is of decreasing order. 3.4. Effect of the calcination dwell time
Fig. 7. Effect of the calcination temperature on (a) SSA and (b) porosity of sol–gel derived Zn-ferrite.
chemical composition of the derived Zn-ferrite remained unchanged (nominal phase pure ZnFe2O4), irrespective of the different calcination temperatures (600–1000 1C). The sharpness of the Zn-ferrite PXRD peaks was observed to increase with the increase in the calcination temperature. This result indicates the higher crystallinity of Zn-ferrite at elevated
Similar to the calcination temperature, the influences of the calcination dwell time (t) on the phase/chemical composition, SSA, porosity, and crystallite size of the sol–gel derived Znferrite were also studied in detail. The as-prepared Zn-ferrite gel (prepared with 20 ml of PO) was aged for 24 h, dried at 100 1C, and then calcined at 600 1C for different dwell times in the range of 1–5 h. The calcined powders obtained were further characterized in terms of the phase/chemical composition via PXRD and EDS analyses. The PXRD (not shown here) and EDS results (Table 4) confirm that the calcination dwell time does not affect the phase/chemical composition of the sol–gel derived Zn-ferrite. Fig. 8 shows that the crystallite size of the Zn-ferrite increases with the increase in the calcination dwell time. As the calcination dwell time increases from 1 to 3 h, the crystallite size increased by 5.77 nm. Similarly, the crystallite size was further increased by 7.11 nm when the calcination dwell time was increased from 3 to 5 h. Variations in the SSA and porosity of the Zn-ferrite due to the increase in the calcination dwell time from 1 to 5 h are
Please cite this article as: R.R. Bhosale, et al., Propylene oxide assisted sol–gel synthesis of zinc ferrite nanoparticles for solar fuel production, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.043
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Fig. 8. Effect of the calcination dwell time on the crystallite size of Zn-ferrite.
Fig. 10. (a) SEM and (b) HR-TEM images of the Zn-ferrite derived via the sol–gel method.
increases, the porous nature of the Zn-ferrite crystal structure collapsed, and hence, both the SSA and the porosity were decreased. 3.5. Microstructural analysis
Fig. 9. Effect of the calcination dwell time on (a) SSA and (b) porosity of Znferrite.
reported in Fig. 9a and b. Due to the increase in the calcination dwell time, both the SSA and the porosity of the Zn-ferrite were decreased. For example, as the calcination dwell time increases from 1 to 3 h, the SSA and the porosity were decreased by 11.09% and 20.09%, respectively. Similarly, with the increase in the calcination dwell time from 3 to 5 h, the SSA and the porosity were decreased by factors of 1.124 and 1.312. It is believed that as the calcination dwell time
Effects of the concentration of PO, aging time, calcination temperature, and calcination dwell time on the phase/chemical composition, SSA, porosity, and crystallite size of sol–gel derived Zn-ferrite indicate that the phase pure ZnFe2O4 with high SSA and porosity and lower crystallite size can be prepared as per the conditions explored in this investigation. Furthermore, the nanoparticle morphology was confirmed by performing the SEM and HR-TEM analysis of the Zn-ferrite powder prepared with 20 ml of PO, aged for 24 h, dried at 100 1C for 2 h, and further calcined in air at 600 1C for 2 h (Fig. 10a and b). According to the SEM and HR-TEM images reported, the average particle size of the Zn-ferrite was in the range of 10–50 nm. These results confirm the formation of ZnFe2O4 nanoparticles via the sol–gel method. The results obtained in this investigation reports successful synthesis of ZnFe2O4 nanoparticles via the sol–gel method. The effect of various synthesis parameters was studied, and an optimized experimental synthesis approach was developed. Furthermore, the experimental investigation of the effectiveness of the sol–gel derived ZnFe2O4 nanoparticles towards
Please cite this article as: R.R. Bhosale, et al., Propylene oxide assisted sol–gel synthesis of zinc ferrite nanoparticles for solar fuel production, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.043
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solar thermochemical H2O and CO2 splitting cycles is under progress. 4. Summary and conclusion In this paper, (Zn-ferrite) ZnFe2O4 nanoparticles were successfully synthesized via the sol–gel method using propylene oxide (PO) as the gelation agent. The effects of various synthesis parameters on physico-chemical properties of the sol–gel derived Zn-ferrite were studied in detail. The results obtained indicated that as the concentration of PO was increased from 5 to 20 ml, the time required for the gel formation was decreased by 296 s. In addition, the SSA and porosity of the Zn-ferrite was enhanced by 72.84% and 63.93% with the increase in the PO concentration from 5 to 20 ml. The phase/chemical composition and crystallite size remained unaffected due to the increase in the PO concentration. The study of the effect of gel aging time indicated no change in the physico-chemical properties of the sol–gel derived Zn-ferrite. The phase and chemical composition of the Zn-ferrite was not affected due to the increase in the calcination temperature (600–1000 1C) and dwell time (1–5 h); however, the SSA and porosity were significantly decreased. For example, the SSA was decreased by 28.73 and 8 m2/g when the calcination temperature and dwell time was increased from 600 to 1000 1C and 1 to 5 h, respectively. Similarly, due to the identical increase in the calcination temperature and dwell time, the porosity was decreased by 0.0513 and 0.0261 cm3/g. In contrast, the crystallite size was increased by 28.14 and 12.88 nm due to the similar increases in the calcination temperature and the dwell time. The SEM and HRTEM analyses indicated the formation of Zn-ferrite nanoparticles in the range of 10–50 nm. Acknowledgments The authors gratefully acknowledge the financial support provided by the Qatar University Internal Grants (QUUGCENG-CHE-13/14-4 and QUUG-CENG-CHE-14\15-10), the Indo-Swiss Joint Research Program (ISJRP, Grant 138852), and the Swiss Federal Office of Energy (SFOE). References [1] P. Loutzenhiser, A. Steinfeld, Solar syngas production from CO2 and H2O in a two-step thermochemical cycle via Zn/ZnO redox reactions: thermodynamic cycle analysis, Int. J. Hydrog. Energy 36 (2011) 12141–12147. [2] A. Stamatiou, P. Loutzenhiser, A. Steinfeld, Syngas production from H2O and CO2 over Zn particles in a packed-bed reactor, AIChE J. 58 (2012) 625–631. [3] A. Steinfeld, Solar hydrogen production via a two-step water-splitting thermochemical cycle based on Zn/ZnO redox reactions, Int. J. Hydrog. Energy 27 (2002) 611–619. [4] A. Steinfeld, Solar thermochemical production of hydrogen—a review, Sol. Energy 78 (2005) 603–615. [5] J. Scheffe, A. Steinfeld, Oxygen exchange materials for solar thermochemical splitting of H2O and CO2: a review, Mater. Today 17 (2014) 341–348.
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Please cite this article as: R.R. Bhosale, et al., Propylene oxide assisted sol–gel synthesis of zinc ferrite nanoparticles for solar fuel production, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.043