Advanced Powder Technology 25 (2014) 1715–1720

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Advanced Powder Technology

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Original Research Paper

Monodisperse nanostructured Fe3O4/ZnO microrods using for wastewater treatment

http://dx.doi.org/10.1016/j.apt.2014.06.0190921-8831/� 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

⇑ Corresponding author. Tel.: +86 29 88494463; fax: +86 29 88492642.E-mail addresses: qli@nwpu.edu.cn, jerrylee57318@hotmail.com (Q. Li).

Mengmeng Xu, Qiang Li ⇑, Huiqing FanState Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China

a r t i c l e i n f o

Article history:Received 16 March 2014Received in revised form 9 June 2014Accepted 23 June 2014Available online 5 July 2014

Keywords:Microwave hydrothermal synthesisFe3O4/ZnO heterostructuresPhotocatalystWaste water treatment

a b s t r a c t

Monodisperse nanostructured Fe3O4/ZnO microrods were successfully prepared by an economic one-stepsynthesis route. The formation of nanostructured Fe3O4/ZnO microrods was evident from the detailedstructural and elemental analysis by field emission scanning electronic microscopy, transmission electronmicroscopy, Raman spectra and X-ray photoelectron spectroscopy measurements. The formation mech-anism of the hexagonal Fe3O4/ZnO microrods was carefully discussed. The removal of toxic metal ionsexperiments display that Fe3O4/ZnO heterostructures show the best removal efficiency compared withpure ZnO and Fe3O4 structures, and the photocatalytic experiments show that the Fe3O4/ZnO heterostruc-tures display the excellent photocatalytic activity decomposing Rhodamine B (100% after 40 min).� 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder

Technology Japan. All rights reserved.

1. Introduction

Today’s world is facing a tremendous set of environmentalproblems related to the remediation of hazardous wastes [1–3],contaminated groundwater, and the control of toxic air contami-nants [4–6]. Moreover, evidence is mounting that such problemsthreaten human health and have already become a serious publicconcern. Therefore, emerging techniques with reference to thetreatment of wastewater and toxic air have been developed,including adsorption, photodegradation, ion exchange, reverseosmosis, chemical precipitation and membrane. Among them,adsorption and photodegradation had been proven to be successfultechniques to remove toxic contaminants from water. In mostcases, these are semiconductor metal oxides [7–9], porous materi-als [10–13], magnetic oxides [14]. Meanwhile, the steady and fastdevelopment of nanoscience and nanotechnology provide opportu-nities for improving the performance of these adsorbents andphotocatalysts.

Zinc oxide (ZnO) one-dimensional (1D) nanostructure [15–18] hasbecome promising photocatalyst for their high surface-to-volumeratio, high photosensitivity, good quantum efficiency, chemicalstability and non-toxic nature. Many techniques including chemicalvapor deposition (CVD), template-guided growth, electrospinningand thermal evaporation have been widely employed over the pastyears to synthesize ZnO 1D nanostructure ZnO. In general, these

synthetic methods involve complex procedures, sophisticatedequipments and rigorous experimental conditions. However, thesetechniques always require high temperatures and long processingtime. Therefore, large-scale and low-cost controllable growth ofZnO 1D nanostructure via a one-step synthetic approach is still cru-cially expected for novel applications. Moreover, Fe3O4 nanoparticleshave superparamagnetic properties and essential functionality[19–22]. The combination of magnetic oxides and ZnO nanocrystalsmay lead to high adsorption efficiency including metal ions, organiccontaminants, meanwhile promote electron–hole pair separation inthe process of photodegradation.

In this paper, nanostructured Fe3O4/ZnO microrods werefabricated by an economic one-step synthesis route. The removalefficiency of toxic metal ions and photocatalytic performance wereinvestigated. Compared with pure ZnO and other oxides/ZnO het-erostructures, Fe3O4/ZnO microrods show enhanced efficiency forthe removal of multiple toxic metal ions (Co2+, Ni2+, Cu2+, Cd2+,Pb2+, Hg2+ and Fe3+) and the photocatalytic degradation of organicpollutant. We expect such a monodisperse Fe3O4/ZnO heterostruc-tures will find its industrial application in the future to removeundesirable contaminants from the waste water.

2. Experimental

2.1. Synthesis

All the chemicals were of analytical reagent (AR) grade andused without further purification. In a typical procedure,

1716 M. Xu et al. / Advanced Powder Technology 25 (2014) 1715–1720

2.5 mmol of Zinc nitrate hexahydrate (Zn(NO3)2 � 6H2O), 2 mmolof hexamethylene tetramine (HMTA) and 1.25 mmol of ureawere added to 40 mL deionized water with stirring for 10 minat room temperature, correspondingly. Then, five differentmolars (0, 0.125, 0.167, 0.25, 0.5 mmol) of Iron (III) chloridehexahydrate (FeCl3�6H2O) were dispersed in the solution. Inorder to obtain superparamagnetic Fe3O4, 3.6 mmol of glucose(C6H12O6 � H2O) acting as a reducing agent [23,24] was simulta-neously added to the solution. After being vigorously stirredfor another 30 min at room temperature, the resulting clear solu-tion was transferred to a Teflon vessel of the MDS-6 (MicrowaveDigestion/Extraction System, Shanghai Sineo Microwave Chemi-cal Technology Co., Ltd.). A temperature program was establishedto make the desired reaction temperature (150 �C). The as-prepared powders were rinsed several times with deionizedwater and pure ethanol and collected by centrifugation, and thenvacuum-dried at 80 �C for 8 h. Finally, the Fe3O4/ZnO hetero-structures were obtained by calcining the precursor at 500 �Cfor 2 h in air.

2.2. Characterization

The phase composition and structure of the samples werecharacterized by X-ray diffraction (XRD; X’pert, Philips, Holland)with Cu Ka1 radiation (k = 1.5406 Å) at 40 kV and 30 mA over 2hof range 20–60�. The morphology of the obtained samples wasinvestigated using field emission scanning electronic microscopy(FE-SEM; JSM-6701F, JEOL, Japan) and transmission electronmicroscopy (TEM; JEM-3010, Questar, New Hope, USA). TheX-ray photoelectron spectroscopy (XPS) measurements were per-formed using Omicron energy analyser (PHI-5400, Perkin Elmer,USA) instrument with Al Ka as radiation X-ray source. Rhoda-mine B (RhB) was employed as a representative dye indicatorto evaluate the UV photocatalytic activity of all samples. Foreach condition, 30 mg of photocatalyst was ultrasonically dis-persed in dye indicator aqueous solution (10 mg/L, 30 mL) andmagnetically stirred in the dark for 30 min to ensure the adsorp-tion/desorption equilibrium of dye indicator on the photocatalystsurface. Then, UV irradiation was carried out with a 300–500 Wfluorescent Hg lamp. After a given irradiation time, about 5 mLof the mixture was withdrawn and the photocatalysts were sep-arated by centrifugation. Photocatalytic degradation process wasmonitored using a UV–vis spectrophotometer (UV-2450; Shima-dzu, Tokyo, Japan) to measure the absorption of RhB at554 nm. In order to investigate the simultaneous removal oftoxic metal ions, the waste-water was prepared by mixing40 ml of water containing different toxic metal ions (290.1 mg/L Ni2+, 290.2 mg/L Co2+, 240.8 mg/L Cu2+, 340.2 mg/L Pb2+,170 mg/L Ag+). The adsorption experiments were conducted bymixing 30 mL waste-water with 30 mg of Fe3O4, ZnO andFe3O4/ZnO heterostructures. The above mixture was kept in awater shaker for 24 h at room temperature (30 �C) with a contin-uously magnetically stirring. The nanoadsorbents with adsorbedmetal ions were separated from the mixture by a permanentmagnet. The concentrations of metal ions were measured byinductively coupled plasma atomic emission spectrometry (ICP-AES, Thermo Electron Corporation, New York, USA). The removalefficiency (%) and equilibrium adsorbed concentration of themetal ions were calculated as follows:

Removal efficiencyð%Þ ¼ ðC0 � CtÞ=Ct � 100 ð1Þ

where C0 and Ct are the initial and residual concentration of metalions (mg/L) in aqueous solution.

3. Results and discussion

3.1. Morphology and composition characterization

Fig. 1(a–e) show the FE-SEM images of samples with differentadded amount (0, 0.125, 0.167, 0.25, 0.5 mmol) of FeCl3 � 6H2Odenoted as ZF1, ZF2, ZF3, ZF4, ZF5, respectively. The insets arethe higher magnification SEM images. As shown in Fig. 1a, pen-cil-like ZnO rods consist of a hexagonal trunk and two hexagonaltips on one side. The trunk has a diameter of about 2 lm in themiddle and a length of about 10 lm. Nanostructured ZnO micro-rods with a regular hexagonal shape and flat end can be clearlyobserved in Fig. 1(b–d) when FeCl3�6H2O is added. In addition,ZnO hexagonal microrods are covered with many nanoparticles.Moreover, with the amount of FeCl3 � 6H2O increases, the coverednanoparticles occur to accumulate. The diameter and length ofthese microrods are in the ranges of 100–200 nm and 1–2 lm,respectively. Especially, when 0.25 mmol FeCl3 � 6H2O was added,monodisperse nanostructured Fe3O4/ZnO microrods wereobtained. Further increase the amount of FeCl3�6H2O to 0.5 mmol,the regular morphology of ZnO microrods was destroyed (Fig. 1e),and we ascribe the results to the introduction of Fe element whichmay enter the ZnO lattice or product oxides leading to the changeof final morphology.

To identify the composition of the heterostructures, HRTEM andselected-area electron diffraction (SAED) pattern images are shownin Fig. 2. The HRTEM image in the Fig. 2(a) displays that the hexag-onal rods have a lattice spacing of about 0.52 nm, corresponding tothe distance between the (002) planes in the ZnO crystal lattice,which demonstrates that the hexagonal rods are composed of wellcrystallized ZnO. The nanoparticles covered the surface of the hex-agonal rods have a diameters of about 10–20 nm and a lattice spac-ing of about 0.295 nm, corresponding to the distance between the(220) planes in the Fe3O4 crystal lattice. Moreover, we can clearlyobserve the bonding of the crystal lattice between the nanoparti-cles and microrods from the HRTEM images. In addition, the SAEDpattern is composed of single-crystalline pattern spots and patternrings (Fig. 2(b)), which can be assigned to ZnO microrods and Fe3O4

nanoparticles, respectively. Similar results were obtained byenergy dispersive spectroscopy (EDS) analysis (Fig. 2(c and d)).The content of Fe in the square region is about seven times greaterthan that of hexagonal region, indicating the heterojunctionformed at the interface.

In order to understand the assembling process of the heteroge-neous nanostructured Fe3O4/ZnO microrods, we studied the influ-ence of different parameters on the morphology and phase ofproducts systematically and found that the content of HMTA andurea added determine the morphology of the final products. Firstly,the role of HMTA was investigated. The SEM images of samplesobtained with different amounts of HMTA (a) 0, (b) 0.5 g addedand the other conditions unchanged from ZF4 are shown inFig. 3(a and b), respectively. Compared with ZF4, when no HMTAwas added, pencil-like ZnO rods with a diameter of about1–2 lm in the middle and a length of about 10–20 lm wereachieved (Fig. 3(a)), and further increasing the quantity of HMTAto 0.5 g, the length to diameter ratio increased (Fig. 3(b)). Addition-ally, the amount of urea also plays a crucial role in determining themorphology of the products. Several experiments with differentamounts of urea such as 0, 0.151 g with the other conditionsunchanged from ZF4 were conducted. The results are shown inFig. 3(c and d). As can be seen, without urea added, irregularlyshaped hexagonal rods with a mass of agglomeration are obtained(Fig. 3(c)). Compared with ZF4, double amount of urea additiondoes not make obvious effect on the morphology of hexagonalrods, but the rods aggregate together. In the reaction process, the

Fig. 1. (a–e) Show the SEM images of ZF1, ZF2, ZF3, ZF4, ZF5. The insets are the higher magnification SEM images.

Fig. 2. (a and b) Show the HRTEM and SAED images of ZF4; (c and d) EDS spectra inthe edge and central areas of ZF4.

Fig. 3. SEM images of samples obtained with different amounts of HMTA or ureaadded and other conditions unchanged from ZF4 were conducted: (a and b) 0 g,0.5 g HMTA added; (c and d) 0 g, 0.151 urea added.

M. Xu et al. / Advanced Powder Technology 25 (2014) 1715–1720 1717

controlled hydrolysis of HMTA and urea in aqueous solutions, it notonly provides the basic alkaline environment for the growth ofZnO, but also acts as surfactant which can tune the growth ratesalong different growing directions. It is proposed that HMTA, beinga long chain polymer and a nonpolar chelating agent, will preferen-tially attach to the nonpolar facets of the ZnO crystal, thereby cut-ting off the access of Zn2+ ions to them leaving only the polar (001)face for epitaxial growth. Meanwhile, urea can promote the growthof the six side facets of hexagonal rods.

3.2. Crystal structure and elements analysis

The XRD study in Fig. 4(a) shows that two sets of diffractionpeaks mixed together can be observed from the spectrum ofsamples, which can be indexed to hexagonal wurtzite ZnO (JCPDSFile No. #36-1451) and cubic Fe3O4 (JCPDS File No. #88-0315).Moreover, the XRD peaks of ZF2–ZF5 shift obviously towardshigher diffraction angles compared to those of ZF1, indicating thatiron Fe are introduced into the ZnO host lattice. The calculatedaverage crystal sizes (D) of Fe3O4 for ZF3 and ZF4 using theDebye–Scherrer formula from XRD patterns are 14.1(1) and

Fig. 4. (a) XRD and (b) room temperature Raman spectra of all samples (ZF1–ZF5).

Fig. 5. (a–c) Show the high-resolution spectra for O, Zn, and Fe elements of ZF1, ZF4,respectively.

Fig. 6. (a) The RhB degradation curves of C/C0 versus time for photodegradation ofFe3O4/ZnO heterostructures photocatalysts with different Fe3O4/ZnO ratios com-pared with commercial TiO2; (b) the time dependent absorption spectra of RhBdegradation of ZF4 under UV irradiation; and (c) removal efficiency of toxic metalions (Co2+, Ni2+, Cu2+, Pb2+, Ag+) of pure ZnO, Fe3O4, and Fe3O4/ZnO heterostructures.

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13.5(6) nm. The result is consistent with that of TEM image.Debye–Scherrer formula for finding the crystallite size (D) is givenas:

D ¼ ðKkÞ=ðb cos hÞ ð2Þ

where K is the Scherrers constant and has the value of 0.9 for hex-agonal crystal structure, k is the wavelength of the radiation(1.5418 Å), b is the forward width at half maximum radian (FWHM)and h is the half value of the Bragg diffraction angle in the XRD pat-tern (degree). The room temperature Raman scattering spectra ofZF1, ZF2, ZF3, ZF4, ZF5 are presented in Fig. 4(b). For pure ZnO,the Raman scattering spectrum consists of conventional modes cen-tered at 333, 383, 438 and 583 cm�1, which can be attributed to theprocesses E2 (H) � E2 (L), A1 (TO), E2 (high), E1 (LO) [25–27], respec-tively. The results demonstrate that the ZnO crystal structure stillretains its wurtzite lattice character. Furthermore, compared withpure ZnO, Fe3O4/ZnO heterostructures show an additional peak 1and the gradually disappeared peak 3. The additional and disap-peared mode relate to the formation of Fe3O4/ZnO heterostructures.In addition, as the increase of the ratio of Fe/Zn, the positions ofpeak 1–4 are all shifted towards lower frequencies. Once again, itproves that a little amount of Fe is incorporated into the ZnO hostlattice.

Then, the XPS analysis is carried out to investigate the surfacefeature of ZF1 and ZF4 as shown in Fig. 5. The high-resolution spec-tra for O, Zn, and Fe species are shown in Fig. 5(a–c), respectively.The typical O1s peak on the surface can be consistently fitted by astrong peak A and a medium partially overlapped peak B. Thepeaks of A and B can be assigned to lattice oxygen and weaklybound oxygen O2� ions in the oxygen deficient region, such asOH groups [28–30], respectively. In addition, it is worth mention-ing that the surface hydroxyls can produce primary active hydroxylradicals, which are capable of trapping photoinduced electrons andholes. Thus, the surface hydroxyls are very important for photoca-talysis. Compared with ZF1, the O1s peaks obviously shift towardslow binding energy. The high-resolution spectra for Fe elements(Fig. 5b) shows binding energy of 710.48 eV, corresponding to Fe2p1/2 [31], in good agreement with the values reported for Fe3O4

in the handbook and literature [32–34].

3.3. Photodegradation and removal of toxic metal ions experiments

The photodegradation of organic pollutant can be considered asa pseudo-first-order reaction [35] and its kinetics can be expressedas follows:

C ¼ C0e�kt ð3Þ

where k is the degradation rate constant, C0 and C are the initial con-centrations of organic pollutant and that at the reaction time t,respectively. Fig. 6(a) displays the �ln (C/C0) � t plots for photodeg-radation of Fe3O4/ZnO heterostructures photocatalysts with differ-ent Fe3O4/ZnO ratios compared with commercial TiO2. The ZF3photocatalysis can rapidly decompose RhB completely in 40 minshowing the highest photocatalytic activity, and the degradation rateconstant k = 0.053(3). Fig. 6(b) shows the time dependent absorptionspectra of RhB degradation over ZF4 photocatalysis under UV irradi-ation. Evidently, the absorbance of RhB decreases rapidly with thereaction time, which is indicative of the reduction of RhB from col-ored aqueous solution to colorless. Moreover, the results of thesimultaneous removal experiments of toxic metal ions are shownin Fig. 6(c). It demonstrates that Fe3O4/ZnO heterostructures show

M. Xu et al. / Advanced Powder Technology 25 (2014) 1715–1720 1719

much better removal activity for metal ions than pure Fe3O4 and ZnO.It is very encouraging to see that almost 100% removal efficiency wasachieved for Ag+, Pb2+ ions and 80%, 89.7%, 91.6% removal efficiencyfor Co2+, Cu2+, Ni2+ using Fe3O4/ZnO heterostructures from waste-water, respectively. It is obvious that the availability of active surfacesites for the adsorption of metal ions increases with the formation ofFe3O4/ZnO heterostructures. In general, the preference of commonhydrous solids for metals has been related to the metal’s electroneg-ativity. The electronegativity values of Co2+, Ni2+, Cu2+, are 1.88, 1.91,1.90, respectively. Due to high electronegativity, Pb2+ and Ag+ exhibitstronger attraction towards the Fe3O4/ZnO heterostructures andhence better removal efficiency.

The reasons why Fe3O4/ZnO heterostructures have a goodefficiency of removal toxic metal ions and photodegradation havebeen widely discussed. The better performances of Fe3O4/ZnOheterostructures over pure ZnO and Fe3O4 could be attributed totheir synergistic effect which provides additional surface activesites for the adsorption of toxic metal ions. In the case of Fe3O4/ZnO heterostructures, the presence of magnetic Fe3O4 nanoparti-cles in the ZnO network provides additional surface active sitesfor the adsorption of toxic metal ions and organic pollutants. Theenhanced photocatalytic performance can be ascribed to the differ-ent band gaps (ZnO: Eg = 3.37 eV; Fe3O4: Eg = 0.1 eV) and workfunctions of ZnO and Fe3O4, which promotes interfacial electron–hole separation in the photocatalytic process, thereby increasingthe number of ‘‘live’’ photocharges and improving the photocata-lytic activity. However, the excess Fe3O4 affects the morphologyof the Fe3O4/ZnO heterostructures, and also reduces the photocat-alytic active sites on the surface of ZnO microrods. Therefore, wecan get optimal photocatalytic activity only when the appropriateFe content is added.

4. Conclusions

We successfully fabricated the heterogeneous nanostructuredFe3O4/ZnO microrods via an economic method. The morphologyand composition were carefully characterized by SEM, TEM, XRD,Raman, XPS, and the results showed the formation of the hexago-nal nanostructured Fe3O4/ZnO microrods. Both photocatalysis andremoval of toxic metal ions experiments indicated that Fe3O4/ZnOheterostructures display the highest efficiency. The present workindicates the suitable nano-heterostructures, increased activity ofZnO nanocrystals and combination of magnetic oxides, should findindustrial application in the future to remove undesirable contam-inants from the waste water. Certainly, the film technology shouldbe developed to overcome the drawbacks related to the use ofpowder materials.

Acknowledgements

This work was supported by National Nature Science Founda-tion (51172187), the SPDRF (20116102130002), the Doctoral fundof Ministry of Education of China (20116102120016), 111 Program(B08040) of MOE, Aeronautical Science Foundation of China(2013ZF53072), Xi’an Science and Technology Foundation(CX12174, XBCL-1-08), Shaanxi Province Science Foundation(2013KW12-02), Shaanxi Province Foundation for Returned Schol-ars, High-level start-up Funding of NWPU.

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