Brown coloration and electrochromic properties of nickel doped TiO2 thin films deposited by nebulized spray pyrolysis technique

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Brown coloration and electrochromic properties of nickel doped TiO2 thin films deposited by nebulized spray pyrolysis technique T. Dhandayuthapani ConceptualizationInvestigation and Writing- Original draft preparation , R. Sivakumar ConceptualizationWriting - Review & Editing and Supervision , R. Ilangovan , C. Sanjeeviraja , K Jeyadheepan , C. Gopalakrishnan , P. Sivaprakash , S. Arumugam PII: DOI: Reference:

S0040-6090(19)30779-5 https://doi.org/10.1016/j.tsf.2019.137754 TSF 137754

To appear in:

Thin Solid Films

Received date: Revised date: Accepted date:

11 October 2018 16 November 2019 12 December 2019

Please cite this article as: T. Dhandayuthapani ConceptualizationInvestigation and Writing- Original draft preparatio R. Sivakumar ConceptualizationWriting - Review & Editing and Supervision , R. Ilangovan , C. Sanjeeviraja , K Jeyadheepan , C. Gopalakrishnan , P. Sivaprakash , S. Arumugam , Brown coloration and electrochromic properties of nickel doped TiO2 thin films deposited by nebulized spray pyrolysis technique, Thin Solid Films (2019), doi: https://doi.org/10.1016/j.tsf.2019.137754

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Highlights 

Ni doped TiO2 films were prepared by nebulized spray deposition method.

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6% Ni doped TiO2 film showed a high coloration efficiency of 267 cm2 C-1.

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Ni doped TiO2 films exhibit a brown coloration and high stability.

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Brown coloration and electrochromic properties of nickel doped TiO2 thin films deposited by nebulized spray pyrolysis technique

T. Dhandayuthapani,1,8 R. Sivakumar,2,* R. Ilangovan,3,* C. Sanjeeviraja,4 K, Jeyadheepan,5 C. Gopalakrishnan,6 P. Sivaprakash,7 and S. Arumugam7 1

Department of Nanoscience and Technology, Alagappa University, Karaikudi 630003, India. Department of Physics, Alagappa University, Karaikudi 630003, India. 3 National Centre for Nanosciences and Nanotechnology, University of Madras, Chennai 600025, India. 4 Department of Physics, Alagappa Chettiar Government College of Engineering and Technology, Karaikudi 630003, India. 5 School of Electrical and Electronics Engineering, SASTRA Deemed to be University, Thanjavur 613401, India. 6 Nanotechnology Research Center, SRM University, Kattangulathur 603203, India 7 Centre for High Pressure Research, School of Physics, Bharathidasan University, Tiruchirapalli 620024, India. 2

Abstract Titanium dioxide (TiO2) is a promising candidate for electrochromic smart window and energy applications. In the present work, an eco-friendly approach to spray deposition technique was used to deposit undoped and nickel doped TiO2 thin films using glycerol as an additive. The effect of nickel doping on the microstructure, morphology, composition, magnetic, and electrochromic properties of TiO2 was investigated in detail. Under the action of negative voltage, the undoped TiO2 films exhibit blue coloration, whereas, the nickel doped TiO2 films exhibit brown coloration due to Ni2+ induced defects. The 6% nickel doped TiO2 film achieved a high reversibility of 82% and high transmittance modulation of 48.5%, with a colouration efficiency of 267 cm2/C, fast response of 2.3 s for colouration and 2.5 s for bleaching, compared to other nickel doped TiO2 films. Key words: Nickel-doped Titanium dioxide; Thin films; Spray pyrolysis; Electrochromism 8

Present address: Key Laboratory of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China

*Authors for correspondence: Tel.: +91-4565-223310; Fax: +91-4565-225202 Emails: [email protected] (R. Sivakumar) [email protected] (R. Ilangovan)

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1. Introduction Titanium dioxide (TiO2) is an extensively studied multifunctional metal oxide having applications in photoelectrochemical cell, gas sensors, electrochromic smart windows and lithium ion batteries [1-7]. Doping of transition metals (such as Nb, W, Mo, Ni and Ta) with TiO2 can modify the defect chemistry and boost the charge storage, sensing, electrical, magnetic and electrochromic properties of TiO2 [8-11]. For instance, Cao et al. prepared Ta doped TiO2 nanocrystals by fluoride assisted synthesis method and the prepared Ta doped TiO2 film exhibited efficient optical modulation of 86.3% at 550 nm and a coloration efficiency of 33.2 cm2/C [9]. Barawi et al. prepared Nb doped TiO2 nanocrystals by solvothermal method [10]. The Nb doped TiO2 electrodes showed a large variation in the optical transmittance close to 64% in the visible and near infrared regions. Similar to these dopants, nickel (Ni) is a promising element which can improve the magnetic, electrical, and electrochromic properties of TiO2 [11, 12]. Many recent findings suggest that doping of nickel with TiO2 can improve the charge storage and photocatalytic performance of TiO2 [13, 14]. Thin films made up of closely interconnected nanosized grains are extensively studied in recent times as it boost the ion insertion/de-insertion kinetics and ion storage properties [15, 16]. Among the reports available in literature on the metal doped TiO2 films, so far no one has reported the electrochromic properties of Ni doped TiO2 films. The present work provides a detailed study on Ni doped TiO2 thin film and influence of nickel doping on the structural, vibrational, morphological, optical, electrical, magnetic, and electrochromic properties of TiO2. 2. Experimental 2.1 Materials

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Titanium tetraisopropoxide (Ti(OC3H7)4, TTIP), glycerol, ethanol (C2H5OH), nickel acetate, lithium perchlorate (LiClO4) and propylene carbonate (PC) are the chemicals used in the present work, which were of analytical grade and used without further purification. The fluorine doped tin oxide (FTO; Rsh = 15Ω/sq) coated glass was used as the substrate. Prior to deposition, the FTO coated glass substrate was gently cleaned with acetone and distilled water and dried in air. 2.2 Preparation of undoped and Ni doped TiO2 thin film In the present work, the precursor solution was made by dissolving 1.5 mL of glycerol to 16 mL of ethanol and further 1 mL of TTIP was added to the same solution. The prepared solution remained transparent colourless. Now, the nickel acetate solution (2, 4, 6 and 10%) was added to the prepared solution. The resultant solution was ultrasonicated for 15 min in order to get a homogeneous dispersion. The pre-cleaned FTO coated glass substrate was kept inside the furnace and the temperature of the furnace was set to 200°C. The prepared ultrasonicated precursor solution was taken into the nebulizer and sprayed on the FTO coated glass substrate through the L tube. The spray head (L tube) and the substrate surface distance was kept at about 10 mm. The solution flow was maintained by keeping the pressure of compressed air at 1 kg/cm2. The spray deposition was done by spraying the solution for 20 s followed by 1 min break, same procedure was repeated for 5 times to get uniform film. The deposited films were further annealed at 450ºC for 1 hr in air. 2.3 Characterization The phase purity and crystal structure of the samples were confirmed by grazing incidence X-ray diffraction (GIXRD) (X’pert Pro PANalytical; Goniometer = PW3050/60 (Theta/Theta); Minimum step size 2Theta: 0.001; Minimum step size Omega: 0.001; Grazing

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incidence angle: 0.5°) with Cu-Kα (λ = 0.154 nm) radiation. The surface morphology of the samples was characterized using field emission scanning electron microscope (FESEM) (Carl ZEISS-ULTRA 55 FESEM) at 5 keV (accelerating voltage). The topography of the samples was studied using atomic force microscopy (AFM; Agilent Pico LE) by non contact mode. The chemical composition of the samples were determined by X-ray photoelectron spectroscopy (XPS) and the measurements were obtained using monochromatic Al Kα X-ray (1486.6 eV) at the vacuum level of 1.33 × 10−7 Pa. The Raman spectra of the samples were recorded at room temperature using Raman spectrometer (Horiba-Jobin, LabRAM HR) with the laser excitation of 514 nm. The transmittance spectra of the films were analyzed in the wavelength range of 300 to 900 nm using Ultraviolet-visible spectrophotometer (Jasco V-640) (FTO coated glass substrate was used as the reference). Magnetization was measured as a function of the applied field and temperature in the magnetic field range of - 3183098 A/m to + 3183098 A/m. Zero field cooled (ZFC) and field cooled (FC) measurement was done at the applied magnetic field of 7957 A/m, using vibrating sample magnetometer module (Quantum Design, USA). The electrochemical properties of the undoped and Ni doped TiO2 films deposited on FTO substrate were measured from cyclic voltammetry (CV), chronoamperometry, chronocoulometry and impedance measurements by designing a three electrode electrochemical set up consists of working electrode (TiO2 thin film on FTO substrate), counter electrode (Pt wire), and reference electrode (Ag/AgCl) using electrochemical workstation (CH Instruments Inc., USA). The working electrode area was 1×1 cm2. The electrochemical properties of TiO2 thin films were determined by intercalating/deintercalating Li+ ions using 1M LiClO4-PC electrolyte solution. 3. Results and discussion 3.1 Structural investigations

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Fig. 1 shows the GIXRD patterns of undoped and Ni (2, 4, 6 and 10%) doped TiO2 thin films prepared using ethanol solvent at the substrate temperature of 200ºC and after calcination at 450°C. For undoped TiO2, the peaks are observed at 2θ = 25.4, 38.1, 48, 55.1 and 62.5º, which correspond to (101), (004), (200), (211) and (204) orientations, respectively. The obtained lattice spacings are indexed with JCPDS data (Card No.: 21-1272), which established the presence of anatase TiO2. It can be seen from Fig. 1 (a) that the intensity of (101) plane decreases with the increase in Ni doping concentration. In the case of 10% Ni doped TiO2, the intensity of (101) peak (anatase phase) was decreased and the coexistence of rutile phase with the presence of (110) and (221) planes along with anatase phase TiO2 was observed. This is due to the promotion of rutile TiO2 phase with increase in Ni doping [13]. In addition to that, the signature of FTO (substrate) was also observed in the films, which may be due to the crystalline nature of FTO [17]. The decrease in peak intensity and the gradual shift in the peak position confirmed the incorporation of Ni ions into Ti site. Full width at half maximum of (101) plane was used to calculate the crystallite size of Ni doped TiO2 film by employing Scherrer’s formula. The lattice parameters “a” and “c” of Ni doped TiO2 films were calculated from the (200) and (004) planes. The evaluated lattice parameters, crystallite size and mass density of films are reported in Table 1. The slight variation in the lattice parameters is expected to result from the larger size of Ni2+ ion (0.69 Å) compared to that of Ti4+ ion (0.60 Å) which confirmed the substitution of Ni2+ into Ti4+ lattice [18]. The crystallite size of Ni doped TiO2 films are varied from 19 to 16 nm. 3.2 Vibrational properties Raman spectroscopy is an excellent tool to identify local structural changes, crystallinity and phase composition due to the transition metal doping into TiO2 lattice. Fig. 1 (b) presents the Raman spectra of undoped and Ni (2, 4, 6 and 10%) doped TiO2 thin films prepared using

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ethanol solvent at the substrate temperature of 200ºC and after calcination at 450°C. Raman spectra of Ni doped TiO2 films are similar to that of undoped TiO2 films. For Ni doped TiO2 films, Raman peaks are located at 144 cm-1, 398 cm-1, 518 cm-1 and 637 cm-1 [19, 20]. It is evident from the figure that the intensity of Raman vibration bands are gradually decreases with increasing Ni doping. This is due to the incorporation of Ni2+ ions into TiO2 lattice. The decrease in intensity might be related to the lattice distortion caused by the Ni doping, which reduces the translational symmetry and results the local distortion in the lattice [21]. 3.3 Surface morphology Fig. 2 presents the FESEM images of undoped and Ni (2, 4, 6 and 10%) doped TiO2 thin films prepared using ethanol solvent at the substrate temperature of 200ºC and after calcination at 450°C. The Ni doped TiO2 films have homogeneously distributed nanograins like features over the entire substrate. No significant variation in the morphology was observed even at high concentration of nickel doping (10%). Fig 3 (a-e) shows the AFM images of undoped and 2, 4, 6 and 10% Ni doped TiO2 thin films. The AFM images of Ni doped TiO2 films reveal evenly distributed pyramidal shaped nanoparticles. The evenly distributed nanograins can generate a uniform electric field which is favorable for electrochemical process as it improves the device efficiency [22]. The thickness of undoped and Ni (2, 4, 6 and 10%) doped TiO2 thin films are found as 600, 350, 470, 470 and 470 nm, respectively. The surface roughness of undoped and Ni (2, 4, 6 and 10%) doped TiO2 thin films are varied as 32, 28, 24, 28 and 20 nm, respectively. 3.4 Compositional analysis XPS study was used to identify the compositional nature of the film and also to know the valence states of elements. The XPS core level spectra of undoped TiO2 thin film are shown in Fig 4. Whereas, Fig. 5 shows the XPS core level spectra of 6% Ni doped TiO2 thin film prepared

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using ethanol solvent at the substrate temperature of 200ºC and after calcination at 450°C. The peaks of Ti 2p3/2 and Ti 2p1/2 are localized at the binding energy values of 458.6 and 464. 4 eV, respectively (Fig. 5) [23, 24]. The energy position of this doublet peak revealed the Ti4+ oxidation state in TiO2. Compared to undoped TiO2, presence of Ti3+ state is realized in the Ti core level spectrum of Ni doped TiO2 [25]. The core level spectrum of Ni is shown in Fig. 5 (b). The Ni 2p3/2 and Ni 2p1/2 peaks were localized at the binding energy values of 855.5 and 873.1 eV, respectively [25, 26]. The energy separation between the Ni 2p3/2 peak and Ni 2p1/2 peak is 17.6 eV, which confirmed the presence of Ni2+ state [25, 26]. The O 1s core level spectrum of the film is shown in Fig. 5 (c). The O 1s core level spectrum is Gaussian fitted and divided into three peaks at 530.4, 531.4 and 532.7 eV. The peak at 530.4 eV is corresponds to lattice oxygen in TiO2. The peak at 531.4 eV is attributed to the oxygen vacancies related to Ti3+ and Ni2+ and the peak at 532.7 eV is ascribed to adsorbed hydroxyl groups on the surface of TiO2 [23, 24]. 3.5 Magnetic properties Magnetic properties of 6% Ni doped TiO2 film was analyzed by vibrating sample magnetometer. The temperature dependence of magnetization of the sample was studied under ZFC and FC conditions with the applied field of 7957 A/m and the curves are shown in Fig. 6 (a). The FC and ZFC magnetization curves are superimposed with each other, confirming the paramagnetic behavior [27, 28]. The hysteresis loop of 6% Ni doped TiO2 film was measured at various temperatures with an applied field of 3183098 A/m and the curves are shown in Fig. 6 (b). The existence of small coercivity in the hysteresis loop of the magnetization curves obtained at all the temperatures is an indicative of paramagnetic behavior or a weak ferromagnetic behavior, which is in agreement with the ZFC/FC magnetization curves. The TiO 2 is an intrinsic diamagnetic material; the change to paramagnetic or weak ferromagnetic behavior may be due to

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the formation of “Ti3+” centers and the substitution of Ni2+ ions in the TiO2 lattice which can be observed from the XPS spectra of 6% Ni doped TiO2 film. 3.6 Electrochemical properties The electrochromic performance of undoped and Ni (2, 4, 6 and 10%) doped TiO2 films was studied using CV measurement by intercalating and deintercalating Li+ ions in 1 M LiClO4 dissolved in PC electrolyte at various scan rates in the potential window of -1.8 V to 0.5 V and the corresponding CV curves are shown in Fig. 7. Initially, the cathodic current density of TiO2 decreased for 2% Ni and later increases for 4 and 6% Ni. Whereas, in the case of 10% Ni, a slight decrease in cathodic current density was observed. This may be due to the formation of denser surface. The maximum cathodic current density of 1.1 mA/cm2 was obtained for 6% Ni doped TiO2 films. The slight variation in the current density of Ni doped TiO2 films are due to Ni doping, which alter the Fermi level near the conduction band and also produce excess of electrons and Ti3+ ions. The undoped TiO2 films exhibit a blue coloration under the action of -3 V, whereas, the Ni doped TiO2 films changed from colourless white to light brownish colour under the action of negative potential of -3 V and changed to colourless white during the bleached state. The double injection of electrons and Li+ ions into the TiO2 electrode leads to the reduction of Ti4+ ionic state to Ti3+ state, which causes the colouration of light brown coloured LixTiO2. The mechanism involved in the colouration/bleaching process is based on the equation given below [29, 30].

The subscript x in LixTiO2 indicates the fractional number of sites which are filled in TiO 2 lattice. It was reported that the Li content could be varied in the 0.5 < x < 1.0 range for LixTiO2 [31]. It is well known that NiO exhibits brown colouration under the action of positive voltage.

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The Ni2+ doping induced alteration of band edge may be responsible for the change in colouration from blue to brown [32]. In addition, Ti3+ species and the oxygen vacancies can also play a crucial role in the observed change in colouration. The electrochemical stability of 6% Ni doped TiO2 film was investigated by performing the CV study in the potential range of –1.6 V to 0.5 V at a scan rate of 75 mV/s for 1000 cycles. Fig. 8 shows the CV curves for 1st and 1000th cycles of 6% Ni doped TiO2 films. It can be seen from the figure that Ni doped TiO2 films exhibit good stability upto 1000 cycles with a slight reduction in cathodic and anodic current density due to the extensive cycling. Colouration and bleaching speed of undoped and Ni (2, 4, 6 and 10%) doped TiO2 films was examined by chronoamperometry measurement in the potential window of -1.8 V to 0.5 V for 20 s and the curves are presented in Fig. 9 (a). The switching time is defined as the time taken for a system to reach 90% of its full modulation [33-35]. Table 2 shows the colouration and bleaching speed, optical density and transmittance modulation of undoped and Ni doped TiO2 films. The undoped and Ni (2, 4, 6 and 10%) doped TiO2 thin films exhibit a similar faster response time of tc (2.5 s) and tb (2.3 s). The chronoamperometric curves for 1st and 1000th cycles of 6% Ni doped TiO2 film was presented in Fig. 9 (b). The 6% Ni doped TiO2 film maintains a fast switching kinetics [tb (2.3 s) and tc (2.5 s)] even for the 1000th cycle. In order to determine the amount of charge intercalated/deintercalated from an electrolyte with respect to time, chronocoulometry measurements were done by sweeping the voltage in the potential region of -1.8 V to 0.5 V with an interval of 10 s and a total duration of 20 s and the curves are presented in Fig. 10 (a). The ratio of deintercalated charges (Qdi) to the intercalated charges (Qi) is called the reversibility and the values are listed in Table 3. The maximum reversibility of 99 and 82% was obtained for undoped and 6% Ni doped TiO2 thin films,

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respectively. The high reversibility of 6% Ni doped TiO2 film is due to the low charge transfer resistance values, which was later ascertained from the Nyquist plot. Fig. 10 (b) shows the chronocoulometry curves of 6% Ni doped TiO2 film for 1st and 1000th cycles. For the 1000th cycle, the intercalated charge density was decreased to 2.6 mC/cm2, the deintercalated charge density was 2.0 mC/cm2 and the obtained reversibility was 77%. The transmittance spectra of coloured and bleached states of undoped and Ni (2, 4, 6 and 10%) doped TiO2 films measured in the wavelength range of 300-1000 nm under an applied voltage of ±3 V are shown in Fig. 11, along with the photographs of coloured and bleached states of Ni doped TiO2 films. The various electrochromic parameters like transmittance difference (ΔT), the photopic contrast ratio (PCR) and the change in optical density (ΔOD) between the bleached and coloured states at 600 nm wavelength were estimated from the relations reported earlier [36] and the results are summarized in Table. 2. The optical modulation (∆T) of Ni doped TiO2 films are varied in the range from 48.5 to 27.5%, which is due to the Ni doping induced variation in anatase TiO2 films. Fig. 12 shows the 1st and 1000th cycles’ transmittance spectra of coloured and bleached states of 6% Ni doped TiO2 films. The optical modulation of 1000th cycle is decreased to 31% from the 1st cycle of 48.6% for 6% Ni doped TiO2 film. Colouration efficiency (CE) is a fundamental figure of merit for an electrochromic device [37, 38]. CE is defined as variation in OD per unit area charge density (ΔQ), where, ΔQ is the intercalated charge during the colouring period of 10 s. The calculated CE of undoped and Ni (2, 4, 6 and 10%) doped TiO2 films are 474, 148, 133, 267 and 123 cm2/C, respectively. The observed colouration efficiency is higher than the ones reported in literature [38-46]. For instance, Haung et al. [38] have reported the higher colouration efficiency of 110.8 cm2/C and faster switching speed (0.7 s and 2.9 s) for WO3/TiO2 core-shell nanostructure. On the other hand, the Ni doped

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WO3 nanostructured film exhibited the maximum CE of 60.5 cm2/C [39]. The CE of undoped TiO2 is much higher than Ni doped TiO2 films, which is due to the combined effect of high optical modulation and low magnitude of intercalated charges (2.36 mC/cm2). Among the Ni doped TiO2 films, 6% Ni doped TiO2 achieved a high CE of 267 cm2/C as it exhibits low charge transfer resistance and a high optical modulation of 48.5%. The CE of 6% Ni doped TiO2 film for 1000th cycle was calculated to be 204 cm2/C. The slight degradation of CE upon extensive cycling can be due to the factors such as ion trapping induced changes and surface modification of films. The Nyquist plot of undoped and Ni (2, 4, 6 and 10%) doped TiO2 films were measured under negative bias voltage -1.5 V, as shown in Fig. 13 (a). The Ni doped TiO2 films exhibit a smaller semicircle in the high to medium frequency region which is an indicative of capacitive behavior that results the fast charge transfer process on the electrode/electrolyte interface [4647]. The inset of Fig. 13 (a) shows the equivalent circuit of impedance analysis. The Randle’s circuit model comprises the charge transfer resistance (Rct), solution resistance (Rs), constant phase element (CPE) and Warburg impedance (W). The charge transfer resistance and solution resistance values of undoped and Ni (2, 4, 6 and 10%) doped TiO2 films are listed in Table 4. The 6% Ni doped TiO2 film exhibits a low charge transfer resistance of 68 Ω. The low charge transfer resistance facilitates the high electronic conductivity and fast charge transfer, which in turn results the excellent electrochromic colouration efficiency. The 6% Ni doping seems to be the optimized concentration for achieving a good electrochromic performance in TiO2 films. The Nyquist plots of 6% Ni doped TiO2 film for 1st and 1000th cycles are shown in Fig. 13 (b). The charge transfer resistance is decreased from 68 Ω in the 1st cycle to 61 Ω in the 1000th cycle. 4. Conclusions

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In summary, undoped and Ni (2, 4, 6 and 10%) doped anatase TiO2 films were prepared by nebulized spray deposition method. XRD and Raman studies confirmed the incorporation of nickel in anatase TiO2 lattice. The Ni2+ substituting Ti4+ in TiO2 anatase lattice was revealed by XPS study. The homogeneous granular morphology was observed from the FESEM and AFM images of Ni doped TiO2 films. Among the samples studied, 6% Ni doped TiO2 films showed a high reversibility of 82% and high transmittance difference of 48.5%, with a colouration efficiency of 267 cm2/C, fast response of 2.3 s for colouration and 2.5 s for bleaching, compared to other Ni doped TiO2 films. Author contribution statement T. Dhandayuthapani: Conceptualization, Investigation and Writing- Original draft preparation, R. Sivakumar: Conceptualization, Writing - Review & Editing and Supervision, R. Ilangovan: Supervision, C. Sanjeeviraja: Visualization, K. Jeyadheepan: Resources, C. Gopalakrishnan: Resources, P. Sivaprakash: Resources, S. Arumugam: Resources

Acknowledgement R.S gratefully acknowledges the Department of Education, Government of India for the financial support under RUSA – Phase 2.0 Scheme (Ref. No.: F. 24-51 /2014-U, Policy (TN Multi-Gen), dt. 09.10.2018). In addition, R.S sincerely acknowledges the Department of Science and Technology, New Delhi, India for the financial support in general and infrastructure facilities sponsored under PURSE 2nd Phase programme (Ref. No.: SR/PURSE Phase 2/38 (G) dt. 21.02.2017).

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Table 1: Structural parameters of undoped and Ni (2, 4, 6, and 10%) doped TiO 2 thin film deposited at 200ºC using ethanol solvent.

Sample

Lattice constant a (Ǻ)

Volume

Crystallite size

Mass density

D

ρ

(Ǻ)3

(nm)

(g/cm3)

c (Ǻ)

TiO2

3.772

9.456

134.5

16

3.94

2% Ni doped TiO2

3.773

9.436

134.33

19

3.94

4% Ni doped TiO2

3.768

9.444

134.06

19

3.95

6% Ni doped TiO2

3.777

9.439

134.69

16

3.93

10% Ni doped TiO2

3.779

9.440

134.81

16

3.93

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Table 2: Response time, transmittance modulation, PCR, ΔOD, and CE values of undoped and Ni (2, 4, 6, and 10%) doped TiO2 films prepared using ethanol solvent at the substrate temperature of 200ºC.

Sample

Response

Transmittance

time

(%)

(s)

(at 600 nm)

ΔOD = ln (Tb/Tc)

CE = ΔOD/Qi

(at 600 nm)

(cm2 C-1)

3.09

1.12

474

PCR = Tb/Tc

tb

tc

Tb

Tc

ΔT

TiO2

2.2

2.2

68

22

46

2% Ni doped TiO2

2

2.5

88.5

61.0 27.5

1.45

0.37

148

4% Ni doped TiO2

2.3

2.5

82.7

53.1 29.5

1.55

0.44

133

6% Ni doped TiO2

2.3

2.5

86

37.5 48.5

2.29

0.83

267

10% Ni doped TiO2

2.3

2.5

88.7

55

33.7

1.61

0.47

123

2.3

2.5

74.9

43.9

31

1.7

0.53

204

1000th cycle of 6% Ni doped TiO2

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Table 3: Qi, Qdi, Qi-Qdi and reversibility values of undoped and Ni (2, 4, 6, and 10%) doped TiO2 films using ethanol solvent at the substrate temperature of 200ºC.

Sample

Qi

Qdi

Qi-Qdi

Reversibility

(×10-3

(×10-3

(×10-3

(%)

C/cm2)

C/cm2)

C/cm2)

TiO2

2.36

2.35

0.01

99.5

2% Ni doped TiO2

2.5

0.60

1.9

24

4% Ni doped TiO2

3.3

1.00

2.3

30

6% Ni doped TiO2

3.1

2.55

0.55

82

10% Ni doped TiO2

3.8

3.00

0.75

80

1000th cycle of 6% Ni

2.6

2.00

0.6

77

doped TiO2

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Table 4: Rs and Rct values of undoped and Ni (2, 4, 6 and 10%) doped TiO2 films deposited using ethanol solvent at the substrate temperature of 200ºC.

Sample

Rs (Ω)

Rct (Ω)

Surface roughness

Thickness

(Rrms) nm

(nm)

TiO2

12.5

107

32

600

2% Ni doped TiO2

58

83

28

350

4% Ni doped TiO2

74

97

24

470

6% Ni doped TiO2

58

68

28

470

10% Ni doped TiO2

78

88

20

470

6% Ni doped TiO2

40

61

-

-

1000th cycle

23

Fig. 1: (a) GIXRD patterns and (b) Raman spectra of undoped and Ni (2, 4, 6 and 10%) doped TiO2 thin films deposited at the substrate temperature of 200°C using ethanol solvent and after annealing at 450°C for 1 h.

24

Fig. 2: FESEM images of pure and Ni doped TiO2 thin films deposited at the substrate temperature of 200°C using ethanol solvent and after annealing at 450°C for 1 h. (a) undoped TiO2, (b) 2% Ni doped TiO2, (c) 4% Ni doped TiO2, (d) 6% Ni doped TiO2, and (e) 10% Ni doped TiO2.

25

Fig. 3: AFM images of pure and Ni doped TiO2 thin films deposited at the substrate temperature of 200°C using ethanol solvent and after annealing at 450°C for 1 h. (a) undoped TiO2, (b) 2% Ni doped TiO2, (c) 4% Ni doped TiO2, (d) 6% Ni doped TiO2, and (e) 10% Ni doped TiO2.

26

Fig. 4: (a) Ti 2p and (b) O 1s core level spectra of undoped TiO2 film.

27

Fig. 5: XPS core level spectra of 6% Ni doped TiO2 thin film deposited at the substrate temperature of 200°C using ethanol solvent and after annealing at 450°C for 1 h. (a) Ti 2p core level spectrum, (b) Ni 2p core level spectrum, and (c) O 1s core level spectrum.

28

Fig. 6: (a) ZFC/FC magnetization curves and (b) M-H plots of 6% Ni doped TiO2 thin film deposited at the substrate temperature of 200°C using ethanol solvent and after annealing at 450°C for 1 h.

29

Fig. 7: Cyclic voltammograms of undoped and Ni (2, 4, 6 and 10%) doped TiO2 thin films deposited at the substrate temperature of 200°C using ethanol solvent and after annealing at 450°C for 1 h.

30

Fig. 8: Cyclic voltammograms of 6% Ni doped TiO2 thin films deposited using ethanol solvent at the substrate temperature of 200ºC for 1st and 1000th cycles.

31

Fig. 9: (a) Chronoamperometry curves of undoped and Ni (2, 4, 6 and 10%) doped TiO2 thin films deposited using ethanol solvent at the substrate temperature of 200ºC and after annealing at 450°C for 1 h. (b) Chronoamperometry curves of 6% Ni doped TiO2 thin film deposited using ethanol solvent at the substrate temperature of 200ºC for 1st and 1000th cycles.

32

Fig. 10: (a) Chronocoulometry curves of undoped and Ni (2, 4, 6 and 10%) doped TiO2 thin films deposited using ethanol solvent at the substrate temperature of 200ºC and after annealing at 450°C for 1 h. (b) Chronocoulometry curves of 6% Ni doped TiO2 thin film deposited using ethanol solvent at the substrate temperature of 200°C for 1st and 1000th cycles.

33

Fig. 11: Coloured and bleached transmittance spectra of undoped and Ni (2, 4, 6 and 10%) doped TiO2 thin films

34

Fig. 12: Coloured and bleached transmittance spectra of 6% Ni doped TiO2 thin films for 1st and 1000th cycles.

35

Fig. 13: (a) Nyquist plots of undoped and Ni (2, 4, 6 and 10%) doped TiO2 thin films deposited using ethanol solvent at the substrate temperature of 200ºC and after annealing at 450°C for 1 h. (b) Nyquist plots of 6% Ni doped TiO2 thin film deposited using ethanol solvent at the substrate temperature of 200°C for 1st and 1000th cycles. Inset shows the equivalent circuit.

36

Declaration of interests

√☐The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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