Fe₃O₄/SiO₂/TiO₂ Core-Shell Nanoparticles as Catalyst for Photoreduction of Ag(I) Ions

Introduction

Silver is one of valuable metals widely used in various industries such as photography, zinc-silver and silver-cadmium batteries, jewelry and electronic devices. Its use as raw material can cause environmental problems since the discharge of silver ions is reported to be very dangerous for living organisms¹. Therefore, treatment of waste containing silver ions is necessary to reduce the hazard it may pose.

Several methods have been applied for treatment of waste containing silver ions that include deposition², adsorption³, emulsion liquid membrane separation⁴ and catalytic photoreduction^5,6.  The photocatalytic reduction is one of the promising technologies for environmental remediation. Among known photoreduction catalysts, titanium dioxide (TiO₂) is the most intensively studied and applied in the waste treatment for organic and inorganic wastes since it is cheap, easy to prepare, stable⁷. However, direct application is not practical because separation of the catalyst suspension with sub-micrometer particle size is not convenient. An additional separation step to remove the material from the treated waste water is needed. Therefore, TiO₂ particles should be modified in order to separate and reuse easily.

Modifications can be realized by coating TiO₂photocatalyst over magnetic material to allow easy separation by using external magnetic field.  Magnetite, Fe₃O₄, is one of many magnetic materials that is promising for the purpose. It is non-toxic and can be prepared[A1]  easily.  Nonetheless, the use of the material alone was not favorable because direct contact between Fe₃O₄ and TiO₂ may generate unwanted heterojunction bond and cause an increase in electron-hole recombination and photodissolution, which can lower its photoactivity^8,9. Therefore, it should be protected first. It can be done by adding a barrier layer of silica (SiO₂).  SiO₂ is stable against acids and bases that can protect the magnetite core. Xueet al. found that the presence of silica layer between Fe₃O₄ and TiO₂ nanoparticles could increase the lifetime of hole (h⁺) product and yield a better photoreactivity¹⁰. The magnetic photocatalyst material has been applied for degradation of organic pollutants^11-14. In this contribution, we report on the magnetic photocatalyst of Fe₃O₄/SiO₂/TiO₂ core-shell for silver ion photoreduction.

Eksperimental

Materials and Methods

Iron(II) chloride tetrahydrate (FeCl₂·4H₂O),  iron(III) chloride hexahydrate, FeCl₃·6H₂O tetraethyl orthosilicate (TEOS, over 99%),  trisodium citrate dihydrate (Na₃C₆H₅O₇.2H₂O) ethanol, ammonia solution (NH₄OH, 25%), silver nitrate (AgNO₃) were purchased from Merck. Titanium(IV) tetraisopropoxide (TTIP, 97%) is obtained from Aldrich. Deionized water was used as the main[A2]  solvent throughout the experiment. The chemicals were used as received without further purification.

Synthesis of Fe₃O₄, Fe₃O4/SiO₂and Fe₃O₄/SiO₂/TiO₂ core-shell nanoparticles were done according to the previous reports^14,15.The as-synthesized Fe₃O₄/SiO₂/TiO₂core-shell nanoparticles were applied for reduction of siver(I) ions.

Preparation of Fe₃O₄

FeCl₃.6H₂O weighed 5.41 g and FeCl₂.4H₂O weighed 2.78 g were dissolved in 100 mL of deionized water under purging of nitrogen gas. The solution was ultrasonicatedand  17 mL of 25% NH₃ was then added drop-wise.[A3]   The produced black solid[A4]  was washed with deionized water until neutral and soaked into 100 mL of 0.5 M [A5] sodium citrate solution for 1 hour.  The precipitate was separated from the liquid by using an external magnetic bar[A6] , washed with deionized water and dried at 80°C for overnight to give the Fe₃O₄product[A7] .

Preparation of Fe₃O₄/SiO₂

The Fe₃O₄product weighed 0.30 g were suspended in 40 mL ethanol and ultrasonicated for 10 minutes. A 2 mL TEOS was added drop-wise to the suspension. A 5 mL of aqueous solution of NH₄OH 25% was added and further ultrasonicated for 3 hours. The precipitates were washed with deionized water to neutrality. The obtained Fe₃O₄/SiO₂ solids were separatedwith external magnetic bar and driedat 80^oC.

Preparation of Fe₃O₄/SiO₂/TiO₂ Core-Shell Nanoparticles

The X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to confirm the crystal structure and nanostructure imaging. The XRD pattern was obtained by using Shimadzu XRD 6000 with Cu-Kα radiation. The TEM images were taken on a (JEOL JEM-1400) with an acceleration voltage [A4] of 120 kV.  A double beam UV-visible spectrophotometer system (Shimadzu 2450, equipped withdiffused reflectance spectroscopic accessories) was applied to record the diffused reflectance spectra.  The amount of Ag(I)[A5]  in the solution was determined by using atomic absorption spectrometry (AAS) (Perkin Elmer 3110).

Photocatalytic Activity 

Photoreduction of Ag(I) was performed in batch system in a closed vessel equipped with a UV lamp[A1]  (40 watts, 220 volts, wavelength 340-390). A 25 mL of a 12.5 mg/L[A2]  Ag(I) solution and 0.025 g of Fe₃O₄/SiO₂/TiO₂ core-shell nanoparticles were placed in the glass vessel. During UV-irradiation, the nanoparticles were dispersed by stirring the suspension continuously using a magnetic stirrer[A3] . After the reaction[A4] , the Ag(I) content was analyzed by AAS. By using the similar manner, the solution pH, and the weight of photocatalyst and the irradiation time[A5]  were optimized. The tested solution pH was predetermined at 2, 4, 6, 8, and 10. The mass of the catalyst was adjusted at 5, 15, 25, 30, and 50 mg. The irradiation times were 0, 15, 30, 60, 120, 150, and 180 minutes.

Result and Discussions

The X-ray diffraction pattern of the samples is presented in Fig. 1.  The Fe₃O₄ diffraction peaks were observed at the 2q of 30.18, 35.70, 43.37, 57.22, and 62.86^o. The Fe₃O₄/SiO₂ solid has similar XRD pattern. The broad peak at 2ϴ about 20^o indicates the presence of the amorphous SiO₂. A new peak of anatase phase of TiO₂ at 25.27^o and reduction of Fe₃O₄ peaks on the diffraction pattern of Fe₃O₄/SiO₂/TiO₂ sample confirm the successful of TiO₂ coating. The result is in good agreement with the previous study¹⁴ and other reports^{10, 15-17}.

Figure 1: XRD patterns of (a) Fe3O4, (b) Fe3O4/SiO2 and (c) Fe3O4/SiO2/TiO2 solids   Click here to View figure

 

Fig. 2 shows TEM image of the produced Fe₃O₄/SiO₂/TiO₂ core-shell nanoparticles. The darker Fe₃O₄ core particles are covered by a bright shell [A1] of SiO₂. However, the TiO₂ outer shell is not so obvious since it has very similar darkness with the SiO₂ inner shell. It is in agreement with the previous[A2]  reports^14,15,18. The solids show the good[A3]  the magneticproperty as it can be drawn well by an external magnetic bar.

Figure 2: TEM image of Fe3O4/SiO2/TiO2 core-shell nanoparticles  Click here to View figure

 

Fig. 3 shows the separation of the product by the use of the magnetic bar[A1] . It shows that the Fe₃O₄/SiO₂/TiO₂ core-shell nanoparticles suspension is attracted to the magnetic bar and it leaves the filtrate clear after 5 min of contact. The magnetic material is attracted by the magnetic bar very well. No remaining solid is seen in the bottom of the container,  which indicates that the prepared material has good intrinsic magnetic properties.

Figure 3: Aqueous suspension of Fe3O4/SiO2/TiO2 core-shell nanoparticles before (a) and after (b) separation by the use of external magnetic bar Click here to View figure

 

Further, we tested the material by Vibrating Sample Magnetometer (VSM) to determine the magnetization of the solids after they were perturbed by external magnetic field. The application of the external magnetic field results in the hysteresis curve between a given magnetic field strength (H) and the magnetic moment of the sample (M) (Fig. 4). The results show that there is a change in the magnetic properties of Fe₃O₄ after coating with SiO₂ and TiO₂. The change in magnetic properties was confirmed by a decline[A1]  in the value of the saturation field (Ms) as a result of coating with the other two oxides. It is believed that SiO₂ coats well the Fe₃O₄ particles and reduces the magnetization by external magnetic field. The decrease in the value of the saturation field (Ms) of Fe₃O₄/SiO₂/TiO₂ core-shell nanoparticles is also observed, as a result of coating the Fe₃O₄/SiO₂ core by TiO₂.  Table 1 reveals the magnetic parameters of the products.

Figure 4: VSM curve of (a) Fe3O4,(b) Fe3O4/SiO2 and (c) Fe3O4/SiO2/TiO2 core-shell nanoparticles Click here to View figure

 

Table 1: Magnetic property of Fe₃O₄, Fe₃O₄/SiO₂, and Fe₃O₄/SiO₂/TiO₂

Material	Ms (emu g-1)	Mr (emu g-1)	Hc (x 10-2 T)
Fe3O4Fe3O4/SiO2Fe3O4/SiO2/TiO2	76,31 32,58 13,01	17,50 10,18   3,45	1,90 1,70 1,53

 

Fig.5 shows the effect of pH on the Ag(I) photoreduction. The catalyst performed well at pH 6. At pH lower and higher than 6, the reduced Ag(I) was lower. It has been known that at pH<6, the TiO₂surface is mostly in the form of TiOH₂⁺. At pH 2-6, there is charge repulsion between TiOH₂⁺ and Ag⁺, which both have a positive charge[A1] . It causes the interactionbetween TiO₂ with Ag(I) ions to decrease, and thephotoreduction reaction becomes less effective. The more alkaline the solution, the less TiOH₂⁺ species forms butthe more TiOH species forms. Consequently, the electrostatic repulsion between TiO₂ and Ag(I) ionsdecreases, leading to the more effective photoreduction.

The reaction can be speculated as follows. After absorbing UV light with appropriate energy, the TiO₂ releases electrons (e^–) and form holes (h⁺). The electrons function as a reducing agent for Ag(I) ions. The ensuing reduction process takes place when Ag(I) ions react with electrons generated from water photolysis as well as TiO₂ photocatalysis reactions of the  Fe₃O₄/SiO₂/TiO₂ core-shell nanoparticles. The reactions are as follows:

H₂O +  hʋ → H⁺  + •OH + e^–

>TiOH + hʋ  → >TiOH (h⁺ + e^–)

Ag⁺ + e^– →  Ag_(s)

Figure 5: Effect of pH on the percent photoreduction of Ag(I) Click here to View figure

 

Meanwhile, the decrease in photoreduction is observed at pH range [A1] of 8-10. It is probably due to the formation of silver oxides such as Ag₂O, Ag₂O₂, and Ag₂O₃ at the base system as shown in the Pourbaix diagram¹⁹.  The presence of these silver oxides caused the increase of solution turbidity. The particles may cause UV light to scatter, which leads to the decline the photoreductioneffectivity[A2] . Fig. 5 shows that in the alkaline solution, the formation of Ag₂O, Ag₂O₂, and Ag₂O₃ compounds becomes obvious. In addition, the formation of silver oxide compounds reduces the electrostatic interaction between TiO₂ and Ag(I) ions, resulting in the reduction of the [A3] effectiveness of photoreduction reaction.

At pH 6, the TiO₂ surface has mostly in the form of TiOH. The absence of Ag(I) and TiOH electrostatic repulsion gives an increase in the photoreduction conversion. Based on the Pourbaix diagram¹⁹, at pH 6 silver oxide compound has not been formed, so the UV light is not blocked and easily to reach the TiO₂ surface, leading to better photoreduction reaction.

Figure 6: Effect of loading of Fe3O4/SiO2/TiO2 core-shell photocatalyst on the photoreduction[A1]  of Ag(I) Click here to View figure

 

To study the effect of photocatalyst load on the photocatalytic reaction, a weight series of 5, 10, 15, 25, 35, and 60 mg were applied. The solution of 25 mL of Ag (I) ions12.5  ppm[A1] was applied at pH 6 with airradiation[A2]  time of  2 hours[A3] . The results of the influence of the mass of Fe₃O₄/SiO₂/TiO₂[A4] on the photoreduction of Ag(I) is shown in Figure 6.

Fig. 6 shows that the catalyst loading affects photoreduction of Ag(I). The larger photocatalyst loading was used, the more Ag(I) being reduced. The increase in catalyst loading causes the increase in the number of generated electrons. The electrons eventually reduced Ag(I) ions to Ag (0). However, when more photocatalyst was added into[A5]  the suspension, it caused the increase of the solution turbidity. The photocatalyst suspension blocked UV beams to enter the reaction mixture. As a result, it reduced the number of electrons generated by the photocatalyst, lowering the photoreduction reaction outcome.  A large amount of photocatalysts used does not [A6] necessarily give a high conversion. The best result was obtained with the catalyst[A7]  loading of 25 mg. In the ensuing trials, the mass of  catalyst loading was predetermined at 25 mg.

Figure 7: Effect of irradiation time on the photoredution of Ag(I)  Click here to View figure

 

Fig. 7 displays the effect of irradiation time on the photoreduction of Ag(I) [A1] ions. In the early time, the reduced Ag(I) is linearly increased along with the irradiation time. As expected, long irradiation time generates more electrons [A2] and causes more Ag(I) to be reduce[A3] d. After 120 minutes, no increase in the reduced Ag(I) was observed.  The Ag (0) solid was attached to the surface of the photocatalyst and may hinder contact between the TiO₂ and Ag (I) ions which has not been reduced and blocking the absorption of light by the photocatalyst. It shows that the [A4] optimum time for the photoreduction is 2 h. When the time is extended, the amount of reduced Ag(I) is relatively constant.

Figure 8: Photoreduction of Ag (I) ion in the presence of photoctalyst (a) Fe3O4/SiO2/TiO2, and (b) TiO2photocatalyst Click here to View figure

 

Photocatalytic activity test under UV illumination of the Fe₃O₄/SiO₂/TiO₂ core-shell nanoparticles for Ag(I) ions reduction was performed and compared with that [A1] of the native  TiO₂. The result is presented in Fig. 8. It shows that the photocatalytic activity of the Fe₃O₄/SiO₂/TiO₂ core-shell nanoparticles is much better than that of unmodified TiO₂. When exposed to the photon energy of UV light, emission of electrons from the valence band to the conduction band of the photocatalyst occurs. The produced electrons are captured by Ag(I) ions to reduce to Ag(0). As shown in Fig. 6, the Fe₃O₄/SiO₂/TiO₂ core-shell nanoparticles shows better photocatalytic activity than that of TiO₂. It may be due to the presence of SiO₂ in the solid material. The SiO₂ coating enhances the surface area and pore volume of the Fe₃O₄ core²⁰. There is more TiO₂ material bonded and the surface area becomes larger, which will lead to the improvement of the photoreduction activity of Ag(I) ions.   In addition, the nanosized TiO₂ particles have a larger band-gap with a higher photoreduction[A2] capability, hence the strong reducing potential of the photogenerated electrons has made by the core-shell nanoparticles for reduction reactions¹⁴.

Figure 9: Plot of in C/C0 versus t for photoreduction reaction of Ag(I) with catalysts of (a) Fe3O4/SiO2/TiO2core shell nanoparticles and (b) TiO2 Click here to View figure

 

The reaction kinetics are studied by developing a plot of ln(C/C_o) versus t, where C_o and C are the concentration of Ag(I) at initial and at time t, respectively. Fig. 9 shows the corresponding plot. The plot suggests that of Fe₃O₄/SiO₂/TiO₂ core-shell nanoparticles have larger reaction rate constant than that of unmodified TiO₂. The plot is fit to the Langmuir-Hinshelwood kinetics model^7,21. The rate of the reaction (v) can be expressed as below.

In the latter equation, k_r is the rate constant at the equilibrium, and K is the equilibrium constant.  If the value of KC_e<< 1, the equation can be reduced to v=k_r kc . And the Langmuir-Hinshelwood kinetics model can be reduced to the first-order kinetics equation to become the following. A linear plot of ln(C/Co) versus t suggests that the chosen model fit well to the reaction mechanism. Extracted kinetic parameters are presented in Table 2.

Table 2: Kinetic parameters of photoreduction of silver(I) ) with different catalysts

Photocatalyst	R2	k(min-1)	K(mgL-1)
Fe3O4/SiO2/TiO2	0,9900	0,0485	5,4174
TiO2	0,9093	0,0094	2,3675

 

Figure 10: Diffused reflectance UV-visible spectra of Fe3O4/SiO2/TiO2 core-shell nanoparticles a) before and b) after photoreduction process Click here to View figure

 

To confirm the progress[A1]  of the photoreduction of Ag(I) to Ag(0), analysis using UV-Visible diffused reflectance (DR-UV) was performed. Fig. 10 displays the DR-UV spectra of Fe₃O₄/SiO₂/TiO₂ core-shell before and after photoreduction process. The Fe₃O₄/SiO₂/TiO₂ core-shell nanoparticles before photoreduction (Fig. 10a) has a strong peak around 200 to 400 nm. The peak is attributed to the band edge absorption of light by titania in the core-shell nanoparticles. This result suggests that the Fe₃O₄/SiO₂/TiO₂ core-shell nanoparticles obtained in this study might be good for photocatalytic reaction under UV irradiation. After photoreduction, however, the Fe₃O₄/SiO₂/TiO₂ core-shell nanoparticles give two strong absorption peaks at 318 and 598 nm. A new absorption is observed at the visible range. The absorbance peak at 598 nm is believed to be the absorption peak for Ag(0) of photoreduction product. It suggests that the Ag(0) particles are formed and dispersed in the TiO₂ shell of the  Fe₃O₄/SiO₂/TiO₂ core-shell nanoparticles. It suggests that the photoreductionreaction of Ag(I) to Ag(0) takes place convincingly.

Conclusion

The Fe₃O₄/SiO₂/TiO₂ core-shell nanoparticles were prepared via sol-gel process followed by microwave assisted treatment. The Fe₃O₄/SiO₂/TiO₂ core-shell nanoparticles show excellent magnetism and higher photocatalytic activity for Ag(I) photoreduction than that of unmodified TiO₂. The nanoparticles were able to reduce 97% of aqueous Ag(I) within 120 minutes at the initial concentration of 12.5 mg/L and working pH of 6. The core-shell nanoparticles can be separated from the reaction mixture by simple magnetic separation. This material may find many applications in the treatment organic and inorganic waste.

Acknowledgments

The financial support provided by the Directorate General of Higher Education, Ministry of Research Technology and Higher Education Republic of Indonesia and UniversitasGadjahMada is greatly appreciated.

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