Insights into the Electrocatalytic Behavior of Defect-Centered Reduced

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Insights Into the Defect-Centred Electrocatalytic Behavior of Reduced Titania (TiO ) 1.23

Jayashree Swaminathan, and Subbiah Ravichandran J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10754 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on December 29, 2017

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The Journal of Physical Chemistry

INSIGHTS INTO THE ELECTROCATALYTIC BEHAVIOR OF DEFECT-CENTRED REDUCED TITANIA (TiO1.23) Jayashree Swaminathan*, Subbiah Ravichandran* CSIR- Central Electrochemical Research Institute, Karaikudi- 630003, Tamilnadu, India E-mail: [email protected]; [email protected]

ABSTRACT As an attempt to explore the instantaneous changes that occur in titania during cathodization process, herein we employ Electron Paramagnetic Resonance (EPR) operando spectroscopy throughout the cathodization. This in-situ probing facilitates an experimentally verifiable clue about the underlying active sites in the highly active catalyst (TiO1.23) resulted from the cathodization. Furthermore, this study correlates the evolution of cathodization driven defects and the resultant polaronic motion with Hydrogen Evolution Reaction (HER). The defect richness and structural diversity in the reduced titania attribute ‘magic effects’ on its charge carrier dynamics and activity. As well, the enhanced electro-catalytic activity of TiO1.23 is explained in terms of its orbital re-construction, electronic and structural modifications. Moreover, work function measurements reveal the shifting of fermi level towards the conduction band by the defect mediated urbach tail and bound excitonic emissions, in line with the findings based on. EPR spectroscopy. The observed phenomenon in TiO1.23 is further validated by its high (negative) surface charge, enhanced hydrophilicity and surface roughness compared to native TiO2. Therefore, these studies are much earmarked for deep insight into the electronic structure modification of titania during the cathodization process, and also provide a better understanding of the HER process occurred in TiO1.23.

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INTRODUCTION Hydrogen from water electrolysis is the most viable route compared to the conventional fossil fuel based energy system that emits greenhouse gases.1,2 But, the current scenario of using expensive platinum based electro-catalysts in water electrolyzer limit the possibility to substitute the subsisting low-cost ‘fossil fuels’.3 Thus, it has long been the aspiration of researchers to obtain an earth-abundant element as an efficient water splitting catalyst by a viable and facile route.4–6 On the grounds, our group has recently developed reduced titania ‘TiO1.23’ by a mere cathodic reduction process and demonstrated as an efficient HER catalyst.7 The referred work manifests the massive increase of oxygen vacancy content in titania and improvement in its HER electrocatalytic activity, when it is cathodically reduced. The stoichiometric composition of titanium oxide samples are calculated from Reitveld refinement and found to be TiO2-x (0.01 < x < 0.49), TiO1.51, TiO1.23 and TiO0.89

during 1, 2, 3 and 4 h cathodization, respectively.7 It also

demonstrates that amorphous titania (TiO2) formed on anodizing titanium is found to be an inactive HER electro-catalyst. However on subsequent cathodization (-1.5 V vs. mercury/ mercurous sulfate electrode), amorphous titania crystallizes into the distorted cubic system (space group: Fm3̅m) with a composition of TiO1.23 and turns to be a highly HER catalyst. On the contrary, this enhanced HER catalytic activity of titania is diminished on extending the cathodization for longer periods. This staggering behavior of TiO2 during the cathodization process needs to be understood well in order to improve its catalytic performance. Besides functioning as a HER catalyst, this reduced titania (TiO2-x) also plays an indispensable role in oxygen reduction reaction for energy conversion systems, electrochemical reduction of nitrobenzene, photo-electrochemical water splitting.8–12 On considering the wide applicability of TiO2-x, it may not be surprising that this vibrant TiO2-x catalyst and its preparation methodologies have been a huge surge of interest and an object of investigations both by theoretical calculations and experimental investigations in recent years, as a curiosity to know its functionality.13–18 Thus, it appears crucial to kindle the catalytic origin in reduced titania and it may be obvious to

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focus deeply on the cathodization strategies as it tunes the intrinsic insulating titania into a highly electrically conductive material. Hence, it may be apparent to monitor the electronic properties of titania during the cathodization, which are central to its electrocatalytic activity. Generally, O 2p and Ti 3d states overlap, hybridize and form a wide bandgap (Eg=3.2 eV) in titania.19 However on cathodic reduction process, the classical reducing agent ‘hydrogen’ adsorbs, which reduces titania and creates oxygen vacancies.20 Consequently, its regular octahedron is distorted owing to the created oxygen vacancies (Fig.1) and results in electron occupancy in Ti 3d-orbitals, which reduces its bandgap.21

Fig. 1: Schematic of defect driven distortion in TiO6 octahedra Thus, these incorporated oxygen defects lead to color centers and impose a great impact on its structural, optical and electronic properties ,22–25 which are crucial factors for catalytic properties. The possible types of color centers (F, F+, F++) formed26,

27

in the event of cathodization are

shown in Fig. 2.

Fig 2: Schematic illustration and description of F center formation in titania26, 27



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Hence during cathodic reduction process, there is a possibility of transformation in titania from F center (diamagnetic)

F+ center (weak paramagnetic)

F++

center

(strong-

paramagnetic). In order to observe the precise changes of color centers in titania, it is worthy to employ a technique that can distinguish the magnetic properties based on spin states. Hence, Electron Paramagnetic Resonance (EPR) has been employed as an in-situ tool during the cathodization process in this study. In general, EPR allows detailed mapping of the chemical environment, electron spin density and unravels the change of metal ion valence state or formation of active sites in the system.28–30 It even provides structural information like point defects, symmetry, atomic and lattice configuration. Hence, an in-situ EPR performed during the cathodization process may ground deeper insight into the formation of electrocatalytically active ‘reduced titania’ (TiO1.23). To envisage the role of titania’s charge state on its HER catalytic activity, linear sweep voltammogram (LSV) has been concurrently taken at periodic intervals during the in-situ measurements.

Though many reports address ex-situ EPR studies once after the reduction process, it has been employed only to detect the presence of trapped electrons/ holes in it.31–37 Moreover, those studies dealt little with the structure-reactivity relationship of the reduced titania. As of now, there is no straightforward relationship between free electrons in reduced titania and its physiochemical properties. But, a systematic knowledge of trapped electrons in reduced titania is essential in order to rationalize the behavior of titania. In this scenario, EPR can play an intellectually demanding role in providing the factual basis on trapped electrons and provide a mechanistic model of the active catalyst TiO1.23. Hence in this study, a systematic attempt has been taken to explain the overarching electrocatalytic behavior of TiO1.23 by its defect geometry and charge distribution through in-situ EPR. In addition, we also investigate the alteration of titania’s surface energetics on the cathodization process through ex-situ surface charge, surface potential, surface morphology, roughness and contact angle measurements. We also depict the electronic energy level diagram of TiO1.23


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catalyst with the aid of absorbance, emission, LSV and Scanning Kelvin Probe (SKP) results. This enables unprecedented insights into the fundamental understanding of the active species in reduced titania and unveils the relationship between electronic structure, surface properties, and its catalytic activity.

RESULTS The LSV and simultaneous in-situ EPR spectrum of titania during cathodization at different time intervals are shown in Fig 3a & 3b, respectively.. Also, the HER overpotential at a cathodic current density of 10 mA.cm-2 (from LSV curves) is represented as η10 mA.cm-2.

Fig 3: (a) LSV of titania and reduced titania samples measured at cathodization interval of 1 h; (b) Room temperature(310 K) in-situ EPR spectra of titania during the cathodization process From fig. 3(b), bare titania (TiO2) shows EPR sub-bands in electron center (g- factor > 2.0) as well as in hole center region (g- factor < 2.0) and the corresponding LSV curve (Fig 3(a)) shows higher η10 mA/cm2~700 mV. This indicates that the electrons and holes are mutually ubiquitous in TiO2 and its characteristic amphoteric nature38 causes the poor HER activity. However, on



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extending the cathodization process for 1h, the EPR sub-bands diminish and an intense signal at a hole center (gx = gy = g┴ =1.96 and gz = g|| = 1.89) is emerged. The observed anisotropic line shape of EPR signal with an axial symmetry is due to tetragonally distorted d1 ion (Ti3+);39 which is attributed to the abstraction of axial O2- ion from titania and subsequent trapping of an electron in the titanium lattice for charge compensation.40 The detected splitting factor values (g┴ > g||) also reveal the tetragonal elongation of TiO6 octahedra.41,

42

It confirms that titania losses its

characteristic octahedral symmetry and leads to the formation of color centers to attain charge neutrality. Thus, the resultant TiO2-x reforms to be an electron rich compound on cathodization and it shows lesser η10 mA/cm2 ~ 570 mV compared to bare TiO2. On continuing the cathodization process further, EPR signal becomes more and more intense in nature and the corresponding LSV curves reflect the enhanced HER electro-catalytic activity (less overpotential). To be noted, the EPR signal diminishes completely on cathodization of titania at 3h, despite the material (TiO1.23) is highly active towards HER. This phenomenon could be due to the formation of delocalized defect complex at longer cathodization. To know the nature of delocalized defect centers, low temperature (140 K) ex-situ EPR analysis is performed exclusively on TiO1.23 and TiO0.89 and the spectrum is shown in Fig. 4.

Fig 4: Low temperature (140 K) ex-situ EPR spectra of TiO1.23 and TiO0.89 As seen from Fig. 4, TiO1.23 and TiO0.89 exhibit isotropic EPR signal at g-factor = 1.924. The switch-over of anisotropic to highly isotropic EPR signal at longer cathodization confirms the


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formation of electron pinned Ti2+ defect complexes in cubic symmetry. i.e., further conversion of Ti3+ polarons (S=1/2) to Ti2+ bipolarons (S=1) on extensive cathodization process.7,

43

Higher

peak-peak EPR signal intensity of TiO0.89 than TiO1.23 indicates the increase in bipolaron concentration on cathodization. From these observations, we can speculate that the presence of metallic44 Ti2+ ion (in place of Ti3+) in reduced titania makes it highly active HER catalyst (least HER overpotential - η10

2 mA/cm

~198 mV) in case of TiO1.23. However, LSV of TiO0.89 shows

higher HER overpotential (η10mA/cm2 ~280 mV) compared to TiO1.23 in spite of increased (Ti2+) EPR signal intensity. The decreased electro-catalytic activity at longer cathodization time (> 3 h) is thought to be related to scattering of mobile electrons by vacancy induced electrons. i.e., prolonged cathodization creates high density of delocalized, charged oxygen vacancy sites (ionized oxygen vacancy with trapped electrons) as schematically represented in Fig. 5. Consequently, the mobile electrons will experience stronger repulsion and scattering due to the charge association and distortion near the Ti ion site, which in-turn weakens the catalytic activity.

Fig 5: Schematic representation of polaronic effect in reduced titania To validate the conducive role of spin carriers and the resultant catalytic behavior, titania’s conductivity is calculated from impedance spectrum45 at different cathodization intervals and its trend on cathodization is shown in Fig. S5. As seen, conductivity is increased with cathodization and high conductivity is observed for TiO1.23 among all titania samples (TiO2, TiO2-x,TiO1.51, TiO1.23, TiO0.89).This also portrays a clear evidence of bi-polaron (Ti2+) formation in TiO1.23. However after 3 h of cathodization, conductivity reduces once again and it could be due to the formation of bi-polaron bands, which can cause scattering within the mobile electrons.



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The varied electronic coupling in reduced titania at different level of cathodization is further confirmed by line shape analysis of EPR signal (Fig S6) using method of moments (see supporting information for further details). As well, the amplitude (intensity) and line width (broadening) of the EPR signals (Ti3+) are analyzed and shown in Fig S7 and S8, respectively. These extensive EPR analyses illustrate that the electrocatalytic activity of titania is tuned significantly at different cathodization periods due to the evolution of defects and the consequent spatial overlap of electronic charge carriers. Even though, oxygen defects in titania markedly improve the catalytic activity in comparison to defect-free titania, it is clear from the above studies that the defect concentration could be a decisive factor in determining the electrocatalytic behavior. Moreover, space distribution of oxygen defects is also critical for the dynamics of charge carriers and hence the catalytic activity. It also clearly demonstrates that the titanium ion (Ti2+), which exhibits optimum coupling of delocalized electrons governs catalytic performance. The optimum disorderliness in reduced titania favors effective exchange pathway for electron transport. The reason is probably that the electron density at a discrete titanium ion in disordered systems can be easily delocalized via d-orbitals overlapping, which is essential for good catalytic performance. Thus, on correlating the EPR and LSV interpretations, we can presume that the highly active defect- rich catalyst ‘TiO1.23’ consists of an effectively coupled TiO6 octahedra with a dominant Ti valence state of +2 (ensures effective electron transport) and a tailored distance between them (for an efficient spin-spin exchange pathway and conductivity). This defect-rich geometry has optimum concentration of bipolarons (Ti2+), which helps hopping (conduction) of electrons. Hence, the in-situ EPR studies have provided a fingerprint for the identification of active species in the catalyst (TiO1.23) and endowed a factual basis for the detailed understanding of the catalyst structure. These interesting observations further motivate us to explore the defect induced optical absorption and emission behavior of reduced titania. Generally, optical absorbance of any material depends on its spin density that varies with the d-d transition.46 Hence, absorbance measurement reveals a clear picture of electronic structure and defects in the materials. As seen


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from the absorbance spectra (Fig. S9), a red-shift in the absorbance edge (~ 380 nm) along with a notable visible absorbance in the range of 420 – 500 nm is observed for TiO2-x, TiO1.51 and TiO1.23, compared to TiO2. Also, a weak absorbance band is noticed for TiO2-x, TiO1.51 in the range of 600 – 700 nm (See supplementary information for further details). The red shift can be visualized as a strain induced band tailing effect, caused by vacancy created charge carriers.47, 48 On the contrary, a slight blue shift is observed exceptionally in the case of TiO0.89 and it could be due to the Moss- Burstein band filling effect by excessive charge carriers.49 To be noted in Fig. S9, the reduction of total absorbance bands from 3 to 2 (i.e., diminishing of the weak band) confirms the transformation of polarons to bi-polarons on cathodization.50 Besides, the increase in intensity of visible absorption with cathodization is the indicative of many closely spaced defect energy levels located within its bandgap. i.e., creation of vacancy band states below the conduction band. Thus, at longer cathodization, all defect states are merged together and the effective bandgap is reduced by `unpinned’ or ‘mobile’ band-edge position. This makes reduced titania to behave like a semi-metal on extensive cathodization. Thus, the incurred changes in defect states of reduced titania on cathodization are well revealed by the measured absorbance spectra. To further exemplify the ensuing changes that occurred on cathodic reduction, kubelka- munk function has been carried out on the absorbance spectra (see supporting information) and the plot is shown in Fig. 6.

Fig 6: Kubelka-munk plot of titania and reduced titania samples measured at cathodization interval of 1 h ACS Paragon Plus Environment

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From Fig. 6, we can infer that except TiO2, all cathodized samples exhibit band tailing effect below its optical bandgap. i.e., an absorption tail is extended deep on both sides of the valence band maxima and conduction band minima. This defect tail is due to delocalized electrons and referred to as ‘Urbach tail’, which is a function of disorder or mid-gap states in the system;51 the energy associated with urbach tail is Urbach energy (Eu) and it is calculated by plotting lnα against E (Fig. S10).52 The urbach energy for TiO2-x, TiO1.51, TiO1.23 and TiO0.89 are 66, 139, 196 and 208 meV, respectively. The remarkable increase in urbach energy magnitude with cathodization eventually indicates the increase in width of the defect bands, which effectively reduces the band gap. To validate the above phenomenon, bandgap of the samples has been calculated and it is found to be 3.25, 2.81, 1.66, 1.55 and 1.57 eV for TiO2, TiO2-x, TiO1.51, TiO1.23 and TiO0.89, respectively. This clearly supports that the number of defect levels below the conduction band increases and leads to larger width defect band as the cathodization progresses. Generally, the extent of hybridization and level of filling (bandgap) depend on its local coordination, defect states and in-turn its crystal structure.53 Thus, diminution of bandgap with cathodization is related to the changes incurred in titania’s crystal structure on cathodization. However, the slightly larger bandgap for TiO0.89 compared to TiO1.23 could be due to the bandfilling effect by excessive charge carriers.49 To well expose the conventional trend, the observed inverse relationship between urbach energy and bandgap in reduced titania is compared in Fig. S11. Thus, from the absorbance measurements, we can infer that the defect densities play an important role in controlling the energetic and electrical properties of the reduced titania. As known, photoluminescence (PL) spectrum provides information on defect centers since emission phenomenon depends on crystal symmetry, stoichiometry and surface states of the materials.54 Hence, the titania samples are excited at emission wavelength of 330 nm and the corresponding PL spectra are shown in Fig. 7. Compared to TiO2, all cathodized samples exhibit broad visible emission; it indicates the presence of structural defects in reduced titania samples as shallow and deep levels below its conduction band. As cathodization time increases, the intensity and width of the excitonic visible PL also increases. These results substantiate the enhancement in defect density on cathodization. These defect states act as donor sites and hence improve the electronic conductivities as well as the overall catalytic performance of the reduced 10 ACS Paragon Plus Environment

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titania. On comparing the emission spectra of all samples, the enhanced luminescence intensity ratio of violet to orange in TiO1.23 and TiO0.89 indicate the transformation of defect states from deep to shallower level (with respect to the CB edges) on extensive cathodization. It also renders a clinching evidence of interplay between the defect states and catalytic activity in the cathodized samples. In order to identify and demonstrate the distinct role of defect centers in highly HER active - TiO1.23 system, the emission spectrum of TiO1.23 is de-convoluted and shown in Fig. S12.

Fig 7: Photoluminescence spectra of titania and reduced titania samples measured at cathodization interval of 1 h

As a further pursuit to understand fundamentally the surface characteristics and functionality of titania during cathodization process, surface roughness of TiO2 and TiO1.23 are characterized and shown in Fig. 8. From the optical microscopic image (Fig. 8), surface morphological features of TiO2 shows non- uniform porous structure with an average roughness of 5.3 µm, while TiO1.23 exhibits uniform needle like structure with an average roughness of 46 µm. Thus, the surface roughness of TiO1.23 is approximately nine times higher than that of TiO2. This disorder induced surface roughness plays a vital role in HER performance, since the roughness is expected to increase the active surface area.



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Fig 8: 3D optical microscopic image of TiO2 and TiO1.23

Complementing the optical microscopic analysis, the localized structural changes induced by cathodization are acquired by TEM imaging of TiO2 and TiO1.23 (Fig. 9). From the images, TiO2 exhibits irregular shape with micron sized particles, whereas TiO1.23 shows uniform nano-porous structure with the average size around 100-150 nm. These results are well correlated with the earlier FESEM studies.7 The observed TiO2 micro-particles could be a fraction of micro-porous amorphous titania formed during anodization, while the characteristic nano-porous structure of TiO1.23 could be resulted from the rapid gas evolution during cathodization process. The corresponding SAED patterns are shown as insets in TEM images. TiO2 diffraction pattern shows the features of amorphous materials, whereas TiO1.23 displays the well-defined (crystalline) diffraction spots. On carefully analyzing the spots, it shows super-imposed patterns of TiO (blue dot) and substrate Ti (yellow dot) diffraction spots along [110] and [001] zone axis, respectively. For better comprehend, [110] zone axis view of TiO structural model is compared with the observed SAED pattern (Fig. S13).


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Fig 9: TEM images and SAED pattern of (a) TiO2 and (b) TiO1.23 The presence of TiO and Ti is further manifested by line profile analysis (Fig. 10) of TiO1.23 SAED pattern. The diffuse streaking in between TiO and Ti Bragg spots are likely due to oxygen vacancies with short-range order.

Fig 10: SAED pattern of TiO1.23 and its corresponding line profile analysis Surface charge and the corresponding zeta-potential is an informative property, since it is directly related to the electro-kinetic charge density and adsorptive properties of the material.56,57 The equilibrated zeta potential (from Fig. 11) of TiO2 and TiO1.23 are found to be negative at all pH values.



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Fig. 11: Zeta potential of TiO2 and TiO1.23 -

This negative surface charge is a consequence of Ti-O formation on the surface of titania ; it is produced by hydroxyl ion dissociation from surface titanol groups in titania, as shown in equation 1.58, 59 The referred titanol (hydroxyl ion adsorption / chemisorption on titania) usually forms to satisfy its surface valence.

-OH -Ti-OH2

-OH -Ti-OH

[-Ti-O]-

1

Thus, the surface under-coordination is illustrated by its negative charge. Compared to TiO2, higher negative charge in TiO1.23 confirms the more under-coordinated surface titanium ion in TiO1.23. On close observation, we can notice that TiO1.23 exhibits significant negative charge especially at lower pH values (acidic pH) and it could be due to the following reason: Generally, O–H bonding strength with co-ordinating titanium ions in titania depends on its electronegativity, i.e., polarization of covalent electron pairs with co-ordinating (singly, doubly, or triply coordinated) titanium ions.59 This strength is favored in an acidic pH, rather than at neutral / alkaline pH and ultimately renders thin diffuse double layer of hydroxyl ions on its surface, particularly at lower pH values.


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The presence of hydroxyl terminated surface in reduced titania plays a key role in faster HER activity by preferential adsorption of H+ ions. Thus, the interfacial structure of titania is modified through the changes in surface energetic on cathodization process and imposes a favorable HER activity at acidic conditions. In addition, high negative charge of TiO1.23 renders -OH group attachment on the bridging sites rather than at terminal sites and favors preferential adsorption of water on titania. The water adsorption characteristics can be easily visualized by contact angle measurement (Fig. 12) taken for titania samples at different cathodization periods. The contact angle shows decreasing trend with cathodization time. i.e. TiO2 > TiO2-x > TiO1.51> TiO1.23. From these observations, we can infer that the reduced titania shows stronger affinity for water than on stoichiometric TiO2.

Fig. 12: Modified reactivity of titania towards water on cathodization

The adsorbed water obviously stimulates the reactant water molecules (beneficial for selectivity), provides higher surface conductance (through the generation of hydroxyl ions), and facilitates the desorption of product, which likely leads to higher HER activity in TiO1.23. To better comprehend the role of defect disorder in reduced titania on catalytic activity, SKP has



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been used to distinguish the disparity in surface electronic potentials of TiO2 and TiO1.23. The mapping of Constant Potential Difference (CPD) value from SKP is an ‘analytical signal’ of surface electronic potential/ work function of the samples (See Supporting Information for further details). The defect-driven restructuring of surface electronic potential of titania is shown in Fig.13; the average CPD value of TiO2 and TiO1.23 is found to be -185 mV and + 137 mV, respectively. Hence, the corresponding work function is calculated to be 4.315 and 4.6367 eV for TiO2 and TiO1.23, respectively. Therefore, we can presume that there is 0.32 eV shift in fermi level of titania on cathodization. This shifting is mainly attributed to the increased defect density in reduced titania (TiO1.23) and it is well agreed with the above discussed results. Furthermore, the increased work function of TiO1.23 compared to TiO2 indicates the energy required for removing an electron in TiO1.23 is higher than that of TiO2. This confirms the presence of highly bound electron in reduced titania and it is an essential requirement for stable HER activity. Besides, the oscillating CPD value of TiO1.23 affirms its distorted character.

Fig 13: Surface electronic potential mapping of (a) TiO2 and (b) TiO1.23 Discussion Based on the above results, it is very apparent that the catalytically in-active titania converts into


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a highly active catalyst (reduced titania) in due course of cathodization process. The reduced titania becomes extremely active only at optimum cathodization interval. It is expected that an optimum level of cathodization produces optimal concentration of metallic Ti2+ ion with oxygen vacancies, which act as active centers. Thus, these results provide an insight into the formed defect centers in reduced titania as well as its escalation on cathodization process. In turn, this makes to suggest a probable scheme on structural evolution of titania. As evidenced from the EPR results, the conception of structural distortions in titaina is the result of transformations of Ti4+ to Ti3+ to Ti2+ ion on cathodization. The sequential alteration in 3d-electron density and the consequent de-localized electronic distribution around Ti ion on cathodization reflect on the symmetry, co-ordination, geometric packing, electro-static field that leads to distortion in reduced titania. Thus, these changes in valence state of titania on cathodization can be related to its structural changes as shown in Fig. 14.

Fig 14: Driven modifications in octahedral packing of titania on cathodization based on EPR observations Hence, the acquired catalytic activity of titania on cathodziation can explicitly be interrelated with its structural evolution, as depicted in Fig. 15.



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Fig 15: Schematic representation of structural evolution of titania on cathodization process The mechanism for the structural changes occurred in titania on cathodization is explained as follows: As cathodization progresses, oxygen vacancies are created in titania that disturb the lattice periodicity, valence state and charge symmetry in its crystal structure. Hence, the local electrostatic stability of titania is rattled; thereby, a pair of electron is distributed at each vacant site for charge compensation. These electrons bound weakly and localized in the lattice due to effective positive charge of the oxygen vacancy sites. Later, one of the electrons from vacancy site is attracted towards neighborhood Ti4+ ion in six-fold coordinated [TiO6] octahedra and yields Ti3+ ion. The ensued Ti3+ ion experiences strong electrostatic repulsion from neighborhood oxygen vacancy (OV) due to effective positive charge. Meanwhile, the surrounding O2− ions on the vertical plane of Ti3+ ion will shift towards the OV due to electrostatic attraction. As a whole, tetragonal distortion of [TiO6] octahedra occurs and leads to five-fold coordinated [TiO5] polyhedra. On continuing the cathodization process further, extensive reduction of titania is triggered; in which the randomly oriented five-fold coordinated [TiO5] polyhedra is re-arranged and converted into ordered six-fold co-ordinated [TiO6] octahedra and leads to the formation of cubic TiO. i. e., further reduction of Ti3+ to Ti2+ occurs. Thus, the local co-ordination has stronger influence on titania’s structure and engineering the


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structure by cathodization process to form a highly symmetric cubic TiO structure (i.e., a perfect octahedral symmetry-Oh) demonstrates a bombastic density of defects in it. This highly symmetric structure is due to its high local geometrical distortion on cathodization, which leads to relaxation and in turn elevate the symmetry of the Ti site. Hence, the coordination number of Ti atoms with O atoms in titania has changed from six to five and again to six during cathodic reduction process. The local site symmetry around Ti ion would also change from D2h to D4h and then to Oh. i.e., large concentration of O vacancies results in weakening of structural rigidity. In such a way, phase transition and catalytically active structure of titania has occurred on cathodization process. Thus, we can speculate that the increase in catalytic activity in reduced titania is attributed to favorable structural changes occurred in titania on cathodization and the inherent strong spin– spin correlation and polaron hopping behavior. At low spin concentration, the isolated polarons are dominant; whereas at high spin concentrations, the polaron spin correlated state prevails and enforces the bipolaron formation. The effective bipolaron formation favors conductivity and hence constructive HER activity. However, very high spin concentration leads to the formation of polar catastrophe, which is detrimental to the catalytic activity. On correlating the construed EPR spectra (Fig. 14) with crystal field theory, d-orbital splitting has occurred to counterbalance the created oxygen voids so as to minimize energy .

In

TiO2

(before cathodization), unoccupied Ti 3d like anti-bonding orbitals are strongly pushed up from O 2p like bonding orbitals on hybridization. Its ground state (electron configuration Ar(3d)0) is a five-fold degenerate orbitals, which is split by the predominant octahedral crystal field into twofold eg orbitals and threefold t2g orbitals, separated by an energy term ∆0. However, on cathodic reduction, an electron is occupied in Ti 3d (t2g state) due to the interaction between Ti ion and the imbalanced static electric field generated by the surrounding oxygen vacancy/ ion. This (uneven) occupancy of electron in t2g state leads to Ti3+ formation and enhances the overall energy of the system. To reduce the energy of titania’s system, it exhibits weak asymmetric crystal field or Jahn- Teller effect in an attempt to maintain stable structure. As a consequence, one of the t2g orbital is lowered with shortening/lengthening of bond along z axis and hence



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tetragonal distortion of titania occurs. Thus, three-fold spin degeneracy (t2g state) in a crystal field of tetragonal symmetry is partly lifted by spin-orbit interaction across weak additional axial fields; therefore t2g state is split into singly degenerated (dxy) and a doubly degenerated (dyz, dzx) orbitals. On continuing the cathodization process, further occupation of vacancy-directed electron in Ti t2g orbitals and results in Ti2+ ion formation. The resulting d orbitals are quantized with respect to three-fold axis and the unpaired electrons dwell in the d orbitals. It accompanies distortion of the titania system with cubic crystal symmetry, where each Ti atoms are in 6-fold coordination with oxygen atoms, while each oxygen is tetragonally surrounded by Ti atoms. Thus, massive distortion has happened in titania on cathodization. Hence, we can infer from the EPR and LSV results that the distortion in reduced titania favors HER activity. However, at longer cathodization time (> 3h), higher distortion may favor scattering and leads to poor catalytic behavior. The change in degeneracy of d-orbitals in presence of ligands and the resultant crystal field splitting is shown in Fig. S14. From the crystal field splitting, we can infer the occurrence of systematic change in crystal-ligand field in titania system on cathodic reduction; which results in new kind of orbital reconstruction that favors HER activity. Furthermore, due to d-electron filling and its increased concentration in reduced titania, dense packing of d-band occurs and makes its electronic structure similar to that of noble metals. In addition, there is a decrease in degree of covalency with an increase in d-d exchange energy. Consequently, the corresponding Ti ions come closer due to electronic interaction that renders metallic behavior, which supports the highly active behavior of reduced titania. Apart from the above d-band structure, oxygen vacancies present in reduced titania can also tune the band gap as well as conductivity of TiO2.48 This is well supported by defect-mediated pronounced visible absorption and emission behavior of cathodized samples (Fig. S9 & Fig. 7) with marked effect on band gap, urbach tail and absorption onset. These modifications are mainly attributed to the extent of orbital coupling between O 2p and Ti 3d states; which is induced by structural modification, especially bond length and TiO6 stacking on cathodization. Thus, Ti-O bonds undergo reorientation, reconstruction and restructuring into variable TiOx polyhedra during the process of cathodization. These factors will collectively increase the potential energy, which in-turn enhance the donor density, conductivity and hence HER activity.


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In addition, the madelung potential from oxygen vacancies causes narrow d-orbital splitting between t2g−eg states on cathodic reduction. It is well known that the decrease in energy gap between t2g and eg states resulted from the reduction of O 2p–Ti 3d hybridized band states shifts the fermi energy level towards the conduction band maxima. Also, the shift in the Fermi level is well correlated with the observed SKP measurements (Fig. 13). Thus, based on the understandings from SKP results together with the absorbance, emission and LSV results, the energy band diagram of TiO2 and TiO1.23 is depicted in Fig. 16 (See supporting information for further details, Fig. S14).

Fig .16: Schematic band energy diagram of TiO2 and TiO1.23 As infer from Fig. 16, there is a remarkable change in the conduction band edge position (Ec) from -3.85 eV (TiO2) to -4.67 eV vs. vacuum (TiO1.23) on cathodization through the occupation of defect states. On analyzing the defect states of TiO1.23, various defect bands from 3.0 to 2.02 eV exist in TiO1.23 that reduces its effective bandgap (Eg) from 3.25 eV to 1.55 eV. It also leads to the enhancement in donor density and shifting of fermi level (EF) from 4.31 to 4.63 eV vs. vacuum with increased work function (ɸ). 21 ACS Paragon Plus Environment


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Above and beyond, the reduced titania becomes outwardly relaxed in configuration with the vacancy defects that further improves the water adsorption through stronger interactions as observed from contact angle results (Fig. 12). Thus, the spatial distribution of unsaturated Ti framework in reduced titania is the beginning of progressive enhancement in its electro-catalytic behavior. These exhaustive analyses and their in-depth understandings enable us to propose the water splitting reactivity model (Fig. S15) of reduced titania. It hypothesizes that the intrinsic oxygen vacancies in reduced titania (TiO1.23) serve as catalytic centers to dissociate H2O by forming hydroxyl groups for every vacancy and hence assist in proton adsorption and hopping. However, at longer cathodization (>3hr), the vacancy induced electrons are very close and represent a highly delocalized electron system. This causes a detrimental effect on its conductivity and hence HER activity. To this end, we can speculate that the inherent electronic and energetic crystal structure associated with TiO1.23 endows overarching electro-catalytic activity.

Conclusion We have systematically used in-situ Electron Paramagnetic Resonance (EPR) in conjunction with the ex-situ analyses such as absorbance, emission, surface charge, roughness, work function and contact angle to provide a detailed insight into the electrocatalytic activity of reduced titania. These systematic approaches enable direct observation of changes in the unsaturated Ti site and their correlation with the catalytic activity and reaction kinetics. We can also surmise that the defect sites density and resultant HER activity is moderated through cathodization by altering the degree of overlap of Ti 3d and O 2p orbitals. For an effective HER catalyst, titania should consists of an effectively coupled TiO6 octahedra with dominant Ti2+ valence state and a tailored distance between them. This originates from engineered oxygen vacancies generated via internal structural modification. It governs favorable electronic distribution, work function, donor density, hydrophilicity and surface roughness, which collectively contribute to an effective electron transport in reduced titania. Moreover, it also facilitates more (negative) charge on the


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surface, which can significantly improve H+ ion adsorption and hence alleviate the HER in an acidic medium. As well, it harmoniously increases the magnitude of defect/ urbach tail, which eventually reduces the bandgap and enhances the excitonic visible PL intensity and disorder induced surface roughness. These findings also open a new direction in designing metal oxide based systems as promising electro-catalysts for water splitting applications.

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Acknowledgement: The authors are so grateful to Dr. Vijayamohanan K Pillai Director-CECRI for his constant support and encouragement. The author S.J. thanks the Department of Science and Technology, New Delhi for the award of DST-INSPIRE fellowship. The author would like to give a reverent honor and heartfelt thanks to Shri. V. M. Shanmugam, Central Instrumentation facility and A. Yamuna for aiding in-situ EPR and SKP studies, respectively. The author S.J. also extends her deep gratitude from the bottom of heart to Dr. S. Vengatesan, Dr. R. Jagannathan and Dr. Kottaisamy for guiding EPR and photoluminescence analysis. Finally, the authors thank the Central Instrumentation Facility, CSIR-CECRI, for the ability to use characterization facilities and Dr. N. Lakshminarasimhan for facilitating photoluminescence studies.

Supporting Information description Experimental section- Characterization and data analysis details- conductivity, spin-spin exchange interaction behavior, EPR linewidth, EPR intensity and absorbance of titania and reduced titania samples with cathodization time, Urbach energy determination – trend of urbach energy and bandgap - De-convolution of photoluminescence spectrum of reduced titania (TiO1.23) - [110] zone axis view of TiO structural model – d orbitals orientation and their crystalfield splitting on cathodization- Conduction band edge determination- Proposed reactivity model of TiO1.23.



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Table of Contents only

Defect-centred HER in reduced titania (TiO1.23) driven by the cathodization of TiO2 (i) Active energy sites from perturbated crystal structure (ii) Localised midgap states induced urbach tail (iii) Disorder induced surface roughness for favourable hydrophilicity (iv) Vacancy stimulated bombastic carrier density (v) Favourable orbital reconstruction and electronic structure (vi) Reactive H+ adsorption sites from surface functional groups (OH-)


Insights into the Electrocatalytic Behavior of Defect-Centered Reduced

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