Anomalous effects of lattice strain and Ti interstitials on room

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C: Physical Processes in Nanomaterials and Nanostructures 3+

Anomalous Effects of Lattice Strain and Ti Interstitials on RoomTemperature Magnetic Ordering in Defect Engineered Nano-TiO 2

Jayaseelan Dhakshinamoorthy, Arun K. Prasad, Sandip Dhara, and Biji Pullithadathil J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09851 • Publication Date (Web): 05 Nov 2018 Downloaded from http://pubs.acs.org on November 6, 2018

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The Journal of Physical Chemistry

Anomalous effects of lattice strain and Ti3+ interstitials on roomtemperature magnetic ordering in defect engineered nano-TiO2 Jayaseelan Dhakshinamoorthy1,2, Arun K. Prasad3, Sandip Dhara3 and Biji Pullithadathil1, 2* 1

Nanosensor Laboratory, PSG Institute of Advanced Studies, Coimbatore-641 004, INDIA. 2

3

Department of Chemistry, PSG College of Technology, Coimbatore-641004, INDIA.

Surface and Nanoscience Division, Materials Science Group, Indira Gandhi Centre for Atomic Research, Homi Bhabha National Institute, Kalpakkam-603 102, INDIA.

*Corresponding Author E-mail: [email protected]. Phone: 0091-422 4344000, (Ext. 4193). Fax: 0091- 422-2573833.

ABSTRACT Defect engineering in n-type undoped metal oxides is a great challenge compared to the often studied surface oxygen vacancies. The present investigation unravels new insights towards defect chemistry and defect engineering in anatase TiO2 nanoparticles. It is demonstrated that using rapid cooling process, high concentration of Ti3+ interstitials and lattice oxygen vacancies can be easily introduced in undoped metal oxides. The structural disorders in anatase TiO2 nanoparticles synthesized under two different argon annealing processes have been comprehensively investigated using spectroscopy and electron microscopic analysis. Though excess of interstitial Ti3+ ions with one unpaired 3d electron in quenched TiO2 introduce local magnetic moments, they could be anti-ferromagnetically coupled via lattice Ti4+ ions, which limit the overall magnetic moment of the quenched materials. Lattice contraction also was found to enhance the ferromagnetic coupling between the defect complexes (Ti3+-F+ center) which helped to reach the saturation moment at lower applied magnetic fields compared to pristine TiO2 nanoparticles.

KEYWORDS: Room temperature ferromagnetism; rapid cooling; surface defects; hydrostatic compressive strain, in-situ carbon layer formation.

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1. INTRODUCTION Diluted magnetic semiconductors (DMSs) are promising materials for spintronics applications, as they can spontaneously generate polarized spins and electrically inject into the non-magnetic semiconductors.1 However, most of the compound semiconductors have low Curie temperature and are difficult to synthesize and dope, which limit their practical applications. Even the most thoroughly studied ferromagnetic Mn doped GaAs loses its magnetization at temperatures above 170K.2 A major breakthrough happened after theoretical prediction of ferromagnetic ordering in wide band-gap Mn doped GaN and ZnO by Dietl and co-workers.3 Later, many groups attempted to realize robust room temperature ferromagnetism (RTFM) in doped wide band-gap semiconducting oxides, so called “diluted magnetic oxide semiconductor” (DMO), such as ZnO, TiO2, SnO2, MgO, CeO2, Al2O3, BaTiO3, etc.4,5 The RTFM in such DMOs became more debatable after finding the room temperature ferromagnetic ordering in undoped HfO2 thin films, which introduced a new concept, ‘d₀ ferromagnetism’.6,7 Triggered by this unexpected result, many groups made considerable theoretical and experimental efforts to understand the possible origin of RTFM in undoped DMOs.8 Especially, undoped anatase and rutile TiO2 nanostructures exhibited room temperature ferromagnetic ordering in presence of defect complex (Ti3+-oxygen vacancy (VO)).9,10 These findings strongly suggest that ferromagnetism is an intrinsic property, originated from the native defects of TiO2 nanostructures. Since, the defects are key ingredients for long range ferromagnetic ordering in undoped metal oxide semiconductors; it can be systematically introduced into the host by rapidly cooling the materials from high reaction temperatures using different quenching media.11-13 This rapid cooling or quenching method allows controlling the type and concentration of defects by adjusting the microstructure of the material. The defect concentration and free electron density of the semiconductor are determined by the initial annealing temperature of the sample used for rapid cooling.14 In TiO2, apart from other structural disorders (such as, VOs and Ti interstitials) lattice parameters have a significant effect on their structural characteristics related to electrical, magnetic, chemical and optical properties at the nanoscale.15-18 As the effect of lattice strain on the crystal properties is noticeable, it is identified as the key factor for ferromagnetic ordering.19 So far, limited number of reports discussed about lattice contraction with respect to their particle size and its effects on electrical and optical properties of anatase TiO2.20, 21 Also, various native defects such as, surface defects, strain and point defects have been reported, which causes RTFM


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ordering in undoped DMOs. However, an intensive investigation of structure-property correlation between magnetic properties and strain in anatase TiO2 nanoparticles does not exist, leaving it as an open question. To the best of our knowledge, there is no report available to elucidate the effect of strain on the magnetic ordering in undoped TiO2 nanoparticles. With this investigation, we demonstrate a systematic method based on liquid argon quenching under a controlled atmosphere to implant Ti3+ self-dopants in TiO2 for studying its influence on magnetic property of the system. TiO2 nanoparticles were directly quenched at 87K after pre-annealing at 673K. Quantitative analyses of the structural disorders were performed by comprehensive spectroscopic and microscopic analyses. The formation of Ti3+ interstitials was found to be related to the lattice contraction. The TiO2 nanoparticles without carbon shell produces more amount of surface oxygen defects rather than interstitial Ti3+ ions during quenching process. Interestingly, the formation of an amorphous carbon shell over TiO2 nanocrystals created a hydrostatic compressive strain leading to the formation of excess interstitial Ti3+ ions, which significantly influenced the magnetic ordering and optical properties of the system. 2. EXPERIMENTAL SECTION All the chemicals used for the synthesis of TiO2 nanoparticles were of analytical grade and used without any further purification. Titanium isopropoxide (TIP) (97%, Sigma Aldrich), 2propanol (99%, Merck) and Acetic acid (99.5%, Merck) were used as precursors for the synthesis of anatase TiO2 nanocrystals. Milli-Q ultra-pure water with a resistivity about 18.2 MΩ·cm was used for all experiments. The glasswares were dried before use and Teflon spatulas and tweezers were used during experiments in order to avoid magnetic impurities. 2.1. Synthesis of anatase TiO2 nanoparticles For the synthesis of TiO2 nanoparticles, TIP (22 ml) and 2-propanol (25 ml) were mixed in a dry round bottom flask and stirred for 30 min at room temperature. Simultaneously, 27 ml of acetic acid was added in 17.5 ml of water separately and stirred for 30 min. Further, the second solution was added drop by drop to the first solution until it formed an off-white opaque monolith gel. The final product was aged at 423K for 24 hours and ground well into a white powder. Finally, the material was annealed under air atmosphere at 673K for 2 hrs and named as pristine TiO2.



2.2. Synthesis of quenched anatase TiO2 nanoparticles The aged TiO2 nanoparticles were further subjected to a controlled atmosphere quenching process under two different conditions. In reaction (i), disordered TiO2 nanoparticles were prepared by purging pre-heated Ar gas (673K) at a flow rate of 100 sccm over TiO2 nanopowder, which was kept inside a quartz tube at 673K in an in-house built controlled atmosphere tilting furnace. The material was subjected to rapid quenching in liquid Ar (87K) under inert atmosphere after 2 hrs of annealing in order to create the excess defects in anatase structure. The material is abbreviated as HA-TiO2. Similarly, during the reaction (ii), the amorphous carbon coated TiO2 material was prepared by purging Ar gas (room temperature (300K)) at a flow rate of 100 sccm over TiO2 nanopowder at 673K. After 2 hrs of annealing, the sample was suddenly quenched into liquid Ar (87K). The material is abbreviated as RTA-TiO2. 2.3. Characterization Techniques The materials were analyzed using various spectroscopic, microscopic and magnetic measurement techniques. X-ray diffraction (XRD) pattern was recorded using Malvern PANalytical diffractometer (XPERT3, UK) with Cu-Kα radiation (λ=1.5406 Å) at a scanning rate of 2o/min. Differential scanning calorimetry (DSC) analysis was performed with 20K/min heating rate using STA 449 F3 (NETZSCH, Germany) thermal analyzer and mass used for the measurements was about 20 mg. A Quantum Designed Magnetic Property Measurement SystemSuperconducting Quantum Interference Device (MPMS-SQUID) (MPMS®3, USA) was used to investigate the magnetic properties at 300 K in the field strength of ±3T. The high-magnification microstructures of the samples were studied by transmission electron microscopy (TEM), highresolution TEM (HRTEM) (JEOL-JEM 2010 operated at 200 kV). Specimens for HRTEM investigations were prepared by dispersing powder particles in ethanol and drop casting on a carbon and holey carbon-coated copper grids. The steady state micro-photoluminescence (PL) spectrum and Raman spectra were acquired at room temperature using an in-Via Renishaw (UK) with He-Cd and Ar laser as the excitation light source at 325 nm and 514.5 nm, respectively, with the help of a spectrometer (used gratings, 150 grooves/mm for PL and 1800 grooves/mm for Raman) in the backscattering configuration. X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra, DLD system) was performed using an Al-Kα radiation dual anode source (1486.6 eV) in an ultra-high vacuum environment. Electron paramagnetic resonance (EPR)


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measurements were taken with a JEOL (JES FA200) instrument by applying an X-band (9.45 GHz, 1 mW) microwave with the sweeping magnetic field at room temperature.

SCHEME 1. Possible reaction pathway leading to various disordered anatase TiO2 nanoparticles via oxolation reactions.

3. RESULTS AND DISCUSSION The nanocrystalline anatase TiO2 was synthesized and stabilized via hydrolytic condensation process of Titanium isopropoxide (TIP) (Ti(OiPr)4). During this reaction, TIP reacts with acetic acid turning solution into less reactive species Ti(OCOCH3)(OiPr)2, in which the isopropoxy groups remain bonded with the octahedral Ti4+ions and which can be easily hydrolyzed by addition of water. The overall reaction is depicted in Scheme 1. In the initial stage, condensation takes place through oxolation and alkoxylation reactions due to -OH, -OiPr, and mono/bi-dentate acetate ligands. The attachment of a third octahedron during condensation step determines the final crystal structure, which depends on whether the octahedron locates in linear or twisted chain forms with the dimer. During this reaction, the relative position in the dimer of two reacting ligands is properly oriented for condensation, which enables the possibility for the third octahedron to share an edge in a twisted chain forming anatase structure rather than rutile or brookite phase.22 The maximum possible number of chelating acetates per Ti4+ ion is limited to



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two under high acetic acid concentration. The bridging (bi-dentate) acetate (CH3COO‾) with two oxygen molecules preferably bond across two fivefold coordinated Ti atoms with C–C bond perpendicular to the surface. In general the bridging acetate groups form titanyl organic compounds like, Ti2O2(OOCCH3)2(OiPr)4 and Ti(OOCCH3)(OiPr)2 as intermediate products.22 Onishi et al. directly observed the decomposition pathway of acetic acid using scanning tunneling microscopy.23 They found that the acetate ions were decomposed via net unimolecular dehydration and releases ketene as main product. During the reaction (i) in Scheme 1, pre-heated argon gas leads to the formation of ketene and ionosorbed H+ species. Also, the adsorbed H+ species interact with surface lattice oxygen and bi-dentate acetates species and release hydrogen, water and acetic acid vapors as byproducts respectively as mentioned in Eq. (1-3)),24 (OOCCH3 )− 𝑎𝑑

→

∆>600𝐾 Ar@673K

+ 2H𝑎𝑑 + O(𝐿𝑎𝑡𝑡𝑖𝑐𝑒) → + (OOCCH3 )− 𝑎𝑑 + H𝑎𝑑 →

+ CH2 CO ↑ + H𝑎𝑑 + O(𝐿𝑎𝑡𝑡𝑖𝑐𝑒) ∆ ∆

(1)

H2 O ↑, H2

(2)

CH3 COOH ↑

(3)

The more surface oxygen vacancies could be created in HA-TiO2 compared to RTA-TiO2 and pristine TiO2 nanoparticles, because of the surface lattice oxygen were utilized for water vapor formation, which was confirmed from the XPS analysis. Whereas during the reaction (ii) in Scheme 1, room temperature argon suppresses the formation of ketene and release the main decomposition products as carbon, carbon monoxide, methane and H+ species (Eq. 4 and 5),24 (OOCCH3 )− 𝑎𝑑

→

∆>600𝐾 Ar@300K

CH3 𝑎𝑑 →

∆

CH3 𝑎𝑑 + CO ↑ + O(𝐿𝑎𝑡𝑡𝑖𝑐𝑒)

(4)

+ CH4 ↑, C𝑎𝑑 , H𝑎𝑑

(5)

These adsorbed carbon atoms form an amorphous shell layer over the TiO2 surface which causes hydrostatic compressive strain in the RTA-TiO2 nanoparticles.

3.1. Structural analysis The crystalline structure of the TiO2 nanoparticles was confirmed by powder XRD analysis (Figure. S1). It can be found that 10 prominent diffraction peaks appeared between 2θ value of 20 to 70o, which corresponds to (hkl) planes (101), (103, (004), (112), (200), (105), (211), (213), (204) and (220) related to anatase TiO2 (Reference pattern No. 98-009-7051). No diffraction peaks were related to secondary phases such as, rutile and brookite were observed. During the


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rapid cooling process, anatase structure was retained, but the intensities of the diffraction peaks in the quenched anatase TiO2 nanoparticles were found to be reduced, implying a decrease in crystallinity. In general, the anatase TiO2 becomes thermodynamically more stable than rutile phase for particles having size below 14 nm.25 It is clear that the phase stability also depends on the impurities, grain size, reaction atmosphere, synthetic process and conditions.26,

27

The

crystallite sizes were calculated as 13 nm, 10 nm and 10 nm for pristine, RTA and HA-TiO2 nanoparticles, respectively, from XRD line width analysis using Rietveld Refinement (Figure. S1a-c). Crystallographic reference data (Reference pattern No. 98-009-7051) were used for Rietveld refinement analysis of the anatase TiO2, (a=b= 3.7850 Å, c= 9.5120 Å). For modeling the profile of the Bragg’s reflection, pseudo-Voigt profile function was used to give the best fit between the experimental and theoretical data. The structure refinement converged rapidly until it reached the goodness of the fit (GoF) in the range 1 to 2 and their results are tabulated in Table S1. Refinement analysis revealed a decrease in the ‘c’ lattice parameter under compressive strain, which suggests shortening of the d-spacing between the Ti―Ti atoms along the direction. The exothermic area of the anatase-to-rutile transition (ART) was determined by differential scanning calorimetry (DSC) (see Figure. S2). The analysis was performed with a constant heating rate of 20K/min in order to understand the role of crystallite size, lattice strain and disorder on the phase stability of TiO2 crystal structure. From PL analysis, it was confirmed that native defects such as, Ti interstitials and oxygen vacancies were created during the rapid cooling process.28,29 The stored internal energy from these disorders was released along with energy absorbed while forming the rutile phase. Therefore, native defects (i.e. entropy) lower the ART temperature. Total energy released during the transition is more pronounced in HA-TiO2 compared to pristine TiO2. Even though RTA-TiO2 exhibited more strain and defect concentration than HA-TiO2, it releases minimal energy than HA-TiO2 and pristine TiO2 nanoparticles. This contradictory observation might be originated from the presence of amorphous carbon layer over the anatase TiO2 nanoparticles. Exothermic energy release during ART of TiO2 core may be absorbed and utilized for the lattice vibration of carbon shell layer and released minimal energy. Consequently, this amorphous carbon layer reduces the effect of the rapid cooling process. Jeong et al. reported similar observation in Si@C core-shell nanostructures and found that carbon shell suppress the intensive exothermic enthalpy of Si core.30



FIGURE 1. Representative (a, d, i) TEM images with insets as the corresponding size distribution; (b, e, j) HRTEM lattice fringes with insets representing the equivalent FFT patterns and (c, f, k) IFFT images of the marked area of the pristine-TiO2, RTA-TiO2 and HA-TiO2 nanoparticles, respectively, (g) One dimensional profile calculated across the blue and red lines in the image (f) and (h) corresponding simulated crystal structure of RTA-TiO2.


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The local microstructures of the nanomaterials were examined with FFT and HRTEM analysis without aberration correction.31 The Fourier transform was performed on a real-space image, masking off the desired frequencies in frequency space to remove unwanted noise. Finally, an inverse Fourier transform (IFFT) was performed on the Fourier masked image in order to get the real image with sub-ångström resolution. HRTEM analysis of pristine and HATiO2 nanoparticles were carried out by using conventional TEM grids, whereas holey-carbon TEM grids were used in order to analyze the amorphous-carbon shell layer formed on TiO2 (i.e. RTA-TiO2) nanoparticles and the corresponding TEM and HRTEM images are shown in Figure. 1 (a-k). The morphology of the TiO2 nanoparticles consisted of highly truncated bi-pyramids and few rod-like structures having average dimension in the range of 9 to 13 nm (Figure. 1(a, d, i)). The crystallite size calculated from the XRD line width analysis was found to be closely matched with the particle size measured from the TEM histogram analysis. The two-dimensional FFTs of the HRTEM images are depicted in the insets of Figure. 1 (b, e, j) which indicates the singlecrystalline anatase TiO2. The IFFT images of nanoparticles show their lattices oriented in different planes as depicted in Figure. 1(c, f, k). The structural models were developed using VESTA software, where the lattice parameters were estimated from the Rietveld refinement analyses of corresponding X-ray diffraction data which matched with the IFFT images. The blue spheres denote Ti atoms at octahedral position and red spheres denote oxygen atoms (Figure. 1(c, f, k)) explicitly show the arrangement of atoms at various atomic planes. The lattice plane and cross-sectional view of the simulated anatase TiO2 structure with appropriate orientation are shown in Figure. 1(c, f, k). The lattice fringe spacing (i.e. d-spacing) of 2.3 Å correspond to planes of the pristine TiO2 nanoparticles as depicted in Figure. 1b and 1c. The lattice fringe spacing of 2.3, 1.8 and 3.5 Å correspond to the , and planes, respectively (Figure. 1e and 1f). Moreover, the angle labeled in the two-dimensional FFT and IFFT images was about 68.5o, which is in good agreement with the theoretical value of the angle between the and atomic planes. Similarly, the angle labeled in the FFT image (43.6o), corresponds to the angle between the and planes of RTA-TiO2 nanoparticles (Figure. 1e inset). It can be observed that two sets of crystal planes are oriented an inclined direction to each other with an equal lattice fringe spacing of 3.5 Å corresponding to the and facets. The angle observed in the Figure. 1j (inset) was 81.8o, which closely matched with the theoretical value 82.1o of the HA-TiO2 nanoparticles.32 Slight deviation in the



plane angles from the theoretical values could be originally attributed from the local distortion in bond length of Ti―O caused by lattice expansion or contraction in the crystal structure. The lattice expansion or contraction of nanocrystalline TiO2 can be often measured by means of Rietveld refinement analysis.33 However, a few evidences also have been reported by HRTEM analysis.34 The IFFT image of the nanocrystalline TiO2 was matched to simulated crystal structure by rotating and aligning the c-axis of simulated structure parallel to real structure (Figure. 1f). For better visualization, the atomic structure was obtained by averaging over 6 unit cells. The bright visible spots in the image (Figure. 1f) are formed by two neighboring ‘heavier’ Ti and two ‘lighter’ O atomic columns that are not completely resolved in strained TiO2. The ‘zig-zag’ like structure represent the forbidden atomic planes with dspacing of ~2.16 Å, was confirmed by the partially overlapped simulated tetragonal structure of anatase TiO2 (Figure. 1f). The IFFT image of a distorted RTA-TiO2 nanocrystal shows reduction in the c-axis length by 7.78 % and 8.2 % in a-axis with respect to the values found for its bulk material counterpart having a=b= 3.78 Å and c= 9.51 Å. This structural abnormality was prudently correlated with the weak diffraction spots like, , which are forbidden and can be detected by FFT processing (Figure. 1f). The enhanced visibility of forbidden planes can be probably due to unusual structural distortion in Ti or O occupancy or lattice strain caused by amorphous carbon shell formed over the anatase TiO2 nanocrystals. This strain induced lattice distortion variation depends on the average thickness of the amorphous carbon layer ranging from ~1 to 5 nm. The blurring effect is probably due to thermal displacements and measurement limitations which prevented resolution of the atomic columns in the bright spots located in direction as well as the neighboring Ti atoms.34 The simulated strained unit cell is depicted in Figure. 1h, which matched well with the IFFT image of the RTA-TiO2 crystal structure and the variations in the spatial distance of atomic columns are portrayed in Figure. 1g, using one dimensional profile analysis. The corresponding values across the blue and red line are marked in Figure. 1f. The analysis confirms the lattice contractions in and direction corresponding to the lattice parameters c and a, respectively. The IFFT of HRTEM images and their simulated crystal structure confirmed the lattice contraction of the defective TiO2 nanoparticles, which helps to understand the atomic-scale origins of change in physical and chemical properties.


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3.2. Raman spectroscopic analysis

FIGURE 2. Raman spectra of (black line) pristine TiO2, (red line) RTA-TiO2 and (blue line) HA-TiO2 nanoparticles. [Insets: (i) pseudo-octahedral position of Ti interstitial, (ii) E1g, (iii) B1g and (iv) A1g vibrational mode].

Raman spectroscopic analysis was carried out in order to understand the structural strain in the anatase TiO2 nanoparticles as depicted in Figure. 2. Five major Raman active peaks were observed around 148, 199, 399, 519 and 640 cm-1 in all materials which correspond to the characteristic Raman modes of anatase TiO2 structure. The anatase phase of TiO2 has a tetragonal structure corresponding to I41/amd space group and can be described as a stacked edge sharing octahedral TiO6 (i.e. Ti4+ cation coordinated with six O2− anions). The 18 zone-center modes are classified as Γvib=1A1g+1A2u+2B1g+1B2u+3Eg +3Eu, in which the terms with subscript u and g are infrared (IR) and Raman active modes, respectively. The B2u mode is silent in both Raman and IR. The minor frequency shifts (i.e. blue shift) due to lattice contraction and the spatial confinement of the phonons has the same sign.18 The quenched TiO2 nanoparticles exhibited comparable size distribution, but lattice strains are independent of their particle size and hence we can neglect the contribution from size effect. In anatase TiO2, the direction (i.e. c-axis) is softer than and which corresponds to Eg and Eu modes, respectively.17 The low frequency of the Eg mode (ʋ6~148 cm-1) implies the interaction between equatorial oxygen (Oeq―Ti―Oeq) and its bending vibration 35 A and B modes are associated with direction and stretch longitudinally as Ti―O eq bond as depicted in Figure. 2 insets (iii and iv). A1g (ʋ2) and B1g (ʋ4) modes are predicted to be the pure O―O vibration and pure Ti―Ti vibration, respectively.36 The presence of amorphous carbon



shell on the TiO2 nanoparticles creates a hydrostatic compressive strain on the crystal structure, which increase the apical bond length of Ti―Oap and decrease the equatorial Ti―Oeq bond length in TiO6 octahedra. In order to overcome the tension experienced by Oeq―Ti―Oeq in and axis, the bond length of (Ti―Oap) was relaxed in direction. The bending vibration frequency of the intense Eg (ʋ6) peak was found to be increased with lattice contraction due to reduced bond length of Ti―Oeq. The difference between B1g (ʋ3), Eg (ʋ6) and other active Raman modes is that the principal vectors fall in the heavier centers (Ti site). Presence of Ti4+ i interstitials causes local bond length distortion by attracting an apical O atom near the pseudo-octahedral site (Figure. 2 inset (i)) and also the neighboring Ti atoms slightly moved outward, due to the electrostatic repulsion between positively charged Ti4+ i ions and finally forms quasi-pyramidal Ti4+ i coordination. Thus, the local distortion in the Ti lattice site severely affects the B1g (ʋ3), Eg (ʋ6) vibrational modes (Figure. 2 insets (ii and iii)). Further, 37 3+ Ti4+ i species spontaneously donate electrons to the neighboring Ti atoms, forming Tii ions.

Since, the E-mode is associated with direction; blue shift in Eg vibrational modes occurs due to the strain induced by the quasi-pyramidal Ti3+ i ions. Also, highly strained RTA-TiO2 nanoparticles showed asymmetrical broadening in A1g (ʋ2) vibrational mode (Figure. 2 inset (iv)) due to the presence of more Ti3+ i species (as highlighted in green in Figure. 2), which affects the vibration of surrounding lattice O atoms in direction rather than TiLattice atoms. Occupancy of Ti3+ i ions in pseudo-octahedral site affects Eg(ʋ1) mode severely rather than B1g (ʋ4) mode due to longitudinal stretching vibration of Ti―Oeq bond in direction as evident from relative peak area of Eg (ʋ1) with respect to B1g (ʋ4) mode. However, the blue shift of Eg frequencies occurs due to the contraction of cell dimensions in , and directions. This indicates the non-uniform hydrostatic compressive strain acting on the anatase structure due to amorphous carbon shell layer and also from the occupancy of quasipyramidal Ti3+ i in pseudo-octahedral position of RTA-TiO2 nanoparticles. The competition between local bond length distortions due to quasi-pyramidal Ti3+ i and non-uniform hydrostatic pressure determines the unit cell volume of TiO2 nanoparticles (Figure. S3).


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3.3. Surface analysis

FIGURE 3. XPS (a) Ti-2p and (b) O-1s core-level spectra of Pristine-TiO2, RTA-TiO2 and HA-TiO2 nanoparticles.

XPS Spectra acquired from materials were charge corrected to give the adventitious C-1s spectral component (C–C, C–H) at a binding energy (BE) of 284.8 eV.38 A standard Shirley background correction was used for all core level spectra. Figure. 3a and 3b show the Ti-2p and O-1s core level spectra and their deconvolution into individual contributions for the materials annealed in the pure Ar environment (Figure. S4) and corresponding B.E.s are tabulated in Table 1. Also, valence band spectra of synthesized TiO2 nanoparticles are depicted in Figure. S5. The Ti-2p core level spectra of the materials consisted of Ti-2p3/2 and Ti-2p1/2 contributions located about 458.5 and 464.3 eV, respectively. The O-1s spectra of the materials were found to be asymmetric and were analyzed in terms of three components, namely, O-I, O-II and O-III. The O-I peak is attributed to O2− ions in metal oxide component (~530eV), O-II is related to hydroxyl group or defective oxide (~531.5eV) and O-III is from adsorbed water molecules (~533.5 eV).39 The effect of compressive strain on the electronic structure of the inner Ti-2p core level state can be investigated using XPS analysis. The quantum confinement effect, lattice strain and the coordination number reduction induce the binding energy shifts in the XPS spectrum.40,

41

In

general, the inner electrons and valence electrons are more strongly affected by the quantum confinement and the coordination reduction respectively. When the particle size decreases, the interplay between the quantum confinement and coordination reduction results in binding energy shifts in various electronic energy levels and thus the inner-core level shifts become more prominent than valence level.42 In the present study, the size of the TiO2 nanoparticle range from 10 to 13 nm. Whereas, RTA-TiO2 shows more lattice contraction and showed a more binding



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energy shift (~0.2eV) in Ti-2p core level, even though the size was comparable with HA-TiO2 nanoparticles, due to the presence of amorphous carbon layer over the TiO2 surface. The Ti-2p3/2 peak shifts towards lower binding energy due to formation of compressive strain in the TiO2 nanoparticles, which significantly affects the electronic structure on the inner orbits of the Ti4+ ion. The outer orbit electrons experiences screening effect on inner orbit electrons, which causes the reduction in effective nuclear charge. If the TiO2 nanoparticles experience the lattice contraction, overlapping of the outer orbit electrons between two neighboring Ti4+ ions increases by decreasing their inter-atomic distance, leading to enhanced screening and lower nuclear charge. The decrease in the inter-atomic distance reduce the BE of inner electrons, resulting in the XPS peak shift towards lower BE. The slight increase in the FWHM in Ti-2p core level of RTA-TiO2 and HA-TiO2 particles is attributed to the increase in the localized unoccupied d-states at the Fermi level and to the phonon excitation.43, 44 Moreover, the quenched HA-TiO2 showed higher non-stoichiometry than RTA-TiO2 and pristine TiO2 nanoparticles. Since, the carbon shell prevented the interior from the quenching medium; the effect of rapid cooling was negligible. Consequently, compressive strain may be due to the formation of Ti3+ interstitials in RTA-TiO2 rather than from rapid cooling process, which is discussed in the later sections. TABLE 1. O-1s and Ti-2p core levels in quenched and pristine TiO2 nanoparticles.

Core level of O-1s (eV) Sample

Core level of

Core level of

ΔB.E. (Ti–O)

O/Ti

O-I

O-II

O-III

Ti-2p3/2 (eV)

Ti-2p1/2 (eV)

TiO2

529.91

531.50

533.56

458.69

464.39

~71.2

1.82

RTA-TiO2

529.74

531.62

533.63

458.53

464.26

~71.2

1.84

HA-TiO2

529.82

531.25

-

458.60

464.29

~71.2

1.76

(eV)

Ratio

3.4. EPR Analysis EPR is a highly sensitive technique used to investigate the presence of paramagnetic species having one or more unpaired electrons either in the bulk or at the surface of various oxides.45 All samples were analyzed under the same instrumental parameters and mass of the sample was kept constant during the measurements. It can be seen that a single sharp peak was observed in the


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FIGURE 4. EPR Spectra of (black line) pristine-TiO2, (red line) RTA-TiO2 and (blue line) HA-TiO2 nanoparticles [Dashed line indicates the corresponding integral form of obtained EPR signals]

resonance field about ~338mT, which may be originated from the paramagnetic Ti 3+ ions (Figure. 4). This can be confirmed by the g value using the relation, ℎ𝜐 = 𝑔. 𝜇B . 𝐻0

(6)

where, h is Planck’s constant, ν is the microwave frequency (~9.4GHz), μB is the Bohr magneton, and H0 is the resonance magnetic field. The value of g is 1.99, which nearly equals the free electron value of 2.0023, indicating the existence of unpaired electrons in the materials which are due to paramagnetic Ti3+ ions in anatase lattice sites.46,47 The relative number of unpaired electron spins (Ns) participating in the resonance was calculated by using the formula,48 Ns ⋍ 𝐼. (Δ𝐻)2

(7)

Where, I is the peak-to-peak height and ΔH is the line width. Figure. S6 displays the relation between the relative Ns, lattice constant ‘c’ and g-factor of the materials. Possibly g-values are assigned to inner electron traps (Ti3+), which are not participating in the surface reaction. These traps are originated from the non-stoichiometric formation of Ti atoms at the grain boundary by heat treatment.49 Also, ab-initio calculations have shown that the compressive strain rapidly reduces the defect formation energies compared to tensile strain in the TiO2 lattice.50 Hence, the combination of lattice contraction and oxygen poor condition favor the formation of Ti interstitials and oxygen vacancies. Especially, excess charge carriers are strongly localized at the



interstitial site as well as three neighboring Ti sites in anatase structure. The pseudo-square [i.e. 4 equatorial + 2 apical] planar coordination stabilizes the 3𝑑𝑧 2 orbital and favors the electron trapping at the interstitial site and distribution of electronic charge can be expressed as [Tii•••+3Ti’Ti] for anatase phase.51 Since, the amorphous carbon coated RTA-TiO2 nanoparticles showed more lattice contraction than pristine and HA-TiO2 nanoparticles, produces large number Tii3+ defects at the core. Consequently, the relative Ns of RTA-TiO2 nanoparticles showed higher value and it resonates at ~338mT, which is corresponding to g value of ~1.997. In contrast, no signature of Ti3+ states was observed in XPS studies. The Tii3+ formed at the core of the particles act as recombination centers, as discussed in PL analysis. The absolute g-factor value was found to be decreased by decreasing the particle size, which is originated from reduction of the orbital contribution through increased quantum confinement. 52 The shift in the g-value was found to be independent of the particle size. The contribution of the lattice contraction on the observed gshift was found to be prominent and was decreased with respect to lattice parameter ‘c’ and unit cell volume (see Figure. S6). Similarly, increased dipolar–dipolar interactions between the particles and more randomly oriented magnetic moments increased the line width of the signal. Likewise, decrease in super-exchange interactions led to broadening of the resonance signal.

3.5. PL Analysis Using EPR spectroscopy, the trapped electrons (Ti3+) and holes (O− and O2−) in TiO2 can be analyzed. However, detection of these delocalized electrons in the conduction band of TiO2 is difficult.53 With PL spectroscopy it is possible to investigate the defect related trap states, electronic band structure, radiative and non-radiative relaxation pathways and quenching mechanisms of the materials.54 Figure. 5 (a-c) illustrates the experimental PL spectra of various anatase TiO2 nanoparticles at 300K. In order to understand the origin of broad visible luminescence, the spectra were deconvoluted as six Gaussian peaks which are represented as three primary color regions (i.e. Red, Green and Blue), and their corresponding emission areas are portrayed in Figure. 5d. The peaks at 2.9 and 2.7 eV correspond to ‘Blue’ emission, which is attributed to the self-trapped excitons (STEs) (i.e. photoexcited electrons are localized in a certain crystal site, subsequently a hole is captured by the localized electron and form STEs) localized at TiO6 octahedra, while the peaks at 2.5 and 2.3 eV causes ‘Green’ emission and is assigned to radiative recombination from the VO related F+ center or associated with 5c-Ti3+ Lattice


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FIGURE 5. PL emission spectra of (a) pristine-TiO2, (b) RTA-TiO2 (c) HA-TiO2 nanoparticles and (d) corresponding relative emission area (in %).

ions formed by the VOs.55 The ‘Red’ emission (originated from 1.8 and 2.1eV energy levels) is 3+ associated with either recombination of trapped electrons at Ti3+ Lattice or bulk interstitial Tii sites

with valence band holes.56 𝒚𝒊𝒆𝒍𝒅𝒔

Oxo →

Vo•• + 2e’ + ½O2 ↑ 𝒚𝒊𝒆𝒍𝒅𝒔

’ V𝑜•• + 2𝑒𝑡𝑟𝑎𝑝 →

𝒚𝒊𝒆𝒍𝒅𝒔

’ V𝑜•• + 𝑒𝑡𝑟𝑎𝑝 →


V𝑜•• → 𝒚𝒊𝒆𝒍𝒅𝒔

𝑚 TiO2 + 2𝑛V𝑜•• →

𝑉𝑜 (F)

(9)

V𝑜• (F + )

(10)

F ++

(11)

(𝑚 − 𝑛)TiO2 + 𝑛Ti•••• + 2𝑛e’ 𝑖 𝒚𝒊𝒆𝒍𝒅𝒔

•••• V𝑜• + Ti•••• 𝑖 (Ti 𝑇𝑖 ) →

••• V𝑜•• + Ti••• 𝑖 (Ti 𝑇𝑖 )


Ti•••• + 3Ti•••• 𝑖 𝑇𝑖 →

(8)

′ Ti••• 𝑖 + 3Ti 𝑇𝑖


(12) (13) (14)


Upon the loss of lattice oxygen atom (Oxo ) in TiO2 lattice, the electron pair remains trapped in the same site forming F centers (Eq. 8 and 9), whereas a F+ center is equivalent to a single electron associated with the VO (Eq. 10). The electron-pair deficient VO is represented as a doubly charged F++ center (Eq. 11). Because of the lower formation energy of Ti4+ i , oxygen vacancies subsequently react with the remaining TiO2 and produces Ti4+ i species via reaction mentioned in Eq. 12.57 The electrons left in the F+ center can interact with the 4+ 3+ 3+ adjacent Ti4+ Lattice or Tii ion and form TiLattice and Tii centers respectively as mentioned in the

Eq. 13. Likewise, the Ti4+ i species spontaneously donate three electrons to the neighboring 3+ (3d1) ions (Eq. 14). Based on the PL analysis and previous three Ti4+ Lattice , forming three Ti

literature, the possible energetic distribution of radiative trap levels present in the anatase TiO2are shown in Figure. S7. The defect states (such as, STE, F+ and Ti3+ i ) are located ~0.3 to 1.6 eV below the conduction band edge.58 The XPS analysis also proves the nonexistence of Ti3+ sites on the surface of the TiO2 nanoparticles. Whereas, EPR studies confirm the presence of paramagnetic Ti3+ ions in the TiO2 lattice sites. The paramagnetic signal in EPR could be originated from the Ti3+ formed at the core of the TiO2 nanoparticles, which act as radiative i recombination centers for ‘Red’ emission. The O1s-I peak in XPS core level spectra is attributed to O2− ions in the TiO2 lattice. The peak area of O1s-I is used to find the oxygen stoichiometry in the TiO2 nanoparticles. The relationship between PL emission area (in %), O1s-I peak area and lattice constant ‘c’ of the TiO2 nanoparticles are depicted in Figure. S8. The RTA-TiO2 nanoparticles showed O1s-I peak area more than 64.5% and their corresponding ‘Green’ emission area was found to be dropped below 25%. The prepared TiO2 nanoparticles exhibited an inverse relationship between O1s-I peak area and ‘Green’ emission peak area, which confirmed that the lattice oxygen vacancies are responsible for ‘Green’ emission rather than surface oxygen vacancies. Subsequently, the ‘Red’ emission peak area was quenched in both pristine and HA-TiO2 nanoparticles. The formation of bulk interstitial Ti3+ is higher in the strained RTA-TiO2 nanoparticles, since their formation i energy was lower under compressive strained condition. The ‘Red’ emission increases while ‘Green’ emission decreases. This inverse relationship confirms the interaction between the F+ ++ 4+ 3+ 3+ center and Ti4+ centers as mentioned in Lattice or Tii ions and formation of TiLattice , Tii and F

the Eq. (8-14). On the other hand, ‘Red’ emission area was dropped to ~10% for HA-TiO2 nanoparticles compared to RTA-TiO2 nanoparticles. The absence of the carbon shell layer may


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reduce the compressive strain and the thermal shock resistance of the HA-TiO2 nanoparticles resulting in more surface oxygen vacancies. The reduction in the TiO2 stoichiometry was confirmed through O1s core level spectrum. In general, during oxygen or air annealing (>400 K), the interstitial Ti3+ ions were migrated from deeper layers to interstitial sites in the layers nearer to the surface either by electrostatic repulsion between F+ center and Ti3+ i ion or via an exchange mechanism and reacts with the adsorbed O2 molecules to form neutral TiO2 islands on the surface.59,60 Here, the Ti3+ i interstitials formed by lattice contraction were retained at room temperature, since their diffusion kinetics was hindered below 360K, which prevented the interaction with adsorbed O2 molecules.61 Therefore, the reduction in the concentration of 3+ Ti3+ i ions would not have originated from the migration and successive annihilation of Tii with

adsorbed O2 molecules, rather from decrease in lattice contraction of HA-TiO2 nanoparticles. Thus, from PL, XPS and EPR analysis, the importance of the lattice contraction on formation + of Ti3+ i , STEs and VO related F defects in anatase TiO2 nanoparticles could be confirmed.

3.6. Room temperature ferromagnetic property analysis Typical room-temperature magnetic moment measured in a quantum design MPMSSQUID magnetometers are presented in Figure. 6a. All materials exhibited ferromagnetic hysteresis added to the linear paramagnetic background. The saturation magnetic moments were in the range of 10−4 emu after subtracting the linear paramagnetic background and were well above the sensitivity limit of SQUID magnetometry. It is the typical characteristic of defectinduced ferromagnetism as per existing literatures. The stoichiometric TiO2 contains only diamagnetic Ti4+ ions, whereas the unpaired 3d electrons in Ti3+ ions can cause magnetic ordering in the non-stoichiometric TiO2.62 Similarly, oxygen vacancies can also create charge imbalance within the system and create Ti3+ lattice ions by gaining electrons from VO site and lead to the formation of magnetic moment. The point defects such as, Ti and O vacancies and Ti interstitials are responsible for the magnetic ordering.63 The presence of Ti3+ ions was confirmed from the PL and EPR spectroscopic analysis. Moreover, the structural arrangement of Ti3+ ions could give rise to hopping of the single 3d electron or a double-exchange mechanism inducing local ferromagnetic (FM) like behavior at nanoscale.64 However, if the density of Ti3+ ions increases, they interact via the exchange mechanism and as a result, one can expect ferri- or ferromagnetism. In addition to the compressive strain, Stoner exchange splitting also can induce



a small chemical potential shift in the core level peak of the Ti 3d band and may provide additional magnetic moment of the Ti atom.65 Yang et al. stated that the classical super-exchange mechanism is not appropriate to explain the anti-ferromagnetic coupling between Ti3+ centers in anatase TiO2, if the oxygen atom in between two Ti atoms is removed.66 The present findings can be correlated with their model, where the anti-ferromagnetic coupling between interstitial Ti3+ ions occurs on the basis of the indirect d-d hopping between the paired interstitial Ti3+ ions via the adjacent lattice Ti4+ ions. If not, Ti3+ ions get isolated and act as paramagnetic centers contributing to the paramagnetism. Therefore, in quenched TiO2 nanoparticles, antiferromagnetic coupling between interstitial Ti3+ ions reduces the overall magnetic moment. In pristine and quenched TiO2 nanoparticles, ferromagnetism arises from the ferromagnetic coupling between two neighboring F+-Ti3+ defect complex happens through, either from spin-up or spin-down conduction electron. 67 Nguyen et al, state that the surface defects are the source of magnetism in nanostructures due to abundant surface states or oxygen vacancies.68 Whereas, in a)

b)

c)

d)

FIGURE 6. (a) Magnetization (M-H) curves of defected TiO2 measured at 300 K. (b-d) χ-T curve of defected TiO2 in the field of 500 Oe and fitted with MFA model.


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the HA-TiO2 sample, even though the higher surface disorder is created from quenching process, it shows lower magnetic moments. This contradictory result indicates that the lattice oxygen vacancies (i.e. F+ center) are playing a major role in determining the ‘d0 ferromagnetism’ rather than the surface defects. The prepared TiO2 nanoparticles require higher magnetic field in order to reach its saturation value (Figure. S9). This could be originated from the isolated spins present in the surface defects. These isolated spins exhibit surface spin-glass (i.e. frustrated surface spins) behavior and rearrange more slowly than interior spins.69 Therefore, particles without carbon shell (HA-TiO2) requires more magnetic field in order to reach their saturation. In the presence of the carbon layer (RTA-TiO2), the surface induced magnetism can be suppressed by exchange interaction between surface atoms and carbon atoms and the overall saturation magnetization was found to be reduced. Also, distance between two carbon atoms determines the FM and antiferromagnetic (AFM) interaction between them.70,71 Since, the C-C atoms exhibit random arrangements over the TiO2 surface, both FM and AFM interactions can happen only on the surface. Perhaps, the surface C atoms have a tendency to form a non-magnetic cluster through the direct C-C bonding interactions.71 Moreover, this shell layer may block the dipoledipole interaction among the nanoparticles and magnetization of the RTA-TiO2 reaches its saturation comparatively under lower magnetic field. Magnetic susceptibility (χ–T) analysis was performed in the temperature range 5K–300K by using field cooling (FC) measurement. The experimental data were fitted into the mean field approximation (MFA), 72 𝑇 𝛽

𝜒 ≈ 𝜒0 (1 − 𝑇 ) 𝑐

(15)

Where, β is the critical exponent, Tc is the Curie temperature; χ is the temperature-dependent susceptibility and 𝜒0 is an arbitrary constant, which is related to electron spin density (N) and μB. The corresponding curves are depicted in Figure. 6 (b-d). Since, the materials showed clear ferromagnetic signals, fitting was performed from 140K. The presence of defects can significantly influence the intrinsic magnetic ordering of semiconducting nanostructures.64 From the MFA fitting, it was predicted that the Tc of the system falls above 330K. A rapid cooling process has a vital role in high Tc of the system, especially, the RTA-TiO2 showed transition temperature about 400K. Even though quenched materials exhibited lower saturation magnetization, their transition temperatures were found to fall above 350K because of its strong exchange interactions between Ti3+ ions.



FIGURE 7. Relationship between Curie temperature, Ms, cell volume and PL emission area of defected TiO2 nanoparticles.

The temperature-dependent magnetization of the nanoparticles was analyzed by zerofield cooling (ZFC) and FC measurements. In FC measurements, the materials were initially cooled down to 5K under an applied magnetic field of 500 Oe and measurement data were recorded from 5K to 300K under 500Oe. In ZFC measurement, the nanoparticles were cooled from 300K to 5K without application of any magnetic field. The ZFC (T) and FC (T) curves did not overlap in the whole temperature range of the measurements, which confirmed the existence of magnetic hysteresis in the M-H curves and implied that the system had not reached the fully super-paramagnetic (SPM) regime even at 300K as depicted in Figure. S10. In FC curve, the magnetization increases by decreasing the temperature from 300K. Below 50K, the magnetic moments of the domains start to freeze in the direction of the external applied magnetic field, giving rise to rapid increase in the FC curve. Whereas, in the ZFC process, upon decreasing the temperature, the potential energy settles to a minimum by aligning the magnetization direction of each nanoparticles along its easy-axis. Because of the random orientation and size dispersion of the nanoparticles, the overall magnetization of the materials showed the lowest value at 5K. When the temperature rises from 5K under an applied magnetic field, the magnetization behavior of the TiO2 nanoparticles followed typical paramagnetic characteristics. The overall magnetization increased as a function of temperature, and reached maximum magnetization at blocking temperature (TB). Above TB (~250K), their overall magnetization obeys the Curie law, where the magnetization decreases with increasing temperature. The appearance of this broad


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distribution of blocking temperatures is a direct consequence from the distribution of size dispersion and anisotropy energy barriers.73, 74 In addition to the point and interstitial defects, the lattice strain also influences the magnetic moment observed in the TiO2 nanoparticles. Chun-gang et al. found that the Curie temperature is proportional to the sum of neighboring exchange energies and is sensitive to the change in lattice constants.75 The lattice strain determines the bond angle as well as bond length, which in turn strongly influence the orbital overlapping and exchange coupling between the neighboring ions.76 Csontos et al. found enhancement in the Curie temperature in (In,Mn)Sb system under hydrostatic strain.77 The relationships between lattice contraction, red emission area, saturation magnetization and Tc are depicted in Figure. 7. Curie temperature highly depends on the hydrostatic compressive strain experienced by unit cell as well as the concentration of interstitial Ti3+ ions. When compressive strain increases, Tc increases because of the strong ferromagnetic exchange mechanism.78 It is possible to approximate the measure of the number of defects or the number of super-exchange interactions site by using M(0) parameter via calculating the number of net spins in the system. The number of net spins give rise to the measured magnetization (at 0K) which can be approximated and calculated from M(0)≈ NgμS, assuming that the orbital contribution is negligible.79 Here, N is the total number of spins between the super-exchange sites, g is the splitting factor, μ = 9.27×10−21 ergs.Oe-1 and S is the spin at T = 0K. Taking S = 1/2 and g = 2, it was found that N≈ 3×1016, 2×1016 and 9×1015 spins.cm−3 for pristine, RTA and HA-TiO2 nanoparticles, respectively. Relative spin concentrations measured from EPR spectra were contradictory from this prediction. RTA-TiO2 showed more number of relative spin concentration in EPR, but magnetization properties predicted that their concentration falls below the pristine TiO2. The lower magnetization value might be originated either from the anti-ferromagnetic coupling between interstitial Ti3+ ions or from isolated paramagnetic centers present in the system due to lack of spin carriers present in between the paramagnetic Ti3+ ions. Even though, the quenched TiO2 nanoparticles possess more compressive strain and strain induced native defects, the lack of localized carriers between the exchange sites hinders the overall saturation magnetization. However, the rapid cooling method has a significant effect on magnetic and optical properties of the anatase TiO2 system.



4. CONCLUSION Based on the microstructural, vibrational and optical studies, it can be concluded that controlled atmosphere rapid cooling process creates more surface as well as core defects in TiO2 nanoparticles. The temperature of the argon gas present during annealing process alters the dissociation pathway of the excess acetate ions and forms volatile organic compounds and amorphous carbon layer. Presence of amorphous carbon shell layer formed on anatase TiO2 nanocrystals (i.e. RTA-TiO2) caused hydrostatic compressive strain and significantly influences 3+ interstitial ions the Ti―Oap bond length and the Ti3+ i defect concentration. Also, excess of Ti

with unpaired electrons may be anti-ferromagnetically coupled via lattice Ti4+ ions, which limits the overall saturation moment of the quenched TiO2 nanoparticles. The compressive strain present in RTA-TiO2 nanoparticles enhances the ferromagnetic coupling between the defect complexes (Ti3+-F+) and helped to reach the saturation moment at lower applied magnetic fields compared to pristine TiO2 nanoparticles. The magnetic property analysis confirmed the lack of 3+ exchange spin carriers between paramagnetic Ti3+ i and TiLattice centers which reduces the long-

range ferromagnetic ordering in the materials. Whereas, HA-TiO2 nanoparticles having more surface oxygen vacancies than RTA-TiO2 due to the absence of the amorphous carbon layer. These surface oxygen vacancies exhibited significant effect on FM ordering and suppress the net magnetic moment, which indicates that the lattice oxygen vacancies (i.e. F+ center) play a major role in determining the ‘d0 ferromagnetism’ rather than the surface defects. To best of our knowledge, there are no clear evidences available to explain how the exchange interaction happening between two neighboring interstitial Ti3+ ions in undoped anatase TiO2 crystal. Overall, the RTFM in TiO2 nanoparticles was found to be originated from the FM coupled defect complexes. Whereas, the reduction in FM ordering in undoped quenched TiO2 nanoparticles was caused by excess interstitial Ti3+ ions. In addition, the strain tuning of the ferromagnetism in TiO2 with intrinsic defects paves the way to development of controllable nanoscale magnetic and spintronic devices.


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ACKNOWLEDGMENTS The authors wish to acknowledge the facilities and support provided by the management, PSG Sons and Charities, Coimbatore. Authors thank Mr. T. Vijayaragavan for TEM analysis and Dr. K. R. Ravi for DSC analysis. J.D. acknowledges Council of Scientific & Industrial Research (CSIR), New Delhi, India for fellowship and financial support. Also, authors thank MNCF, IISc and SAIF, IIT Madras, India for providing facilities for SQUID and EPR measurements.

SUPPORTING INFORMATION XRD data, Rietveld refinement analysis, DSC curves, deconvoluted XPS core-level spectrums, valance band spectrum, Raman shift, bond length, cell volume, g-factor, Ns and lattice constant-c, PL emission areas (in %), O1s-I peak area, M-H and M-T curves of the synthesised TiO2 nanoparticles. Table show the Crystallographic and microstructural parameters extracted from Rietveld refinement analysis. Notes The authors declare no competing financial interest.

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GRAPHICAL ABSTRACT

Anomalous effects of lattice strain and Ti3+ interstitials on roomtemperature magnetic ordering in defect engineered nano-TiO2 Jayaseelan Dhakshinamoorthy1,2, Arun K. Prasad3, Sandip Dhara3 and Biji Pullithadathil1,2* 1

Nanosensor Laboratory, PSG Institute of Advanced Studies, Coimbatore-641 004, INDIA. 2

3

Department of Chemistry, PSG College of Technology, Coimbatore-641004, INDIA.

Surface and Nanoscience Division, Materials Science Group, Indira Gandhi Centre for Atomic Research, Homi Bhabha National Institute, Kalpakkam-603 102, INDIA. *E-mail:[email protected] Fax: +91-42-2257-3833: Tel:+91-422 4344000, Extn (4193)

FM

O

Ti

FM

1.0

0.5

AFM 0.0

M (x10-3 emu/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.5

H (T)

-1.0 -1.0


-0.5

0.0

0.5

1.0

Anomalous effects of lattice strain and Ti interstitials on room

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