Preparation and photocatalytic application of Zn-Fe2O4@ZnO core-shell structured spheres

In this work, nanohollow Zn-Fe2O4 spheres were prepared from ZnCl2, FeCl3.6H2O and ammonium acetate using the hydrothermal method. Then, Zn-Fe2O4@ZnO core-shell structured spheres were synthesized by using immobilization of ZnO nanoparticles on surface of Zn-ferrite spheres via sol-gel rout. SEM images showed that the Zn-Fe2O4 and Zn-Fe2O4@ZnO core-shell structured spheres are made of the spherical shape particles. Results of VSM reveal that the ZnFe2O4 nanoparticles are superparamagnetic. Photocatalytic activity studies confirmed that asprepared Zn-Fe2O4@ZnO core-shell structured spheres had excellent photodegrading behavior to methylene blue (MB) compared to pure ZnO nanoparticles. Also Zn-Fe2O4@ZnO core-shell structured spheres due to the magnetic properties of environment can be easily removed. Furthermore, Zn-Fe2O4@ZnO core-shell structured spheres could be also served as convenient recyclable photocatalysts because of their magnetic properties.


Introduction
Dyes can give various products beautiful colors, and therefore they are widely used in many fields such as textiles, paper, plastic, food, painting, and medicine [1]. Quantities of dye effluent are produced in the dye manufacturing industries and during the dyeing process. According to incomplete statistics, they are more than 10,000 types of dyes in commercial circulation [2]. Dye contamination in wastewater causes problems in several ways: the presence of dyes in water, even in very low quantities, is highly visible and undesirable; color interferes with penetration of sunlight into waters; retards photosynthesis; inhibits the growth of aquatic biota and interferes with gas solubility in water bodies [3,4].
The alarming question is the disposal of effluent discharged by these industries. The basis of ISO-14000 is mainly for the disposal of residual contents of these industries. But unfortunately the industries are lacking somewhere in following the procedure recommended for this purpose [5].
Some of the worrying problems related to it including. One of the high consuming materials in the dye industry is Methylene Blue (MB) which is used for cotton and silk painting [6].
Chemical structure of MB is illustrated in Fig. 1 [7]. The separation and recycling of the photocatalysts are still a serious problem. Fortunately, magnetic separation offers a convenient way to remove and recycle the magnetic composite materials by applying an external magnetic field [8]. Recently, research on core-shell structured nanoparticles has become an active field because of their unique chemical and physical properties and potential applications in many areas [9,10]. ZnO is an important wide band-gap II-IV semiconductor, which has a wide and direct band-gap of 3.37 eV at room temperature with the large exciton binding energy of 60 meV [11].
Although the ZnO nanoparticles have been used as a catalyst for photocatalytic degradation, the Zn-Fe 2 O 4 @ZnO core-shell has not been sufficiently investigated. In the present investigation, a new nanohallow Zn-Fe 2 O 4 @ZnO core-shell catalyst was synthesized tocombine both advantages of Zn-Fe 2 O 4 and ZnO.

Preparation of Zn-Fe 2 O 4 nanoparticles
A mixture of 70 ml ethylene glycol, iron (ІІІ) chloride and zinc chloride was stirred in a mechanic stirrer to achieve a clean solution. Then, the above solution was added during stirring 2.312 g NH 4 Ac. With continued practice of mixing, the color turns to dark yellow and palms appeared. This solution for 40 min was sonicated. The solution is then placed in an oven at 215 ºC for 4 h to obtain ablack precipitate.

Preparation of Zn-Fe 2 O 4 @ZnO core-shell
The resulting black precipitation was re-dispersed in deionized water before 0.08 g Zn-Fe 2 O 4 was added to it. Then, we sonicated for 2 h and freeze-thaw action of ammonia added drop wise until the pH reached 11. The precursor solution was transferred into a round-bottom flask and kept at 120 ºC for 3 h. After cooled to room temperature, the precipitate was placed in an oven at temperature of 80 ºC for 24h and then placed into a vacuum oven at 80 ºC for 24 h.

Characterization of Zn-Fe 2 O 4 @ZnO core-shell
The particle morphologies of the ZnO powder were observed by an AIS2100 (Seron Technology) scanning electron microscopy (SEM). The FT-IR analyses were carried out on a Shimadzu FTIR-8400S spectrophotometer using a KBr pellet for sample preparation. DRS spectra were prepared via a Shimadzu (MPC-2200) spectrophotometer. Also a commercial HH-15 model vibrating sample magnetometer (VSM, Lake Shore 7410) was used at room temperature to characterize the magnetic properties ofZn-Fe 2 O 4 particles and Zn-Fe 2 O 4 @ZnO nanohallow spheres.

Photocatalytic experiments
Photocatalytic activity studies of the prepared Zn-Fe 2 O 4 @ZnO core-shells were evaluated by the degradation MB solution. LED lamp was used as Shimadzu spectrophotometer devices.
Subsequently the mixture was poured into a quartz glass beaker and began the photocatalytic degradation tests. In the experiment, the reaction solution was mixed by magnet stirrer which was placed right under light source.
The concentration of MB was determined by measuring the absorption intensity at its maximum absorbance wavelength of MB by using a UV-vis spectrophotometer with a 1 cm path length spectrophotometric quartz cell. The degradation percentage of the dyes wastewaters was defined as: Where PDP is the abbreviation of the photocatalytic degradation percentage, C 0 is the initial dye concentration; C t is the dye concentration at certain reaction time t (min), A 0 UV-Vis absorption of original solution and A t is the UV-vis absorption of degraded solution at the certain minutes [12].

FT-IR spectroscopy
The FT-IR spectra of Zn-Fe 2 O 4 , pure ZnO and composite are shown in Fig. 1.

Magnetic characterization
The magnetic hysteresis loops for the synthesized sample is shown in Fig. 6