Low voltage colour modulation in hydrothermally grown Ni‐Co nanoneedles for electrochromic application

Abstract

A nanostructured film of NiCo₂O₄ has been prepared using a hydrothermal technique by simply using separate precursors to obtain nanoneedle‐like architecture for electrochromic applications. A homogeneous film consisting of packed nanoneedles with moderate density, appearing translucent white in colour, has been obtained and characterized using XRD and Raman spectroscopy techniques for confirming the composition and structure. Electrochemical analysis of the film reveals that the film shows good electrochromic properties under the anodic scan of potential with strong stability. The mechanism of the electrode under the transformation from natural white to opaque dark brown colour has been understood with the help of an in situ optical absorption spectroscopy technique. The electrode is found electrochromically active with a bias of up to 2 V and shows 50% optical contrast which makes it a good candidate for application in a solid state electrochromic device.

INTRODUCTION

Currently, there are enormous innovations in transition metal oxides (TMOs) [1–5] and their hydroxide family especially in their ‘nano': form [6–10] due to the breakthrough of their extraordinary properties in electrical and optical field. The advent of nano dimension [11, 12], underlines that different nanostructures of the same material also exhibit the property with greater improvement [12]. Therefore, a worldwide parallel study is conducted in either finding the new nano materials or designing new morphologies for achieving better performance of different applications [5, 13, 14]. A huge number applications of TMO like supercapacitor [15], field emission [16], sensors [17], photocatalytic [18], hydrogen/oxygen evolution [19], electrochromic [4] and so forth demonstrates excellent performance by incorporating the different dimension-based nanostructures like nanodot [20], nanosheet, nanorod [21], nanoneedle, nanoflakes [21] and so forth. Among these, electrochromic (EC) application [22–27] also depends upon morphologies for obtaining the higher surface area of the particular architecture to make contact with the electrolyte immensely. The higher surface area can be related to the presence of larger active sites; hence the rate of the reaction will increase. Since the electrochromic application is defined by the chromic modulation under an applied electrical bias which also needs higher surface area to activate the charge intercalation/de-intercalation reaction within the electrolyte [14, 28].

There are many electrochromic active materials in organic, inorganic or even in the herbal [29] family, classified based on their redox activity and fall under either ‘cathodic': or ‘anodic': electrochromic active species [30]. In general, inorganic electrochromic active species are known for their poor EC performances in terms of large switching time and weak stability [14]. Different approaches have been incorporated to resolve such bottlenecks in the inorganic family by modifying the electrochromic device (ECD) with the formation of nano morphology-based EC active material, double layer ECD, hybrid ECD and so forth [4, 14, 31]. Therefore, every study for fabricating the EC active electrode should be in favour of modifying such EC parameters. The inorganic materials, though very robust, prove to be disadvantageous due to their poor cyclic life and high required biases. In this context, a modified nanostructure based inorganic electrochromic materials could be a good idea to address the above two drawbacks to exploit the robust nature of these materials, which makes the basis of the current study.

Hydrothermal method has been used to deposit the NiCo₂O₄ films in nanoneedle morphology over a conducting FTO (fluorine-doped tin oxide) glass substrate for exploring its electrochromic properties. The electrochemical performance of the NiCo₂O₄@FTO electrode shows the redox and reversible nature of EC active material with the chromic modulation from parent translucent white colour to opaque dark brown colour with an application of a small bias of up to 2 V. In situ optical spectroscopy has been conducted for spectroelectrochemistry measurement of the NiCo₂O₄ electrode to understand the optical property under the influence of external biasing. An overwhelming electrochromic performance of NiCo₂O₄ nanoneedle film has been observed with very good colour contrast and reversible cyclic behaviour. This opens a door for exploration of inorganic materials for fabrication of robust and power efficient solid state electrochromic device.

EXPERIMENTAL DETAILS

A simple and very efficient bottom up approach, named as the hydrothermal method [13, 32], due to its ease as compared to other techniques like etching, spray pyrolysis and so forth [33–36], has been used for preparing the nanostructured film consisting of Ni and Co backbone. The raw precursor of Ni(NO₃)₂.6H₂O and Co(NO₃)₂.6H₂O bought from Sigma Aldrich (>99% purity) and mixed in 1:2 M ratio in a 30 ml aqueous medium. After complete mixing of these two precursors, 0.25 g urea was added gradually and left for 1 h with continuous stirring. This solution, appearing transparent pink in colour, is further transferred into a 50 ml Teflon beaker to process further. A conducting Fluorine doped tin oxide (FTO) coated glass substrates, facing downward towards the bottom of beaker, has been placed inside. The beaker is sealed tightly inside a stainless steel chamber and kept for 12 h at 120^oC followed by cooling that occurs gradually for 24 h until the room temperature is achieved. The prepared film over the FTO substrate is rinsed several times with DI water and annealed for 2 h at 100^oC. The film morphology has been analysed with the help of Scanning Electron Microscopy or SEM (supra Z5 Zeiss make) and characterized by X-ray diffraction or XRD (Bruker D2 Phaser) and Raman spectroscopy (Horiba LABRAM HR evolution model). The Image J software has been used to find the sharpness of the nanoneedle and surface plot. The electrochemistry was performed using Keithley 2450 electrochemical workstation and in situ absorption measurements have been carried out using Lambda 365 (Perkin Elmer make) spectrophotometer.

RESULTS AND DISCUSSION

As mentioned above, a convenient hydrothermal method has been used to deposit the film from Ni and Co precursors in different molar concentrations and characterized initially through XRD and Raman spectroscopy. A thick white translucent film was apparent over the FTO substrate and XRD was performed in Bragg Brentano mode. The XRD scan was fixed at 10°/minutes in the 15° to 80° diffraction angle range of 2θ. The XRD pattern obtained (Figure 1a) yields diffraction peaks at positions 36.5, 64.9, 59.2, 44.6, 31.0, 55.5, 19.1° in order of decreasing intensity. The peak position completely matches with the JCPDS no 20-0781 and reveals the existence of NiCo₂O₄ phase in the inverse spinal crystal structure [37]. Further, to understand the vibrational mode and validation of XRD results, Raman spectroscopy has been performed on the film using 633 nm excitation wavelength due to its versatile nature [38–44]. The Raman signal was recorded in backscattering geometry under with 30 s integration time. The vibration of Ni and Co atoms at tetrahedral and octahedral positions of crystal structure NiCo₂O₄ forms five different Raman bands [45] and are identified as F2g (192.5 cm⁻¹, 525.5 cm⁻¹, 613.2 cm⁻¹), E2g (481.8 cm⁻¹) and A1g (682.2 cm⁻¹) as shown in Figure 1b. Thus, these two basic characterizations confirm the phase purity with the occurrence of inverse spinal NiCo₂O₄ crystal structure.

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The morphology of white translucent film of NiCo₂O₄, as revealed by XRD and Raman measurements, have been studied using SEM (Figure 2) revealing a film consisting of nanoneedles. Figure 2a is showing the top view image of the film revealing vertically aligned nanoneedle architecture with sharp tips. The SEM image also reveals that the nanoneedles are deposited homogenously over the whole substrate (Figure 2a). Moreover, all the nanoneedles are well separated from each other which means the availability of larger surface areas to be exploited for specific application as will be discussed later. In other words, a better morphology coverage with porous nanostructures signifies the presence of larger contact area of the nanoneedle to activate the charge transfer reaction (in the case of electrolytic medium) for further application and hence a higher rate of reaction. The ImageJ^TM software has been used to estimate the average tip size of nanoneedle as well as the surface plot of nanostructures at randomly selected area on the film. Figure 2b shows that the average tip size is 9 nm as estimated from line profile meaning that nanosized NiCo₂O₄ film has been grown using hydrothermal method to study some interesting phenomena. The nanoneedles are well separated leaving most of the nanostructure exposed which can be clearly seen from the surface plot shown in the inset (Figure 2b). Surface plot is also showing the moderate density of nanostructures with significant separation between the individual nanoneedles.

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Since these NiCo₂O₄ films have been prepared for intended application as electrochromic electrodes, the electrochemical measurement has been performed using three electrodes setup. The three electrode setup consists of the reference electrode, counter electrode and working electrode as Ag/AgCl, a Pt-wire and NiCo₂O₄@FTO film respectively. The electrochemistry was performed in a cell consisting of 1 M aqueous KOH electrolyte. The Cyclic Voltammetry (CV) measurements [46] were done using a reversible voltage scan from −0.2 V to +0.6 V applied across the working (NiCo₂O₄) electrode and reference (Ag/AgCl) electrode and the corresponding current is measured across the counter electrode (pt-wire) and working electrode. Figure 3a shows the CV scan curves obtained with a scan rate of 50 mV/s corresponding to 1st, 25th and 50th cycle of CV scans along with the actual images of the electrode at different bias values. Under the anodic scan on sweeping the voltage from −0.2 V to +0.6 V, an anodic current peak appears near +0.45 V due to the OH⁻ ion intercalated to the NiCo₂O₄. The intercalation of OH⁻ ion to the working electrode switches the electrode from its parent translucent white colour state to opaque dark brown coloured state. After that, the working electrode get back to its natural colour, showing reversibility of the colour change, state under the cathodic scan from +0.6 V to −0.2 V (on reversing) with generating the cathodic current peak around +0.1 V is due to the OH⁻ ion de-intercalation from the electrode. Since the NiCo₂O₄ nanoneedle type morphology possesses the larger atomic site in every isolated single nanoneedle, which gives more possibility for intercalation of the OH⁻ ion and hence does not allow much time delay during reduction/oxidation processes. The higher surface area of morphology makes larger contact to the OH^- ion in KOH electrolyte medium which provide more sites for redox reaction and thus helps for colour modulation at low operating voltage. Owing to this correlation, a low bias colour switching appears to be a direct consequence of deposited nanoneedle architecture. Therefore, a complete cycle of CV measurement shows the reversible colour switching from whitish colour to dark brown colour under the charge intercalation and de-intercalation to the NiCo₂O₄ electrode. Thus, the electrode exhibiting two-state colour switching, shown in the background of CV diagram (real picture), is said to be the electrochromic electrode and the redox behaviour of the electrode making it a potential counter electrode candidate for other electrochromic active species. The stability of the redox behaviour of electrode under colour modulation has also been checked for 50 cycles and the data corresponding to 1st, 25th and 50th cycle is being presented in Figure 3a showing little variation in current. It signifies that the electrode is exhibiting a stable electrochromic performance when used as electrode in an electrochemical cell with liquid electrolyte. Therefore, the colouration (white → brown) and bleaching (brown → white) state of the electrochromic active NiCo₂O₄ electrode can be named as the ON and OFF state under the switching mechanism respectively for the discussion purpose.

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To better understand the chromic modulation of electrochromic electrode during colour switching, in situ UV-Visible spectroscopy has been performed in two electrode systems due to experimental constraints to use three electrode arrangements. The UV-Vis experiments have been done by applying voltages up to 2 V between the electrochromic NiCo₂O₄ electrode and Pt-wire as the two-electrode system need relatively higher raw voltages to switch the electrode form beached state to coloured state as compared to the three electrode system. The bias-dependent absorption spectra have been recorded in the whole range of the visible wavelength for OFF and ON states of the electrode (Figure 3b). The absorption spectrum (black curve, Figure 3b) taken from the electrode in a natural state shows a smooth absorption spectrum explaining the translucent nature of the electrode. The absorbance throughout the whole wavelength range increases on applying a +2 V bias to the electrode which indicates the appearance of the dark brown colour of the electrode shown by the blue curve (Figure 3b). The reversibility, an important aspect in electrochromism, was checked during CV measurements and the same state has been seen using absorption spectrum by reversing the applied bias (−1 V) to the NiCo₂O₄ electrode (red curve, Figure 3b). It is apparent that a −1 V bias is sufficient to completely reverse the visible spectrum to reinstall the original colour of the electrode (the bleaching process) confirming the reversible nature of the electrochromic effect. Therefore, the electrode found in the active state for the chromic modulation in the potential domain of +2 V and −1 V which is itself a less operating voltage in the family of all inorganic TMO. Overall absorption spectrum in the whole visible range has been shown in the inset (Figure 3b) to appreciate the bias-dependent colour modulation of the solid state nanoneedle electrode. A close analysis of the bias-dependent spectra (Figure 3b) showing ∼ 50% change in the absorbance indicates that the electrode acquires an optical contrast of 50% corresponding to the 500 nm wavelength which is one of the best for an inorganic electrochromic material. In situ optical absorption spectroelectrochemistry has also been carried out (Figure 3c) obtained by applying a toggling potential pulse between -1V and +2V. Corresponding absorption spectra during the colouration and bleaching response of electrode have been measured as shown in Figures 3a,b. Little variation in the colour modulation for four consecutive absorption cycles is a clear signature of electrode stability for its electrochromic device engineering. Zoomed portion of one absorption cycle (Figure 3d) reveals that the NiCo₂O₄@FTO nanoneedle takes only 3.4 s to switch from the bleached state to the coloured state (i.e. ON state) and nearly takes 2.8 s for reverse chromic modulation (i.e. OFF state), which reflects the importance of the nanoneedle structure towards electrochromic application. Therefore, the inorganic NiCo₂O₄ material in its nanoneedle structure is displaying the electrochromic property with better performance. A large optical contrast with small operating voltage as low as 1 V can be achieved for the NiCo₂O₄ electrode likely due to the deposited nanoneedle structure which makes a larger active site exposed to allow the charge intercalation, while the de-intercalation reaction speedily makes the redox process easy for colour modulation. It is clear from the above discussion that the new nanoneedle architecture enables one to get electrochromic modulation easily in terms of switching voltage and colour contrast which is one of the best in the family of inorganic electrochromic materials. It opens a possibility of new paradigm for revisiting power efficient electrochromic devices using robust inorganic materials.

CONCLUSIONS

A film consisting of NiCo₂O₄ nanoneedles, as confirmed using structural characterization, can be grown using the hydrothermal process by simple combination of separate cobalt- and nickel-based precursors. Electrochromic performance of the obtained inverse spinal NiCo₂O₄ structure film electrode shows bias-dependent colour switching from translucent white to dark brown colour states reversibly. The colour switching appears very efficient as it shows a colour contrast of ∼50% with an applied bias of as low as 1 V. The obtained nanoneedle geometry of NiCo₂O₄ with moderate density makes a large connection of the prepared film electrode to the electrolyte ion; hence the rate of the reaction from each nanoneedle increases. The impact of this, displayed in terms of the better performance of NiCo₂O₄ electrode towards electrochromic phenomenon opens a new paradigm for efficient and robust inorganic electrochromic devices.

ACKNOWLEDGEMENTS

The authors acknowledge the financial support from Science and Engineering Research Board, Government of India (Grant CRG/2019/000371). The authors are thankful to IIT Indore for providing SIC facility. One of the authors (D.K.P.) acknowledges the Council of Scientific and Industrial Research (CSIR) for financial assistance (File 09/1022(0039)/2017-EMR-I). Author T.G. acknowledge IIT Indore for fellowship. The DST, Govt. of India, is acknowledged for providing fellowship to M.T. (DST/INSPIRE/03/2018/000910/IF180398) and C.R. (DST/INSPIRE/03/2019/002160/IF190314). S.K. acknowledges UGC (Ref. 1304-JUNE-2018-513215), Govt. of India for providing fellowships. Facilities received from the Department of Science and Technology (DST), Government of India, under FIST scheme (Grant SR/FST/PSI-225/2016) is highly acknowledged. Partial funding under the TEQIP collaborative scheme is also acknowledged. Useful discussion with Dr. K.D. Shukla (Ujjain) Dr. P.R. Sagdeo (IIT Indore) is also acknowledged.

References

Wu, H.B. et al.: ‘Mixed transition‐metal oxides: design, synthesis, and energy‐related applications':Angew. Chem. Int. Ed Engl. 53(6), 1488–1504 (2014)

Poizot, P. et al.: Nano‐sized transition‐metal oxides as negative‐electrode materials for lithium‐ion batteries': Nature. 407(6803), 496–499 (2000)

Chaudhary, A. et al.: ‘Polythiophene‐nanoWO 3 bilayer as an electrochromic infrared filter: a transparent heat shield': J. Mater. Chem. C. 8(5), 1773–1780 (2020)

Pathak, D.K. et al.: ‘Nano‐cobalt oxide/viologen hybrid solid state device: electrochromism beyond chemical cell': Appl. Phys. Lett. 116(14), [eLocator: 141901] (2020)

Mishra, S. et al.: ‘Fast electrochromic display: tetrathiafulvalene–graphene nanoflake as facilitating materials': J. Mater. Chem. C. 5(36), 9504–9512 (2017)

Wang, L., Song, X.C., Zheng, Y.F.: ‘Electrochromic properties of nanoporous Co₃O₄ thin films prepared by electrodeposition method': Micro. Amp. Nano Lett. 7(10), 1026–1029 (2012)

Tanwar, M. et al.: ‘Mapping longitudinal inhomogeneity in nanostructures using cross‐sectional spatial Raman imaging': J. Phys. Chem. C. 124(11), 6467–6471 (2020)

Tanwar, M. et al.: ‘Unintended deviation of Fermi level from band edge in fractal silicon nanostructures: consequence of dopants': zonal depletion': J. Phys. Chem. C. 124(30), 16675–16679 (2020)

Tanwar M., Deconvoluting diffuse reflectance spectra for retrieving nanostructures’:size details: an easy and efficient approach. J. Phys. Chem. 123(16), 3607–3614 (2019). http://dx.doi.org/10.1021/acs.jpca.9b01935

Yogi, P. et al.: ‘Fano scattering: manifestation of acoustic phonons at the nanoscale': J. Phys. Chem. Lett. 7(24), 5291–5296 (2016)

Service, R.F.: ‘Nanotechnology grows up': Science. 304(5678), 1732–1734 (2004)

Book review of what is what in the nanoworld: a handbook on nanoscience and nanotechnology, completely revised and enlarged 2nd ed.': J. Am. Chem. Soc., 2008, 130, (50), pp. [eLocator: 17204]–17204

Pathak, D.K., et al.: ‘Precursor concentration dependent hydrothermal NiO nanopetals: tuning morphology for efficient applications': Superlattices Microstruct., 2019, 125, pp. 138–143

Chaudhary, A., et al.: ‘Prussian blue‐cobalt oxide double layer for efficient all‐inorganic multicolour electrochromic device': ACS Appl. Electron. Mater. 2(6), 1768–1773 (2020)

Bi, Z. et al.: ‘Large‐Scale multifunctional electrochromic‐energy storage device based on tungsten trioxide monohydrate nanosheets and Prussian white': ACS Appl Mater Interfaces. 9(35), 29872–29880 (2017)

Pathak, D.K. et al.: Improved field emission from appropriately packed TiO₂ nanorods. Designing the miniaturization': Superlattices Microstruct. 126, 1–7 (2019)

Mishra, S. et al.: ‘Mesoporous nickel oxide (NiO) nanopetals for ultrasensitive glucose sensing': Nanoscale. Res. Lett. 13, 16 (2018)

Wang, Y. et al.: ‘Controllable synthesis of ZnO nanoflowers and their morphology‐dependent photocatalytic activities': Sep. Purif. Technol. 62(3), 727–732 (2008)

Nail, B.A., et al.: ‘Nickel oxide particles catalyze photochemical hydrogen evolution from water‐‐nanoscaling promotes p‐type character and minority carrier extraction': ACS Nano. 9(5), 5135–5142 (2015)

Pacholski, C., Kornowski, A., Weller, H.: ‘Self‐Assembly of ZnO: from nanodots to nanorods': Angew. Chem. Int. Ed. 41(7), 1188–1191 (2002)

Pathak, D.K., et al.: ‘Improved analytical framework for quantifying field emission from nanostructures': Mater. Chem. Phys. 245, [eLocator: 122686] (2020)

Mortimer, R.J.: ‘Electrochromic materials': Ann. Rev. Mater. Res. 41(1), 241–268 (2011)

Chaudhary, A. et al.: ‘Polythiophene ‐viologen bilayer for electro‐trichromic device': Sol. Energy. Mater. Sol. Cells. 188, 249–254 (2018)

Moon, H.C. et al.: ‘Multicolored, low‐power, flexible electrochromic devices based on ion gels': ACS Appl. Mater. Interfaces. 8(9), 6252–6260 (2016)

Moon, H.C., Lodge, T.P., Frisbie, C.D.: Solution processable, electrochromic ion gels for sub‐1 V, flexible displays on plastic': Chem. Mater. 27(4), 1420–1425 (2015)

Zhang, S.: et al.: Preparation, characterization, and electrochromic properties of nanocellulose‐based polyaniline nanocomposite films. ACS Appl. Mater. Interfaces 9(19), 16426–16434 (2017). https://pubs.acs.org/doi/10.1021/acsami.7b02794

Roy, S., Chakraborty, C.: ‘Nanostructured metallo‐supramolecular polymer‐based gel‐type electrochromic devices with ultrafast switching time and high colouration efficiency': J. Mater. Chem. C. 7(10), 2871–2879 (2019)

Mjejri, I. et al.: ‘Double‐sided electrochromic device based on metal–organic frameworks': ACS Appl. Mater. Interfaces. 9(46), 39930–39934 (2017)

Chaudhary, A. et al.: ‘Raw hibiscus extract as redox active biomaterial for novel herbal electrochromic device': Sol. Energy Mater. Sol. Cells. 215, [eLocator: 110588] (2020)

Pathak, D.K., et al.: Nickel Cobalt Oxide Nanoneedles for Electrochromic Glucose Sensors. ACS Appl. Nano Mater. 4(2), 2143–2152 (2021). https://pubs.acs.org/doi/abs/10.1021/acsanm.0c03451

Tanwar M., et al.: Pseudo‐Anomalous Size‐Dependent Electron–Phonon Interaction in Graded Energy Band: Solving the Fano Paradox. J. Phys. Chem. Lett., 12(8), 2044–2051 (2021). [DOI: https://dx.doi.org/10.1049/nde2.12011]

Mavi, H.S. et al.: ‘Formation of Si and Ge quantum structures by laser‐induced etching': Phys. Status Solidi Appl. Mater. Sci. 203(10), 2444–2450 (2006)

Sahu, G., Kumar, R., Mahapatra, D.P:. ‘Raman scattering and backscattering studies of silicon nanocrystals formed using sequential ion implantation': Silicon. 6(1), 65–71 (2014)

Khalid, S. et al.: ‘Microwave assisted synthesis of porous NiCo₂O₄ microspheres: application as high performance asymmetric and symmetric supercapacitors with large areal capacitance': Sci. Rep. 6(1), [eLocator: 22699] (2016)

Shukla, A.K., Kumar, R., Kumar, V.: ‘Electronic Raman scattering in the laser‐etched silicon nanostructures': J. Appl. Phys. 107(1), [eLocator: 014306] (2010)

Gupta, V. et al.: ‘Raman imaging for measuring homogeneity of dry binary blend: combining microscopy with spectroscopy for technologists': Anal. Sci. Adv. 1(2), 89–96 (2020)

Gupta, V. et al.: ‘A state of the art review to enhance the industrial scale waste utilization in sustainable unfired bricks': Constr. Build. Mater. 254, [eLocator: 119220] (2020)

Anusuya, T., et al.: ‘Porous graphene network from graphene oxide: facile self‐assembly and temperature dependent structural evolution': Mater. Today Commun. 26, [eLocator: 101930] (2021)

Sharma, M. et al.: ‘Manifestation of anharmonicities in terms of phonon modes': energy and lifetime in multiwall carbon nanotubes': Carbon, vol. 171, pp. 568–574 (2021)

Mishra, S., et al.: Live spectroscopy to observe electrochromism in viologen based solid state device': Solid State Commun. 261, 17–20 (2017)

Tiwari, D. et al.: ‘Temporal in vitro Raman spectroscopy for monitoring replication kinetics of Epstein–Barr virus infection in glial cells': ACS Omega. 5(45), 29547–29560 (2020)

Umeshbabu, E. et al.: ‘Magnetic, optical and electrocatalytic properties of urchin and sheaf‐like NiCo₂O₄ nanostructures': Mater. Chem. Phys. 165, 235–244 (2015)

Elgrishi, N. et al.: ‘A practical beginner': s guide to cyclic voltammetry': J. Chem. Educ. 95(2), 197–206 (2018)