Thermally reduced graphene oxide showing n- to p-type electrical response inversion with water adsorption

doi:10.1016/j.apsusc.2020.147285

Applied Surface Science

Volume 531, 30 November 2020, 147285

https://doi.org/10.1016/j.apsusc.2020.147285 Get rights and content

Highlights

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Synthesis of high quality thermally reduced Graphene oxide coatings.
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Excellent humidity sensing characteristics of the GO sheets.
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An n- to p-type conductivity inversion was observed under humidity.
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The conductivity inversion phenomenon is elucidated with basic characterization.

Abstract

Graphene oxide (GO) is regarded as one of the potential candidates for gas and humidity sensor applications owing to its excellent water adsorption capabilities. However, there is a lack of understanding the response of GO to humidity. Some authors believe the water adsorption leads to p-type response while other think it to be n-type. The present work demonstrates the mechanism of water adsorption and n- to p-type response inversion of GO to humidity. The GO suspension was synthesised by modified Hummers’ method and then drop-casted onto the substrates having already patterned gold interdigital electrodes (IDE). The sensors can detect wide humidity range (5–95 %RH) with a low voltage 0.1 V leading to low power consumption <10⁻⁵ W. It is also found that rGO samples annealed at temperature below 150 °C show n-type (e.g. R_air/R_RH ~ 1.29 × 10⁵ for rGO-40) while the samples annealed above this temperature demonstrate p-type humidity response (e.g. R_RH/R_air ~ 40 for rGO-150). The static test response time is 8 s, while recovery time is 13 s with excellent long-term stability and RH accuracy value (±4% RH). Further characterization are performed to elucidate the physio-chemical model of the sensing mechanism as well as n- to p-type conductivity inversion under humidity.

Graphical abstract

Introduction

There is an urgent demand to develop new materials for next-generation humidity sensors to be used in various applications. To meet this demand, scientists around the globe are putting huge efforts in industrialized research yielding novel class of humidity sensors with excellent sensing characteristics [1], [2], [3], [4]. These sensors are also used in manufacturing processes, food and environment control, disease diagnostics, security system, aerospace and automotive industry. Generally, a good sensor possesses basic characteristics and improved performance, such as low-cost fabrication, high response and selectivity, fast response/recovery time (below 10 s), both short- and long-term stability, compact in size, and low power consumption (<1 μW). Thus, novel 2D materials like Graphene, Graphene Oxide (GO), reduced Graphene oxide (rGO) and functionalized Graphene oxide (fGO) are nowadays preferred as they owe enhanced sensing performance due to inadmissible defects which surprisingly give rise to exciting properties.

Graphene oxide (GO) is generally produced from graphite by using strong acid and oxidizer agents treatment. The process was introduced by Hummers and Offeman [5], also widely known as an oxidation method, which make use of potassium permanganate (KMnO₄) and concentrated sulfuric acid (H₂SO₄) to produce high quality GO. For sensing applications, the structure of GO is generally thought to be based on single layer highly intact carbon sp² and sp³ hybridized form of framework and contains various oxygen-containing functional groups such as epoxy groups (=O), hydroxyl groups (–OH) and carboxyl groups (–COOH) at the basal plane and edges of this carbon framework [6], [7], [8]. Recent research has shown that oxygen containing functional groups on GO based sensing layers (such as GO, rGO and fGO) are key to enhance sensing characteristics [9], [10], [11], [12], [13], [14], [15], [16]. For instance, in our previous study we found that hydroxyl group play a critical role in improved humidity sensing properties of GO [9]. In another study, Jelinek et al. [12] fabricated a capacitive-type vapor sensor via two step freeze-drying technique, which showed the possibility to selectively detect the presence of molecules like water, NH₃, toluene, ethanol and so on. Zhang et al. [14] assembled a flexible resistive-type humidity sensor with nano self-assembly and chemical reducing method of rGO and a Polyelectrolyte. Hu et al. [15] developed a dimethyl methylphosphonate (DMMP) sensor based on p-phenylenediamine functionalized rGO with, which showed higher response to DMMP vapors than that of rGO (4.7 times at 30 ppm and 3.3 times at 10 ppm).

Despite, there are several issues related to practical implications of GO based sensors. One of them is the knowledge of the nature of charge carriers in GO. Several publications discussed about n- or p- type GO based sensor, but only few researchers focused on n- to p-type conversion and mechanism of GO [17], [18], [19], [20], [21], [22].

For gas sensing and many other applications, knowing the exact nature of charge carrier of materials is a key parameter. To this context, in this work, we found that the electrical response of rGO inverses from low n-type to high p-type electrical conductivity under humid conditions with the increase in annealing temperature. The GO was synthesized using modified hummers method [9] and subsequently annealed at 40 °C, 80 °C, 100 °C, 150 °C, 200 °C and 250 °C. The annealed rGO flakes were then dissolved in ethanol and drop-casted onto sensor substrate to measure the humidity sensing response. The SEM, TEM, XRD, FTIR, Raman and XPS analysis were performed to understand the sensing mechanism.

Section snippets

Materials

Unless otherwise specified, all the chemicals and reagents in this study are high purity analytically (AR) and used without further purification. For synthesis of GO, we used graphite powder (Squama Carbon 80–99.95%, particle size of 700 mesh, Qingdao Jinhui Graphite Co. Ltd. China), sodium nitrate (NaNO₃, Sinopharm Chemical Reagent Co. Ltd. China), sulfuric acid (H₂SO₄, 98% pure, Nanjing Chemical Reagents Co., Ltd. China), potassium permanganate (KMnO₄, 99.5% pure, Sinopharm Chemical Reagent

Morphology characterization

The morphology of all the samples annealed at different temperature (room temperature RT, 40 °C, 80 °C, 100 °C, 150 °C, 200 °C) was characterized by SEM and TEM, as shown in Fig. 2. It can be clearly seen that the GO flakes are highly electron transparent and layered morphology of the graphene oxide with wrinkled and folded feature is found in Fig. 2(a), which is in agreement with previous report [6], [19], [26]. From the selected area electron diffraction (SAED) patterns shown in the inset low

Discussion

In previous studies, GO-based humidity sensors were reported with controversial discussion of the GO sensing mechanism; some studies report GO behave like n-type material [37], [38], while other report as p-type material [39], [40]. In the present work, a combination of n-type and p-type GO sensing mechanism is proposed based on the previously recognize sensing mechanism shown in Fig. 8.

Basically, the gaseous molecules act as donors or acceptors when adsorption and desorption on the surface of

Conclusion

We demonstrated GO based humidity (5–95 %RH) sensors which can operate at 0.1 V and lead to significantly reduce the power consumption (1.314 × 10^-4 kWh/year). The response of rGO-40 sample is n-type and high (~1.29 × 10⁵) that inverses to low p-type response (~40) when the annealing temperature reaches 150 °C. The estimated values of response (8 s) and recovery (13 s) times are fast. The XRD, SEM, TEM, FTIR, Raman and XPS analysis are performed to elucidate the sensing and conductivity

CRediT authorship contribution statement

Azhar Ali Haidry: Conceptualization, Methodology, Writing - original draft, Writing - review & editing. Zhe Wang: Methodology, Data curation, Investigation, Writing - review and editing. Qawareer Fatima: Investigation, Supervision, Visualization. Ali Zavabeti: Resources. Lijuan Xie: Validation. Hao Zhu: Data curation. Zhong Li: Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank the Funding “Natural Science Foundation of Jiangsu Province” (BK20170795), “National Natural Science Foundation of China” (51850410506) and "Central University Basic Scientific Research Business Expenses Special Funds" (NG2020002) for providing major financial support. This work was also supported “Postgraduate Research and Practice Innovation Program of Jiangsu Province” (KYCX18_0281).

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Thermally reduced graphene oxide showing n- to p-type electrical response inversion with water adsorption

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Morphology characterization

Discussion

Conclusion

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgements

Microp. Mesop. Mater.

Sens. & Act. B: Chem.

Polymer

Nanoscale Adv.

Carbon

Sens. Actuator B-Chem.

Sens. Actuator B-Chem.

Mater. Lett.

Appl. Surf. Sci.

Ceram. Int.

Carbon

Carbon

Carbon

J. Colloid Interface Sci.

Chem. Eng. J.

Carbon

Appl. Phys. Lett.

J. Am. Chem. Soc.

Chem. Rev.