Thermal-based Zinc-Oxide-Coated Smart Fabric for Thermochromic Applications

The present study focuses on developing a thermochromic device with a flexible substrate through the coating of cotton fabric with ZnO (Zinc oxide) by solvothermal synthesis technique. Here, ZnO is used as the thermochromic layer for the fabrication work, and it is suitable for textile and wearable applications as it is non-toxic to human skin. This device is designed and fabricated in order to gain better insight into the role of ZnO in thermochromic applications. Here, 3D nanostructures of ZnO are grown on the surface of cotton fabric using a simple and cost-effective solvothermal synthesis approach. The coated fabrics are investigated to determine their structure, morphology, composition, electrical, optical and emissivity properties using an X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM), current-voltage (I-V) characteristics, ultraviolet protection factor (UPF) etc. From the morphology study, uniformly packed ZnO nanorods with growth in the c-axis direction are observed. The ZnO nanostructures are known to have excellent UPF when exposed to solar radiation and showed UPF value of 112.48. It is found that coated fabrics have increased electrical conductivity under optical excitations and also enhanced the reflectance. Moreover, based on the emissivity analysis coated ZnO cotton fabric showed the emissivity of 0.95, which is higher and has greater radiation protection than that of bare cotton fabric. Hence, the developed thermochromic device has potential for use in the future in textile and wearable based thermochromic application.

Energy in today's world is the foremost crucial problem humankind is progressing to confront by the year 2050.Energy prices have risen dramatically as a result of the rising population, decreasing natural resources, and a lack of sufficient energy provided from renewable energy sources.Hence, by introducing smart materials, we can improve the energy efficiency in residential, commercial buildings, spacecraft applications, etc Smart materials are materials that have the capability to respond to an external stimulus to make necessary changes in its properties like optical properties.The smart materials that change their colour and transparency upon applying the external stimulus are known as chromogenic polymers.Based on the external stimulus, these chromogenic polymers are classified as electrochromic and thermochromic, respectively for electric field and temperature stimuli.In recent years, electrochromic and thermochromic materials have been growing interest due to their functional properties.
Electrochromism is a non-emissive technique that uses electrical impulses to adjust optical characteristics. 1,2Electrochromic materials have been used for a wide range of applications in the fields of displays like electronic paper, 3,4 smart windows with electrically tuneable reflection or transmission properties, automobile rear-view mirrors, 5 helmet visors, 6 and light shutters. 7Similarly, when the temperature of thermochromic materials vary, the colour changes reversibly.This thermal stimulus results in changing the material's molecular structure, as well as, in its crystalline phase. 8The advent of thermochromic materials (TCMs) for outdoor applications would further assist the building, vehicles, and textile sectors in future.Maintaining the storage temperature might be vital in the case of thermal storage; therefore, any TCM utilized in this application must feature a rapid change of colour. 9Some of the potential applications of TCMs are Sunlight control window glazing with transmittance modulation (smart window glass coatings), 10,11 textiles, 12 luminous thermo sensors, 13 and colour indicators. 14The majority of prior thermochromic devices were made on rigid substrates, usually transparent glass substrates.The pressing demands of next-generation electronics, on the other hand, would necessitate flexible, stretchable, and even wearable electrochromic and thermochromic devices. 15,16TCM layer on the exterior surface of residential and commercial buildings might alter the amount of solar radiation absorbed or reflected by the building, enhancing its energy efficiency.In general, based on the distinct material properties and operating conditions, thermochromic materials are divided into four categories: inorganic, organic, polymeric, and hybrid.Inorganic compounds like titanium oxide (TiO 2 ), vanadium oxide (VO 2 ), and tungsten oxide (WO 3 ) have shown interest in chromic application in recent years.In comparison to organic materials like TiO 2 , VO 2 , and WO 3 , 17 the inorganic counterparts of these materials are more durable and resistant to environmental variables such as high temperatures, UV light, and mechanical wear. 18,19When subjected to numerous external stimuli such as UV irradiation from Sunlight and ambient environmental factors such as temperature, pressure, and humidity fluctuations, TCMs frequently degrade severely.TCMs' properties and physical behaviours alter as a result of this deterioration. 20Several synthesis methods and coating techniques are used to improve the thermochromic performance of the materials and preserve the core TCMs from getting damaged.The inorganic compound ZnO is being used as the thermochromic layer by replacing the existing TCM.ZnO is an n-type semiconductor metal oxide with a wide direct bandgap that ranges from ∼2.8-3.4 eV, which is similar to the bandgap of those existing thermochromic materials.Here, ZnO is used as the thermochromic layer due to its material property, such as high UPF value, low cost for production, high thermoelectric efficiency, easy fabrication in large quantities, and non-toxic to human skin.z E-mail: suha.sathiya@gmail.com;jayabal@iiitdm.ac.in; pandiyarasan@yahoo.co.in ECS Advances, 2024 3 012003 Pandiyarasan Veluswamy et al. used a simple, cost-effective hydrothermal method to synthesize ZnO -coated carbon fabric.The as-fabricated material's structural, morphological, chemical states, optical, electrical, and thermopower properties were studied.The nanostructured ZnO-coated fabric has a significant role in enhancing UV shielding compared to bare carbon fabric.It is found that the ZnO-composite fabrics have increased electrical conductivity. 21iroya Ikeda et al. reports the fabrication and characterization of low-cost and large-area flexible thermoelectric materials with nanocrystalline ZnO applicable for the wearable power generator.The ZnO nanostructures were grown on cotton fabric and carbon fabric by the solvothermal synthesis method. 22Hyeongkeun Kim et al. investigated that, to date, forming highly crystalline and stoichiometric VO 2 on a flexible substrate has been a challenge, due to the high-temperature condition for VO 2 growth, and hence, this has not been demonstrated. 23Chanil Park et al. fabricated an electrochromic device on a PET film by a solution process using a multifunctional conducting polymer synthesized by oxidation polymerization.PEDOT: PSS and PANI: PSS were employed as the polymer layers, which may act as both an electrode and an EC layer without the requirement of additional electrode materials (ITO, FTO, etc).Because of the balanced surface charge capacity, the ECD displayed outstanding optical properties and cycle stability. 24Dipta Mukherjee et al. used a spin coating method with four different rpm was used to create smooth, homogeneous, and crystalline VO 2 thin films on quartz, which were then post-annealed at 350 °C, 450 °C, and 550 °C in a vacuum.Thermo-optical and electrical characteristics of several thin films, including solar transmittance, reflectance, absorptance, infrared (IR) emittance, and sheet resistance were examined. 25M. Benkahoul et al. used the RF reactive magnetron sputtering technique to deposit thermochromic VO 2 thin films over various substrates; quartz, aluminium, and silicon.7][28][29] From the literature review, thermochromic materials have traditionally been placed on rigid substrates, and the inorganic compound ZnO has not been employed before on flexible substrates to the knowledge of the authors for thermochromic applications.Hence, in this study, the synthesis of ZnO nanostructure on cellulose cotton fabric using solvothermal seed process and growth process is attempted for thermochromic applications.It is also confirmed with Emissivity analysis.

Experimental
Materials.-Thechemical reagents used are as follows: Triton X-100, Citric acid, Sodium hydroxide, Hexamethylenetetramine (Hexamine)[C 6 H 12 N 4 ], and Zinc nitrate hexahydrate [Zn (NO₃)₂•6H₂O], purchased from Sigma Aldrich, India, and used without any further purification.The cotton fabric of dimension (6.5 cm × 4.5 cm) is cut for the experiment and undergoes a scouring process.The cellulose of the cotton fabric treats with non-ionic and anionic detergents; sodium hydroxide (NaOH), citric acid, Triton X-100, and water.The scouring process removes the fatty compounds, greasy materials, wax, and other impurities.
Synthesis.-A3D ZnO nanostructure was formed on the scoured cotton fabric by a two-step growth method consisting of a seed creation process and a nanostructure growth process.Figures 1a, 1b, shows the sample preparation for solvothermal growth.The typical solvothermal seeding process was done as follows; 0.1 M concentration of Zn(NO₃)₂•6H₂O dissolved in 50 ml of double-distilled (DD) water under magnetic stirring at room temperature.Similarly, to the 50 ml of DD water, a 0.2 molar concentration of C 6 H 12 N 4 was added and dissolved, as shown in Fig. 1c.While adding the C 6 H 12 N 4 solution, the change of color was visualized from clear to salty color, confirming the nucleation process.Then the C 6 H 12 N 4 solution was added very slowly to the previous solution.After some time, dried CF was immersed in the mixture solution for 1 h and then ultrasonicated for 30 min.The whole process was done at 50 °C.The above process was repeated for the molarity ratio of Zn (NO₃)₂•6H₂O and C 6 H 12 N 4 from 1:1 to 2:1.The fabric with mixture solution was taken and placed inside an autoclave having an inner volume of 200 ml (TEFLON, F-1029-06) and heated in the hot air oven for 1 h at 120 °C for the solvothermal seeding process.Then the fabric was collected after 4-5 h of the heating process.The solvothermal seed coated fabric was washed with distilled water and ethanol.Finally, the ZnO-seed-coated cotton fabric was placed in a hot air oven at 60 °C for 1 h.
In the second step, the nanostructure growth process of ZnO was carried out by taking 100 ml of DD water, to which 0.1 molar concentration of Zn(NO₃)₂•6H₂O and 0.2 molar concentration of C 6 H 12 N 4 was added and dissolved using magnetic stirring at 50 °C.Then the mixture solution with cotton fabric was transferred into the autoclave, which was kept inside the hot air oven for the ZnO nanostructure growth at 120 °C for 5 h.Then cotton fabric in the autoclave was allowed to reach room temperature inside the oven itself.Then the ZnO nanostructured coated cotton fabric was rinsed with deionized water and ethanol three times each.Finally, the fabric was placed inside a hot air oven for 1 h at 60 °C for drying.The ZnO nanostructure cotton fabric was taken for several analyses to check its quality and properties.The ZnO-seed (ZSCF) and ZnO-growth coated fabrics (ZGCF) used for further study.

Characterization
The sample surface morphology was characterized by FESEM, JOEL JSM 6390, with an acceleration voltage of 15 kV.
The UV shielding effect of as-prepared fabric was investigated using scattering and absorption spectroscopy (Lapsphere UV1000F) in the wavelength range 250-450 nm.
I-V characteristics were measured by ENLITECH SS-X solar simulator equipped with a Xe short-arc lamp as the broadband light source.
The solar reflectance was evaluated by 2' Integrating Sphere (THORLABS IS236A-4) and UV-vis and NIR Spectrometers (Ocean Optics USB4000) with Deuterium Halogen light source.
The emissivity study was conducted using a device working based on the Stefan Boltzmann Law.

Results and Discussion
Morphological study.-FE-SEMwas utilized to get insight into the morphology and microstructures of the as-prepared fabric, bare and scoured cotton fabric.The images of each sample were taken in four scales − 100 μm, 20 μm, 10 μm, and 1 μm.Figures 2a-2d shows the magnified FE-SEM image of bare cotton fabric, which clearly shows the presence of impurities.The fabrics were treated with anionic and non-ionic detergents as they went through the scouring process.
Figures 2e-2h shows scoured cotton fabric, where the surface of the fibres is smooth and the series of ridges, furrows or linear marks are no longer apparent.All wax, grease and other impurities were removed during the scouring process.The nano-crystalline seeds coated the cotton fibre surface uniformly with strongly connected and dense hexagonal shapes as shown in Figs.2i-2l.On the cotton fabric, high density and high aspect ratios are clearly seen after coating ZnO nanostructure on seed coated cotton fabric.The Zn (NO₃)₂•6H₂O and C 6 H 12 N 4 were used as the precursors in 1:2 proportion.From the Figs.2m-2p, the FE-SEM images reveal the growth of ZnO nanorods array on the surface of a single fiber and it is densely packed on the surface of cotton fabric, haphazardly aligned with preferred axis of growth of nanorods perpendicular to the fiber.Since the hexamine concentration is more compared to Zn (NO₃)₂•6H₂O, the nanorods were grown on the cotton fabric.The proportion of C 6 H 12 N 4 is responsible for the length of nanorods grown over the cotton fabric.If the proportion of Zn(NO₃)₂•6H₂O is more than C 6 H 12 N 4 , nanosheets along with nanorods can be observed.
ECS Advances, 2024 3 012003  ECS Advances, 2024 3 012003 Structural analysis.-Thecrystalline phase constitution of the as-synthesized sample was investigated using XRD studies.Figure 3, shows the XRD patterns of ZnO-coated and bare cotton fabric.Both the samples exhibited significant diffraction peaks at 22.94°, 14.82°, and 16.64°, which could be well indexed to (1 2 0), (1 1 0), and (0 0 2) planes of hexagonal carbon (JCPDS card no-50-0926) and can be attributed to the presence of cellulose substrate.The presence of carbon peaks in both samples indicates that the ZnO nanostructure formation had not affected the crystallinity of the cellulose fabric.All the prominent diffraction peaks of ZnO are in good agreement with previously reported data with hexagonal structure.Relatively intense solid peaks were observed at 36.238°, 31.786°and34.495°which is consistent with (1 0 1), (1 0 0) and (0 0 2) planes (JCPDS card no-75-0576).Among them, the ZnO (1 0 0) and (1 0 1) diffraction peaks, on the other hand, had a much higher intensity, indicating that the preferred growth direction is along the c-axis.Zinc nitrate hexahydrate and hexamine in the ratio 1:2 were used for the synthesis of ZnO nanostructure.When the hexamine concentration is higher than zinc nitrate, hexamine plays an active role in delivering a regulated supply of OH -anions by reacting with water, promoting the growth in the c-axis direction leading to the formation of nanorods on the fabric surface.The perfect alignment of the ZnO nanorods array on the surface of cotton fabric exhibited in the FE-SEM image as shown in Fig. 2, was fairly consistent with the XRD result when considering the growth direction of the ZnO nanorods.Furthermore, no impurity peaks were found in either the ZnO nanostructure coated cotton fabric or the bare cotton fabric, reaffirming that the ZGCF was pure.
Evaluation of UV protection factor (UPF).-The UV protection factor shows the ability of textiles to protect against ultraviolet (UV) rays influenced by a number of elements, including weave, fiber chemistry, finishing procedures, fabric colour, additives, and washing.UPF is one of the most important criteria to consider in protective textile and garment industries.The bare cotton fabric had a UPF value around 6.8, which is considered as having very poor capability for blocking UV rays.The UPF value of the ZnO coated nanostructure on CF(ZGCF) is shown in Fig. 4. UV blocking is classified into UV-A and UV-B rays.If the UPF value is greater than 15, it can block UV-B rays not UV-A rays.And from the studies, it is understood that if UPF value is greater than 45, it can block UV-A rays.For the bare cotton fabric, the UPF value lies below 15, so it cannot block UV rays and cannot protect human skin.If UPF value is more than 40, then it is considered to be having excellent UV blocking capability.It is observed that the prepared sample possessed UPF value of 112.48, indicating excellent UV blocking property.
I-V characteristics.-TheI-V characteristics of ZGCF are shown in Fig. 5 and found to be linear, indicating an ohmic conduction mechanism.The conductivity of the inorganic compound ZnO coated on the cotton fabric was measured using a solar simulator under varying conditions.A semiconductor acts as an insulator at 0 K, and the electrons will occupy the lowest energy states available to them.The conduction band (CB) will be totally devoid of electrons, as every state in the valence band (VB) will be occupied.The optical or thermal excitation and the addition of impurities can change the conductivity of a ZnO semiconductor.
Here, the focus was on optical excitation (photon energy).It is found that the conductivity of as-coated fabric varies under different conditions; without light (WOL) and with light (WL), as shown in Figs.5a and 5b, respectively.At room temperature, only a significant amount of electrons gets transported from the VB to the CB for smaller bandgap (∼1 eV) material.So, in the case of ZnO, electrons cannot transport from the valence band to the CB at room temperature due to its wider bandgap.In Fig. 5c, two samples of ZGCF were randomly taken out from a set of samples and named ZGCF-1 and ZGCF-2.Here, green and red colors represent the currents produced when the voltage range was between −3 and 3 V.
There is a negligible amount of current produced due to less mobility of charged particles since ZnO has a wider bandgap.The electronhole transport is not possible between the VB and CB.So, to overcome this issue or increase the conductivity, an external excitation is to be done.When the Xenon lamp is ON, the blue and violet colors in Fig. 5c show the linear graph of the currents produced on the ZnO coated cotton fabric.The current incrases due to optical excitations; photons' energy causes electrons to move from the lower band to the higher band, making the system conductive.Here, WL: ZGCF-1 and WL: ZGCF-2 clearly show that the number of electrons available for conduction can be significantly increased in ZnO semiconductors by optical excitation.Almost both samples have similar results demonstrating the repeatability and reproducibility of the samples.
Tauc plot.-ZnO has a wider bandgap in the range of 2.8-3.4 eV, which is somewhat similar to the bandgap of TiO 2 and tungsten oxide used for thermochromic applications.Hence, the Tauc plot was used to calculate the energy bandgap of the material.The Tauc plot shows the quantity hν, that is, the photon energy on the abscissa, and the quantity (αhν) 2 on the ordinate, where α is the absorption coefficient of the material, h is Plank's constant and ν frequency of  light.Extrapolating the energy of the optical bandgap of the ZnO growth coated on the cotton fabric from this linear area to the abscissa yields the energy of the optical bandgap of the ZnO growth coated on the cotton fabric.The Fig. 6 shows the energy bandgap of the ZnO growth coated material with a magnitude of 3.16 eV.
Optical characteristics.-Theoptical characteristics were studied to evaluate the interaction of the material with light.Among the optical properties, transmittance, optical density (OD) and reflectance were investigated.
Optical density.-Theoptical density of the coated fabric was measured in order to comprehend its optical behaviour.The wavelength of the light wave has the greatest influence on optical density.The OD of the samples was measured at five different locations and were labelled OD-1, OD-2, and so on.Here, the optical density of transmittance in solid mode is plotted and shown in Fig. 7.The OD value is notably low in the visible light spectrum's early region, say up to 390 nm, affirming that the light is blocked.Beyond 390 nm, the higher the wavelength, the higher is the OD value, indicating increased transmittance through the coated sample.two heaters to provide heat to the samples, three T-type thermocouples for measuring temperature of both samples and surroundings.Initially heat is supplied to the black body sample with voltage of 8.28 V and current of 1.20 A. Then heat is supplied to the bare cotton fabric sample to reach its temperature around 80 °C which took 8.3 V and 1.18 A. Once the temperature got settled, the emissivity of the bare cotton fabric sample is calculated using Stefan Boltzmann law using Eq. 1, which gave the value of 0.87 and it is also confirm its validity through the journal-"Mid-infrared emissivity of nylon, cotton, acrylic, and polyester fabrics as a function of moisture content" by Raymond G Belliveau and others.Generally, when thermal radiation is absorbed by a fabric, the energy is converted into and re emitted back out from the surface of the fabric.In this case, based on the emissivity values, ZGCF have higher emissivity value than bare cotton fabric which means it gives more radiation protection compared to bare cotton fabric.Figure 9b shows the comparison between the emissivity of nylon, cotton, acrylic, polyester with fabricated samples.

Conclusions
In this work, ZnO nanostructures were grown on the surface of cotton fabric using the solvothermal synthesis technique.The morphology of fabricated cotton fabrics was investigated using FE-SEM.From the morphology study, uniformly packed ZnO nanorods with growth in the c-axis direction were observed.The UPF value of ZnO nanostructured coated fabric was 112.48.The high UV shielding characteristic of ZnO is responsible for increasing UPF.The calculated band gap of ZnO-coated fabric was 3.16 eV, which lies within the ZnO semiconductor bandgap range.The   obtained E g value of ZGCF is similar to that of thermochromic materials such as TiO 2 and tungsten oxide.It was demonstrated that ZnO coating could convert an insulator, a cotton fabric, into a conductive fabric.The optical characteristics of the as-coated fabric are enhanced to a considerable extent.The higher the optical density, the higher is the transmittance of ZnO-coated fabric in the visible region.The reflectance of the ZnO nanostructured fabric increased significantly in the UV region compared to the bare cotton fabric.It is suggested from this study that ZnO can be a potential candidate for thermochromic materials due to their relatively high electrical, optical, UV shielding with emissivity properties.Hence, the fabricated thermochromic device can be used in textile and wearable based applications.

Figure 1 .
Figure 1.(a), (b) Method for solvothermal growth.(c) Schematic representation of fabrication process of ZnO nanostructure coated on cotton fabric.

Figure 3 .
Figure 3. X-ray diffraction pattern of bare cotton fabric and ZnO nanostructure coated cotton fabric.

Figure 4 .
Figure 4. UV Shielding property of bare CF and ZGCF.
Reflectance.-Thereflectance properties of bare cotton fabric and ZnO growth coated cotton fabric are shown in Fig. 8. Three samples were chosen randomly from the ten sets of ZnO growth coated samples (ZGCF-1, ZGCF-2, and ZGCF-3), and their reflectance properties were studied.From the figure, it is evident that ZnO nanostructured cotton fabric shows enhanced reflectance properties.While comparing to the bare cotton fabric, the ZnO nanostructured fabrics reflect more UV-B and a high percentage of UV-A radiations.The maximum reflectance was observed in the range of 200-300 nm region.At the beginning of visible region, a sudden peak is observed in ZnO coated fabric due to plasma resonance of electrons in the UV region.All ZGCF samples yielded similar results indicating the repeatability and reproducibility of the samples.Emissivity analysis.-Theemissivity analysis was carried out to check the measure the materials ability to emit and absorb thermal radiation.Emissivity of the samples is obtained by vacuum sustainable device as shown in Fig. 9a, which work basis on the Stefan Boltzmann law.ZGCF and bare cotton fabric along with black body sample are taken at same cross section area.The device consists of

Figure 5 .
Figure 5. Material under solar simulator (a) without light (b) with light conditions (c) I-V characteristics of ZGCF.
, B V T and I , B I T are voltage and current provided to black body sample and testing sample-bare cotton fabric sample and ZGCF sample in this case.σ is Stefan's constant with value of 5.6703 × 10 −8 W/m 2 K 4 .A is the area of cross section of the sample.T, T S are the temperatures of sample and surroundings., B ε T ε are the emissivity of black body sample and testing sample.The same experimental procedure is repeated to ZGCF.The voltage and current provided were 8.3 V, 1.2 A for black body sample and 8.3 V and 1.19 A. The emissivity of the ZGCF sample is 0.95, obtained after calculating using the Stefan Boltzmann law.

Figure 9 .
Figure 9. (a) Schematic representation of the emissivity device (b) Comparison between the emissivity of nylon, cotton, acrylic, polyester with fabricated samples.