Ultrasonic doping and photo-reduction of graphene oxide films for flexible and high-performance electrothermal heaters

Highlights

• Nitrogen-doped rGO heater prepared by ultrasonic doping and laser writing.

• Inelastic phonon-electron collisions enhanced by improved lattice ordering.

• The N-LrGO heater shows high heating rates of 107 °C/s, good temperature distribution, high thermal, and mechanical stability.

• Heater is compatible with portable energy storage devices and suitable for next-generation flexible electronics.

• The N-LrGO heater was successfully applied as the thermotherapy patch.

Abstract

Thermotherapy has emerged as one of the most promising treatments for arthritis, a prevalent, crippling, painful bone disease. This demands more flexibility, energy-efficiency, safety, and light-weight in thermotherapy packs and hot clothing. Heteroatom doping is a metal-free, cost-effective way to improve carrier concentration and hence electrical and thermal conductivity in reduced Graphene Oxide (rGO) thus rendering it suitable for wearable joule heaters. However, the current doping techniques result in complex chemical structures that hinder phonon propagation and suffer other problems such as low yield, low scalability, and rigidity of the final product. Here, we disclose a novel and facile, low-temperature technique for nitrogen doping and photoreduction of graphene oxide (GO) films for high-performance, flexible graphene-based electrothermal heaters. The nitrogen atoms are introduced into the GO lattice with the aid of ultrasonic power in a wet chemical doping phase and maskless, automated, rapid CO₂ laser scanning is used for the concurrent removal of oxygen-containing functional groups and the rearrangement of nitrogen atoms in the graphene lattice. X-ray Photoelectron Spectroscopy (XPS) studies reveal up to 5.43% nitrogen dopant concentration with a high carbon to oxygen ratio of 16, while Raman studies uniquely show improved atomic ordering with I_D/I_G ratio of 0.51 in the nitrogen-doped Laser reduced graphene oxide (N-LrGO) films. The fabricated N-LrGO heater has a sheet resistance of 26 O/sq. and attains a higher steady-state temperature of up to 245.7 °C at a low driving voltage of 9 V with a low power demand of 0.7 Wcm⁻² and a heating rate of 103 °C/s. Its excellent temperature distribution and high flexibility join with the scalability of the preparation technique to demonstrate great potential for its incorporation with next-generation wearable electronics powered by low voltage portable energy storage devices.

Introduction

Electrothermal heaters are resistive structures capable of self-heating by a joule effect wherein electrical energy is transduced to thermal energy [1]. Modern times have seen an ever-increasing demand for better performances in electrothermal heaters in terms of transduction efficiency, flexibility, weight, and power management [1]. Moreover, wearable applications such as hot clothing, thermotherapy packs, and other healthcare appliances make an extra demand for flexibility, stretchability, and energy savings in the next generation electrothermal heaters. The quest to satisfy all these performance features explains why flexible thin-film heaters (TFH) have gained substantial research grounds in recent years [1], [2], [3], [4], [5], [6], [7], [8].

Low heater resistance allows for high steady-state temperatures to be attained at relatively lower driving voltage. This renders the heater highly efficient in its function as an energy transducer [1]. Many research efforts have already been invested in the line of providing reliably innovative, flexible heating solutions. Some of these solutions include the preparation TFH from metal nanoparticles and nanowires [9], [10], [11], carbon nanotubes (CNT) [2], and graphene [12] on flexible or even stretchable substrates [13], [14]. Embedding metal nanostructures [8], [15], CNT [16], or reduced graphene oxide (rGO) [17] within conductive or non-conductive polymers matrices have also been proposed.

Along with poor adhesions to polymer substrates [6], metals nanostructures and CNT have proved capable of penetrating the human skin, which may yield toxic effects and are thus categorized as not suitable for wearable applications [18]. Moreover, at high temperatures, they also suffer contamination due to oxidation at the particle junctions. This increases their resistivity, thus deteriorating their performance [5]. As a result, there has been a shift in attention toward highly conductive noble metals such as gold and silver, wherein their nanoparticles have been embedded in transparent polymer matrices to form high performance transparent flexible heaters [9], [10]. However, the high cost of these precious metals makes them unattractive for wearable applications such as hot clothing, and thermotherapy packs wherein transparency is not needed.

Overall, the potentially low cost, lightweight, high electrical, and thermal conductivity of carbon-based materials such as graphene makes it a suitable candidate for joule heaters [5]. This makes graphene preparation routes that arrive at flexible heaters with high-performance features of paramount importance. Chemical exfoliation of graphite into GO, followed by reduction to rGO is one the most popular processing routes in research today because of its high yield and great potential for scalability [19]. Laser scribing has shown to be a chemical-free, rapid, low-temperature method to reduce GO deposited on flexible polymer substrates while affording the possibility for patterning for various applications [20], [21], [22], [23], [24], [25], [26] and the doping in the presence of precursor molecules [27], [28], [29], [30]. Photo-irradiation has also been used to induce graphene for polyimide films [3], [31] and from coal [32] via laser scribing. The heaters from laser-induced graphene (LIG) [3] showed excellent performance. However, they had slow heating rates and suffered degradation within extended periods of high applied power [3]. Zhang et al. [33] prepared flexible pattered heaters from Laser reduced graphene oxide (LrGO), which attained as a steady-state temperature of up to 247.3 °C at 18 V applied voltage. However, the heaters needed a longer response time of 20 s. Lin et al. [5] added silver nanoparticles to the GO films before laser reduction in attempts to build in conductive bridges between the graphene sheets in the c-direction. The heaters prepared from this nanocomposite attained a steady-state temperature of up to 229 °C after 5 s of applying 18 V. Even though the response time of these heaters were improved, their reliance upon high concentrations of costly precious metals make these films financially unattractive for large-scale wearable electronics. Also, more research efforts geared towards lowering the driving voltages of these flexible heaters are still necessary to render them compatible with portable electrochemical power sources.

Furthermore, doping graphene with elements such as boron, nitrogen, and sulfur has been proposed as a metal-free means of increasing charge carrier concentration [34], thus improving conductivity. Various methods have already been established for doping graphene, which can be separated into two categories. Firstly, the in-situ approaches, such as introducing the dopant atoms during graphene deposition by using CVD [28], [35], [36], [37]. The second approach includes post-treatment methods of GO, such as hydrothermal methods [38], thermal annealing in the presence of dopant molecules [39], wet chemical methods [40], with heteroatom precursors [34]. While the in-situ approaches suffered low yield and high cost, the post-treatment methods involve harsh processing conditions incompatible with the use of flexible substrates, and theses procedures are too complex to industrialize. Besides, these doping techniques result in excessive lattice defects in the 2D structures which inhibit the propagation of phonons in the lattice. While electrons still can jump over minor vacancy and substitutional defects, phonons do not.

Consequently, the mean free path for phonons is much shorter than that for electrons. High electron density in the π*state is just as relevant as a high degree of atomic ordering in electrothermal materials for adequate heat generation and the propagation of the generated heat, which occurs principally by the movement phonons in carbon-based materials [41]. For, joule effect is caused by the inelastic collisions between phonons and electrons in a material when a potential difference is applied [42]. This means that even though doping represents a low cost, metal-free, means to improve upon the electrical and electrochemical properties of graphene, more efforts are still required to improve upon the electrothermal behavior of doped graphene-based materials and to render them more compatible with flexible substrates.

In this work, present a novel low-temperature doping and reduction technique in which nitrogen dopants are driven into the graphene lattice in a wet chemical process with the aid ultrasonic cavitation energy and further processed by maskless, automated, rapid laser irradiation. Unlike other doping techniques, this method uniquely preserves order in the sp² network of the graphene sheets while a high dopant concentration is attained. This not only allows for large amounts of heat to be generated by the heaters but also offers the doped films a larger capacity to propagate the generated heat. Consequently, a significant improvement in electrical and electrothermal behavior was observed in comparison with the undoped LrGO films. The low-temperatures applied in this approach contribute to its scalability and compatibility with flexible polymer substrates. This paper is structured as follows. In Section 2 of the article, the fabrication and characterization methods are described along with the performance measurement procedure. The evolution of chemical structure is presented in the first subsection of Section 3. A second subsection describes the electrical and transient electrothermal performance of the nitrogen-doped graphene-based heater while benchmarking it against other heaters presented in previous works. Thermal stability, robustness, and potential applications of the heaters are examined in a later subsection of Section 3, while Section 4 summarizes our key findings.