Simple hydrothermal synthesis of metal oxides coupled nanocomposites: Structural, optical, magnetic and photocatalytic studies

Simple hydrothermal synthesis of metal oxides coupled nanocomposites: Structural, optical, magnetic and photocatalytic studies

Accepted Manuscript Title: Simple Hydrothermal Synthesis of Metal Oxides Coupled Nanocomposites: Structural, Optical, Magnetic and Photocatalytic Stud...

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Accepted Manuscript Title: Simple Hydrothermal Synthesis of Metal Oxides Coupled Nanocomposites: Structural, Optical, Magnetic and Photocatalytic Studies Author: Ayyakannu Sundaram Ganeshraja Antoni Samy Clara Kanniah Rajkumar Yanjie Wang Yu Wang Junhu Wang Krishnamoorthy Anbalagan PII: DOI: Reference:

S0169-4332(15)01465-8 http://dx.doi.org/doi:10.1016/j.apsusc.2015.06.118 APSUSC 30638

To appear in:

APSUSC

Received date: Revised date: Accepted date:

24-3-2015 15-6-2015 19-6-2015

Please cite this article as: A.S. Ganeshraja, A.S. Clara, K. Rajkumar, Y. Wang, Y. Wang, J. Wang, K. Anbalagan, Simple Hydrothermal Synthesis of Metal Oxides Coupled Nanocomposites: Structural, Optical, Magnetic and Photocatalytic Studies, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.06.118 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Simple Hydrothermal Synthesis of Metal Oxides Coupled Nanocomposites: Structural, Optical, Magnetic and Photocatalytic

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Studies

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Ayyakannu Sundaram Ganeshrajaa,b1*[email protected], Antoni Samy

[email protected], Kanniah Rajkumara, Yanjie Wangb,c1, Yu Wangd,

a

an

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Junhu Wangb1*[email protected], Krishnamoorthy Anbalagana*

Department of Chemistry, Pondicherry University, Pondicherry 605014, India

b

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Tel: +91 413 2654509

Mössbauer Effect Data Center & Laboratory of Catalysts and New Materials for

University of Chinese Academy of Sciences, Beijing 100049, China

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c

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116023, China

d

Aerospace, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian

d

Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics,

Chinese Academy of Sciences, Shanghai 201204, China

Abstract

The present article is focused on recent developments towards the preparation of room temperature ferromagnetic nanocomposites using better photocatalytic performance. These nanocomposites were successfully prepared by a simple hydrothermal method and their

molecular

formulas

were

confirmed

as

Ti0.90Sn0.10O2

(S1),

0.2CuO-

Ti0.73Sn0.06Cu0.21O2-δ (S2), and Ti0.82Sn0.09Fe0.09O2-δ (S3). The ICP, XRD, DRS, FTIR,

Page 1 of 37

Raman, XAFS, XPS, EPR, SEM-EDX, HRSEM, HRTEM, photoluminescence and vibrating sample magnetometric measurements were employed to characterize the phase structures, morphologies, optical and magnetic properties of the photocatalysts. The local 119

Sn and

57

Fe Mössbauer analysis. The

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structures of Sn4+ and Fe3+ were confirmed by

photocatalytic activities of the samples were evaluated by the degradation of methyl

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showed the best photocatalytic performance and stability.

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orange in water under visible light irradiation. Among the samples, tin doped TiO2 (S1)

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Keywords

Nanocomposites, Doping, Coupling, Room temperature ferromagnetism, Photocatalytic

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1. Introduction

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activity.

In recent years, there has been an extensive interest in developing semiconductor

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photocatalysts with high activities for environmental applications such as air purification, water disinfection, hazardous waste remediation, and water purification [1,2]. TiO2 is one of the most attracted materials in nanoscience and nanotechnology because of it has a lot of interesting properties from fundamental and practical point of view [3,4]. Although many striking results have been achieved by using TiO2 based nanomaterials in the field of photocatalytic degradation of contaminations or in the photo-electrochemical solar-cell fabrication, efforts of scientists to improve performances of this material continuously increase day by day [5,6]. However, TiO2 photocatalysts have an inherent and significant drawback: the photogenerated charge carriers (electron-hole pairs) could recombine

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rapidly [7]. Therefore, to increase the photocatalytic activity of TiO2, it is important to suppress the recombination of photogenerated charge carriers. A variety of approaches have been applied to improve the photocatalytic activity of TiO2. One interesting

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approach is to couple the TiO2 with other semiconductor material with different energy levels, which suppresses the recombination of photogenerated charge carriers in a

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semiconductor system and thus improving the efficiency of net charge transfer at the

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semiconductor/electrolyte interfaces [8]. It has been previously demonstrated that TiO2 coupling with SnO2 semiconductor can facilitate the charge separation and transportation

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because of the proper band edge alignment and higher electron mobility of SnO2 and thus boost up the photo-conversion efficiency [9]. Another route is the metal cation doping,

M

which reduces the absorption threshold of TiO2 and extends of its optical absorption

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range from UV to visible region. Yu et al. [10] and Wang et al. [11] reported that the

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incorporation of Sn-Fe cations into the TiO2 lattice resulted in a noticeable red shift of the absorption edge and a significant enhancement of photocatalytic activity.

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Another emerging field is the development of room temperature ferromagnetic

materials. Realizing ferromagnetic interactions in dilute magnetic semiconductor (DMS) and dilute magnetic metal oxides (DMO) is a central theme for the development of great scientific and next-generation spin-based information technologies [12,13]. Magnetic nanocomposites have attracted a great deal of attention, not only from a fundamental perspective but also for their potential applications due to their acquisition of novel magnetic properties of grain size dependency. Doping transparent conducting oxides, such as In2O3, ZnO, TiO2 and SnO2 with 3d transition metal ions can be the promising candidates for DMO [12,13]. Anbalagan [14] and Ganeshraja [15] recently reported the

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ferromagnetic ordering in photoinduced MxTi1-xO2-δ (M = Co, Fe and Zn) samples can be justified by the creation of some defect sites in the sample. The low concentration of transition metal impurity excludes a direct exchange process as the origin of the

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ferromagnetic ordering. At the same time the important role played by the defect concentration in determining the magnetic properties became evident, making them

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strongly dependent on the intrinsic disorder and preparation technique.

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Herein we report the preparation and characterization of room temperature ferromagnetic metal oxides coupled nanocomposites by simple hydrothermal method for

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the first time. The photocatalytic activities of Ti0.90Sn0.10O2 (S1), 0.20CuOTi0.73Sn0.06Cu0.21O2-δ (S2), and Ti0.82Sn0.09Fe0.09O2-δ (S3) have been studied using

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photocatalytic degradation of methyl orange (MO) in water under visible light irradiation.

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We believe that our research results are useful to further improve the photocatalytic

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2. Experimental

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activities for ferromagnetic materials.

2.1. Chemicals

Titanium isopropoxide, tin(IV) chloride, copper(II) nitrate, iron(III) nitrate,

nanocrystalline titanium dioxide (surface area = 200-220 m2g-1 and particle size = 25 nm), P25 (titanium dioxide, particle size = 27 nm, 99.7%) and tin(IV) oxide were purchased from Sigma-Aldrich. Ammonium hydroxide, hydrogen peroxide, oxalic acid, nitric acid, MO and all other chemicals were purchased from Himedia and SD Fine Chemicals. Water was triply distilled using alkaline KMnO4 over an all glass apparatus.

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2.2. Instrumentation Ti, Sn, Cu and Fe contents in the nanocomposites were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) on an IRIS Intrepid II XSP

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instrument (Thermo Electron Corporation). UV-vis diffuse reflectance spectra (UV-vis DRS) were recorded in absorbance mode at room temperature in the range 200-800 nm

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on a Shimadzu, UV 2450 double-beam spectrophotometer equipped with integrating

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sphere attachment (ISR- 2200) using BaSO4 as the reference. Powder X-ray diffraction (XRD) patterns were recorded in the 2θ range 20-80o with step size 0.02o using

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CuKα radiation from X-ray diffractometer (X-Pert PANalytical). Steady state fluorescence emission spectra was recorded on Spex FluoroLog-3 spectrofluorometer

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(Jobin-Yvon Inc.) using 450 W xenon lamp and equipped with a Hamamatsu R928

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photomultiplier tube. The instrument works on the principle of time-correlated single

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photon counting (TCSPC) technique. Time-resolved fluorescence decay measurements were carried out using nano-LED (λexc = 295 nm) source for excitation (repetition rate 10

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kHz). The photons were collected from the front face of the sample with TBX-4-X single-photon-counting detector. Life times were determined by fitting the data to exponential decay models using software packages of the commercially available DAS6 v6.2-Horiba Jobin Yvon. The goodness of fit was assessed by minimizing the reduced chi-squared function (χ2). Ti, Cu and Fe K-edge X-ray absorption fine structure (XAFS) spectra were taken at the BL14W1 beam line of SSRF, SINAP (Shanghai, China), with the use of a Si(1 1 1) crystal monochromator. Cu and Fe K-edge XAFS measured by fluorescence mode, while Ti K-edge XAFS measured by transmittance mode. The X-ray photoelectron spectra (XPS) were measured on ESCALAB 250Xi X-ray photoelectron

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spectrometer with monochromatic source Al Kα. Binding energy (B.E.) was calibrated using contaminated carbon as an internal standard (C 1s B.E. 284.6 eV) with a precision of 0.1 eV. JEOL Model JES FA200 instrument is the state of the art Electron Spin

transition metal ions in the nanocomposites.

119

Sn and

57

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Resonance (ESR) spectrometer used for the measurement of species that contain Fe Mössbauer spectra were

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separately collected on a Topologic 500A system at room temperature. Ca119mSnO3 and

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Rh(57Co) sources were separately moved in a constant acceleration mode. Magnetization measurements were carried out using vibrating sample magnetometric (VSM) in powder

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form on Lakeshore-7404, sample vibration frequency 82.5 Hz, dynamic range (1 × 10-7) to 103 emu. Surface morphology and chemical mapping were examined by scanning

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electron microscope-energy dispersive X-ray detector (SEM-EDX, Hitachi, S-3400N

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microscope), operating at 0.3-30 kV with Thermo SuperDry II attachment of SEM (FEI-

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Quanta FEG-200). The HRSEM (high resolution-SEM) and HRTEM (HR-transmission electron microscope) images were taken on FEI-Quanta FEG-200 and JEOL JEM-

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2000EX microscopes, respectively. Raman spectra were collected on the Jobin Yvon Horibra LABRAMR1100 micro-Raman spectrophotometer with measured at room temperature. The FTIR spectra were taken on Thermo Nicolet 6700 FTIR spectrophotometer using the KBr disk technique.

2.3. Preparation of nanocomposites Firstly, 5 mol precipitates of titanium(IV) hydroxide prepared by 5 mL titanium isopropoxide (C12H28O4Ti, Sigma-Alrich, 97%) and concentrated ammonium hydroxide (30%, 50 mL) in water were dissolved into nitric acid (6 M, 50 mL) and hydrogen

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peroxide (30%, 5 mL) to produce a reddish brown titanium(IV) nitrate solution. Next, SnCl4 solution (1 M, 1 mL) was added into the titanium(IV) nitrate solution under magnetic stirring. Then oxalic acid solution (0.6 M, 100 mL) was introduced drop wise

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within 2 h under magnetic stirring to ensure complete precipitation. The resultant mixture was transferred into a Teflon lined stainless steel autoclave, sealed and heated at 383 K

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for 4 h with the pressure maintained at 18 psi. Finally, the precipitates were filtered,

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washed with distilled water and ethanol, and dried in air at 363 K for 12 h. The dried precipitates were calcined at 673 K for 4 h in a muffle furnace to get Ti0.90Sn0.10O2 (S1).

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For the preparation of 0.2CuO-Ti0.73Sn0.06Cu0.21O2-δ (S2) and Ti0.82Sn0.09Fe0.09O2-δ (S3), only 50 mL, 1 M of copper(II) nitrate and iron(III) nitrate solution were extra-added into

M

the prepared reddish brown titanium(IV) nitrate solution, respectively. The other

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d

treatments were all the same as that of S1.

2.4. Photocatalytic activity

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The photocatalytic activity of the samples were evaluated by photodegradation of

MO using HEBER Visible Annular Type Photoreactor, model HVAR1234 (Heber Scientific, India), under visible light irradiation using 300 W Tungsten lamp as a light source. In a typical process, 80 mL of MO aqueous solution with initial concentration 2.5 × 10-6 M and 100 mg of catalyst were taken in a cylindrical-shaped glass reactor at room temperature in air and at neutral pH conditions. The flow rate of air was kept at a constant value of 80 mL min-1. The mixture solutions were kept in dark for 15 min before irradiation. Furthermore, prior to irradiation, the mixture solution were continuously aerated by a pump to provide oxygen and for complete mixing. The samples (4 mL) were

Page 7 of 37

taken out every 10 min, and were analyzed by UV-vis spectrophotometer. The photodegradation efficiencies (PDE) were calculated via the formula PDE = (A0 - At/A0)

MO solution measured at various irradiation time at 465 nm.

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3. Results and discussion

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× 100%, where A0 is the absorbance of initial MO solution and At is the absorbance of

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The nanocomposites of Ti0.90Sn0.10O2 (S1), 0.2CuO-Ti0.73Sn0.06Cu0.21O2-δ (S2), and Ti0.82Sn0.09Fe0.09O2-δ (S3) were prepared by simple hydrothermal method. The chemical

an

formulas were confirmed from ICP elemental analysis (Table S1). The co-doping and coupling processes changed not only the phase composition but also the surface structure,

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3.1. Powder XRD analysis

d

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optical property, crystalline, magnetic and photocatalytic behaviors of the samples.

Fig. 1 shows XRD patterns of S1-S3 samples calcined at 673 K. The narrow and

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intense peaks at 2θ = 25.5°, 37.9°, 48.2°, 53.8°, and 55.0° are the standard XRD peaks of anatase TiO2, while the narrow and intense peaks at 2θ = 27.6°, 36.1°, 41.2°, and 54.3° are attributed to the rutile TiO2 [16]. The intense peaks appeared at 2θ = 35.7° and 38.8° in S2 can be assigned to CuO component, which indicates CuO coupled with anatase and rutile particles in S2. The intense anatase phase combined with impurity phases of rutile (2θ = 27.26°) and brookite (2θ = 30.8°, 40.3°, 42.3°, 64.3° and 67.7°) phases in S3 [17]. Therefore, it can be concluded that Sn doped into TiO2 (S1) is to get pure anatase phase, while Cu and Fe separately co-doped into (Ti,Sn)O2 tend to get anatase-rutile and anatase-rutile-brookite mixed phases, respectively. Average particle sizes of samples are

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calculated from the broadening of the XRD peaks based on the Scherrer equation, which is approximately 8-18 nm [18].

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3.2. DRS analysis

To study the optical absorption, DRS spectra were investigated, and the results

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were shown in Fig. S1. The prepared nanocomposites exhibited a significant absorption

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of both ultraviolet and visible light, revealing that the synergistic effect of Sn4+ addition and Cu2+ or Fe3+ modification made the band gap of TiO2 narrowed. It can be ascribed to

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the charge-transfers between Sn4+ electrons and the TiO2 conduction band [19], and the incorporation of metallic ions such as Cu2+ or Fe3+ serves as a delocalization energy level

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in the energy gap, which could extend the absorbance of TiO2-based photocatalyst to the

d

visible light region [20]. The band gap energy (Eg) values estimated from Kubelka-Munk

te

curve for S1, S2 and S3 are 3.13, 2.61 and 3.05 eV, respectively. For photocatalytic activity is tightly related to the band gap energy, the decrease of the band gap energy

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after doping and coupling should be beneficial for the enhancement of photocatalytic activity.

3.3. Emission and life time studies To investigate the effects of Sn4+ and co-doped metal ions (Cu2+ and Fe3+)

incorporation in the TiO2 lattice on the native point defects and optical properties of S1S3 nanostructures, the photoluminescence (PL) spectra were measured at room temperature under an excitation wavelength of 292-337 nm (inset of Fig. S2). It was found that the steady state emission spectra contain a narrower UV emission located near

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the position of 390 nm and a widened emission range from 450 to 491 nm [21]. Indeed, some researchers reported that PL peaks around ~480 and ~468 nm were attributed to oxygen vacancy [22]. In general, the UV emission is due to the near band-edge emission

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(NBE), and the emission in the visible region could be induced by deep level emission (DLE) as a result of the presence of structural defects (e.g., oxygen vacancies and TiO2

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interstitials), which is also commonly referred to as green emission [23]. Therefore,

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emissions likely originate from surface defects, such as oxygen vacancies or titanium vacancies. In this investigation, a decrease in PL intensity at 450 and 468 nm emissions

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observed for S1-S3 nanocomposites are with respect to Sn and co-dopants. In comparison with pure TiO2, the decrease of PL intensity for S1-S3 samples indicates the inhibition of

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recombination of charge carriers [24]. To characterize this relative change in emission

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process, the plausible channel is that the dopant and co-dopant impurity induces

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geometrical distortions (contractions) of the neighbor oxygen vacancies [14,25], which are bound to Sn4+, Cu2+ or Fe3+.

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Fig. S2 displays life time decays of S1-S3 nanocomposites. This is confirmed that

tri-exponential kinetic decays. The tri-exponential decay indicates the involvement of equilibrated excited states responsible for emission. A part of the photogenerated charge carriers quickly recombines radiatively in the band edge and gives rise to the near band edge emission of TiO2. The fast component τ3 may be due to this near band edge relaxation of TiO2. Another part of the photogenerated charge carriers relax to the shallow-trap levels, which radiatively recombine with the lifetime of τ1 (Table 1). The rest of the photogenerated charge carriers could be relaxed to the deep-trap levels related to the oxygen vacancies of the nanostructure and consequently recombine radiatively

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with a much longer lifetime of τ2 [26]. The initial intensity originates from photogenerated electrons after excitation and is proportional to the number of the electrons in conduction band and/or shallowly trapped states. As shown in the Fig. S2,

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the initial life time emission intensity of mixed phases of samples S2 and S3 decreases with the relative content of anatase phase. The life time intensity of pure anatase phase

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(S1) is more than that of anatase-rutile mixed phase, which confirmed that anatase phase

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exhibits the slowest recombination rate. Therefore, S1 is expected to exhibit higher

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photocatalytic acitivity compare to mixed phases of S2 and S3.

3.4. FTIR, Raman, XAFS and XPS spectral analyses

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Fig. S3a depicts FTIR spectra of S1-S3 nanocomposites. The entire spectra

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display two characteristics broad band centered at ~3488 and ~1628 cm−1 which are

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assigned to the stretching and bending modes of vibrations of physical adsorbed water on titania surface [27] or to hydroxyl groups exist on the surface of the oxides, respectively.

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A remarkable broad band in the region 400–680 cm−1 is associated with the stretching modes of vibrations of bridged Sn-O-Sn, Ti-O-Ti and Ti-O-Sn bonds [28]. It should be emphasizing to notice two weak bands at ~1400 and ~1050 cm−1 which are assigned to the hetero Ti-O-Sn bonds. The strong sharp band centered at 669 cm-1 is assigned to the Cu–O stretching mode for S2 sample [29]. Raman spectroscopy as a sensitive technique has been employed to examine phase composition and surface homogeneity [30]. Phase structures of S1-S3 samples are confirmed by Raman spectroscopy, as shown in Fig. S3b. Sample S1 showed the characteristic Raman bands at 149, 404, 520, and 638 cm−1, which are corresponding to

Page 11 of 37

the Eg, B1g, A1g, and Eg modes of the anatase phase of TiO2, respectively [31]. For the sample S2, anatase Eg peak shifted from 149 to 152 cm-1, while CuO peaks appeared at 254, 332, 611 cm-1 and rutile peaks at 209 and 460 cm-1 [32]. This is the direct evidence

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for CuO particle coupled with anatase and rutile particles, which is in agreement with XRD. Sample S3 showed the Raman spectrum of anatase as the six characteristic bands

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with small shift at 151, 254, 402, 519 and 636 cm-1, the rutile peaks predicted at 448 and

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462 cm-1. The existence of the brookite phase is evidenced in Fig. S3b by the Raman peaks at 254, 284, 324 and 367 cm-1 [33]. S3 exhibited the characteristic Raman bands of

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anatase, rutile and brookite mixed phases. No Raman lines due to iron oxide can be observed in the S3 sample. This is because the iron content of this sample is lower

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compared with that of the other reports [34,35].

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To further study the coordination structures of Cu and Fe over (Ti,Sn)O2

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nanocomposites, we supplied the X-ray absorption fine structure (XAFS) technique [11]. The XANES spectra of the pure CuO and S2 nanocomposites are shown in Fig. 2a. Their

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spectra revealed the well-defined pre-edge peaks at 8977.0 eV, the main characteristic shoulder features at 8985.3 eV and sharp intense white lines at 8997.7 eV which are attributed to the 1s → 3d, 1s → 4pz (shakedown) and 1s → 4p (continuum) features for Cu2+, respectively [36]. This result clearly indicated that the ionic states for Cu in S2 are +2. Moreover, it could be seen that the spectral shape (Fig. 2b) and features of S2 were similar to that of CuO suggesting that comparing Cu2+ doping, more amounts of CuO species were coupled with (Ti,Sn)O2. As for S3 sample, by comparing with the reference spectra of α-Fe2O3, Fe3O4, and FeO, it was confirmed that the Fe K-edge XAFS of the S3 sample was relevant to Fe3+ doped into the lattice of (Ti,Sn)O2 (Fig. 2c). Based on the

Page 12 of 37

crystal structure, the coordination peaks shown in the Fig. 2d can be assigned. For the pure TiO2, the first peak located at a distance of 1.44 Å corresponds to the Ti-O coordination, which covers the Ti-O1 and Ti-O2, while the second peak is assigned to the

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Ti-Ti (2.55 Å) coordination. It is obvious that the magnitude of the Ti-O peak is much higher than that of the Ti-Ti peak. For α-Fe2O3, the magnitude of the Fe-O coordination

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at the distance of 1.44 Å is much lower than that of the Fe-Fe coordination. This

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difference indicates that the radial structural function (RSF) curve can be used to distinguish whether Fe3+ ions enter the (Ti,Sn)O2 lattice or exist in α-Fe2O3 phase

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[37,38]. From the Fig. 2d, the RSF curve of the S3 sample is similar to that of the TiO2, indicating that the doped Fe3+ ions substituted the octahedrally coordinated Ti4+ sites,

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which is crucial for extending the absorption of TiO2 to visible light region [39,40]. The

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Ti K-edge XANES analysis and the RSF curve for the pure TiO2 and S1-S3 samples are

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shown in Fig. S4. The three low intensity pre-edge peaks (A1, A2 and A3) for all samples are attributed to the transition from 1s → 1t1g, 1s → 2t2g and 1s → 3eg, respectively, and

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are related with the octahedral coordination of Ti4+ in TiO2 lattice [41]. The presence of tetrahedrally coordinated Ti4+ would result in a single peak between the A1 and A2 peaks. Comparing with pure TiO2, a small increase of the shoulder peak intensity at ~4971 eV for S2 is indeed observed, indicates that a small amount of tetrahedrally coordinated Ti4+ is probably formed upon high Cu co-doping. The onset of the Ti K-edge XANES spectra is defined by the first maximum of the first derivative for the main edge-jump. No shift of this peak (4987 eV) is observed upon Sn doping and Cu and Fe co-doping (Fig. S4a), indicating that the titanium ions remain in the 4+ valence state.

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The surface composition and chemical states of S1-S3 nanocomposites were further characterized by XPS (Fig. 3). The XPS peaks observed at 458.7, 464.4 and 530 eV were attributed to Ti 2p3/2, Ti 2p1/2, and O 1s (Fig. 3a,d) and the two peaks at 486.5

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and 494.9 eV corresponded to Sn 3d5/2 and Sn 3d3/2 (Fig. 3b), respectively, confirming that the bulk and main dopant forms were Ti4+ and Sn4+ [42,43]. Furthermore the broad

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peak centered at about 716 eV present in Fig. 3c corresponds to Sn 3p in the form of 4+

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oxidation state [44]. The Cu 2p XPS for S2 sample is present in Fig. 3c. The two main peaks appearing at 936.7 and 956.4 eV were attributed to Cu 2p3/2 and Cu 2p1/2. These

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peaks are higher B.E. shifts than pure CuO powder with Cu 2p3/2 = 933.7 eV and Cu 2p1/2 = 953.6 eV (2:1 area ratio) [45]. The presence of characteristic “shake up” satellites ~8

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eV above the Cu 2p peaks, further confirmed that paramagnetic Cu2+ (formally 3d9) was

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the dominant copper species in the samples, while the absence of metallic copper (932.4

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eV) and Cu2O (Cu 2p3/2 = 932.0 eV and Cu 2p1/2 = 952.0 eV) [45]. Notably, very weak Fe3+ signal was detected at 711 eV for S3 (Fig. 3c), probably because of the very low

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amount of Fe3+ ions in the surface. Quantitative XPS analyses were performed using peak areas of the Ti 2p, Sn 3d, Cu 2p, Fe 3p and O 1s signals, results for which are presented in Table 2. The XPS results proved the successful incorporation of Sn4+ in the TiO2 lattices in the form of substitutional doping, Fe3+ co-doped into (Ti,Sn)O2 lattice and more amount of CuO coupled with less amounts of Cu2+ doped (Ti,Sn)O2.

3.5. Surface morphology analysis The surface morphology and EDX spectra of the S1-S3 nanocomposites are shown Figs. S5-S7 and it reveals that the agglomeration of nanocrystals to form particles

Page 14 of 37

or grains, which are expected to contribute more grain boundary effects. Fig. 4 shows EDX spectra and elemental mapping of S3 sample and confirmed that the iron is combined with Ti, Sn and O. Table 2 gives the weights and atomic percentages of Ti, O,

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Sn, Cu and Fe elements obtained from EDX and XPS analyses. Element mappings of S1S3 provide direct evidence that Sn and co-dopant ions are uniformly dispersed among the

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TiO2 crystallites. Fig. 5 shows the HRSEM images of S1 and S2 and the uniform size of

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the particles can be observed. The sizes of most particles are below 25 nm.

Surface morphology investigations were further studied by TEM, HRTEM images

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and selected area electron diffraction (SAED). TEM observations were used to investigate the particle size, crystalline and morphology of the synthesized

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nanocomposites as shown in Fig. 6a,b. A relatively narrow size distribution of mono-

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dispersed nanoparticles with spheroid morphology was revealed by the TEM analysis.

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Mean particle diameters of 8–11 and 5–8 nm were observed for samples S1 and S3, respectively. The SAED patterns revealed a predominant characteristic ring

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corresponding to anatase (1 0 1) phases and high crystallinity. Representative HRTEM images of S1 and S3 samples are depicted in Fig. 6c,d. In both nanocomposites, TiO2 anatase nanoparticles with 10 nm in size (identified by (1 0 1) crystallographic planes of interplanar distance 0.35 nm) can be observed, as well as high crystallinity.

3.6. 119Sn and 57Fe Mössbauer, ESR and VSM analyses 119

Sn Mössbauer spectra of S1-S3 nanocomposites, recorded at room temperature

are displayed in Fig. 7a. The Mössbauer parameters are summarized in Table 3. The values of isomer shift (IS) in the range from 0.14 to 0.15 mm s-1 are unambiguously

Page 15 of 37

characteristic of a Sn4+ oxidation state in 6-fold coordination. Quadrupole splitting (QS) in the range of 1.12-1.30 mm s-1 indicated a relatively large distortion of the local tin coordination in comparison of the SnO2 (QS = 0.58 mm s-1) [11,46]. The relatively large

57

Fe Mössbauer spectrum for S3 is shown in Fig. 7b. The spectrum can be

fitted with a paramagnetic doublet.

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Fe Mössbauer parameters of QS and IS for the

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temperature

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QS indicated the existence of oxygen vacancies around Sn4+ ions [47]. The room-

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doublet are 0.32 and 0.16 mm s-1, respectively, which can be assigned to paramagnetic Fe3+ ions doped into (Ti,Sn)O2 lattice and have relatively high geometry symmetry 119

Sn and

57

Fe Mössbauer results together, it

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[48,49]. Taking into consideration of

probably indicates that the oxygen vacancies existed in S1-S3 nanocomposites are mainly

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located around the dopant of Sn4+ ions, the co-dopants of Fe3+ and Cu2+ ions are highly

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possible to exist in six-oxygen coordinated octahedral positions.

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ESR is a suitable technique to study the doping of paramagnetic transition metal ions in an oxide matrix. The ESR was used to characterize the oxidation state of co-doped

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metal ions and change of oxygen vacancies in the prepared samples. All the curves in Fig. 8 have a formant and the g value equals to 2.003. Generally, g value equaling to 2.003 is known as the characteristic value of the single-electron oxygen vacancy [50]. It can be seen clearly that the samples display a stronger signal intensity than that of TiO2, implying that whether Sn addition or co-doped metal ions (Cu2+, Fe3+) modification could create more single-electron oxygen vacancies. Fig. 8a shows ESR of S1, we can clearly see multiple paramagnetic resonance absorption signal at g = 1.99, which is that typical of Ti3+ doped oxide center [50], the signal at g = 2.003 was titanium dioxide that is free electrons in the conduction band or oxygen anion vacancies (O-) and this could lead to

Page 16 of 37

expect higher photocatalytic activity than S2 and S3 nanocomposites [51]. S2 sample, a sharp Cu2+ ESR signal was observed. One new signal appeared at g = 4.30 for S3 nanocomposites (Fig. 8b), which shows an anisotropic feature and can be generically

ip t

assigned to Fe3+ cations in a rhombic environment, attributable to the presence of oxygen vacancies in the anatase-like close environment of the cations [52]. What is more

cr

important is that there is a synergistic enhancement between Sn4+ addition and co-doped

us

metal ions modification.

The magnetic moment as a function of magnetic field are presented in Fig. 9,

an

which reveals typical weak ferromagnetic behavior at room temperature for all of the

te

d

M

samples. A well-defined hysteresis loop with relatively high external field ∼3000 G to

Ac ce p

achieve the saturation level is seen for the magnetization curve. The diamagnetic background of the TiO2 matrix was determined by measuring an undoped TiO2 sample used as a reference [53,54]. The saturation magnetic moments (Ms) of the samples were estimated from the linear extrapolation of high field data. The Ms were calculated after the paramagnetic and diamagnetic contributions were corrected. The magnetization data collected for the S1-S3 samples as present in Table 3. The room temperature ferromagnetic saturation moment is high for S2 compare to other two samples, since copper co-doped sample creates more oxygen vacancies than other metals. This was agreed with PL and Raman spectral analyses. The observed ferromagnetic ordering in S1S3 samples can be justified by the creation of some sort of defect sites in the sample. Li

Page 17 of 37

et al. showed weak ferromagnetism in metal-doped semiconductor, but rule out the effect of metallic cluster or metal oxides [55]. Accordingly, the ferromagnetism is addressed by the presence of oxygen vacancies. This is similar to observations in transition metal

ip t

doped titanium dioxide films, nanotubes, and nanoparticles, where the presences of oxygen vacancies along with transition metal ions were found to be essential for the

cr

observation of room temperature ferromagnetic ordering [56]. The observed

us

ferromagnetism of metal oxide (like ZnO, TiO2) nanoparticles could also be associated with the formation of bound magnetic polarons or defect related magnetic ordering [57].

an

A similar situation is encountered when one tries to ascertain the origin of ferromagnetic order in S1-S3 nanocomposites. Namely, doped elements with different valence and size

M

can be incorporated into host crystal lattice to create vacancies and other ionic defects,

te

nanocomposites.

d

which should be responsible for the novel physical and magnetic properties of S1-S3

Ac ce p

3.7. Photocatalytic activities of different samples The photocatalytic activities of the pure SnO2, P25, anatase TiO2, and prepared

S1-S3 nanocomposites were evaluated by the degradation of MO under visible light irradiation, and the photoefficiencies were shown in Fig. 10. Presently reported nanocomposites showed the better photocatalytic performances than pure SnO2 and P25 under visible light irradiation. A typical adsorption process was performed before photoreaction in same reaction condition in absence of visible light as present in Fig. S8. Compared with S2 nanocomposites, S1 and S3 exhibited little higher adsorption capacity, and this is beneficial for the improvement of photocatalytic activity. Repetitive scan

Page 18 of 37

spectra (Fig. S9) indicates MO dye was completely degraded at 240 min. Compared with pure anatase TiO2, all the other samples show a shoddier photocatalytic performance, except S1 which quickened the degradation of MO remarkably. Although the band gap of

ip t

SnO2 is wider than that of TiO2, its Fermi level is lower than that of TiO2. Hence, for (Ti,Sn)O2, it could be expected that the photogenerated electrons would transfer easily

cr

from TiO2 to the SnO2 underlayer, [(TiO2) eCB- → (SnO2) eCB-], and holes oppositely flow

us

into the TiO2 over layer, [(SnO2) hVB+ → (TiO2) hVB+] [58-60]. The photogenerated electron-hole pair will migrate to the surface of the catalyst and react with the species

an

adsorbed on the surface to form active hydroxyl radicals [61-65]. These hydroxyl radicals will decompose the organic dyes into CO2 and water. The electrons could be rapidly

M

transferred to molecular oxygen to form superoxide radical anion (•O2 − ), hydroxyl

d

radicals and H2O2. The whole process can be described using the equations (1) - (4).

te

Catalyst + h+ → e− (CB) + h+ (VB)

Ac ce p

e− (CB) + O2 → •O2 − + catalyst

(1) (2)

h+ (VB) + H2O → •OH + H+

(3)

•OH + MO → CO2 + H2O

(4)

Besides, a greatly quantity of oxygen vacancies on the surface could capture the photogenerated electrons, which will result in the following two opposite aspects. On one hand, it will prevent the recombination of electron-hole pairs, which is beneficial to improve the photocatalytic efficiency [66]. On the other hand, the amount of electrons reacting with O2 will decreases with the increase of oxygen vacancies, which resulted in the decrease of photocatalytic activity. Therefore, the real photocatalytic activity will not

Page 19 of 37

be improved or debased monotonously with the increase of the oxygen vacancies. Since oxygen vacancy concentration has a direct relationship with doping metal ions amount, it can be deduced that the final photocatalytic activity will not be improved or debased

ip t

monotonously with the increase of metal ion proportions [67,68]. That is why the sample

cr

S1 possessed the best photocatalytic performance in the degradation of MO.

At high concentration of 9.10 at.% Cu co-doped (Ti,Sn)O2, weak luminescence

us

and photocatalytic activity could be observed, which can be explained as follows: in low

an

copper concentrations, the copper ions are randomly distributed in the host lattice and the ion-ion distance is too far apart. But, in case of high concentrations, the distances

M

between the ions are shortened, which results in energy transfer between nearby ions, so, the concentration quenching process will be the predominant nonradiative decay process

d

at higher concentrations [69]. Zhang et al. reported the p-n heterojunction photocatalyst

te

CuO/F-TiO2 has higher photocatalytic reduction activity, but lower photocatalytic

Ac ce p

oxidation activity [70]. Similar phenomenon is applicable for high concentrated iron codoped (Ti,Sn)O2 nanocomposites. Therefore the photocatalytic activities were decreased for S2 and S3 samples compare to pure anatase TiO2 and S1 nanocomposites. But pure SnO2 is more stable photochemical properties, but the wide band gap (3.62 eV), make it impossible to absorb visible light. Therefore obtained S1-S3 nanocomposites showed higher photocatalytic activity than pure SnO2 nanoparticles under visible light irradiation. Variations of the Ms and photocatalytic efficiencies of S1-S3 are plotted as a function of the surface oxygen vacancies in percentage (SOV %) calculated from XPS analysis in inset Fig. 9. The high Ms value with decreasing oxygen content supports the role of oxygen vacancies in producing ferromagnetism in S2 nanocomposites. However,

Page 20 of 37

photocatalytic efficiency shows an opposite behavior with increasing surface oxygen vacancies [71]. This may be due to the removal of surface redox species when Cu2+ was co-doped into (Ti,Sn)O2. Thus, the direct correlation observed between photocatalytic

ip t

activity and magnetism might suggest a major role of surface oxygen vacancies and charge carriers [72]. It is also important to note that our experiment points to the oxygen

cr

vacancies as the most likely origin of the decrease in photocatalytic activity and

us

increasing magnetic moment.

an

4. Conclusions

We have synthesized metal oxides doped and coupled nanocomposites by a

M

simple hydrothermal method calcined at 673 K. Optical properties of the nanocomposites

d

were performed by DRS and PL measurements. SEM-EDX, HRSEM and HRTEM

te

results characterize the elemental composition and the formation of uniform metal oxides coupled nanocomposites with typical diameter 15-25 nm. It was also observed that, the

Ac ce p

shape and uniformity of the nanomaterials changes with different co-doped metal ions. The ICP, XRD, FTIR, Raman, XAFS and XPS studies confirmed the formation of doped, co-doped and metal oxide coupled nanocomposites such as Ti0.90Sn0.10O2 (S1), 0.2CuOTi0.73Sn0.06Cu0.21O2-δ (S2), and Ti0.82Sn0.09Fe0.10O2-δ (S3). In addition, PL and life time measurements observed oxygen vacancies, structural defects and excitation life time in the prepared nanocomposites. This was also confirmed from ESR and Mössbauer analyses. The room temperature VSM measurement shows a weak ferromagnetic behavior. The Ms is higher for S2 sample compare to other two samples. The obtained nanocomposites were used as photocatalyst for degradation of MO dye in aqueous

Page 21 of 37

solution under visible light irradiation. The pure anatase phase of S1 nanocomposites had better photocatalytic performance than other two nanocomposites. The photocatalytic activity and magnetism studies of prepared nanocomposites might suggest a major role of

ip t

surface oxygen vacancies and charge carriers. While the high Ms value with decreasing oxygen content supports the role of oxygen vacancies in producing ferromagnetism and

cr

inhibits photocatalytic activity for S2 nanocomposites. While the reported room

us

temperature ferromagnetic nanocomposites using better photocatalytic performance than

an

diamagnetic SnO2 and P25 under visible light irradiation.

Acknowledgements

M

Prof. KA is thankful to CSIR, New Delhi (Lr: No. 01 (2570)/12/EMR-II/3.4.2012) for

d

financial support through a major research project. Prof. J. Wang is grateful to National

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te

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70. J. Zhang, Y. Yang, W. Liu, Preparation, characterization, and activity evaluation of

an

CuO/F-TiO2 photocatalyst, Int. J. Photoenergy, 2012 (2012) 139739-1–9. 71. L. Jing, B. Xin, F. Yuan, L. Xue, B. Wang, H. Fu, Effects of surface oxygen

M

vacancies on photophysical and photochemical processes of Zn-doped TiO2 nanoparticles

d

and their relationships, J. Phys. Chem. B 110 (2006) 17860–17865.

te

72. K. Dodge, J. Chess, J. Eixenberger, G. Alanko, C.B. Hanna, A. Punnoose, Role of oxygen defects on the magnetic properties of ultra-small Sn1−xFexO2 nanoparticles, J.

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Appl. Phys. 113 (2013) 17B504-1–3.

Fig. 1 Powder XRD patterns of S1-S3 nanocomposites. Fig. 2 (a) Cu K-edge XANES, (b) Cu K-edge XAFS, RSF curve, (c) Fe K-edge XANES and XAFS, RSF curve of reference samples and prepared nanocomposites at room temperature. Fig. 3 (a) Ti 2p (b) Sn 3d (c) Cu 2p, Sn 3p and Fe 2p and (d) O 1s XPS spectra for S1-S3 nanocomposites.

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Fig. 4 EDX spectrum and element mapping for S3 nanocomposites. Fig. 5 HRSEM images of (a) S1 and (b) S2 nanocomposites. Fig. 6 TEM images (a) and (b) and HRTEM images (c) and (d) of S1 and S3

shows ~0.34 and ~0.26 nm interplanar distances, respectively. 119

Sn Mössbauer spectra for S1-S3 nanocomposites and (b)

Fe Mössbauer

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spectra for S3 nanocomposites at room temperature.

57

cr

Fig. 7 (a)

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nanocomposites. Inset figure shows corresponding SAED patterns. Red and blue rings

Fig. 8 ESR signals for (a) S1, (b) S2 and (c) S3 nanocomposites at room temperature.

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Fig. 9 Magnetic hysteresis loops of S1-S3 nanocomposites at room temperature. The inset shows the variation of Ms and PDE (%) of S1-S3 nanocomposites as a function of the

M

SOV (%) calculated from XPS analysis.

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Fig. 10 MO photodegradation efficiency under visible light irradiation of SnO2, P25,

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anatase TiO2, and prepared S1-S3 nanocomposites at various irradiation time at room

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temperature.

Table 1 Life time data for S1-S3 nanocomposites. life time (ns)

sample

τ1

τ2

τ3

χ2

S1

2.88

37.37

0.34

1.11

S2

1.87

31.68

0.08

1.04

S3

3.12

42.84

0.40

1.15

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Excitation at 510 nm, R928 Detector HV, 950 V, coaxial delay = 0 ns, TAC range = 50 ns, room temperature preset + 0 sec, peak preset = 10,000 counts, repetition rate = 1 MHz, sync delay = 90

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te

d

M

an

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cr

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ns at room temperature.

Table 2 Surface elemental analysis data by EDX and XPS spectra for S1-S3 nanocomposites. EDX, atomic %

samples

Ti

S1

Sn

XPS, atomic %

SOV

M

O

Ti2p

Sn3d

Cu2p or Fe3p

O

(%)

28.49 3.44

-

68.07

23.27

8.10

-

68.63

-9.5

S2

14.27 1.90

9.10 74.73

11.31

4.49

19.69

64.52

0.05

S3

30.81 2.68

0.40 66.11

17.95

13.75

1.64

66.66

9.0

Page 34 of 37

Based on the XPS fitting results, the surface oxygen vacancies (SOV) of S1-S3 samples can be calculated by the following equation: SOV = (2 − x)/2 × 100 %, where x is the molar ratio of OL

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M

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cr

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and (Ti + Sn + M) (where M = Cu and Fe for S2 and S3, respectively).

Table 3 VSM and 119Sn Mössbauer parameters of S1-S3 nanocomposites at room temperature. χm × 10-2

Hc

Mr (memu

IS (mm

QS (mm

LW (mm

g-1)

(emu g-1 G-1)

(G)

g-1)

s-1)

s-1)

s-1)

S1

129.56

2.12

57.12

6.23

0.14

1.30

2.53

S2

146.04

1.26

51.34

7.39

0.14

1.12

2.47

S3

93.78

1.14

57.77

4.58

0.15

1.28

2.53

sample Ms (memu

Page 35 of 37

magnetic susceptibility (χm), ferromagnetic saturation moment (Ms), coercivity (Hc), remanences

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(Mr), isomer shift (IS), quadrupole splitting (QS), line width (LW).

For the first time, ferromagnetic metal oxides coupled nanocomposites were prepared by

In situ XAFS and 119Sn and 57Fe Mössbauer spectral analyses were used to characterize the local structures of nanocomposites.

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The photocatalytic activity and magnetism studies of nanocomposites suggest a major role

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of surface oxygen vacancies and charge carriers.

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M



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simple hydrothermal method at the room temperature.

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cr

Highlights

Page 36 of 37

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M

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