Preparation of Bi2Fe4O9 particles by hydrothermal synthesis and functional properties

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Preparation of Bi2Fe4O9 particles byhydrothermal synthesis and functionalpropertiesFelicia Gheorghiu a , Radu Tanasa a , Maria Teresa Buscaglia b ,Vincenzo Buscaglia b , Cristina G. Pastravanu c , Eveline Popovici c

& Liliana Mitoseriu aa Department of Physics , Alexandru Ioan Cuza University , Blv.Carol I, nr.11, 700506 Iasi , Romaniab Institute for Energetics & Interphases-CNR , Via de Marini no.6,Genoa I-16149 , Italyc Department of Materials Chemistry , Alexandru Ioan CuzaUniversity , Blv. Carol I, nr.11, 700506 Iasi , RomaniaPublished online: 09 Nov 2012.

To cite this article: Felicia Gheorghiu , Radu Tanasa , Maria Teresa Buscaglia , Vincenzo Buscaglia ,Cristina G. Pastravanu , Eveline Popovici & Liliana Mitoseriu (2013) Preparation of Bi2Fe4O9 particlesby hydrothermal synthesis and functional properties, Phase Transitions: A Multinational Journal,86:7, 726-736, DOI: 10.1080/01411594.2012.741238

To link to this article: http://dx.doi.org/10.1080/01411594.2012.741238

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Preparation of Bi2Fe4O9 particles by hydrothermal synthesis andfunctional properties

Felicia Gheorghiua*, Radu Tanasaa, Maria Teresa Buscagliab, Vincenzo Buscagliab,Cristina G. Pastravanuc, Eveline Popovicic and Liliana Mitoseriua

aDepartment of Physics, Alexandru Ioan Cuza University, Blv. Carol I, nr.11, 700506 Iasi,Romania; bInstitute for Energetics & Interphases-CNR, Via de Marini no.6,

Genoa I-16149, Italy; cDepartment of Materials Chemistry, AlexandruIoan Cuza University, Blv. Carol I, nr.11, 700506 Iasi, Romania

(Received 9 October 2012; final version received 14 October 2012)

In the present study, particles with different Bi2Fe4O9 micro/nanostructures witha few particular morphologies (flower-like nanoplatelets, hierarchical microstruc-tures, perfectly square platelets single crystals, etc.) obtained under specifichydrothermal synthesis conditions were investigated. The role of the processingparameters (such as NaOH concentration, reaction temperature, and reactionduration time) on the phase formation mechanism and on the microstructuralcharacteristics was investigated. All the Bi2Fe4O9 morphologies showed ortho-rhombic symmetry with space group Pbam. The photocatalytic properties andmagnetic behavior as a function of the micro/nanostructural characteristics ofvarious Bi2Fe4O9 powders were determined. In the presence of Bi2Fe4O9, adegradation rate of Rose Bengal in the range of 52–61% was determined after180min under UV light irradiation (�¼ 254 nm). Magnetic activity withantiferromagnetic behavior and a transition at �240K slightly dependent onthe microstructures was found. The role of Bi2Fe4O9 microstructures in thephotocatalytic activity and magnetic properties was discussed.

Keywords: Bi2Fe4O9 powders; photocatalysis; magnetic properties; hydrothermalsynthesis

1. Introduction

Over the last decade, multiferroic materials have been intensively studied, due to theirunique property of coupling ferroelectric and antiferro/ferromagnetic orders in the samephase [1,2]. The challenge in searching for new multiferroics is given by the discovery ofthe magnetoelectric effect which driven to a whole range of new phenomena andapplications. Consequently, there is strong interest in exploiting multiferroic materialstowards developing new types of devices. The number of room-temperature single phasemultiferroics is very limited [3] and for practical reasons, it is important to search for newcompounds. The perovskite BiFeO3 is one of the few and best known room-temperaturemultiferroics. Although its use was expected to produce an applicative breakthrough, thissingle-phase multiferroic still shows poor dielectric and ferroelectric properties at room

*Corresponding author. Email: [email protected]; [email protected]

Phase Transitions, 2013Vol. 86, No. 7, 726–736, http://dx.doi.org/10.1080/01411594.2012.741238

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Phase Transitions 727

Preparation of Bi2Fe4O9 particles by hydrothermal synthesis andfunctional properties

Felicia Gheorghiua*, Radu Tanasaa, Maria Teresa Buscagliab, Vincenzo Buscagliab,Cristina G. Pastravanuc, Eveline Popovicic and Liliana Mitoseriua

aDepartment of Physics, Alexandru Ioan Cuza University, Blv. Carol I, nr.11, 700506 Iasi,Romania; bInstitute for Energetics & Interphases-CNR, Via de Marini no.6,

Genoa I-16149, Italy; cDepartment of Materials Chemistry, AlexandruIoan Cuza University, Blv. Carol I, nr.11, 700506 Iasi, Romania

(Received 9 October 2012; final version received 14 October 2012)

In the present study, particles with different Bi2Fe4O9 micro/nanostructures witha few particular morphologies (flower-like nanoplatelets, hierarchical microstruc-tures, perfectly square platelets single crystals, etc.) obtained under specifichydrothermal synthesis conditions were investigated. The role of the processingparameters (such as NaOH concentration, reaction temperature, and reactionduration time) on the phase formation mechanism and on the microstructuralcharacteristics was investigated. All the Bi2Fe4O9 morphologies showed ortho-rhombic symmetry with space group Pbam. The photocatalytic properties andmagnetic behavior as a function of the micro/nanostructural characteristics ofvarious Bi2Fe4O9 powders were determined. In the presence of Bi2Fe4O9, adegradation rate of Rose Bengal in the range of 52–61% was determined after180min under UV light irradiation (�¼ 254 nm). Magnetic activity withantiferromagnetic behavior and a transition at �240K slightly dependent onthe microstructures was found. The role of Bi2Fe4O9 microstructures in thephotocatalytic activity and magnetic properties was discussed.

Keywords: Bi2Fe4O9 powders; photocatalysis; magnetic properties; hydrothermalsynthesis

1. Introduction

Over the last decade, multiferroic materials have been intensively studied, due to theirunique property of coupling ferroelectric and antiferro/ferromagnetic orders in the samephase [1,2]. The challenge in searching for new multiferroics is given by the discovery ofthe magnetoelectric effect which driven to a whole range of new phenomena andapplications. Consequently, there is strong interest in exploiting multiferroic materialstowards developing new types of devices. The number of room-temperature single phasemultiferroics is very limited [3] and for practical reasons, it is important to search for newcompounds. The perovskite BiFeO3 is one of the few and best known room-temperaturemultiferroics. Although its use was expected to produce an applicative breakthrough, thissingle-phase multiferroic still shows poor dielectric and ferroelectric properties at room

*Corresponding author. Email: [email protected]; [email protected]

temperature, one of the reasons being the presence of the common undesired impurityphase Bi2Fe4O9, often resulted as secondary phase in the synthesis of BiFeO3. However,this compound itself is expected to possess multifunctional properties (magnetic,semiconductor gas sensor, and photocatalyst) and it was recently proposed as a possiblenew multiferroic [4]. Therefore, the investigation of Bi2Fe4O9 became even more attractive.

Bi2Fe4O9 shows at room temperature a crystallographic structure with orthorhombicsymmetry which belongs to the space group Pbam, No. 55, with lattice parameters:a¼ 7.94 A, b¼ 8.44 A, c¼ 6.01 A, and �¼ �¼ �¼ 90� and with two formula units per unitcell [5]. It is considered to be paramagnetic at room temperature and exhibits a transitionto an antiferromagnetic state at low temperatures below TN� 240–265K [5–7]; itsproposed magnetic structure was confirmed by neutron scattering experiments [5,8].

Semiconductor-based photocatalysis, which shows great potential for decomposingorganic contaminants, has attracted more and more attention in the last years.The preparation of nanostructured powders is the basic challenge in order to achievegood photocatalytic activity, since nanostructures provide a large surface area to volumeratio, which is crucial for the photocatalytic reaction which occurs mostly on the catalystsurface [9]. The preparations of Bi2Fe4O9 material were reported by different methods:sol–gel [7], traditional solid state [4,10], sol–gel using template [11], molten salt technique[12] and via hydrothermal synthesis [13–18]. Among all these techniques, below�200�C without a further the hydrothermal approach is an important method to obtainBi2Fe4O9 pure phase powders because the syntheses of the crystalline material is possibleat low temperature calcinations step. Other fabrication methods require a hightemperature treatment during or after synthesis process. Moreover, it is difficult toobtain single-phase Bi2Fe4O9 materials by high temperature treatment, because thevolatilization of Bi2O3 leads to an incomplete reaction. In the case of hydrothermalsynthesis such low processing temperatures prevent the volatilization of reactants andminimize the amount of impurities. It is already known that the functional properties ofmultiferroic materials are strongly dependent on their method of preparation, morphol-ogy, microstructure, particle size and shape, and also on their crystallinity.

In the present work, the preparation of Bi2Fe4O9 powders with various types of micro/nanoparticles size and shape, resulted by tuning hydrothermal synthesis parameters ispresented. The photocatalytic and magnetic properties of various Bi2Fe4O9 powders withdifferent microstructural characteristics were investigated. In the future, dense Bi2Fe4O9

ceramics obtained by the present micro/nanostructures will be checked from the point ofview of their functional properties (dielectric, ferroelectric, magnetic, and magnetoelectric)for the confirmation of the expected multiferroic character and magnetoelectric couplingby a complex nano-macroscale investigation.

2. Experimental details

Bi2Fe4O9 powders with different morphologies were prepared by the hydrothermalsynthesis technique, according to the preparation flowchart presented in Figure 1.All chemicals were purchased from Aldrich (Milan, Italy) and were of standard purity(98%). The chemical reagents used for preparation of these powders were: bismuth nitrate(Bi(NO3) � 5H2O), iron nitrate (Fe(NO3) � 9H2O), sodium hydroxide (NaOH), and hydro-gen peroxide (H2O2). Bi(NO3) � 5H2O and Fe(NO3) � 9H2O reagents were dissolved in50mL distilled water under vigorous stirring. 12.5mL of NaOH solution (6 or 9M) wereadded to this solution. After 30min stirring, 2mL H2O2 (30wt% solution) were finally

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728 F. Gheorghiu et al.

added and the suspension was poured into a Teflon-lined steel acidic digestion bomb.The final volume of the suspension was 62.5mL. The bomb was placed inside the oven andheated at the reaction temperature with a heating rate of �1�Cmin�1.

The hydrothermal treatments were conducted in the temperature range of 180–220�Cfor 8–72 h. The pressure inside the bomb was that corresponding to equilibrium watervapor pressure at the given temperature. At �200�C the value of this pressure wasp� 18 bar. The produced powders were collected at the bottom of the autoclave after thetemperature was reduced down to the room temperature. The products were washed atleast five times with distilled water and then dried at �50�C in a freeze-drier. After thefreeze-drying process, the phase composition was determined by X-ray diffraction and themicrostructure was examined by Scanning Electron Microscopy (SEM) in order toconfirm the successful preparation of Bi2Fe4O9 powders.

The phase purity of the powders was checked by X-ray diffraction(XRD, PANanalytical CubiX PRO, Type 1017) using Cu K� radiation with wavelengthof 1.54 A and operating at 40 kV and 40mA in the 2� range of 10–90�, with a step size of2�¼ 0.02� and sampling time of 6 s under a measurement temperature of 25�C. Themorphology of the powders was investigated by SEM (LEO, Model 1450VP) analysis.

Porosity and surface area were obtained from the N2-sorption isotherms performed ona Quantachrome NOVA 2200e system using nitrogen as adsorbate at liquid nitrogentemperature (�196�C). All the samples were outgassed under vacuum, for 2 h at 200�Cbefore the adsorption measurements. The surface area was calculated using the BETmethod in the range of relative pressure 0.05–0.35. Pore volume was calculated at the

Starting reagents: Bi(NO3)·5H2O, Fe(NO3)·9H2O

Weight, dissolving in distilled water, mixing under stirring at

room temperature

Mixing of the two solutions

Preparation of NaOH solution(Weight NaOH, dissolving in distilled water, mixing under stirring at room temperature)

Stirring 30 min at room temperature

Checking the pH=14

2 mL H2O2 addition

Hydrothermal synthesis

Filtration, washing, drying

Bi2Fe4O9powder

XRDSEM



room temperature




Checking the pH=14

2 mL H2O2 addition



Bi2Fe4O9powder

XRDSEM

Figure 1. Preparation flowchart for the Bi2Fe4O9 powders.

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added and the suspension was poured into a Teflon-lined steel acidic digestion bomb.The final volume of the suspension was 62.5mL. The bomb was placed inside the oven andheated at the reaction temperature with a heating rate of �1�Cmin�1.

The hydrothermal treatments were conducted in the temperature range of 180–220�Cfor 8–72 h. The pressure inside the bomb was that corresponding to equilibrium watervapor pressure at the given temperature. At �200�C the value of this pressure wasp� 18 bar. The produced powders were collected at the bottom of the autoclave after thetemperature was reduced down to the room temperature. The products were washed atleast five times with distilled water and then dried at �50�C in a freeze-drier. After thefreeze-drying process, the phase composition was determined by X-ray diffraction and themicrostructure was examined by Scanning Electron Microscopy (SEM) in order toconfirm the successful preparation of Bi2Fe4O9 powders.

The phase purity of the powders was checked by X-ray diffraction(XRD, PANanalytical CubiX PRO, Type 1017) using Cu K� radiation with wavelengthof 1.54 A and operating at 40 kV and 40mA in the 2� range of 10–90�, with a step size of2�¼ 0.02� and sampling time of 6 s under a measurement temperature of 25�C. Themorphology of the powders was investigated by SEM (LEO, Model 1450VP) analysis.

Porosity and surface area were obtained from the N2-sorption isotherms performed ona Quantachrome NOVA 2200e system using nitrogen as adsorbate at liquid nitrogentemperature (�196�C). All the samples were outgassed under vacuum, for 2 h at 200�Cbefore the adsorption measurements. The surface area was calculated using the BETmethod in the range of relative pressure 0.05–0.35. Pore volume was calculated at the



room temperature




Checking the pH=14

2 mL H2O2 addition



Bi2Fe4O9powder

XRDSEM



room temperature




Checking the pH=14

2 mL H2O2 addition



Bi2Fe4O9powder

XRDSEM

Figure 1. Preparation flowchart for the Bi2Fe4O9 powders.

relative pressure of 0.95. Pore sizes distributions were calculated from the adsorptionbranches of the N2 adsorption isotherms using the Barett–Joyner–Halenda [19].

The photocatalytic activity of the Bi2Fe4O9 powders was evaluated by degradation ofRose Bengal (RB) solution under UV light irradiation (�¼ 254 and 365 nm). For the UVirradiation, an 8W lamp (VL-4.LC) supplied by Vilber Lourmat, was used at natural pHvalue. The initial concentration of RB was 20mgL�1, with a catalyst loading of 0.5 gL�1.

The magnetic properties in the range of temperatures (10–350)K under fields of(0–30) kOe were determined with a superconducting quantum interference device SQUIDmagnetometer MPMS-XL-7AC (Quantum Design).

3. Results and discussion

3.1. Phase purity and microstructural characteristics

The synthesis parameters and the corresponding denomination of the as-prepared samplesare listed in Table 1. The temperature and time as well as the NaOH amounts were thereaction control parameters. The purity of the as-prepared Bi2Fe4O9 powders wasexamined by XRD. Figure 2 displays the XRD patterns of the Bi2Fe4O9 powderssynthesized with 1.2M or 3.6M NaOH concentration at different reaction temperatures of180, 200, and 220�C with a reaction time between 48 and 72 h.

All the XRD patterns correspond to the orthorhombic symmetry (space group Pbam)of the Bi2Fe4O9 phase, identified with ICSD Ref. Code. No. 01-072-1832. The XRDpatterns show that all the powders (Figure 2a) are well crystallized and exhibit amajoritary Bi2Fe4O9 phase for all the employed reaction parameters. In the case of BF3and BF4 samples, the presence of small amounts of BiFeO3 phase was also noticed.

The SEM images of the resulted powders are presented in Figure 3(a)–(d) and theyshow the morphologies of the as-prepared Bi2Fe4O9 powders. The SEM images reveal thatthe size and shape of Bi2Fe4O9 particles strongly depend on the processing parameters ofthe hydrothermal synthesis, which lead to different morphologies: flower-like nanoplate-lets (Figure 3a), hierarchical micro/nanostructures (Figure 3b), or perfectly squareplatelets single crystals (Figure 3c, d).

According to Figure 3(a) the morphology of the Bi2Fe4O9 particle corresponds toaggregates of flower-square nanoplatelets. These nanoplateles with a lateral average sizearound 1.6 mm appear quite transparent under the electron beam, indicating that theirthickness is only of some tens of mm. Figure 3(b) shows the SEM image of Bi2Fe4O9

prepared using a different concentration of the metal ions but keeping the samestoichiometric ratio and the same temperatures and reaction time parameters. In this case,a hierarchical microstructure that consisted in combined Bi2Fe4O9 irregular sheets shapeswith some Bi2O3 rod-like structures was obtained. The sheets appear transparent under theelectron beam and they have nanosized thickness.

Table 1. Sample identifications and employed reaction parameters.

Samplenotation

[Bi]¼ [Fe](moles L�1)

[NaOH](M)

Reactiontemperature (�C)

Reactiontime (h)

BF1 0.005 6 200 72BF2 0.05 6 200 72BF3 0.001667 6 220 48BF4 0.001667 8 180 48

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When the reaction temperature increased to 220�C and reaction time was about 48 h,uniform square single crystal nanoplatelets were produced (Figure 3c) with a lateral size of�0.8 mm. It can be seen that the size and shape of the resulted Bi2Fe4O9 particles stronglydepend on the reaction temperature. A lower reaction temperature (Figure 3d) resulted in

Figure 3. SEM images of Bi2Fe4O9 powders prepared under various reaction conditions as indicatedin Table 1: (a) BF1, (b) BF2, (c) BF3, and (d) BF4.

10 20 30 40 50 60 700

5000

10000

15000

20000

25000

30000

2θ - CuKα (degrees)

- Bi2Fe4O9 - BiFeO3

Inte

nsity

(a.u

) BF1 BF2 BF3 BF4

ICSD Ref. Code. No. 01-072-1832

10 15 20 25 30 35 40 45 500

5000

10000

15000

20000

25000

(411

)(330

)(4

10)

(400

)(1

40)(0

22)

(221

)

(200

)

(212

)(311

)(1

22)

(202

)(1

31)

(320

)

(310

)(1

30)

(112

)(2

20)

(002

)(2

11)

(121

)(2

01)

(021

)(2

10)

(120

)

(111

)(0

20)

(110

)


Inte

nsity

(a.u

)

BF1

(001

)

(b)(a)

Figure 2. (a) XRD patterns of the Bi2Fe4O9 micro/nanopowders and (b) indexed XRD pattern ofthe BF1 powders (ICSD Ref. Code. No. 01-072-1832).

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When the reaction temperature increased to 220�C and reaction time was about 48 h,uniform square single crystal nanoplatelets were produced (Figure 3c) with a lateral size of�0.8 mm. It can be seen that the size and shape of the resulted Bi2Fe4O9 particles stronglydepend on the reaction temperature. A lower reaction temperature (Figure 3d) resulted in

Figure 3. SEM images of Bi2Fe4O9 powders prepared under various reaction conditions as indicatedin Table 1: (a) BF1, (b) BF2, (c) BF3, and (d) BF4.

10 20 30 40 50 60 700

5000

10000

15000

20000

25000

30000


- Bi2Fe4O9 - BiFeO3

Inte

nsity

(a.u

)

BF1 BF2 BF3 BF4

ICSD Ref. Code. No. 01-072-1832

10 15 20 25 30 35 40 45 500

5000

10000

15000

20000

25000

(411

)(330

)(4

10)

(400

)(1

40)(0

22)

(221

)

(200

)

(212

)(311

)(1

22)

(202

)(1

31)

(320

)

(310

)(1

30)

(112

)(2

20)

(002

)(2

11)

(121

)(2

01)

(021

)(2

10)

(120

)

(111

)(0

20)

(110

)


Inte

nsity

(a.u

)

BF1

(001

)

(b)(a)

Figure 2. (a) XRD patterns of the Bi2Fe4O9 micro/nanopowders and (b) indexed XRD pattern ofthe BF1 powders (ICSD Ref. Code. No. 01-072-1832).

smaller and thinner square single crystal nanoplatelets with lateral size of about �1 mm.As expected, a significant change of the particle morphology was observed with themodification of the reaction parameters such as reaction time, reaction temperature, metalion concentration, and NaOH concentration.

3.2. Porosity properties

The N2-sorption measurements were used to characterize the textural properties of thesynthesized materials. The obtained isotherms and their corresponding pore sizedistributions are shown in Figure 4 and the summary of the physico–chemical propertiesof synthesized Bi2Fe4O9 powders resulted from these measurements are presented in theTable 2.

The analysis of the adsorption isotherms, as resulted for Bi2Fe4O9 with different micro/nanostructural characteristics are shown in Figure 4(a) provided an approximateassessment of the micro-meso-macroporosity. The isotherms of type II (S-shaped),

0,0 0,2 0,4 0,6 0,8 1,00

1

2

3

4

5

6

7

III

II

Relative pressure, p/p0

Ads

orbe

d vo

lum

e, c

c/g

STP Bi2Fe4O9

BF1 BF2 BF3 BF4

I

1 2 3 4 5 6 7 8 9 10 11 12 13

0246012

024024

Pore diameter(nm)

3.33 BF1

dV (

x10

-3)

(d)

4.15 BF2

1.97

1.61 BF3

BF4

(a) (b)

Figure 4. (a) Representative adsorption-desorption isotherms and (b) pore size distribution for theas-synthesized Bi2Fe4O9 powders.

Table 2. Summary of the physico–chemical properties of synthesized Bi2Fe4O9

powders.

Sample specification

Parameters BF1 BF2 BF3 BF4

BET Surface area (m2 g�1)a 12.85 15.52 10.26 11.69Pore diameter (nm)b 3.34 4.15 1.62 1.97Pore volume (cm3 g�1)c 0.11 0.18 0.02 0.03

Notes: aCalculated using the BET method in the range of relative pressure 0.05–0.35.bCalculated from the adsorption branches of the N2 adsorption isotherms usingthe Barrett–Joyner–Halenda model [19].cCalculated at the relative pressure of 0.95.

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are normally associated with monolayer-multilayer sorption on the nonporous ormacroporous surface of the powder. Types II isotherms show a significant uptake atlow partial pressures followed by small adsorption at intermediate vapor concentrationand again a high uptake at elevated partial pressures.

For the samples BF1 and BF2 the adsorption started at 1 cm3 g�1. This behaviorindicated a significant development of the microporosity (region I), unlike samples BF3and BF4. No further adsorption activity was observed in the range 0.15 p/p05 0.8 forany of the analyzed samples. In this range the adsorption reached equilibrium (region II),meaning that the synthesized samples do not exhibit porous characteristics. The thirdregion (III) corresponds to the adsorption in wide mesopores and macropores character-izing interparticle voids. The large amount of pores of 2–8 nm in diameter and theBET surfaces values of 10–15.5m2 g�1 are in accordance with the conclusions statedabove. The samples BF2 and BF1 have the highest BET surface area and pore volume,while the powders BF3 and BF4 have lower similar BET surface area and pore volume.

3.3. Photocatalytic activity

Figure 5 shows the photocatalytic activity of the Bi2Fe4O9 as-prepared micro/nanopowders with various microstructures for the RB degradation. The decolorizationand photocatalytic degradation rate have been expressed using the normalized concen-tration values (C/C0), in which C0 is the initial RB concentration and C is dyeconcentration measured at different moments of time of exposure to irradiation. In thefirst stages of the degradation process, there is not a major difference between the BF1-4powders for both types of radiations. Differences can be noticed after longer time ofdegradation. After 180min of UV light irradiation of �¼ 254 nm the RB degradation ratewas in the range 52–61% for all the synthesized samples. However, better degradationproperties are shown by the flower-like samples BF1 (61%) while BF4 showed a smallerdegradation rate of 52%. When 365 nm UV light irradiation was used, only the samplesBF4 and BF2 showed significant decolorization rates (of 39% and 32%) after 180minirradiation, while the other two samples BF1 and BF3 a small rate of only 15% wasnoticed. The best decomposition rate under the UV irradiation with �¼ 365 nm

0 20 40 60 80 100 120 140 160 180

0,4

0,5

0,6

0,7

0,8

0,9

1,0 BF1 BF2 BF3 BF4

C/C

0

Bi2Fe4O9

λ = 254 nm

Time (min) 0 20 40 60 80 100 120 140 160 180

0,6

0,7

0,8

0,9

1,0 Bi2Fe4O9

BF1 BF2 BF3 BF4

C/C

0

λ = 365 nm

Time (min)

(a) (b)

Figure 5. Photocatalytic activity of the Bi2Fe4O9 microstructures for RB degradation under UVlight irradiation at: (a) �¼ 254 nm and (b) �¼ 365 nm.

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are normally associated with monolayer-multilayer sorption on the nonporous ormacroporous surface of the powder. Types II isotherms show a significant uptake atlow partial pressures followed by small adsorption at intermediate vapor concentrationand again a high uptake at elevated partial pressures.

For the samples BF1 and BF2 the adsorption started at 1 cm3 g�1. This behaviorindicated a significant development of the microporosity (region I), unlike samples BF3and BF4. No further adsorption activity was observed in the range 0.15 p/p05 0.8 forany of the analyzed samples. In this range the adsorption reached equilibrium (region II),meaning that the synthesized samples do not exhibit porous characteristics. The thirdregion (III) corresponds to the adsorption in wide mesopores and macropores character-izing interparticle voids. The large amount of pores of 2–8 nm in diameter and theBET surfaces values of 10–15.5m2 g�1 are in accordance with the conclusions statedabove. The samples BF2 and BF1 have the highest BET surface area and pore volume,while the powders BF3 and BF4 have lower similar BET surface area and pore volume.

3.3. Photocatalytic activity

Figure 5 shows the photocatalytic activity of the Bi2Fe4O9 as-prepared micro/nanopowders with various microstructures for the RB degradation. The decolorizationand photocatalytic degradation rate have been expressed using the normalized concen-tration values (C/C0), in which C0 is the initial RB concentration and C is dyeconcentration measured at different moments of time of exposure to irradiation. In thefirst stages of the degradation process, there is not a major difference between the BF1-4powders for both types of radiations. Differences can be noticed after longer time ofdegradation. After 180min of UV light irradiation of �¼ 254 nm the RB degradation ratewas in the range 52–61% for all the synthesized samples. However, better degradationproperties are shown by the flower-like samples BF1 (61%) while BF4 showed a smallerdegradation rate of 52%. When 365 nm UV light irradiation was used, only the samplesBF4 and BF2 showed significant decolorization rates (of 39% and 32%) after 180minirradiation, while the other two samples BF1 and BF3 a small rate of only 15% wasnoticed. The best decomposition rate under the UV irradiation with �¼ 365 nm

0 20 40 60 80 100 120 140 160 180

0,4

0,5

0,6

0,7

0,8

0,9

1,0 BF1 BF2 BF3 BF4

C/C

0

Bi2Fe4O9

λ = 254 nm

Time (min) 0 20 40 60 80 100 120 140 160 180

0,6

0,7

0,8

0,9

1,0 Bi2Fe4O9

BF1 BF2 BF3 BF4

C/C

0

λ = 365 nm

Time (min)

(a) (b)

Figure 5. Photocatalytic activity of the Bi2Fe4O9 microstructures for RB degradation under UVlight irradiation at: (a) �¼ 254 nm and (b) �¼ 365 nm.

wavelength is still shown by the micro/nanoplatelets BF4 (39%) after 180min. Thepossible role of small amounts of BiFeO3 in the degradation process of BF3 and BF4samples is not excluded; however, the main effect is determined by the differentmorphologies of the Bi2Fe4O9 majoritary phase.

It can be concluded here that a shorter wavelength (�¼ 254 nm) has a higher impact onthe photocatalytic activity with significantly faster rate of degradation than thatcorresponding for the longer wavelength of �¼ 365 nm. For a fixed wavelength UVirradiation, the photocatalytic properties are slightly dependent on the nano/microstruc-tural characteristics of the present Bi2Fe4O9 powders, but some optimum microstructuresin terms of the best photocatalytic properties for this compound are shown by the flower-and micro/nanoplatelet-like Bi2Fe4O9 powders.

The size and shape-dependent photocatalytic activity of the Bi2Fe4O9 micro/nanocrys-tals can be understand by considering the results of ab initio calculations [5], which indicatethat Bi2Fe4O9 is a multiband semiconductor. For improved photocatalytic performance theefficiency of electron–hole separation should increase, considering the middle energy bandthat may act as electron–hole recombination centers. The size-dependent photocatalyticproperties of Bi2Fe4O9 micro/nanostructures can be ascribed to the lower recombinationrate of the photogenerated electron/hole pair within smaller nanocrystals in complexhierarchical structures, which allow for stronger photon absorption on the surface ofsmaller crystallites. However, the present study shows that not only the size of the micro/nanocrystallites tune their photocatalytic properties, because in such a case we couldexpected the highest photocatalytic activity on the BF2 and BF4 samples. It results thatmicro/nanocrystallite shape also strongly influence photocatalytic activity, becausedifferent crystal facets have different surface energy levels of the conduction and valencebands. As demonstrated recently [20], various crystalline facets have different reactivity(e.g., the (001) facet has a higher reactivity than the (221) facet) and this might explain thedifferent photocatalytic response of various morphologies, i.e., the observed shape-dependent photocatalytic properties of the Bi2Fe4O9 micro/nanocrystals.

Only few papers reported the photocatalytic data of the Bi2Fe4O9. Zhang et al. [9]reported the photocatalytic activity of the Bi2Fe4O9 nanoparticles evaluated by thedegradation of methyl red (MR) under UV and visible irradiation. The measurementsreveal a marked photocatalytic decomposition of the MR using Bi2Fe4O9 nanoparticles:after 6 h UV irradiation, the MR degradation rate reaches values between 73% and 77%.It can be clearly seen that the using of RB leads to a degradation rate between 52% and61% only after 3 h (180min). Ruan and Zhang [13] reported the photocatalytic activity ofwell-crystallized Bi2Fe4O9 samples using the degradation of methyl orange (MO) underUV and visible irradiation. After 90min UV irradiation with Bi2Fe4O9 samples, the MOdegradation rate was about 86–93%. In [20], the authors pointed out that under visiblelight irradiation for 3 h, the degradation efficiency of the Bi2Fe4O9 sample with randomorientation toward MO is about 52%. Our values obtained for RB degradation in UVirradiation using Bi2Fe4O9 micro/nanopowders are comparable with the reported ones.A size and shape-dependent photocatalytic activity can be confirmed for the presentBi2Fe4O9 micro/nanostructures.

3.4. Magnetic properties

In order to check the role of the particle morphology on the magnetic properties, thezero field cooling (ZFC) thermomagnetic response recorded in the temperature range of

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10–300K, under external magnetic field of H¼ 100Oe was measured. The magneticresponses of the as-prepared powders are shown in Figure 6: (a) represent thethermomagnetic dependences (ZFC curves) while (b) show the M(H) hysteresis loops ata few selected temperatures for the BF3 powders. The ZFC data clearly shows that themagnetic response in Bi2Fe4O9 is morphology-dependent, the maximum magnetizationcorresponding to the BF1 and the lowest tot the BF3 powders, while BF2 and BF4 havevery similar thermomagnetic responses and magnetization values. However, we do notexclude the role of possible small amounts of secondary phases which are below the XRDdetection limit. The inset of Figure 6(a) show the ZFC/FC curves for the BF3 powders.The ZFC and FC magnetization values start to clear split below the temperature of about230K and a divergence between the ZFC and FC magnetization curves increases withdecreasing temperature. Both ZFC and FC curves present a broad maximum corre-sponding to the TN¼ 240K and then suddenly decreasing with reducing temperature untilthe temperature of about 160K is reached. The clear anomaly at TN indicates a phasetransition to the antiferromagnetic state which is only slightly dependent on themicrostructures. The ZFC/FC continues with a slightly increasing until temperaturearound 50K. Below this temperature, the magnetic moment suddenly increases again forfurther lowering temperature. The rise of the magnetization in the ZFC/FC curves below50K indicates a change in the spin ordering at low temperatures, which probably are dueto spin reorientation leading to an additional magnetic moment (coming fromuncompensated magnetic spin). The magnetic field dependences of magnetic moment atthree different temperatures (Figure 6b) are almost linear and non-hysteretic, indicatingthe antiferromagnetic character of the BF3 powders. More detailed investigations arenecessary for a better understanding of the role of the morphology on the magneticcharacteristics of the Bi2Fe4O9 micro/nanopowders.

4. Conclusions

In summary, we present the results of the preparation by hydrothermal-assisted synthesisof Bi2Fe4O9 micro/nanostructures with different morphologies. Different Bi2Fe4O9

0 50 100 150 200 250 300

0.001

0.002

0.003

0.004

0.005

BF3

BF4BF2

0 50 100 150 200 250 300

0.00090

0.00095

0.00100

0.00105

0.00110

0.00115

BF3

m (e

mu/

g)T (K)

ZFC FC

H = 100 Oe

BF1

m (

emu/

g)H = 100 Oe

T (K)

ZFC

-30000 -15000 0 15000 30000-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

-3000 -1500 0 1500 3000-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015

H (Oe)

m (

emu/

g)

50 K 160 K 240 K

BF3

(b) (a)

Figure 6. (a) Temperature dependence of the magnetization for Bi2Fe4O9 powders subjected toa ZFC curve under the magnetic field of H¼ 100Oe. Inset: ZFC/FC cycle for the BF3 powders.(b) The m(H) dependences of the BF3 powders at the three different temperatures corresponding tothe curves anomalies. Inset: m(H) larger area curves.

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10–300K, under external magnetic field of H¼ 100Oe was measured. The magneticresponses of the as-prepared powders are shown in Figure 6: (a) represent thethermomagnetic dependences (ZFC curves) while (b) show the M(H) hysteresis loops ata few selected temperatures for the BF3 powders. The ZFC data clearly shows that themagnetic response in Bi2Fe4O9 is morphology-dependent, the maximum magnetizationcorresponding to the BF1 and the lowest tot the BF3 powders, while BF2 and BF4 havevery similar thermomagnetic responses and magnetization values. However, we do notexclude the role of possible small amounts of secondary phases which are below the XRDdetection limit. The inset of Figure 6(a) show the ZFC/FC curves for the BF3 powders.The ZFC and FC magnetization values start to clear split below the temperature of about230K and a divergence between the ZFC and FC magnetization curves increases withdecreasing temperature. Both ZFC and FC curves present a broad maximum corre-sponding to the TN¼ 240K and then suddenly decreasing with reducing temperature untilthe temperature of about 160K is reached. The clear anomaly at TN indicates a phasetransition to the antiferromagnetic state which is only slightly dependent on themicrostructures. The ZFC/FC continues with a slightly increasing until temperaturearound 50K. Below this temperature, the magnetic moment suddenly increases again forfurther lowering temperature. The rise of the magnetization in the ZFC/FC curves below50K indicates a change in the spin ordering at low temperatures, which probably are dueto spin reorientation leading to an additional magnetic moment (coming fromuncompensated magnetic spin). The magnetic field dependences of magnetic moment atthree different temperatures (Figure 6b) are almost linear and non-hysteretic, indicatingthe antiferromagnetic character of the BF3 powders. More detailed investigations arenecessary for a better understanding of the role of the morphology on the magneticcharacteristics of the Bi2Fe4O9 micro/nanopowders.

4. Conclusions

In summary, we present the results of the preparation by hydrothermal-assisted synthesisof Bi2Fe4O9 micro/nanostructures with different morphologies. Different Bi2Fe4O9

0 50 100 150 200 250 300

0.001

0.002

0.003

0.004

0.005

BF3

BF4BF2

0 50 100 150 200 250 300

0.00090

0.00095

0.00100

0.00105

0.00110

0.00115

BF3

m (e

mu/

g)

T (K)

ZFC FC

H = 100 Oe

BF1

m (

emu/

g)

H = 100 Oe

T (K)

ZFC

-30000 -15000 0 15000 30000-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

-3000 -1500 0 1500 3000-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015

H (Oe)

m (

emu/

g)

50 K 160 K 240 K

BF3

(b) (a)

Figure 6. (a) Temperature dependence of the magnetization for Bi2Fe4O9 powders subjected toa ZFC curve under the magnetic field of H¼ 100Oe. Inset: ZFC/FC cycle for the BF3 powders.(b) The m(H) dependences of the BF3 powders at the three different temperatures corresponding tothe curves anomalies. Inset: m(H) larger area curves.

powders with particular geometries and microstructural characteristics were produced bycontrolling the processing parameters, such as NaOH concentration, metal ion concen-tration, reaction temperature, and reaction duration time. The Bi2Fe4O9 powderscrystallize in the orthorhombic phase with space group Pbam. The modification ofdifferent processing parameters in the formation of Bi2Fe4O9 particles lead to a fewdifferent morphologies: flower-like nanoplatelets, hierarchical microstructures, perfectlysquare platelet-like single crystals, etc. The photocatalytic properties are dependent on themicro/nanostructural characteristics induced by various processing parameters. Althoughboth UV light irradiation wavelengths (�¼ 254 and 365 nm) with Bi2Fe4O9 samples revealsphotocatalytic degradation of the RB, the degradation representations for the shorterwavelength show a higher decolorization and photocatalytic activity with different rates inthe range 52–61% in various time regions for different microstructures. The magneticcharacterization demonstrated a predominant antiferromagnetic character with a Neeltemperature TN in the range of 230–240K and blocking temperature of about 230K,identified accurately on the magnetic data for BF3 sample. The highest magnetization istypical to the samples with flower-like morphology (BF1).

In conclusion, the processing parameters have a strong influence on the phaseformation and particle morphology as well in the photocatalytic activity and magneticbehavior of Bi2Fe4O9, for which size and shape-dependent effects were observed.The various morphologies can induce other different functional properties, which willbe further investigated.

Acknowledgements

The financial support of the CNCS-UEFISCDI project PN-II-ID-PCE-2011-3-0745 is highlyacknowledged. FG also acknowledges the MP0904-COST Action (STSM mobility at IENI-CNRGenoa).

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Documents

Preparation of Bi2Fe4O9 particles by hydrothermal synthesis and functional properties