Polyvinylpyridine–carbon dots composite-based novel humidity sensor

Applied Physics A (2023) 129:691 https://doi.org/10.1007/s00339-023-06908-3 Polyvinylpyridine–carbon dots composite‑based novel humidity sensor Khouloud Jlassi1 · Shoaib Mallick2 · Ahmed Ben Ali4 · Hafsa Mutahir3 · Sayma Akhter Salauddin3 · Zubair Ahmad2 Lahcene Tennouga5 · Mohamed Chehimi6 · Received: 26 December 2022 / Accepted: 7 August 2023 © The Author(s) 2023 Abstract This work describes the rational design of thin films based on PVP-modified carbon dots for potential resistive humidity sensing application, prepared via spin coating on ITO substrates. The modified carbon dots were manufactured from graphite waste and modified with PVP to test the synergetic effect of the two materials. The surface hydrophilicity, morphology, and sensing properties were studied. AFM has been performed to investigate the prepared films’ texture and distribution over the surface. Overall, the hydrophilicity of the prepared films increases with concentration, leading to enhanced water vapor absorption on the surface of the sensing film. As a result, the sensor’s sensitivity is improved with the increasing concentration of PVP–CDs. The electrical response of the PVP–CDs composite film sensor shows a higher sensitivity level above 80% RH sensor with an irregular response; however, the concentration of 0.5 wt%, higher sensitivity, and linear change in impedance response was noted compared to other concentrations. Keywords Polyvinylpyridine · Carbon dots · Humidity sensor · Sensing mechanism 1 Introduction Humidity sensors are fabricated using a variety of materials and are of three types: ceramic type [1], organic [2], and hybrid polymers [3]. Humidity sensors mostly rely on * Khouloud Jlassi * Zubair Ahmad 1 Center for Advanced Materials, Qatar University, P.O. Box 2713, Doha, Qatar 2 Qatar University Young Scientists Center (QUYSC), Qatar University, P.O. Box 2713, Doha, Qatar 3 Gas Processing Center, College of Engineering, Qatar University, P.O. Box 2713, Doha, Qatar 4 Department of Chemical Engineering, College of Engineering, Qatar University, 2713 Doha, Qatar 5 Laboratoire d’Application des Electrolytes et des Polyélectrolytes Organiques (LAEPO), Département de Chimie, Université de Tlemcen, P.O. Box 119, 13000 Tlemcen, Algeria 6 Université de Paris, CNRS, ITODYS (UMR 7086), 75013 Paris, France the electrolytic properties of the material and should be manufactured to have high sensitivity, low hysteresis, and quick response and recovery [4]. A wide range of materials have been studied to achieve the desired characteristics of a suitable humidity sensor. Some of these materials are metal oxides [5], ceramics [6], carbon-based materials [7] such as graphene [8], and synthetic and natural polymers [9]. Several modern industries have recognized the need for efficient and low-cost humidity sensors. One of these is the agricultural industry, which requires effective monitoring of moisture content in air and soil. The healthcare sector also requires humidity sensors to monitor the moisture in medical equipment, such as incubators and Intensive Care Units [10]. One of the polymers used is conductive polymers, which are mainly inexpensive to synthesize and highly sensitive, proving excellent materials for humidity sensors [11]. The most widely used conductive polymers are polyaniline(PANI) [12] and polypyrrole (PPY) [13]; however, more recently, PVP and PVA have been used in humidity sensing applications due to their hydrophilic groups causing them to have a high affinity for water [14] due to its environmental stability and good electrical properties [15]. It is also non-toxic and has previously been used together with metal oxides [16], mainly tungsten [17], titanium [18], and 13 Vol.:(0123456789) 691 Page 2 of 8 zinc [19], for humidity sensing applications. It has, however, not previously been used with carbon dots. This paper uses quaternized PVP to fabricate humidity sensors in interaction with carbon quantum dots. Carbon quantum dots have been widely used for humidity sensing applications in combination with metals or by themselves [20]. Their increasing popularity is that they have several oxygen-containing functional groups on their surface, enabling them to create hydrogen bonds with water molecules and allow electrons movement between carbon dots and the water molecules [21]. The carbon dots used in this combination have been prepared using graphite waste as the carbon source; hence, they are easy to prepare and highly economical, whereas other methods of graphene quantum dots are traditionally more cost-intensive [22]. This study aims to design hybrid humidity sensors by exploring the synergetic effect of quaternized PVP with carbon quantum dots, as both materials have been used separately or with other combinations. Still, it has never been used together [23]. By carrying out the response, recovery, and hysteresis tests, the aim is to fabricate a humidity sensor with the characteristics of an ideal sensor. This work’s novelty lies in using solid waste from graphite processing and combining it with PVP to create functioning humidity sensors. These sensors fulfill the criteria required for a suitable humidity sensor and help turn process waste into value-added products, increasing the economic benefit and reducing the environmental impact of graphite processing. In this manuscript the PVP–CDs composites properties such as structure, morphology, surface roughness, hydrophilicity were studied. The electrical characterization of the sensors with different concentrations of PVP–CDs nanocomposites was investigated as well. 2 Experiment 2.1 Chemicals and materials 4-Vinyl pyridine, benzoyl peroxide, and hexyl bromide, all used organic solvents, were from analytical grade and purchased from Sigma-Aldrich, and ITO/glass electrode (S161) from Ossila UK. K. Jlassi et al. by dissolution/precipitation in ethanol/ether, respectively, then dried for several days at 60 °C and stored in a desiccator. The chemical structure of PVP is presented in Fig. 1a. Measurements were conducted in absolute ethanol by an Ubbelhode viscosimeter in a thermostat bath at 25 ± 0.1 °C. The Mv of the polymer was estimated at 1.24 × 105 g/mol using the empirical power law [η] = 6, 0.8 × 10−4 Mv 0.61. The quaternization was prepared by refluxing 3 g of PVP in 50 ml of methanol with 0.03 mol of hexyl bromide. The reactions were conducted in thermostated water at 70 °C for 5 days. The copolymer PVPC6Br was dissolved in chloroform and precipitated in hexane many times, then dried at 70 °C until there was no change in weight. The chemical structure of PVPC6Br is shown in Fig. 1b. The quaternization degree of PVP was found to be 65% by conductimetric titration of bromide ions with silver nitrate AgNO3 using a CDM 210 conduct meter (Radiometer, Meter Lab); we confirmed the results by 1H-nuclear magnetic resonance NMR with a Bruker 300 MHz (Rheinstetten, Germany) in CDCl3 solvent at room tempe (...truncated)