Theoretical Insights into the Experimental Observation of Stable p

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Article

Theoretical Insights into the Experimental Observation of Stable p-Type Conductivity and Ferromagnetic Ordering in Vacuum-Hydrogenated TiO

2

Divya Nechiyil, Manoharan Muruganathan, Hiroshi Mizuta, and Ramaprabhu Sundara J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04397 • Publication Date (Web): 16 Jun 2017 Downloaded from http://pubs.acs.org on June 18, 2017

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The Journal of Physical Chemistry

Theoretical Insights into the Experimental Observation of Stable p-Type Conductivity and Ferromagnetic Ordering in Vacuum-Hydrogenated TiO2 Divya Nechiyil1, Manoharan Muruganathan2*, Hiroshi Mizuta2 and Sundara Ramaprabhu1 1

Alternative Energy and Nanotechnology Laboratory (AENL), Nano Functional Materials

Technology Centre (NFMTC), Department of Physics, Indian Institute of Technology Madras, Chennai – 600036, India 2

School of Materials Science, Japan Advanced Institute of Science and Technology, Asahidai 1-1, Nomishi, Ishikawa 923-1292, Japan *

Email: - [email protected]

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Abstract Tuning of electrical and magnetic properties to achieve stable p-type conductivity and room temperature ferromagnetism in undoped TiO2 is quite challenging. Here both are attained simultaneously through a facile method of vacuum-hydrogenation, wherein vacuum annealing as well as hydrogenation play crucial role. The p-type conductivity in hydrogenated TiO2 is investigated through the Hall measurement studies, which show considerable enhancement in Hall mobility and electrical conductivity. The high and low pressures of hydrogenation show strong and weak ferromagnetic ordering respectively, whereas the pristine TiO2 NPs manifest paramagnetic behavior. In order to understand the mechanism of these characteristics changes, density functional theory (DFT) calculations are performed. DFT calculations reveal that the smaller amount of hydrogenation leads to gapstates above valance band maximum (VBM) due to the effect of hydrogen atoms 1s orbitals and by the formation of ~Ti-H and ~O-H bonds. Further increase in the hydrogenation changes the ~O-H bond to the ~H2O bond and these H2O molecules will be easily detached during the next vacuum annealing step. These processes will lead to the formation of excess oxygen vacancies and cause the localization of excess electrons on Ti atoms. This results in emergence of well pronounced mid-gap states in the forbidden bandgap. These mid-gap states are mostly contributed by the 3d orbitals of Ti atoms. DFT studies also disclose that the higher spin polarization for the high hydrogen concentration, which is reflected as the ferromagnetic ordering in the experimental results.


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Introduction TiO2 is a well-known and intensively studied wide band gap metal oxide. Exploring its properties continues due to its outspread applications in solar assisted energy applications like photovoltaics and photocatalysis, with many other applications such as gas sensors, batteries, antireflection and protective coatings.1–4 In spite of these extensive applications, the wide band gap of TiO2 limits the solar driven energy utilization. In order to enhance the visible light absorption and to improve the electrical transport properties, introduction of defects into the TiO2 lattice has gained unique attraction. Defects can be created by various methods such as doping, thermal treatment and exposure to oxidizing or reducing gases.5–11 Mostly intrinsic defects are preferred over the extrinsic defects because of the recombination centers generated by the extrinsic dopants, which can have a negative impact on the electronic properties.12 Recently, Chen et al. has reported hydrogenated black TiO2 with enhanced solar absorption, which has given a groundbreaking startup for the research in hydrogen induced disordered TiO2.11 Hydrogenated TiO2 has received substantial scientific recognition since it manifests remarkable changes in optical, electrical and magnetic properties thereby finding wide applications.12–17 Here, the drastic changes in physical and chemical properties can be attributed to the highly reactive nature of hydrogen and its small atomic size eases the interaction with different lattice sites of host material. Amphoteric nature of hydrogen also accredits for various types of interaction with a semiconductor, such as donor (H+), acceptor (H-) and neutral (H0) and can counteract the prevailing conductivity.18,19 Even though there are enormous efforts to unravel the exact physics behind the hydrogen induced disordereness of TiO2, it is not yet fully understood. It demands further systematic experimental as well as theoretical studies to explore the unseen black magic of hydrogenated TiO2. There are different explanations for the hydrogenation puzzle and for the immense change in properties after hydrogenation. The nature of hydrogenated TiO2 and the 3 ACS Paragon Plus Environment


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explanations for the enhanced properties varies from one research group to the other, since the interaction of hydrogen with the host material and the defect formation are highly dependent on the experimental conditions, morphology of the sample, applied pressure and temperature conditions.20,21 According to Chen et al, Ti3+ centers do not play a vital role in enhancing the visible and infrared light absorption. They have suggested the formation of mid-gap states above the valence band maximum (VBM) due to the surface disorder induced during the hydrogenation.13 Mo, L.-B et al. proposed that hydrogen impurity indirectly affects Ti 3d and O 2p electronic states and thereby changes the electronic properties.22 Lei Liu.et al. explained the formation of localized mid-gap states based on their density functional theory calculations.23 The present work illustrates a novel method of hydrogenation which combines vacuum annealing and hydrogenation, resulting in unusual stable p-type conductivity in TiO2. Herein, systematic experimental as well as theoretical investigations are carried out to explore the significant changes in optical, electrical as well as magnetic properties of vacuumhydrogenated TiO2. In the present study, anatase TiO2 nanoparticles (TiO2 NPs) are vacuum annealed at 400 ᴼC, and then hydrogenated at different pressures (1 bar and 10 bar). Compared to other reported hydrogenation experiments, this low pressure hydrogenated TiO2 show p-type conductivity with one order increase in electrical conductivity and considerable enhanced hall mobility. Here the vacuum annealing plays an important role in introducing excessive oxygen deficiencies, which further eases the hydrogen interaction with the TiO2 lattice. Thus the hydrogen induced defects results in stable mid-gap states above the VBM. DFT simulation results disclose the strong spin-polarization for the hydrogenated nanocrystals which have oxygen vacancies, from where the ferromagnetic ordering originates. There are few reports, which explain the n→p transition of TiO2 by varying temperature, oxygen partial pressure and exposure to reducing gases or doping with extrinsic 4 ACS Paragon Plus Environment

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impurities.4,24–27 In spite of all these reported p-type TiO2, hydrogen induced disordered TiO2 gives a new way for getting stable p-type conductivity which exhibits room temperature ferromagnetism. Experimental Anatase TiO2 NPs were synthesized by sol-gel method using the precursor titanium tetraisopropoxide (TTIP). Briefly 42 ml of TTIP was poured into 135 ml of DI water while vigorous stirring and then the mixture was kept for 12 h at room temperature. The formed precipitate was centrifuged and washed with DI water and ethanol, and dried in the vacuum oven to get amorphous TiO2 NPs. The crystalline anataseTiO2 NPs were obtained by annealing the sample at 400 ᴼC for 1 h in air atmosphere. The anatase TiO2 NPs have the advantage of high surface to volume ratio, which supports more surface as well as lattice disorder during the hydrogenation experiments. The hydrogenation experiment was carried out using high-pressure Sieverts apparatus.28 Each time, prior to hydrogenation, the sample was activated by vacuum annealing, thereby creating highly reactive oxygen vacancies, which enhance the hydrogen interaction with the sample. During activation, the sample was evacuated to high vacuum of the order of 10-6 mbar and then heated at 400 ᴼC for 2 h. During hydrogenation, the sample was vacuum heated for 2 h; subsequently 1 bar hydrogen gas was allowed and kept for 5 h. The process was repeated for another cycle to get TiO2 NP-1H. To obtain TiO2 NP-10H, the above procedure was repeated except that the hydrogen pressure was 10 bar. The difference between 1H and 10H samples is in the allowed hydrogen pressures, i.e 1 bar and 10 bar respectively. After hydrogenation the white colored TiO2 changed into grey in color which confirms the defect formation. Powder X-ray Diffraction (XRD) pattern of the samples were recorded using Rigagu X-ray diffractometer. Field emission scanning electron microscopy (FESEM) images were obtained 5 ACS Paragon Plus Environment


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using FEI QUANTA 300. High-resolution transmission electron microscopy (HRTEM) images of the samples were acquired using FEI Tecnai. UV-Visible optical absorption spectra of the samples were obtained with Agilent Cary 100 UV-Visible absorption spectrometer. Raman spectra of the samples were measured using Horiba HR-800-UV Raman spectrometer consist of He-Ne laser with 633nm excitation wavelength. Brunauer-Emmett-Teller (BET) surface area measurement for TiO2 NPs was carried out using Micromeritics ASAP 2020 instrument. Electron paramagnetic resonance (EPR) spectra were recorded with JES-FA200 EPR spectrometer. Solid state 1H Nuclear magnetic resonance measurements were done in Bruker 400 MHz NMR spectrometer. Hall measurements were carried out in Resi Test 8400 Hall Effect measurement system. For Hall measurements the samples were pressed to pellets using hydraulic pressing unit. Magnetic measurements were carried out using Quantum Design SQUID-VSM. The X-ray photoelectron spectroscopy (XPS) measurements were carried out with SPECS X-ray photoelectron spectrometer, where MgKα is the X-ray source and PHOIBOS 100MSD is the energy analyzer. The XPS analysis was carried out by using CASA XPS software. Results and discussion In order to reveal the remarkable changes in electronic properties of hydrogenated TiO2, DC hall measurement studies were carried out. Here the hydrogen induced disorder causes the unexpected carrier type switching of n→p, by contradicting the prevailing n-type conductivity of pristine TiO2 NPs. Hall parameters measured at room temperature for pristine as well as hydrogenated TiO2 NPs are tabulated in Table 1. Pristine TiO2, which is an n-type semiconductor, exhibits negative Hall voltage as expected. The n-type conductivity of TiO2 at room temperature is due to the inbuilt O vacancies and Ti interstitials present in the system. Counteracting to this, the p-type conductivity in TiO2 can be explained by the defect induced mid-gap states above the VBM. Hydrogenated TiO2 NPs shows positive Hall voltage for 6 ACS Paragon Plus Environment

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TiO2 NP-1H and TiO2 NP-10 H, which is a clear indication of p-type conductivity. The highly reactive hydrogen atoms can interact with Ti atoms, O atoms and can also occupy the interstitial sites of TiO2 lattice. The interaction of hydrogen with Ti atoms can result in Ti-H bond formation and thereby can cause a repulsive force on oxygen atoms.23 Consequent lattice distortion in Ti-O bonds results in anion dangling bonds, whose energy lies very near to VBM.19,29,30 In addition to that, prior vacuum annealing of the samples creates excess oxygen vacancies and they can further act as the reactive sites for hydrogen. The increased hydrogen interaction with O sites can cause O-H bond as well as H2O bond formation.31 This leads to the production of excess oxygen vacancies by the removal of H2O in further vacuum annealing and the subsequent charge imbalance results in localization of electrons at Ti4+ sites. Thus, the bonded hydrogen in TiO2 lattice and Ti3+ centers can collectively contribute to more prominent mid-gap states near to VBM as compared to that of CBM (Conduction Band Minimum). This mid-gap states lead to the band gap narrowing as well as carrier type switching in vacuum-hydrogenated TiO2, which is further explained in detail by the DFT calculations. Vacuum-hydrogenated TiO2 NPs exhibit higher Hall mobility and lower sheet resistivity. The increase in Hall mobility is a clear indication of the newly incorporated hydrogen atoms.22,32 TiO2 NP-1H exhibits highest Hall mobility (93.9 cm²/V·s) and lowest sheet resistivity as compared to the TiO2NP-10H as well as pristine TiO2 NP. Here, the newly incorporated hydrogen atoms in TiO2 lattice act as the source for enhancing electrical conductivity and Hall mobility. The hydrogenation at high pressures has a deteriorating effect in the electrical properties. This is attributed to the excess defects created in the system, which increases the scattering centers; thereby decaying the electronic properties. As hydrogen concentration increases, the probability for Ti-O bond reformation increases and thereby enhancing the lattice deformation.



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Table 1 Hall parameters measured at room temperature Sample

Hall mobility

Sheet resistivity

Hall voltage

Type of

[cm²/V·s]

[Ω/square ]

[V]

conductivity

TiO₂ NP

27.5

1.55E+09

-1.44E-02

n-type

TiO₂ NP-1H

93.9

9.38E+07

1.53E-02

p-type

TiO₂ NP-10H

51.3

2.98E+08

7.13E-03

p-type

Manifestation of room temperature ferromagnetism by undoped metal oxide is unusual. Magnetic metal oxides are at the forefront research due to dilute magnetic semiconductors (DMS) and their potential applications in multifunctional devices, where electron’s spin and charge play significant role. The emergence of ferromagnetic ordering in TiO2 and in other metal oxides even in the absence of magnetic impurities is controversial.33 The room temperature ferromagnetism in TiO2 is mostly explained by oxygen vacancies, Ti3+ ions and titanium vacancies.34–36 Figure1 illustrates the magnetic moment vs. magnetic field (M-H) curves of pristine as well as hydrogenated TiO2 NPs at room temperature after subtracting the diamagnetic background of Teflon. Hydrogenated TiO2 NPs exhibit ferromagnetic ordering with clear hysteresis loop (inset of Figure 1), whereas the pristine TiO2 NPs exhibit paramagnetic behavior. TiO2 NP-10H depicts higher saturation magnetic moment (Ms) of 5.33×10-3 emu g-1, with a remanence and coercivity of 2.37×10-4 emu g-1 and 77 Oe respectively. TiO2 NP-1H also shows the ferromagnetic behavior with Ms of 1.76 X 10-3 emu g-1, which clearly explains the role of hydrogen induced defect formation in ferromagnetic ordering. Figure S1 (supporting information) represents the field cooling (FC) magnetic moment vs. temperature (M-T) curves in the temperature range of 5-300 K at 5000 Oe. It 8 ACS Paragon Plus Environment

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confirms the enhanced magnetization after hydrogenation, as compared to the pristine TiO2, for which magnetization is near to the Teflon background. It also shows that the magnetic moment is high at low temperature due to the orientation of spins and increase in the temperature distorts the spin alignment, thereby causes decrease in the magnetic moment. Figure S2 depicts the increased magnetization at 5K for TiO2 NPs, TiO2 NP-1H and TiO2 NP-10H. Hereby, we establish the strong correlation between the hydrogen induced defects and ferromagnetic ordering in TiO2. Saturation of magnetic moment at high fields with small hysteresis loop in M-H curve and the M-T curves reassert enhanced magnetization of hydrogenated TiO2 NPs. Hydrogen is a highly reactive reducing gas, which has an active role in inducing excess oxygen vacancies. The excess oxygen vacancies results in the redistribution of electrons and thereby causes the formation of oxygen vacancies occupied with single electrons. The formation of such oxygen vacancy clusters induce stable room temperature ferromagnetic ordering in vacuum hydrogenated TiO2 NPs. The vacuumhydrogenation can result in H2O bond formation and its removal increases the oxygen vacancies, which in turn leads the localization of more electrons at Ti4+ sites and resulting in the formation of Ti3+ centers. Thus the exchange interaction of paramagnetic Ti3+centers with one unpaired electron in 3d orbital contributes to the enhanced magnetization of hydrogenated TiO2 NPs. The pristine TiO2 NPs shows paramagnetic behavior, owing to the isolated electrons occupied in the native oxygen vacancies.34,35,37,38 Thus, counteracting the decreasing trend of electronic transport properties with respect to the increase in hydrogen induced disorderness, the magnetization is enhanced due to the defect cluster formation. Supporting the magnetic measurements, room temperature electron paramagnetic studies (Figure S2, supporting information) clearly show the peak corresponding to Ti3+ centers with g value of ~1.99 for hydrogenated TiO2 NPs and the same is absent for pristine TiO2 NPs.39


0.006 0.004

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TiO2 NP TiO2 NP-1H TiO2 NP-10H

0.002

-1

0.000 Magnetic moment (emu g

-1

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Magnetic moment (emu g )


-0.002 -0.004

0.0005

TiO2 NP-1H TiO2 NP-10H

0.0000

-0.0005

-200

-0.006 -40000

-20000

0

0

Magnetic field (Oe)

20000

200

40000

Magnetic field (Oe) Figure 1. Magnetic moment vs. magnetic field (M-H) curves of pristine and vacuum hydrogenated TiO2 NPs at room temperature Density functional theory (DFT) calculations were performed to understand the origin of gapstates above VBM as well as to probe the characteristics change of TiO2 from weak ferromagnetic (TiO2 NP-1H) to higher saturation magnetic moment (TiO2 NP-10H) with increase in the hydrogenation. In order to clarify the experimental results, spin-unrestricted, and Hubbard U or hybrid levels of DFT simulations (DFT+U) is required.40 Experimental results of antiferromagnetism in anatase TiO2 and gap states of ~1 eV below conduction band are well reproduced by DFT+U approach.41–43 Finazzi et al. has reported that in bulk anatase TiO2, U range of 3−4 eV best reflects the experimental data, and recently Ha et al. explored U value of 3.6 eV for surface anatase.40,42 Here, we have used DFT+U approach with U value of 3.6 eV. Theoretical calculations were performed using DFT code Atomistic Tool kit

44,45

, which is

based on a linear combination of numerical atomic orbitals and norm conserving TroullierMartins pseudo potentials. Generalized gradient approximation (GGA) + U method was used with Perdew-Burke-Ernzerh of exchange correlation functional. The geometric optimization was performed with a criterion of maximum force on each atom less than 0.01 eVÅ−1. 10 ACS Paragon Plus Environment

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Double zeta plus polarized basis sets were used in all of these calculations. Around the anatase TiO2 nanocrystal, 10 Å vacuum distances was maintained to avoid any spurious interactions with the neighbor super cell. Nanocrystalline anataseTiO2 has high surface to bulk ratio over the bulk TiO2 and the surface atoms have more degrees of freedom than the bulk material to interact with external environment. In order to explore the hydrogenation in TiO2 nanocrystal, we consider ~1 nm size TiO2 of Ti25O50. Four singly coordinated O atoms dangling bonds are passivated with H atoms (Ti25O50H4) and this passivation can occur during the synthesis procedure23 (Figure 2a). Most importantly, it has been demonstrated that the interaction of H2 molecule with TiO2 nanocrystal is exothermic rather than endothermic and the interaction of H2 molecule with TiO2 nanocrystal leads to ~Ti-H and ~Ti-OH bonds.23 Based on these results, the effect of hydrogen concentration on anatase TiO2 is considered by placing different number of hydrogen atoms around the nanocrystal structure as shown in Figure 2b and then the resultant structure is fully relaxed. During vacuum annealing oxygen vacancies will be created34 which are taken care in the DFT simulation by introducing two oxygen vacancies at the surface shown as empty spheres (Ti25O48H4) in Figure 2b. As the experimental results shows weak ferromagnetic to higher saturation magnetic moment with the increase in hydrogenation, 12 and 18 hydrogen atoms are placed around Ti25O48H4 structure to study the impact of different hydrogen concentration on anatase TiO2 nanocrystal. These structures are denoted as Ti25O48H4-H12 and Ti25O48H4-H18, respectively.



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Figure 2. (a) Ti25O50H4 anatase nanocrystal structure, where the light grey, red and light blue balls represent Ti, O and H atoms respectively, (b) Two surface oxygen vacancies introduced atomic structure Ti25O48H4. Vacancies are highlighted by empty spheres and 11 hydrogen atoms are placed around the nanocrystal before geometrical optimization. Figure 3a illustrates the density of states (DOS) of bulk anatase TiO2 and different hydrogenated nanocrystals. Valance band maximum (VBM) of bulk TiO2 is located at -1.6 eV. In case of bulk TiO2, valance bands are contributed by O atoms 2p orbitals and conduction bands are dominated by Ti atoms 3d orbitals (Figure S4, supporting information). In the nanocrystal model without oxygen vacancy Ti25O50H4-H11, gap states are induced above bulk VBM. These states are induced by the hydrogen atoms 1s orbitals. Different orbitals projected DOS plot is given in Figure S5. As the hydrogen atoms are bonded to Ti and O atoms (Figure 4a), their respective 3d and 2p orbitals are partially influenced by H atom 1s orbital. However, these gap states are mainly from the valance band states as O 2p orbital is dominant over Ti 3d orbital, which is consistent with the reported mid-gap states in black TiO2.11, 23 After introducing oxygen vacancies into the nanocrystals, one can see slight spin-polarization along with gap states above the bulk VBM for Ti25O48H4-H12 (Figure 3a). Further increase in hydrogenation (Ti25O48H4-H18) leads to the strong spin-polarization as 12 ACS Paragon Plus Environment

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shown in Figure 3b, where spin-down state is not present around -0.5 eV. These spin polarization is mainly induced by Ti 3d orbitals along with O 2p orbitals. These mid-gap states are not directly contributed by H atom, which indicates that the hydrogen induced lattice disorder along with oxygen vacancy is responsible for these states.

Figure 3. (a) Calculated DOS plots for bulk and hydrogenated anatase nanocrystals, (b) Hydrogenated Ti25O48H4-H18 nanocrystal DOS, decomposed Projected DOS and different orbitals of Ti, O, and H atoms. In these plots, positive and negative values denote spin-up and spin-down states, respectively. In order to analyze the details of spin-polarization, difference between spin-up and spin-down electron density for these structures are plotted in Figure 4. This difference density is superimposed on geometrically relaxed atomic structure. Hereby, we are trying to correlate the oxygen vacancy, hydrogenation as well as the role of H2O bond formation. During hydrogenation OH bond formation and transformation of existing OH bond to H2O bond will be taking place. Once stable H2O bonds are formed, it has a tendency to detach from the TiO2 lattice. Thus, ~ H2O bonding leads to localization excess electron on Ti atom since ~H2O bonding makes 2.167 Å distance from nearest Ti atom as compared to ~OH bond distance of 1.898 Å. Figure 4a clearly demonstrates that, when oxygen vacancy is not present then the spin-polarization is almost absent (Ti25O50H4-H11) except at ~ H2O bonding site (highlighted 13 ACS Paragon Plus Environment


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by arrow in Figure 4a). Furthermore, introduction of oxygen vacancies on TiO2 surface clearly leads to more localization of excess electrons on Ti atoms as shown for Ti25O48H4-H12. When six coordinated Ti6c bonds are terminated by O2c or H atoms, then Ti3+ center is not formed. With the increase in the hydrogenation, possibility of forming of ~ H2O bond is enlarged as indicated by Figure 4b, c. This loosely bonded H2O will be easily removed in further vacuum annealing, which will lead to more oxygen vacancies in TiO2 nanocrystal. Thus, hydrogenation induces excess oxygen vacancies by forming ~ H2O bonding and thereby results in localization of electron on Ti atoms. Briefly DFT studies concludes that when there is no oxygen vacancy, then the hydrogenation of TiO2 nanocrystals leads to formation of gap states induced by hydrogen 1s orbitals above VBM, and with the increase in hydrogenation, gap-states above VBM and mid-gap states are formed. The increase in hydrogenation leads to excess oxygen vacancies by H2O bond formation and the localization of excess electrons on Ti atoms, which is reflected as room temperature ferromagnetism in the experimental results.

Figure 4. Difference of spin-up and spin-down electron densities isosurface superimposed on the relaxed atomic structure of different hydrogenated TiO2 nanocrystals. Isovalue is 0.1 Å-3. Formation of ~H2O bonds is highlighted by arrows.


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In order to study other characteristic changes in hydrogenated TiO2, morphological analyses of pristine and vacuum-hydrogenated TiO2 NPs were carried out using SEM and HRTEM analysis. Figure S6 (Supporting information) shows the SEM images of TiO2 NPs and hydrogenated TiO2 NPs and which depicts the clusters of TiO2 NPs. Figure 5a, b illustrates the HRTEM images, which demonstrate TiO2 NPs of particle size of the order of 10 nm and the inter planar spacing of anatase TiO2 (101) plane is 0.353 nm. Figure 5c, d shows the lattice fringes correspond to anatase (101) plane of TiO2 NP-1H and TiO2 NP-10H. The uneven interplanar spacing and lattice strains of hydrogenated TiO2 NPs unravel the lattice disorder caused by hydrogenation. Here, TiO2 NPs with small particle size and high surface to volume ratio enhance the hydrogen interaction and hence increases the probability for the defects formation. The specific surface area of pristine TiO2 NPs was calculated from the nitrogen adsorption-desorption isotherms and was found to be 131.30 m2g-1 (Figure S7, supporting information). Powder XRD patterns and Raman spectra (Figure 6) confirm the anatase phase of TiO2 NPs and vacuum-hydrogenated TiO2 NPs. Powder XRD technique reveals that even after hydrogenation, TiO2 NPs retains the anatase phase which exhibits prominent anatase peaks. Raman spectra show prominent anatase phase peaks corresponding to the Raman active modes of vibrations. Anatase TiO2 has six Raman active vibration modes (A1g+2B1g+3Eg) 46,47

, where, Eg, B1g, and A1g peaks arise due to the symmetric stretching vibration of O-Ti-O,

symmetric bending vibration of O-Ti-O and anti-symmetric bending vibration of O-Ti-O bonds respectively.48 The blue shift in Eg (1) peak and the broadening after the hydrogenation disclose the lattice disorder induced in the system.49 The vacuum annealing and the subsequent hydrogenation increase hydrogen interaction with the TiO2 NPs, which have high specific surface area. Therefore highly reactive hydrogen can have a tendency to have bonding with Ti or O sites thereby cause lattice distortions in TiO6 octahedrons. Thus 15 ACS Paragon Plus Environment


hydrogen-induced disorder affects the molecular vibrations and thereby results shift of Eg (1) peak.

Figure 5. (a, b) High resolution transmission electron microscope (HRTEM) images of TiO2 NPs, (c) TiO2 NP-1H (d) and TiO2 NP-10 H, illustrates interplanar spacing of (101) plane.

(b)

(a) TiO2 NP-10H

TiO2 NP

Eg (1)

TiO2 NP-1H

TiO2 NP-1H

TiO2 NP-10H

10

20

30

40

60

70

(303)

(215)

(204)

(116) (220)

(200)

50

(105) (211)

(004)

(101)

Intensity (a.u.)

TiO2 NP

Intensity (a.u.)

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

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80

90

2θ (degree)

100

B1g

Eg (2)

200

300

400

A1g +B1g

500

Raman shift (cm-1)


Eg (3)

600

700

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Figure 6. (a) Powder X-ray diffraction pattern shows the anatase phase of pristine as well as hydrogenated TiO2 NPs, (b) Raman spectra of pristine and hydrogenated TiO2 NPs Figure 7a illustrates the enhanced ultraviolet (UV)-visible absorption of TiO2 NPs after hydrogenation. Hydrogenation results in shift of the absorbance peak towards higher wavelength, which reveals that the band gap narrowing due to the defect induced mid-gap states. Figure 7b represents the Tauc plot 50,51 for band gap calculation, where TiO2-10H and TiO2-1H shows gradual decrease in band gap compared to pristine TiO2 NPs. The calculated band gaps for TiO2 NP, TiO2 NP-1H and TiO2 NP-10H are 3.01 eV, 2.85 eV and 2.75 eV respectively. Thus, it concludes that hydrogen induced defects have critical role in band gap narrowing and thereby enhances optical absorption.52 Stability of hydrogenated TiO2 in atmospheric conditions is most important for the multifunctional applications. Even after storing the sample in ambient conditions for more than 90 days, we could repeat the p-type conductivity and ferromagnetic behavior in hydrogenated samples. It affirms the stable p-type conductivity and ferromagnetic ordering in vacuum-hydrogenated samples. To reaffirm the stable defect formation in the sample, solid state 1H NMR measurement was carried out after 90 days. It confirms that the incorporated hydrogen in TiO2 makes stable bonding, which plays here a major role in tuning the electrical and magnetic properties. Figure 7c illustrates 1H NMR spectra of pristine as well as hydrogenated TiO2 NPs. Pristine TiO2 NPs, TiO2 NP-1H and TiO2 NP-10H exhibit chemical shift of 5.704 ppm, 5.713 ppm and 5.722 ppm respectively. The increase in line width and peak intensity after hydrogenation reasserts the disorder formation. The emergence of a new peak near to ~1 ppm as reported in literature, further confirms the new hydrogen bonding in TiO2 lattice.13,53,54 Thus 1H NMR spectra give strong confirmation for the stable bonding of hydrogen with different lattice sites, which results in mid-gap states and consequently stable p-type conductivity. XPS analysis was carried to understand the chemical states and to confirm the presence of doping species after vacuum-hydrogenation (Figure S8). It reveals the formation of Ti3+ after vacuum hydrogenation for TiO2 NP-1H, which is attributed to the removal of H2O bond and due to the localization of excess electrons at Ti4+ sites. Figure S8a represent the deconvoluted spectra of Ti 2p3/2, which elucidates the peaks corresponding to 17 ACS Paragon Plus Environment


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Ti3+ and Ti4+ states.27 Figure S8b depicts the XPS analysis of O 1s and deconvoluted to two peaks corresponding to Ti-O and Ti-OH.17 The peak positions and atomic % concentration are tabulated in Table S1. The estimated atomic % concentration of Ti3+ is 23.08 % and OH is 20.01 % and affirms that both has significant role in hydrogen induced properties.

Figure 7. (a) UV-Visible absorption spectra of TiO2 NPs and hydrogenated TiO2 NPs, (b) Tauc plot elucidating band gap narrowing of hydrogenated TiO2 NPs, (c) 1H NMR spectra of TiO2 NPs, TiO2 NP-1H and TiO2 NP-10H, depicts the bonded hydrogen in hydrogenated TiO2

4. Conclusion


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In conclusion, the present work elucidates stable p-type conductivity and room temperature ferromagnetism in TiO2 through a novel method of vacuum-hydrogenation which combines vacuum annealing as well as hydrogenation. The disorder induced by hydrogenation has an important role in p-type conductivity of hydrogenated TiO2, which counteracts the prevailing n-type conductivity owing to its native defects. This carrier type switching is attributed to the stable bonding of hydrogen in TiO2 lattice. The room temperature ferromagnetic ordering of hydrogenated TiO2 is due to the combined effect of Ti3+ centers and single electron occupied oxygen vacancy clusters. DFT simulation reveals that the excess oxygen vacancies are formed due to H2O formation during vacuum annealing and hydrogenation. This results in localization of electrons around Ti sites which in turn results in the emergence of well pronounced mid-gap states above VBM. These mid-gap states are mostly due to the 3d orbitals of Ti atoms. DFT calculations also show the strong spin polarization induced by Ti 3d orbitals, which causes ferromagnetic ordering. Thus the p-type hydrogenated TiO2, which exhibits room temperature ferromagnetism and enhanced optical absorption gives a new opening towards the foreseen photocathode and sensing applications which needs to be investigated further. Acknowledgements Authors thank Indian Institute of Technology Madras, India for supporting this work. For Solid state 1H NMR and EPR measurements, we acknowledge Department of chemistry, IIT Madras and SAIF, IIT Madras. The authors thank Masashi Akabori for his help with Hall measurement system.

Supporting Information Field cooled M-T curves of Teflon, pristine TiO2 and hydrogenated TiO2 NPs at 5000 Oe; MH curves of pristine and hydrogenated TiO2 NPs at 5 K; Room temperature EPR spectra of 19 ACS Paragon Plus Environment


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TiO2 NP, TiO2 NP-1H and TiO2 NP-10H; Bulk Anatase DOS - Hydrogenated Ti25O50H4-H11 nanocrystal DOS; Field Emission Scanning Electron Microscopy images; Nitrogen adsorption-desorption isotherms of pristine TiO2 NPs; X-ray photoelectron spectra of (a) Ti 2p

3/2

and (b) O 1s of TiO2 NP-1H; Distribution of oxidation states and doping species

obtained from XPS analysis.

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Table of Contents Graphic: Graphical abstract illustrates the hydrogen bonding with Ti and O atoms along with H2O bond formation, which results in localization of electrons at Ti sites, following mid-gap formation


Theoretical Insights into the Experimental Observation of Stable p

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