Photocatalytic and bactericidal activities of hydrothermally and sonochemically prepared Fe2O3–SnO2 nanoparticles

Materials Science in Semiconductor Processing 16 (2013) 818–824

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Photocatalytic and bactericidal activities of hydrothermally and sonochemically prepared Fe2O3–SnO2 nanoparticles C. Karunakaran n, S. SakthiRaadha, P. Gomathisankar Department of Chemistry, Annamalai University, Annamalainagar 608002, India

a r t i c l e in f o

abstract

Available online 9 March 2013

Fe2O3–SnO2 nanocomposites (NCs) were prepared by hydrothermal and sonochemical methods. Transmission electron micrographs confirmed that the composites comprise nanoparticles. Energy-dispersive X-ray analyses revealed compositions of 25 at.% Sn and 22 at.% Fe for hydrothermally prepared NC (HNC) and 4 at.% Sn and 56 at.% Fe for sonochemically prepared NC (SNC). X-Ray diffractograms revealed rutile SnO2, g-Fe2O3, and FeO(OH) as components of HNC, and rutile SnO2, g-Fe2O3, b-Fe2O3 and FeO(OH) of SNC. Both NCs absorb visible light and display red emission. The solid-state impedance spectrum for HNC is a half-semicircular arc and SNC exhibits a quasi-linear relationship between Z0 0 and Z0 . Both NCs are ferromagnetic. The saturation magnetization of HNC is much less than that of the SNC, which in turn is far less than that of the g-Fe2O3 precursor. Both NCs display visible light photocatalysis and HNC is a better photocatalyst than SNC. Furthermore, both NCs exhibit bactericidal activity. & 2013 Elsevier Ltd. All rights reserved.

Keywords: Nanocomposite Magnetic separation Visible light photocatalysis Bactericide

1. Introduction SnO2 is the most studied semiconductor nanomaterial for sensor applications [1]. Its use as photoelectrodes in solar cells is well established [2]. Owing to its unique photoelectronic properties [3,4], non-toxicity, and chemical stability, it is also used as a photocatalyst [5–7]. The problem of nanopowder recovery after photocatalysis is a limitation for the adoption of this technology in industry. Magnetic separation of the photocatalyst is a possible solution [8] and Fe2O3–SnO2 nanocomposites (NCs) are suitable for this purpose. Furthermore, SnO2 is a widebandgap n-type semiconductor (  3.8 eV) and requires UV light of o330 nm for photocatalysis. Fe2O3 is a narrow-bandgap n-type semiconductor (  2.2 eV) and is photoactivated by visible light. However, the photogenerated hole in the valence band (VB) of Fe2O3 is less

n Corresponding author. Tel.: þ 91 9443481590; fax: þ91 4144238145. E-mail address: [email protected] (C. Karunakaran).

1369-8001/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.12.030

anodic than those in widely used photocatalysts such as TiO2, ZnO, and SnO2 and its oxidizing power is low [9]. By contrast, the hole in the VB of SnO2 is more anodic (high oxidizing power) than in most widely used semiconductors [9]. Furthermore, the conduction band (CB) electron in photoexcited SnO2 is less cathodic than that of Fe2O3 [9]. The difference in VB and CB edge potentials between SnO2 and Fe2O3 leads to separation of photogenerated charge carriers across the heterojunction, which enhances photocatalytic activity. Fe3 þ -doped SnO2 nanoparticles have been prepared by sol–gel calcination and sol–gel hydrothermal methods and the absorption edge is redshifted with increasing dopant content [10]. Fe-doped SnO2 has also been prepared using an aqueous solution method [11]. However, it has been reported that Fe3 þ doping of SnO2 suppresses sensing of methane [12]. By contrast, sol–gel-prepared Fe2O3-SnO2 can sense gas concentrations of ethanol, carbon monoxide, and methane [13–15]. Fe2O3–SnO2 obtained by mechanical alloying showed greater selectivity in sensing ethanol gas compared to carbon monoxide and hydrogen [16]. Fe2O3– SnO2 prepared by a wet chemical method was used to

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sense hydrogen sulfide [17]. Liquefied petroleum gas can also be sensed by SnO2–Fe2O3 films [18]. Hydrothermally prepared 0.1 SnO2–0.9 a-Fe2O3 was used to sense methane and carbon monoxide [19], where Fe3 þ was substituted by Sn4 þ . Fe2O3–SnO2 obtained by a coprecipitation method showed efficient electron–hole separation [20]. SnO2–Fe2O3 can also be obtained by chemical precipitation [21,22]. SnO2–a-Fe2O3 composite nanowire arrays have been obtained by electrophoretic deposition [23]. a-Fe2O3–SnO2 NC showed lattice mismatch at the interface [24]. Sol–gel-prepared a-F2O3– SnO2 film showed greater photoelectrochemical properties [25]. SnO2–a-Fe2O3 heterostructures fabricated by chemical vapor deposition showed enhanced photocatalytic activity due to improved electron–hole separation [26]. Fe2O3–SnO2 NC obtained by co-precipitation exhibited visible light photocatalysis [27]. Branched SnO2–a-Fe2O3 semiconductor nanoheterostructures were recently synthesized hydrothermally. The heterostructures showed excellent visible light and UV photocatalytic abilities that were remarkably superior to those of the aFe2O3 precursor, mainly owing to effective electron–hole separation at the SnO2–a-Fe2O3 interface [28]. Sonochemical and hydrothermal methods are solution synthesis techniques. In the latter, chemical transformation occurs in supercritical aqueous solution. In the former, the chemical effects of ultrasound arise due to acoustic cavitation, which involves the formation, growth, and implosive collapse of bubbles in aqueous solution. Localized hotspots are produced through adiabatic compression. The transient temperature and pressure associated with these hotspots are as high as 5000 K and 1800 atm., respectively, and the cooling rate is as great as 1010 K s  1 [29]. These extreme conditions lead to chemical reaction. Although there are reports on the synthesis of Fe2O3–SnO2 NCs using the methods mentioned above, there are no reports on the synthesis of Fe2O3–SnO2 NC by sonochemical and hydrothermal solution methods using the same precursor and reagents for comparison of their photocatalytic and bactericidal activities. Our results show that for Fe2O3–SnO2, hydrothermally synthesized NC (HNC) exhibit greater photocatalytic and antibacterial activities than sonochemically prepared NC (SNC).

(for HNC) or sonication (for SNC; Toshcon SW2 ultrasonic bath, 100 W at 3773 kHz) to obtain a suspension. Then 20 ml of a 1:1 aqueous ethanolic alkaline (1.8 M NaOH) solution of H2O2 (7.5% v/v) was added under stirring (for HNC) or sonication (for SNC). To the resulting dispersion, 30 ml of an ethanolic solution containing 1.4 g SnCl2 and 1.0 g hexamine was added dropwise with stirring (for HNC) or sonication (for SNC). The mixture was either sonicated for 30 min to obtain a brown solid or transferred to a Teflon-lined stainless steel autoclave for hydrothermal treatment. The autoclave was sealed, kept at 180 1C for 12 h and allowed to cool to room temperature naturally to obtain a brown solid. In both cases the solid was separated, washed several times with water and absolute ethanol, and finally dried at 120 1C for 4 h. The product was calcinated at 500 1C for 2 h in a muffle furnace fitted with a PID temperature controller. The heating rate was 10 1C min  1. A schematic diagram of the synthesis methods is shown in Fig. 1. Pristine SnO2 nanocrystals were synthesized hydrothermally and sonochemically using the same procedure but without Fe2O3. 2.3. Characterization techniques A JOEL JSM 5610 scanning electron microscope (SEM) equipped with BE detector was used to examine the sample morphology. Samples were placed on an adhesive carbon slice supported on copper stubs and coated with 10-nm-thick gold using a JOEL JFC-1600 auto fine-coater prior to measurement. Transmission electron microscopic (TEM) analysis was carried out using a JEOL 100CX II

PEG 20,000 in ethanol (2 g/10 mL) Nano Fe2O3 (0.37 g) Sonication / stirring H2O2 (7.5 %) + NaOH (1.8 M) in 20 mL 1:1 aq. ethanol Sonication / stirring

2. Experimental 2.1. Materials Stannous chloride (Qualigens), hydrogen peroxide (SD Fine), sodium hydroxide (SD Fine), poly(ethylene glycol) (PEG, 20 000; Himedia), hexamine (Himedia) and rhodamine B (SD Fine) of LR grade were used as received. Phenol (AR, SD Fine) was distilled before use. Fe2O3 nanopowder (Sigma Aldrich) was used as received. Distilled ethanol and deionized distilled water were used for the experiments. 2.2. Hydrothermal and sonochemical preparation To an ethanolic solution of PEG (2 g in 10 ml) was added 0.37 g of Fe2O3 nanopowder under constant stirring

819

Sonication / stirring

SnCl2 (1.4 g) + Hexamine (1.0 g) in ethanol (30 mL)

Sonication (30 min) / Hydrothermal treatment (180°C, 12 h )

Filteration & drying (120°C, 4 h) Calcination (500°C, 2 h) Fig. 1. Scheme for hydrothermal (H) and sonochemical (S) preparation of Fe2O3–SnO2.

820

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instrument. Samples was dispersed in acetone and spread on Formavar-coated copper grids for imaging. A PANalytical X’Pert PRO diffractometer with Cu Ka radiation ˚ tube current 30 mA at 40 kV) was (wavelength 1.5406 A, used to obtain powder X-ray diffraction (XRD) patterns of the samples in the 2y range 20–801. UV-visible diffuse reflectance (DR) spectra were recorded on a Perkin Elmer Lambda 35 spectrophotometer. A PerkinElmer LS 55 fluorescence spectrometer was used to record photoluminescence (PL) spectra at room temperature. The nanoparticles were dispersed in carbon tetrachloride and excited using light of 625 nm in wavelength. Solid-state electrochemical impedance spectra of the oxides were recorded in the frequency range from 1 MHz to 1 Hz using a CH Instruments 604C electrochemical analyzer at room temperature. The disk area was 0.5024 cm2 and the pellet thickness for HNC and SNC samples was 3.00 mm. Magnetic measurements were made at room temperature with a Lakeshore vibrating sample magnetometer at a maximum magnetic field of 10 kOe. 2.4. Photocatalytic study Photocatalytic experiments were carried out in an immersion-type photoreactor. A 150-W tungsten halogen lamp was fixed to a double-walled borosilicate immersion well of 40 mm in outer diameter with an inlet and outlet for water circulation. NaNO2 solution (2 M) was used for circulation to filter UV light of 320–400 nm [30]. After NC addition to phenol or rhodamine B solution, air was bubbled through the solution to keep the particles under suspension and in constant motion. Dissolved oxygen was measured using an Elico PE 135 dissolved oxygen analyzer. After illumination the NC was separated using a small magnet and undegraded phenol or dye was analyzed spectrophotometrically at 270 or 554 nm, respectively. The results were corrected for adsorption. Calibration curves were constructed by measuring the absorbance of phenol and rhodamine B at different concentrations. 2.5. Bactericidal study Nutrient broth culture medium at pH 7.4 was prepared by dissolving 13 g of nutrient broth (5 g of peptone, 5 g of NaCl, 2 g of yeast extract, 1 g of beef extract) in 1 l of distilled water and sterilizing it in an autoclave at 121 1C.

Fe2O3-SnO2 H

MacConkey agar plates were prepared by dissolving 55 g of MacConkey agar (20 g of peptic digest of animal tissue, 10 g of lactose, 5 g of sodium taurocholate, 0.04 g of neutral red, 20 g of agar) in 1 l of boiling distilled water and pouring the medium into Petri dishes after sterilization in an autoclave at 121 1C. Escherichia coli bacteria were inoculated in 10 ml of nutrient broth and incubated for 24 h at 37 1C. The cultured bacteria were centrifuged at 3500 rpm for 5 min, washed twice with autoclaved NaCl (0.9%) solution, and suspended in 50 ml of autoclaved 0.9% NaCl solution. For counting of E. coli colonies [as colony-forming units (cfu) ml  1], the bacterial solution was serially diluted by a factor of 107 using 0.9% NaCl solution to achieve approximately 100 colonies on a Petri dish. A 10-ml aliquot of the diluted E. coli was streaked onto a MacConkey agar plate using a loop and incubated at 37 1C for 24 h, after which viable colonies were counted. To 25 ml of E. coli solution in a 60-ml bottle, 20 mg of NC was added and the bottle was shaken continuously without any direct illumination. At specified time intervals, a finite volume of the solution was removed using a small magnet. Samples were serially diluted and colony counting was carried out as described above.

3. Results and discussion 3.1. Characterization SEM images of HNC and SNC samples are shown in Fig. 2. The image of the HNC shows its particulate nature. However, the particles are devoid of any finite shape. The SNC material appears as lumps. However, TEM images of both materials confirm that the NCs are composed of nanoparticles (Fig. 3). EDX spectra for HNC and SNC samples presented in Fig. 4 confirm the presence of iron and tin; the unlabelled peak at 2.2 keV corresponds to sputtered gold nanoparticles. The data were used to determine Fe and Sn content in the NCs. The composition determined at two different sample locations is consistent and indicates a uniform distribution of elements in the NC samples. HNC contains 25 at.% Sn and 22 at.% Fe and SNC contains 4 at.% Sn and 56 at.% Fe. The results indicate that the sonochemical method is less efficient than the hydrothermal technique at incorporating tin in the NC.

Fe2O3-SnO2 S

Fig. 2. SEM images of hydrothermally (H) and sonochemically (S) prepared nanocomposites.

C. Karunakaran et al. / Materials Science in Semiconductor Processing 16 (2013) 818–824

Fe2O3-SnO2 H

821

Fe2O3-SnO2 S

Fig. 3. TEM images of hydrothermally (H) and sonochemically (S) prepared nanocomposites.

20

[440]

[511]

SnO2 H (301)

(221)

(002)

(301)

(211)

SnO2 S

(211) (220)

[422]

[400]

[311] (200)

(101) [311] (200) [222]

[440]

{600} {600} [422] (220) (002) [511] [511]

Fe2O3-SnO2 S [400]

Fe2O3-SnO2 H

40

50

60

[440] (301)

[220]

[211] (101)

[220] (101) 30

( ) SnO2

γ-Fe2O3

[311] (200)

[220]

(101)

(110) Fig. 4. EDX spectra for hydrothermally (H) and sonochemically (S) prepared nanocomposites.

XRD patterns for HNC, SNC, the Fe2O3 precursor and hydrothermally and sonochemically prepared pristine SnO2 are presented in Fig. 5. The pattern for HNC shows the presence of rutile SnO2, g-Fe2O3 and FeO(OH). The 110, 101, 200, 220, 002, and 301 peaks at 2y values of 26.61, 34.01, 38.01, 54.81, 57.61 and 65.71 correspond to tetragonal SnO2 (JCPDS No. 88-0287) with crystal parameters a ¼0.4737 and c¼0.3186 nm. The XRD lines at 30.41, 35.71, 57.61 and 63.11 correspond to the 220, 311, 511 and 440 peaks of maghemite cubic Fe2O3 with a cell length of 0.8352 nm (JCPDS No. 39-1346). The strong XRD line at 52.11 is characteristic of the 600 peak of tetragonal FeO(OH) with cell parameters a ¼1.048 and c¼0.3023 nm (JCPDS No. 75-1594). The XRD pattern for SNC reveals the presence of rutile SnO2, g-Fe2O3, b-Fe2O3 and FeO(OH). The 110 and 101 peaks at 2y values of 26.61 and 34.11 correspond to tetragonal SnO2 (JCPDS No. 88-0287). The lines at 30.41, 33.31, 35.81, 37.41, 43.41, 53.91, 57.51 and 63.11 correspond to the 220, 211, 311, 222, 400, 422, 511 and 440 peaks of maghemite cubic Fe2O3 (JCPDS No. 39-1346). The intense line at 20.91 corresponds to the 020 peak for orthorhombic Fe2O3 (JCPDS No. 08-0093) with cell parameters a ¼1.026, b¼1.058 and c ¼0.304 nm. The weak 600 peak for tetragonal FeO(OH) at 52.21 confirms

(110) (110)

*020*

Intensity

(110)

Fe2O3-SnO2 H

[ ] γ-Fe2O3 { } FeO(OH)

β-Fe2O3

**

Fe2O3-SnO2 S

70

2θ Fig. 5. XRD patterns for hydrothermally (H) and sonochemically (S) prepared nanocomposites.

its presence. The XRD pattern for the Fe2O3 precursor is in agreement with that for maghemite cubic Fe2O3. The peaks at 30.41, 35.81, 43.41, 53.91, 57.51 and 63.11 correspond to 220, 311, 400, 422, 511 and 440 planes (JCPDS No. 39-1346). The diffraction patterns for hydrothermally and sonochemically synthesized SnO2 nanocrystals match the standard JCPDS pattern (41–1445) for tetragonal rutile SnO2 with cell lengths of a ¼0.47382 and c¼ 0.31871 nm. DR spectra for the NCs are shown in Fig. 6. The reflectance data are presented as F(R), according to the Kubelka–Munk equation F(R)¼(1–R2)/2R, where R is the measured reflectance. DR spectra for HNC and SNC and the Fe2O3 precursor are in agreement with the literature. The bandgap widely reported for SnO2 is 3.8 eV [9] and UV-A light is required for photoactivation. Pristine Fe2O3 is a narrow-bandgap semiconductor (2.2 eV) [9] and

C. Karunakaran et al. / Materials Science in Semiconductor Processing 16 (2013) 818–824

16

Fe2O3-SnO2 H Fe2O3-SnO2 S SnO2 H SnO2 S Fe2O3

K-M

12

8

4

0 200

300

400

500

600

700

800

900

Wavelength, nm Fig. 6. Diffuse reflectance spectra for hydrothermally (H) and sonochemically (S) prepared nanocomposites.

Fe2O3-SnO2 H

PL Intensity

717 nm

Fe2O3-SnO2 S

660

680

700

720

740

760

780

Wavelength, nm Fig. 7. Photoluminescence spectra for hydrothermally (H) and sonochemically (S) prepared nanocomposites.

absorbs visible light. The HNC and SNC samples absorb visible light because of the presence of Fe2O3. Fig. 7 shows PL spectra for HNC and SNC samples. The bandgaps of the SnO2 and Fe2O3 NC constituents correspond to absorption edges of 330 and 565 nm, respectively. To gain an insight into crystal defects that influence photocatalytic activity, the composites were excited with light of 625 nm. Both NCs showed red emission at 717 nm. This red emission is due to crystal defects, most probably arising from oxygen vacancies in the Fe2O3 lattice. The majority of the constituent ions in nanocrystalline semiconductors reside in the grain boundary or within a couple of layers from the boundary. A high degree of defects such as vacancies are present in the grain boundary. Defects in semiconductor nanocrystals have a strong influence on the electrical properties and photocatalytic activity of the material. Solid-state impedance spectroscopy is a relatively new and powerful technique for

probing the electrical properties of semiconductors. It can be used to investigate the dynamics of mobile and bound charges in interfacial or bulk regions of semiconductors. In polycrystalline materials, the overall resistance is a combination of intragranular or bulk crystal resistance and intergranular or grain boundary resistance. Fig. 8 shows solid-state complex impedance spectra for HNC and SNC samples. The curve for HNC is a half-semicircular arc, while SNC shows a quasi-linear relationship between Z00 and Z0 . There are no solid-state impedance spectra for Fe2O3–SnO2 NC in the literature for comparison. In general, the solid-state complex impedance spectrum of nanocrystalline SnO2 is complex; SnO2 nanocrystals obtained by chemical precipitation show a double semicircular arc at 200 and 250 1C and a single semicircular arc at 350, 400 and 450 1C [31]. However, at 300 1C the variation of Z00 with Z0 is exponential. Furthermore, the solid-state impedance of SnO2 nanocrystals is highly sensitive to humidity [32]. Magnetic hysteresis curves recorded at room temperature for HNC, SNC and the g-Fe2O3 precursor are presented in Fig. 9. The hysteresis loops all exhibit ferromagnetic behavior, with saturation magnetization (MS) of 0.3, 3.5 and 31.4 emu g  1, coercivity (HC) of 410, 200 and 140 Oe, and remanant magnetization (MR) of 0, 0 and 5.6 emu g  1 for HNC, SNC and g-Fe2O3, respectively. MS determined for g-Fe2O3 is greater than the value reported by Wang et al. (25 emu g  1) and HC is less than the value reported (166 Oe) [33]. Furthermore, MS and MR values for a-Fe2O3 prepared by precipitation (1.2 and 0.8 emu g  1 and 0.5 and 0.25 emu g  1 at 40 and 60 1C, respectively) are far less than those observed for our precursor [34]. Moreover, MR (0.115 emu g  1) and HC (431 G) values for hydrothermally synthesized a-Fe2O3 hexagonal microplatelets are much less and much greater than, respectively, those for our precursor [35]. The magnetic properties depend on crystal size. Bi et al. reported MS values of 20.9 and 76 emu g  1 for nanocrystalline and bulk g-Fe2O3, respectively [36]. The nanostructure also influences the magnetic properties; HC values of 839 and 1808 Oe were

0.3

Fe2O3-SnO2 H Fe2O3-SnO2 S

-Z Im, MΩ

822

0.2

0.1

0

0

0.1

0.2

0.3

0.4

Z Re, MΩ Fig. 8. Solid-state complex impedance spectra for hydrothermally (H) and sonochemically (S) prepared nanocomposites.

C. Karunakaran et al. / Materials Science in Semiconductor Processing 16 (2013) 818–824

40

5.0

0.2

Fe2O3-SnO2 H 0

20

- 0.2

4.5

-1

Fe2O3-SnO2 H Fe2O3-SnO2 S γ-Fe2O3

- 0.4

10

-10

-5

0

5

10

0 4 2

-10

Fe2O3-SnO2 S 0

-20

[Rhodamine B], ppm

30

Fe2O3-SnO2 H Magnetization, emu g

823

-2 -30

-4

Fe2O3-SnO2 S

4.0 3.5 3.0 2.5

-10

-5

0

5

10

2.0

-40 -10

-5

0

5

0

10

15

30

45

60

75

90

Time, min

Field, kOe Fig. 9. M–H hysteresis loops for hydrothermally (H) and sonochemically (S) prepared nanocomposites and Fe2O3.

Fig. 11. Visible-light photocatalytic degradation of rhodamine B by hydrothermally (H) and sonochemically (S) prepared nanocomposites. Conditions: catalyst loading, 0.05 g; air flow, 7.8 ml s  1; 9.3 mg l  1 dissolved O2; 1650 W m  2; 75 ml of dye solution.

5.00

Fe2O3-SnO2 H

2.0x10

11

1.5x10

11

1.0x10

11

0.5x10

11

Fe2O3-SnO2 S Survival of E.coli, CFU mL-1

[PhOH], mM

4.75

4.50

4.25

4.00

3.75 0

30

60

90

120

150

Fe2O3-SnO2 H Fe2O3-SnO2 S

180

Time, min Fig. 10. Visible-light photocatalytic degradation of phenol by hydrothermally (H) and sonochemically (S) prepared nanocomposites. Conditions: catalyst loading, 0.02 g; air flow, 7.8 ml s  1; 9.3 mg l  1 dissolved O2; 1650 W m  2; 60 ml of phenol solution.

measured for a-Fe2O3 nanospheres and so-called nanojujubes, respectively [37]. In the present study, MS values for the NC samples were much lower than for the g-Fe2O3 precursor. Incorporation of SnO2 in our nanomaterials decreased the saturation magnetization. The MS value was much lower for HNC than for SNC. The low Fe2O3 content in HNC is the reason for this result. 3.2. Photocatalytic activity The visible-light photocatalytic activity of HNC and SNC samples was evaluated by testing the degradation of phenol, an industrial pollutant generally used as a standard in testing photocatalytic efficiency. Phenol degradation profiles for HNC and SNC samples are shown in Fig. 10. The time profiles clearly show that HNC has greater photocatalytic activity than SNC. Degradation of rhodamine B, a dye widely used to test photocatalytic

0 0

5

10

15

Time, min Fig. 12. Bacterial inactivation by hydrothermally (H) and sonochemically (S) prepared nanocomposites with a composite loading of 0.02 g for 25 ml of E. coli solution, pH 7.

activity, confirms this finding, as shown in Fig. 11. A possible reason may be the NC composition. The low Sn content in SNC does not significantly enhance separation of photogenerated charge carriers. The CB edge of Fe2O3 is more cathodic than that of SnO2, which enables electrons to flow from the CB of Fe2O3 to the CB of SnO2 [9]. CB electrons in Fe2O3 and SnO2 correspond to the reduced forms of Fe3 þ (Fe2 þ ) and Sn4 þ (Sn3 þ ). Transfer of an electron from the CB of Fe2O3 to that of SnO2 involves an electron jump from Fe2 þ to Sn4 þ . This charge separation enhances the photocatalysis. 3.3. Bactericidal activity Fig. 12 show E. coli inactivation as a function of time for HNC and SNC samples in aqueous suspension in the absence of direct illumination. In the absence of NC, the

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E. coli population remained unaffected during the experimental period. E. coli in 0.9% saline was used to evaluate bactericidal activity. The cell population was determined by colony counting on MacConkey agar plates after serial dilution. The temporal profiles reveal that both NCs are bactericides. Furthermore, the bactericidal activity of HNC is greater than that of SNC. A recent in vitro evaluation of the cytotoxicity of ZnO, CuO, Al2O3, La2O3, Fe2O3, SnO2 and TiO2 nanoparticles towards E. coli showed that SnO2 and Fe2O3 are less toxic oxides (LD50 1046 and 638 mg l  1, respectively) and ZnO is the most toxic (LD50 21 mg l  1) [38]. However, our study shows that Fe2O3– SnO2 NCs exhibit significant bactericidal activity. This is the first report of Fe2O3–SnO2 bactericidal activity. The mechanism of bacterial inactivation requires further investigation. 4. Conclusions Hydrothermal preparation of SnO2–Fe2O3 nanoparticles led to incorporation of a larger amount of SnO2 in the NC compared to the sonochemical method. Both NCs absorb visible light and are ferromagnetic but they differ in their electrical properties. The saturation magnetization was much lower for HNC than for SNC. HNC is a better visible-light photocatalyst than SNC. Both NCs are bactericidal.

Acknowledgements We thank the Science and Engineering Research Board (SERB), Department of Science and Technology (DST), New Delhi for funding (SR/S1/PC-41/2011). C.K. acknowledges financial support by the Council of Scientific and Industrial Research (CSIR), New Delhi, under Scheme 21(0887)/12/EMR-II. P.G. is grateful to CSIR for the award of a Senior Research Fellowship (SRF). We also thank Dr. J. Jayabharathi, Annamalai University, for DRS and PL facilities. References [1] E. Comini, C. Baratto, G. Faglia, M Ferroni, A Vomiero, G. Sberveglieri, Prog. Mater. Sci. 54 (2009) 1–67. [2] P.V. Kamat, J. Phys. Chem. C 111 (2007) 2834–2860. [3] Z.W. Chen, Z. Jiao, M.H. Wu, C.H. Shek, C.M.L. Wu, J.K.L. Lai, Prog. Mater. Sci. 56 (2011) 901–1029. [4] M. Batzill, U. Diebold, Prog. Surf. Sci. 79 (2005) 47–154. [5] Y. Han, X. Wu, Y. Ma, L. Gong, F. Qu, H. Fan, CrystEngComm 13 (2011) 3506–3510.

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