Unveiling the non-innocence of vanadium dopant in TiO2 nanocrystals for advanced energy storage and smart windows

Vanadium doped TiO2 NCs stand out as a promising candidate for energy storage applications due to its high electrical conductivity and redox properties. However, the thermodynamical behavior of the material under working conditions has not been explored and the reasons for its superior performance remain unlocked. This study explores the use of a combination of advanced in situ spectroscopy techniques, including x-ray absorption spectroscopy (XAS), spectro-electrochemistry (SEC), and electrochemical impedance spectroscopy (EIS) to provide unprecedented insights into the intricate electrochemical reaction mechanisms within these nanocrystals. Density functional theory calculations and EIS reveal the active role of substitutional V ions in the TiO2 anatase network as electron donors, enhancing surface charge and carrier density and improving pseudocapacitive properties. Cyclic voltammetry and in situ SEC reveal that V-doped TiO2 NCs exhibit significantly improved charge storage capacities, particularly in the pseudo-capacitance storage mechanism. In situ SEC and XAS analyses indicate that a more effective reduction of Ti4+ ions occurs during the electrochemical process in doped NCs, leading to higher charge capacitance and faster processes. Furthermore, in situ XAS measurements of the V K-edge revealed that the vanadium ions, beyond improving the redox behavior of the host, also actively participate in the reduction process. The significant changes in the V K-edge XANES and extended x-ray absorption fine structure spectra observed under reduction conditions can be ascribed to a change in the structure and oxidation state of the vanadium ions during the electrochemical reaction.


Introduction
The search for an advanced renewable energy storage technology is essential for the transition towards a more sustainable energy system.This holds significance not only in the storing of grid energy for future use but also in many other energy saving and utilisation applications such as supercapacitors or electrochromic devices.The main breakthroughs in this area are going to come in the form of novel high performance materials and the understanding of their underlying optoelectronic mechanisms.In the last two decades, significant advances have been made in the field of nanomaterial-based energy storage and conversion technologies and, in special, through the precise control of their structural and morphological characteristics at the nanoscale level [1][2][3][4].Recent synthetic advances, particularly in colloidal wet chemistry routes, have led to the development of engineered free-standing inorganic nanocrystals (NCs) in solution, for which high monodispersity, controlled crystalline-structure, and high doping control can be achieved [5][6][7][8].The precise control of the processed colloidal NCs with synthetically tunable physico-chemical properties has allowed the preparation of thin film electrodes with tailored architectures and great potential in key optoelectronic applications.
Titanium dioxide (TiO 2 ) have been extensively explored due to its remarkable electrochemical energy conversion properties and its chemical stability, low cost, non-toxicity and abundance [9].In particular, TiO 2 NCs have recently attracted great attention as an efficient lithium-ion (Li + ) anode material due to its high specific capacity and long term cyclic stability at high charging rates [10,11].Moreover, a significant improvement in the accessible charge/discharge rates is observed with the reduction of the TiO 2 particle dimensions [10].In this sense, TiO 2 NCs have found broad applicability in several energy applications, including batteries [11][12][13], supercapacitors [14,15], and other electrochemical devices such as electrochromic or smart window devices [16,17], that share the same underlying principles of operation [18,19].During their operation, the Li + ion intercalation or insertion process is accompanied by a redox process, as some of the Ti 4+ is reduced to Ti 3+ to compensate for the incorporation of Li + , leading to a color change in the material, that is useful in electrochromic applications and that has been ascribed to the generation of polaron states [20,21].
Despite the improvements as electrodes for energy storage applications achieved by nanostructuring TiO 2 into NCs, there are some limitations that cannot be overcome by only this strategy, such as the restricted specific capacitance and low electrical conductivity, which hinder their use in energy storage applications [22][23][24].In this regard, several transition metal dopants have been used to improve these TiO 2 NCs properties.Ion doping with metal elements containing multiple redox couples can increase the amount of available electrons for electrochemical reactions.Besides, ion doping can influence bond strength, local lattice environment, size of ion migration channels, valence state of cations, and the introduction of new defects, gaps, or vacancies in the host material [25].
In this sense, TiO 2 has been doped with Ni 2+ [26], Co 2+ [27], and Mn 2+ [28], among others, leading to a general improvement in conductivity and structural stability.Moreover, aliovalent Nb 5+ , Mo 6+ or V 5+ ion-doping can increase the population of electrons in the conduction band and originate oxygen defects within TiO 2 , thereby improving the conductivity of TiO 2 and creating more Li + transmission channels [8,29,30].These doping strategies can broaden the ion migration channels and/or increase the proportion of Ti 3+ , and subsequently improve electronic conductivity and the diffusion coefficient of Li + ions.
Among the possible dopants, vanadium (V) stands out as a promising candidate due to its high electrical conductivity and redox properties [31][32][33].Vanadium oxides have been widely studied as electrochemical energy storage materials for supercapacitors and Li-ion batteries [34,35] because of their ability to undergo redox intercalation thanks to the various available oxidation states of vanadium (V-II).However, bare vanadium oxides exhibit low lithium ion diffusion coefficients that slow the intercalation process.Nevertheless, vanadium stands as a very attractive dopant to improve other more promising networks.In fact, recently, high storage capacity and fast charge transfer kinetics have been observed in vanadium doped TiO 2 and attributed to the better conductivity of the doped material.The mechanisms behind the improved storage capacity are complex and can be deconvolved into bulk (diffusion-controlled lithium ion intercalation) and surface (pseudocapacitance storage mechanism) processes [10,36].Therefore, detailed investigation is required to gain a deeper understanding of the mechanisms that lead to the higher efficiency of these nanocrystals and in particular, to elucidate the true role of the dopant.
In situ electrochemical spectroscopy has proven to be an invaluable tool for probing the thermodynamics of chemical and electrochemical processes as they occur in real-time.This work focuses on the application of advanced in situ spectroscopy techniques to unravel the intricate electrochemical mechanisms taking place within vanadium-doped TiO 2 NCs.By utilizing a combination of techniques such as x-ray absorption spectroscopy (XAS), spectro-electrochemistry (SEC) and electrochemical impedance spectroscopy (EIS), we provide unprecedented insights into the response of vanadium doped TiO 2 NCs under electrochemical conditions and unveil, for the first time, the non-innocent role of the vanadium dopant in these materials.

Synthesis of colloidal nanocrystals and general characterisation
A synthetic route, previously used in our laboratory to prepare TiO 2 -doped NCs [8], was adapted (see experimental details in ESI) and used to obtain nanocrystals (NCs) of both 10% vanadium-doped and pristine TiO 2 anatase [8].The doping level was selected to optimise the electrochemical properties, avoiding the appearance of segregates and reducing the dispersity of the TiO 2 particles [37].
A first glance to the Raman spectra of TiO 2 and V doped TiO 2 NCs samples (figure 1(a)), confirms the presence of the anatase phase and rules out the presence of other vanadium compounds that could be segregated during the synthesis procedure.The pristine TiO 2 NCs sample exhibits characteristic Raman vibrational modes associated with tetragonal anatase TiO 2 , showing distinct peaks at 149, 396, 518, and 639 cm −1 corresponding to Eg, B1g, A1g/B1g, and Eg vibrational modes, respectively.The spectra of the doped sample is composed of the same peaks, with no appearance of additional signals related to secondary species.Besides, it exhibits an enlargement of the most intense peak, attributable to the Ti-O single bond bending vibration (figure 1(b)) [38].There is also a slight shift in its position, suggesting the incorporation of V in the crystal structure, in good agreement with previous reported works [39][40][41].This could be related to the distortion of the crystal lattice structure and an increase in the strength of Ti-O single bonds caused by the presence of V 5+ ions.The Raman results clearly indicate that the anatase crystal structure of TiO 2 is well maintained even after the doping of vanadium cations.Also in relation to the crystal phase of vanadium doped NCs, TEM images from vanadium doped NCs exhibit a lattice fringe with a distance of ∼0.34 nm (figure 1(e)) which corresponds to the (101) plane of anatase TiO 2 [42,43].In summary, the combination of the Raman spectra and TEM images confirm the successful substitution of V 5+ for Ti 4+ in anatase TiO 2 .
The analysis of the TEM images also confirm the nanometric size of the colloidal TiO 2 NCs, with crystals of around 5 nm (figures 1(c) and (d)), as seen in previous works [8].Upon closer examination of the TEM images of the V-TiO 2 NCs, a slight decrease in their crystal size is observable, which is in agreement with previous studies where the incorporation of V into the anatase structure has been shown to cause a slight reduction in crystal size [37,44,45].The smaller ionic radius of V (V) compared to that of Ti (VI) [46,47], introduces ions with smaller sizes into the crystal lattice that can induce lattice distortion, preventing the orderly growth of crystals and impeding the formation of larger crystals.Consequently, smaller crystals are formed due to the disturbance in the lattice structure resulting from this substitution.
X-ray photoemission spectroscopy (XPS) was also used to analyse V doped TiO 2 samples.The quantitative analysis of the Ti 2p and V 2p high resolution spectra (figures 2(e) and (f) respectively) confirm a V doping of 10% for V doped TiO 2 NCs.The Ti 2p spectrum of the doped V sample is the same without error as the sample of the pristine TiO 2 sample with a Ti 2p 3/2 peak at 458.8 eV.The V 2p spectrum can be deconvoluted into two main contributions with binding energies characteristic of V 5+ (517.5 eV) and V 4+ (516.5 eV) [48].The surface of vanadium oxides is known to be dominated by V 5+ signals, regarding of the oxidation state of the bulk.Thus, these XPS results confirm the presence of V 4+ in the bulk of the sample but cannot be used to confidently calculate the V 5+ /V 4+ ratio in the sample.Moreover, the valence band scan (figure 2(g)) suggests a decrease in the distance between the valence band and the Fermi level in the vanadium-containing sample, which agrees with the decrease in the energy bandgap already determined in several previous works [44,49].
To better understand the local structure of the doped material, a thorough analysis was conducted through x-ray absorption (XAS).Figure 2 shows the Ti K-edge XANES spectra of undoped and V-doped TiO 2 thin films.The spectral features agree with those reported previously for the anatase phase [50,51] (more details about the analysis in ESI).Relevant changes are observed in the pre-edge peaks when doping TiO 2 with V.The region around 4971 eV shifts towards higher energy and becomes more intense compared to the undoped TiO 2 structure.Similar changes in the pre-edge peak region were also observed for other dopants concluding that Vanadium dopants are effectively incorporated in the anatase structure.The rising edge shifts slightly towards lower energy for V-doped TiO 2 .This could be associated with a partial reduction of Ti 4+ ions to accommodate V 5+ species and/or the presence of oxygen vacancies.Other relevant changes are observed in the XANES features in the range 4988-5004 eV.These features are due to multiple scattering processes and constitute a specific phase fingerprint.The resonance around 4987 eV decreases in intensity for the V-doped sample and the resonance around 4995 eV disappear in this sample.Even though the general envelope of these features is very similar, the difference in the relative intensity of these resonances indicates that important changes occurred in the electronic structure of Ti when adding V.
The analysis of the extended x-ray absorption fine structure (EXAFS) for undoped and V-doped TiO 2 NCs is shown in figure 2(b).The Fourier Transform moduli are quite similar and in agreement with the structure of the anatase phase [52,53].The most relevant change is observed in the first shell, where a reduced Ti-O coordination is observed for the V-doped TiO 2 compared to the undoped sample (table 1 in ESI).This corroborates the distortion in the network that was initially observed in the Raman spectra and suggests the formation of oxygen vacancies upon V addition which are supported by the blue shift of the XANES spectrum towards lower energy.
DFT calculations for V-TiO 2 doped system were carried out to investigate its structural and electronic properties.Firstly, to determine the relative stability of V doped TiO 2 , the substitution energy (E s ) was calculated, considering that four Ti atoms are replaced, using use equation ( 1): where E V TiO2 and E b TiO2 are the total energies of the doped system and bare TiO 2 , respectively, while E Ti and E V are energies per atom for metal Ti and V.This substitution energy is negative (E s = −2.32eV), indicating that this system is thermodynamically stable, in agreement with other works [54].To determine intrinsic thermodynamic stability of doped TiO 2 , the formation energy of V-doped TiO 2 was calculated through equation ( 2): where E O is the energy per oxygen atom.Thus, the formation energies for the tetra-substituted V-TiO 2 phase is-147.65 eV, which is consistent with previous works [54].
The lattice parameters of V-TiO 2 (table S2) show small differences with bare TiO 2 .Thus the a = b axis depicts a similar value, with a very slight increase, while the c-axis show a diminution in respect to TiO 2 which is consistent with reported works [37,44,45].The near neighbours Ti-O, V-O, Ti-O-V and Ti-O-Ti mean distances for bare TiO 2 and V-TiO 2 are summarized in table S3.V doping leads to an increase in the Ti-O bonds in the apical positions, while equatorial ones are almost unchanged.The HSE06 density of states (DOS) analyses were carried for V-doped TiO 2 (figure S1).The valence band (VB) is dominated by the overlapping between O2p and Ti 3d orbitals, while the conduction band minimum (CBM) is mainly formed by Ti 3d orbitals with a small contribution of V 3d orbitals.In addition, we found a defect states in the forbidden gap of the pristine TiO 2 leading to the formation of intermediate bands (IB).Through the detailed analysis of the valence band spectrum (figure 2(g)) a small contribution in the signal at approx.1.5 eV can be seen which could be confirmation of the existence of these intermediate bands.Due to de presence of these IB two sub-gaps are observed.This band gap reduction was previously reported [55,56].In addition, the energy difference between CBM and IB maximum is higher than the VBM and IB minimum, so the probability for a hole in the VBM to pump up into the IB will be larger than the probability for an electron in the CB to combine with a hole in the IB [54].

Electrochemical and in situ spectroelectrochemical characterisation
For electrochemical and spectro-electrochemical (SEC) investigations, thin films were prepared by doctor-blading an organic, viscous paste containing the synthesized NCs (further details in ESI) on ITO covered glasses.The dispersion of the NCs in this paste and subsequent annealing, enabled the creation of highly porous and transparent films with precise thickness control.The high specific surface area and mesoporosity of these thin films notably increase the pseudo-capacitive contribution, providing a favorable morphology to study the influence of surface effects on electrochemical charge storage mechanisms [57].Cross-section FESEM images (figure S2) reveal highly homogeneous films with a thickness of 300 ± 20 nm of both pristine and V doped TiO 2 thin films.Energy dispersive x-ray spectroscopy (EDX) measurements (figures S3 and S4) detect the presence of the expected elements in the electrode (Ti, O and V in doped sample) and the substrate (In, Sn and Si).No C and N signals are observed by EDX, confirming the absence of organic compounds present in the viscous paste used to prepared the NCs thin films.
In ESI, figure S5 shows the cyclic voltammetry's performed at 1, 5, 10 20 and 50 mV s −1 .Figure 3(a) shows the cyclic voltammetry of the doped and undoped samples at low scan rates (1 mV s −1 ).In this measurement, a slight increase in the doped material's charge capacity is observed, along with the appearance of a new redox peak at lower potentials, which can be attributed to the presence of vanadium in the crystal lattice of TiO 2 .Vanadium exhibits multiple redox couples that can increase the amount of available electrons for electrochemical reactions [25].By XPS analyses we could detect the presence of V 4+ and V 5+ in the V-TiO 2 NCs, which can be potentially reduced to V 3+ and V 2+ states [58,59].At 50 mV sec −1 (figure 3(b)) the increase in the charge capacity starts at lower potentials and is significantly higher when doped.The general storage capacity can be deconvolved into bulk (diffusion-controlled lithium ion intercalation) and surface (pseudo-capacitance storage) processes.As it is well-known, it is possible to assign the contribution from the bulk processes when scanning at low scan speeds, while surface processes dominate at high scan speeds [10,24].These results, indicate that there is an improvement in the total charge of the NCs but especially in the superficial processes, in line with what has already been seen in the literature [33].This contribution is correlated with the pseudo-capacitance storage mechanism previously reported for pristine [10] and other aliovalent doped TiO 2 NCs [10,29].Besides, V (V) based oxides are also known to exhibit this type of surface capacitance [60].
For the in situ SEC, smart window devices containing the pristine and the V doped TiO 2 NCs were fabricated using previous methods (see in section S7 device fabrication).Figures 3(c) and (d) show the transmittance spectra of the pristine and V doped TiO 2 devices respectively, while applying a bias range from −1 to −4 V (two electrodes configuration).Both devices exhibit high transmittance, reaching around 75% across almost the entire spectrum.However, there is a decrease in transmittance in the range of 300-600 nm in the V doped TiO 2 original device, attributable to the optical properties of the material itself.A transmittance reduction is observed in both devices when a negative voltage is applied related to the reduction of TiO 2 .Moreover, there are significant differences in the total transmittance and the effect of the applied voltage at different working potentials.At a first glance, the transmittance at −4 V is lower across the entire wavelength range in the case of the V doped TiO 2 .More interestingly, in the case of undoped TiO 2 , there is no significant variation in transmittance until −2.75 V, while for doped NCs, it occurs at −2 V.In the same line, it is observed that the transmittance observed in the doped device is much lower at lower potentials.To understand this difference in electrochemical behaviour, we performed XAS and EIS in situ measurements of both systems.

In situ electrochemical XAS analysis
In-situ electrochemical XAS measurements in three electrode configuration were performed at the Ti and V K-edges in TiO 2 and V-TiO 2 NCs.An electrochemical cell manufactured in our laboratory using 3D printing was used [8].The samples were first measured in an open circuit potential (OCP), then reduced (−1.6 V Ag/Ag + ), and finally re-oxidized to return to the OCP stage.Figure 4(a) shows the Ti K-edge XANES spectra for the V-doped TiO 2 sample measured at the reduction and oxidation potentials.The inset shows the spectra at the reduction potential for V-doped and undoped TiO 2 samples.The spectra measured initially at OCP and at oxidation potentials are practically identical, as shown in figure S6 and in agreement with previous results [9], demonstrating the reversibility of the electrochemical reaction.During the reduction reaction, the absorption edge shifts towards lower energies as the TiO 2 anatase phase converts into a structure very similar to Li + -intercalated TiO 2 , as confirmed by the presence of two well-defined resonances around 4987 and 5001 eV [61].The pre-edge region also undergoes important changes.Only a broad pre-edge peak is observed after reduction and its maximum shifts by more than 1 eV towards lower energy.The inset of figure 4(a) shows a comparison of Ti K-edge for the undoped and V-doped samples after the reduction reaction.For the V-doped sample, the spectrum is slightly shifted towards lower energy compared with the undoped TiO 2 , and the integrated area up to 0.5 in intensity (in the normalized scale) increases by 10% for the V-doped TiO 2 .It can be concluded that part of the Ti 4+ ions converts into Ti 3+ due to Li + intercalation in the anatase structure and that this reduction process seems to be more effective in the V-doped sample.This increase in reduction has been observed before when doping TiO 2 with Nb 5+ [8], but to a significantly lower degree.Figure 4(b) shows the Fourier Transform (FT) moduli of the Ti K-edge EXAFS spectra of the V-doped TiO 2 sample at oxidation and reduction potentials.Only the first shell was analysed in order to obtain the Ti-O bond distance and coordination.The results of the fit are summarized in table S4.In both conditions, Ti ions are found in octahedral coordination and the Ti-O bond distance is 1.99(1) Å and 1.96(1) Å at reducing and oxidizing potentials, respectively.The value obtained after re-oxidation is similar to the value obtained for the ex-situ measurements, meaning that no permanent changes occurred during the reduction.This observation, in combination with the similarity also observed for the XANES spectra at OCP and oxidation potentials (figure S6) further confirm the reversibility of the process.
V K-edge XANES spectra of V-doped TiO 2 were measured in-situ at the same reducing and oxidizing potentials, see figure 4(c).The spectrum at oxidation potentials presents a pre-edge peak at around 5470 eV with an intensity of around 0.3 on a normalized scale.The pre-edge peak is strongly dependent on oxidation state and coordination of the V ions.For V 5+ ions, the pre-edge peak is generally found between 5470 and 5471 eV but with much higher intensity going from 0.6 to 0.9 for penta-and tetra-coordinated V.The pre-edge intensity is consistent with the presence of V 4+ ions, as reported by other authors [62].The pre-edge peak and other spectral features, such as the resonances at 5487 and 5502 eV, resemble the spectra reported by other authors for V-doped TiO 2 [63].These results are in line with the Raman spectra analysis and suggest that the V ions are incorporated in the TiO 2 structure.After reduction, the spectrum shifts towards lower energy by around 3.5 eV (value measured at 0.5 of the absorption in the normalized scale).The pre-edge peak is strongly reduced in intensity after reduction and only a weak shoulder is observed around 5468 eV.The XANES features also show significant changes during reduction and the white line, around 5486 eV, is red-shifted by 1 eV compared to the oxidized sample.The second resonance, around 5502 eV in the oxidized sample, is also red-shifted by 2 eV under reduction conditions and it is also accompanied by a significant change in spectral shape.All these changes indicate that V ions suffer important structural and chemical changes during reduction as further confirmed by the analysis of the EXAFS spectra shown in figure 4(d).The EXAFS signals were analysed in the R-range 1-2.2 Å in order to obtain the different V-O bond distances as a consequence of Li + intercalation in the TiO 2 structure (figure 4(d) and table S5).Similar results on the effect of distorted octahedra coordination for V in TiO 2 were also reported in previous works [64].The presence of a quite long V-O distance in tetrahedral coordination could be related to the incorporation of V 4+ in the TiO 2 structure while the presence of a much shorter length bond could be related to the presence of V 5+ ions.The presence of both V 4+ and V 5+ in V-doped TiO 2 was also discussed in other works [13].After reduction, the EXAFS signal changes remarkably, and V ions are found in octahedral coordination with an average V-O bond distance of 2.05(2) Å in line with the abrupt changes in the XANES spectra.

In situ EIS
To gain further insights on the electrochemical performance of TiO 2 and V doped TiO 2 samples, EIS measurements were performed (figure 5) [65].A simple Randles´circuit was employed to fit the EIS data because a single arc was observed in the Nyquist plots (inset figure 5(a)) [66].This circuit is composed by three electrical components: series resistance (R S ), representing the substrate, the electrolyte resistance and the wiring; the charge transfer resistance (R ct ) of the TiO 2 /electrolyte interface and the capacitance (C) associated with the same interface.Figure 5(a) shows the series resistance values of the two analysed samples giving values between 100-180 Ω cm 2 along the whole potential window.On the other hand, figure 5(b) shows the extracted values of the R ct at different applied potentials (V app ).As it can be observed, the values of the R ct decrease with the applied potential from 0.5 to −2 V vs Ag/Ag + except in the region where the capacitive current domains the performance, i.e. between −0.25 to −1.25 V vs Ag/Ag + approximately, as it can be seen in the cyclic voltammetry shown in figure 4. It also important to notice how the V doped sample shows a lower R ct along the whole potential window in good agreement with the higher current densities observed in this sample and previous studies [67].
Figure 5(c) shows the extracted capacitances for both TiO 2 and V doped TiO 2 NCs samples.Two main differences can be observed induced by the V doping.First, the V doped sample shows a higher capacitance along all studied potentials than the bare TiO 2 .The second one, the V doped TiO 2 NCs shows the maximum of the capacitance, which is the state of maximum accumulated charge, at around −0.5 V vs Ag/Ag + which is at a lower applied potential than the undoped TiO 2 (around −1.3 V vs Ag/Ag + ), in good agreement with the j-V curves measured by cyclic voltammetry (figures 4(a) and (b)) and the SEC results (figures 4(c) and (d)) in which the transmittance of the doped sample noticeably decreases at lower applied potentials.The DOS was calculated (figure 5(d)) from the extracted chemical capacitances by EIS through C = e • DOS, where e is the elementary charge [68].Interestingly, the V doped TiO 2 samples shows a higher DOS than the pristine TiO 2 , as previously seen in other modified and doped TiO 2 electrodes [69] Additionally, a Mott-Schottky analysis was performed (figure S7) in order to shed light into the semiconducting behaviour of the analysed samples [70].From this analysis, the n-type semiconducting behaviour of both samples was confirmed, as it can be observed from the positive slope of the C −2 sc vs V app plot.Using the Mott-Schottky equation (see supporting information), the values of the flatband potential and the donor densities were extracted and are summarized in table S4.As observed, the donor densities increase from 5.85 × 10 17 to 1.12 × 10 19 , an increase of two orders of magnitude.This carrier density also explains the significant increase in surface charge and therefore in the pseudo-capacitance storage mechanism of the doped NCs.

Conclusions
V-doped and pristine TiO 2 anatase nanocrystals with an average size of 4 and 5 nm have been successfully synthesised and fully characterised by Raman, TEM and XPS analysis.An in-depth analysis by in situ SEC, XAS and EIS of the response of these NCs to electrochemical stimuli has been carried out.
The electrochemical characterisation of the pristine and doped TiO 2 materials shows that the V doped TiO 2 NCs exhibit significantly higher charge storage capacities, especially in the pseudo-capacitance storage mechanism.In the in-situ SEC measurements, the occurring redox processes are indicated by a colour change in the device from transparent to blue at lower voltages and measured as a significantly lower reduction in transmittance in the case of doped crystals.This effect can be attributed to diffusion-controlled Li + intercalation, a process that is more related to bulk charging.The dopant seems to provide a higher electron density as corroborated by the DFT results and the carrier density measured by EIS.This suggests that substitutional V 5+ ions act as electron donors, increasing the surface charge and providing the NCs with unique pseudocapacitive properties.Finally, EIS reveals a higher charge capacitance over the entire measured potential range for V doped NCs.Furthermore, in good agreement with the results observed by SEC, it was observed that the charging capacity starts at much lower potentials.Besides, V doped electrode shows a lower R ct along the whole potential window in good agreement with the higher current densities observed in this sample and previous studies.
In addition, in-situ XAS analysis revealed a more effective reduction of Ti within the doped anatase and, more interesting, a substantial change in coordination and oxidation state of the V ions during the electrochemical process.The analysis of the V K-edge during operation conditions expose the real active role of the dopant for the first time, not only modifying the electronic structure of the host, but also working as an active part in the reduction process.

Figure 2 .
Figure 2. (a) Ti K-edge XANES spectra for undoped and V-doped TiO2 thin films together with Ti metal foil as reference.(b) Fourier transform moduli (solid line) and fit (dashed line) of Ti K-edge EXAFS spectra for undoped and V-doped TiO2 thin film.(c) DFT models for TIO2 and V-doped TiO2.Atom colours: Ti (blue), O (red), V (dark red).(e) Ti2p high resolution XPS spectrum, (f) V2p high resolution XPS spectrum and (g) valence band spectrum of V-doped TiO2 NCs.

Figure 3 .
Figure 3. Cyclic voltammetry of pristine (grey) and V doped TiO2 (purple) at (a) 1 mV sec −1 and (b) 50 mV sec −1 in 1 M LiClO4 in propylene carbonate.Spectroelectrochemical measures of lab scale smart windows devices of (c) bare and (d) V doped TiO2 in 1 M LiCl + 0.1 M LiI in dimethyl sulfoxide.

Figure 4 .
Figure 4. (a) Ti K-edge XANES spectra for V-doped TiO2 thin films at oxidation and reduction potentials.The inset shows the XANES spectra for undoped and V-doped TiO2 at reduction potential.(b) Fourier transform (FT) moduli (solid line) and fit (dashed line) of Ti K-edge EXAFS spectra of V-doped TiO2 thin films at oxidation and reduction potential.The dashed grey line is a guide for the eye and highlights the phase shift of the real part of the FT.(c) V K-edge XANES spectra for V-doped TiO2 thin films at oxidation and reduction potential.(d) Fourier transform moduli (solid line) and fit (dashed line) of V K-edge EXAFS spectra for V-doped TiO2 thin films at oxidation and reduction potential.The dashed grey line is a guide for the eye.

Figure 5 .
Figure 5. EIS results along the whole potential window on the TiO2 and V: TiO2 samples showing: (a) series resistance, (b) charge transfer resistance, (c) capacitance and (d) calculates DOS.The inset in figure 5(a) shows the employed Randles´circuit considering the TiO2/electrolyte interface.