Synthesis and Fabrication of Graphite/WO₃ Nanocomposite-Based Screen-Printed Flexible Humidity Sensor

Sodium tungstate dihydrate (Na₂WO₄.2H₂O), cetyltrimethylammonium bromide (CTAB), hydrochloric acid, diacetone alcohol C₆H₁₂O₂, 98%), and graphite c, extra pure) were purchased from LOBA Chemie, as were the salts, CH₃COOK, MgCl₂, K₂CO₃, (MgNO₃)₂, CuCl₂, NaCl, KCl, KNO₃ , and K₂SO₄ required for the humidity sensing application. Cellulose acetate propionate C₇₆H₁₁₄O₄₉, extra pure) was bought from Sigma-Aldrich and utilized as received. All resistance and current measurements were carried out using a Fluke 179 True Multimeter.

For the preparation of WO₃ nanosheets, a simple chemical reduction precipitation approach has been used, as reported elsewhere.⁵⁰ In a nutshell, CTAB (capping agent, 0.0521 g) was dissolved in an appropriate quantity of water to provide a clear solution for the WO₃ synthesis. A suitable quantity of Na₂WO₄.2H₂O was added to the reaction mixture, followed by the incorporation of dilute HCl solution dropwise, and vigorous stirring produced a pale-yellow precipitate, which was centrifuged, completely rinsed with water, and then set aside to dry. To create pure WO₃ nanosheets, the obtained powder was calcined at a temperature of 500 °C.

For the preparation of the graphite/WO₃ (GW) nanocomposite, the synthesized WO₃ powder and pure graphite were physically mixed and added to ethanol. Ultrasonication was then carried out to ensure the proper mixing of the materials and an even distribution of nanoparticles over the nanosheet, as demonstrated in Fig. 1. The prepared inks were used to print the humidity sensor using screen-printing. The graphite and synthesized WO₃ were added in various ratios (1:1, 2:1, and 1:3, which were named as GW-1, GW-2, and GW-3 as shown in Table I. This simple, efficient, inexpensive, and mass-producible method was employed in synthesizing a composite intended to develop a humidity sensor. The nanocomposites were mixed with vehicle resin (mixture of solvent and binder), composed of diacetone alcohol (solvent) and cellulose acetate propionate (binder) in the ratio of 3:1. Furthermore, a few drops of triton-X100 was added to the solution followed by continuous stirring for 45 min to obtain graphite/WO₃ ink, as shown in Fig. 2.

Screen-printing of the device was carried out using polyester-based screen fabric mesh having a mesh count of 120/cm. First, commercial silver conducting ink was used to print the electrode, which resulted in good continuity with a very low resistance of 3 \(\Omega .\) After drying the silver ink, graphite/WO₃ inks with various compositions were printed. A metal connection was made to avoid harm and discontinuity during the characterizing sensor, as shown in Fig. 3. The complete overview of screen-printed humidity sensor is given in Fig. 4.

The flexible humidity sensor characterization was carried out using a saturated solutions approach, allowing relative humidity levels to be consistently maintained. In this procedure, saturated salt solutions were made in cylindrical sealed Teflon jars to provide the necessary relative humidity (RH) values using a saturated solution,⁵¹ and, furthermore, the RH of the sealed jars was measured using a commercially available humidity sensor. The RH associated with the saturated salts employed in this experiment were LiCl (11% RH), CH₃COOK (23% RH), MgCl₂ (33% RH), K₂CO₃ (43% RH), (MgNO₃)₂ (54% RH), CuCl₂ (62% RH), NaCl (75% RH), KCl (85% RH), KNO₃ (93% RH), and K₂SO₄ (97% RH) at room temperature.⁵² Furthermore, the salt solution was kept overnight to reach the state of equilibrium (optimum relative humidity condition), and each salt solution’s humidity was cross-checked using the commercial humidity sensor. Meanwhile, the humidity sensor was connected to a Tektronix TBS 1102B oscilloscope to ensure real-time resistance change. The device was placed in air-tight Teflon containers 2 cm above the solution to analyze the change in resistance, as depicted in Fig. 5.

X-ray diffraction XRD) analysis was carried out using a Rigaku Ultima IV diffractometer, with CuK \({\upalpha }\)radiation using a monochromator at 10-kV voltage, and scanning was carried out in the range of 10–90°/min, at a scan rate of 2°/min. XRD was used to determine the phase analysis and crystallinity of the Graphite, Tungsten oxide (WO₃), and their nanocomposites. As given in Fig. 6a and b, the sharp diffraction peaks observed are clear evidence for the monoclinic and orthorhombic phases of WO_3.⁵⁰ Figure 6a shows peaks at 23.18, 23.65, 24.33, 28.76, 33.18, 34.22, 50.14, and 55.96, which correspond to the planes at (002), (020), (200), (112), (002), (220), (040), and (100) respectively. The resultant data were further matched with the standard JCPDS cards nos. 00-024-0747 and 01-071-0131. The triplet peaks which exhibit the planes of (002), (020), and (200) with moderate intensity imply the crystallinity of the WO₃. Another sharp diffraction peak at 26.4\(^\circ\) corresponds to the (002) plane of the graphite-layered structure, as per the standard JCDPS data card (00-012-0212); the peak is shown in the (002) plane in Fig. 6b. As the concentration of graphite increased, the diffraction peak intensity increased, as shown in the XRD patterns of GW-1 and GW-2. In contrast with GW-3, the composition is increased by three-fold with respect to graphite, which leads to an increase in the triplet characteristic peak for WO₃ nanoparticles. The crystalline size of WO₃ was calculated using the Debye–Scherrer equation^53,54:where D represents the crystalline particle size, K stands for the Scherrer constant, \(\lambda\) denotes the x-ray wavelength, and \(\beta\) represents the full width at half-maximum of the diffraction peak. The calculated average crystalline size of the WO₃ nanoparticles is 236 nm.

The study of the molecular bond structure of the synthesized WO₃ nanoparticles was examined using Fourier-transform infrared spectroscopy (FT-IR) within 400–4000 cm⁻¹. The spectrum of the synthesized WO₃ nanoparticles is shown in Fig. 7, which shows the FT-IR spectrum of pure WO₃ crystalline nanoparticles with vibrational bands at 3592, 807, and 684. The stretching and bending vibrations of O-H molecules in the absorbed water molecules from the surroundings are attributed to the detected IR bands at 3592 cm⁻¹ and 1530 cm⁻¹. The stretching and bridging vibrations of W-O_int-W in the WO₃ nanoparticles are also shown by large IR bands at 807 cm⁻¹ and 684 cm⁻¹.^55,56

From the XRD analysis,, it was found that the tungsten oxide is present in two phases, the monoclinic and the orthorhombic. The SEM images of tungsten oxide and its nanocomposites also correlate with the dual phase nature of the nanoparticles of tungsten oxide, and, for the determination of the sizes and morphologies of the synthesized WO₃ and its nanocomposite, are given in Fig. 8a–e. The SEM image of WO₃ exhibits a granular nanoparticle morphology, as shown in Fig. 8a, which reveals the distinct nanoparticles with remarkable continuity of the particles, which in turn reduces the internal resistance of the device. Figure 8b–d demonstrates the composites of pristine graphite and WO₃ nanoparticles, which can be easily differentiated based on their structural morphologies. The accumulated WO₃ nanoparticles on the graphitic sheet are circled in Fig. 8b. As the concentration of the graphite increases, it causes agglomeration and restacking of the nanosheets, as seen in Fig. 8c, whereas the optimum distribution of the WO₃ nanoparticles over the graphite nanosheet can be easily visualized in Fig. 8d. The screen-printed silver (current collector) is even and finely printed over the PET substrate while maintaining good continuity and having exceptionally low resistances of \(\sim\) 3 \({\Omega }\) , which is highly advantageous when it comes to a flexible sensor to ensure its continuity while twisting and bending, as shown in Fig. 8e.

The sensing process is based on the change in the resistance of composite materials caused by the water molecules which were adsorbed on the surfaces, and the humidity can be effectively determined by measuring the resistance change across the sensor.⁵⁷ Tungsten oxide is considered a promising sensing material for humidity-sensing applications because of its merits, such as being sensitive in a wide relative humidity range, showing a fast response toward variations in relative humidity, and providing physical and chemical stability. The sensing phenomenon for tungsten oxide nanoparticles can be understood by considering that electrons get trapped on the surface defects such as ionized oxygen vacancies, and then get released when molecules of water are adsorbed onto the defect sites.⁵⁸ The resistive response of sensing material depends upon the concentration of the moisture content which can be driven by two factors, i.e., chemisorption and physisorption of water molecules adhering to the surface of the sensor material. Under a low moisture content, only chemisorption takes place, which implies the adsorption of the first water molecule layer over the sensing material surface, which strongly adheres to the oxide surface at around 30–40% RH concentration.⁵⁹ The present oxygen absorbs in the form of ions like O⁻, O²⁻, and O₂⁻²:

As the relative humidity (RH%) concentration increases, it leads to the formation of a physio-absorbed layer over the chemisorbed and the process continues over the numerous layers. However, all the physio-absorbed layers can be removed by either raising the temperature or lowering the RH% concentration because they are volatile (unstable) in nature, causing the device’s resistance to increase:

At this phase of the process, H₃O⁺ ions with a significant number of H⁺ protons develop and begin to serve as a charge carrier, also known as the Grotthuss mechanism.^60‐62 These charge carrier (H⁺) protons start hopping from one to another water molecule, through the simultaneous making and breaking of hydrogen bonds with an adjacent water molecule, and release an excess of protons, which leads to a dramatic decrease in the resistance. Furthermore, water molecules begin condensing into the underlying porous structural arrangement within the bulk of the sensor material, and a new carrier channel for ion migration is produced:

The variation of resistance with respect to the RH of nanocomposites was carried out at room temperature (25 °C) in RH% conditions is shown in Fig. 9a and b. The resistance of the GW-2 (168–211 \( {\Omega }\)) device is comparatively less than the GW-1 (313–353 \( {\Omega }){ }\) device, while the GW-3 device has a resistance varying in the range of 4.2–6.47 k\(\Omega\) due to an increase in the content of WO₃ in composite GW-3. To account for the sensing performance of the device, various parameters such as sensitivity, hysteresis, and dynamic response awerrere carried out, namely relative resistance with respect to resistance, as shown in Fig. 9a. It can be seen in Fig. 10 that the GW-3 composite shows a better performance and remarkable improvement during the determination of the real sensitivity compared to the other composites. This may be a huge change in resistance in accordance with the RH, and due to enhanced physical parameters like bonding between WO₃ and graphite, and water adsorption capacity. The sensitivity of the flexible humidity sensor was calculated using:where S (%) represents the sensitivity, R_lowest symbolizes the device’s resistance at lower humidity (11%), and R_current represents resistance at the current humidity, The sensitivity of the GW-3 composite shows an essentially linear rise with the increase in humidity, which is evident in the case of the high RH% values, as can be seen in Fig. 10. As the RH% increases, the resistance and sensitivity of the sensor shows a direct increasing trend.

For the estimation of hysteresis in the humidity sensor, the resistance is measured throughout the device in various RH conditions: RH% 11–97% response and from 97% to 11% recovery. The resultant output differs from the initial response at the same RH value, which is nothing but defined as hysteresis. During the hysteresis, the difference is observed which depends upon the rate of absorption and desorption of water vapor on the sensing layer of the device during the sensing of humidity. In GW-1 and GW-2, the sensors showed high hysteresis compared to GW-3, which is nearly superimposed over each other at various RH (%) ranges and shows the excellent reliability of the sensors. The absorption and desorption curves of the humidity sensor are shown in Fig. 11a–c. The dynamic response curve of the humidity sensor is measured concerning the resistance change for time (s), as shown in Fig. 12a–c. For the GW-3, Fig. 12c has a steady change in resistance compared to the other nanocomposites. As the concentration of graphite is greater in these composites, this leads to a decrease in the response change towards the RH condition.

The printed humidity sensor (GW-3) was subjected to different bending angles \(0^\circ ,\;30^\circ ,\;60^\circ ,\;90^\circ ,\;{\text{and 120}}^\circ\) using a customized device, as reported elsewhere.⁶³ As demonstrated in Fig. 13a, the absolute resistance of the screen-printed GW-3 at each angle was measured using a digital probe multimeter (Fluke 179). There is a very minimal change observed during the electrical resistance of the sensor at a different bending angle, as shown in Fig. 13b. Table II compares the fabrication and sensing methods between previously reported work and the present work.