Nanoporous naphthalene diimide surface enhances humidity and ammonia sensing at room temperature

https://doi.org/10.1016/j.snb.2021.130972Get rights and content

Highlights

  • NDI derivative based humidity and ammonia sensors are investigated.

  • The sensors show good sensitivity, quick response/recovery, and excellent recyclability.

  • Breath analysis and non-contact humidity applications of the sensor has been investigated.

  • The NDI based sensors are simple, cost-effective, efficient and portable.

Abstract

We report fabrication and characterisation of humidity and ammonia sensors based on a nanoporous naphthalene diimide (NDI) thin layer, the active material coded as NDI-1, operating at room temperature. A thin layer of NDI-1 as an organic semiconducting sensing material was deposited onto the interdigitated electrodes using the spin-coating technique. Surface morphology investigations of the thin film using atomic force microscopy and scanning electronic microscopy revealed a uniform nanoporous surface. Structural and thermal behaviour studies of the NDI-1 using X-ray diffraction and thermogravimetric analyses confirmed the crystallinity and thermal stability, respectively. The capacitive type NDI-1 humidity sensor displayed a sensitivity of 10.13 pF/%RH, quick response and recovery (20.1/6.28 s), low hysteresis (0.72%), long-term stability and more importantly, a wide working range of relative humidity (10–95%) at 250 Hz. Interestingly, the developed sensor was sensitive enough to monitor respiration rate and demonstrated an excellent non-contact skin humidity sensing performance. The amperometric response of the sensor towards ammonia was also investigated. An increase in the sensor current was recorded when exposed to different ammonia concentrations (25, 37.5 and 50 ppm). The sensor showed high selectivity and good sensitivity (27.7%) towards 50 ppm ammonia with 200 s and 350 s response and recovery times, respectively.

Introduction

Porous materials with micro- to nano-dimensional pore sizes, and subsequently high surface areas, are essential for manufacturing, scientific, and domestic applications where an adsorption mechanism is favoured. Long recognised and extensively employed porous inorganic materials such as, alumina, fumed silica, zeolites, or activated charcoals have inspired the development of novel materials with definite pore sizes, high surface areas and functionalities [1], [2]. Over the past few years, highly engineered organic-inorganic composites and organic porous materials have also emerged and are currently being studied with the hope that they can compete with their inorganic equivalents in a variety of applications [3], [4], [5]. Nanoporous materials have found use in an array of applications such as energy and gas storage [6], [7], solar cells [8], catalysis [9] and sensors [10], mainly due to their distinctive physical and chemical properties including high surface area [11]. Such materials provide promise in chemical and biological sensing because they offer tunable pore/void space and large surface area, which offer enrichment of the adsorbate effects and high activity in surface chemical interactions. Studies have shown that the electrical and optical properties of porous semiconductors may vary substantially upon adsorption of molecules to their surfaces and/or by filling the pores. Therefore, surface adsorption and capillary condensation effects in nanoporous materials can be applied in the preparation of efficient sensor systems [12], [13], [14].

Among the variety of chemical and biological sensors, simple humidity sensors are applied to monitor relative humidity (RH) and have become crucial for weather prediction, historic preservation, environmental studies, semiconductors, agriculture applications, food processing, pharmaceutical preparations, textiles, paper, medicinal apparatus’s, and heating, as well as ventilation and air-conditioning [15], [16], [17], [18]. Various materials, including small organic molecules, organic polymers, graphene, and inorganic composite materials, have been employed for humidity sensing applications with various effects [19], [20], [21], [22], [23], [24]. However, water vapour isn’t the only potential use for such sensors. Considering environmental pollution factors, it is critical to identify and measure the precise concentration of hazardous gases such as ammonia, which has extensive applications in fertilisers, synthetic fibres, drugs, generating ammonium salts, and the manufacturing of plastics [25]. Ammonia is a highly corrosive and poisonous gas that can be easily propagate into the ecosystem [26], [27]. Prolonged exposure can lead to severe health effects from airway swelling or damage through burning. In high concentrations, it can be lethal. While the lower limit of human sensitivity by smell is about 50 ppm for ammonia [28], even at this limit, it is irritating to the skin, eyes, and the respiratory system, leading to cough and nose/throat irritation [29]. The permissible thresholds of 25 ppm over 8 h and 35 ppm over 10 min periods have been recommended and legislated by Health and Safety Executive (HSE), London [30].

Commercially available sensors for humidity and ammonia detection are usually based on porous metallic oxide materials, like ZnO, SnO2, WO3 and Fe2O3 [31], [32], [33]. However, the sensors based on these materials have certain drawbacks such as high working temperature, low selectivity, and the solid substrate brittleness limit their suitability in wearable electronics [34], [35]. Polymers, graphene and organic semiconductor-based sensors which can surmount these existing weaknesses are considered as favourable candidates [36], [37]. Currently, small organic molecules, including phthalocyanines (Pcs), perylene diimides (PDIs), and naphthalene diimides (NDIs), have gained significant attention in various humidity and gas sensing applications [24], [27], [38]. Small organic molecules should provide benefits such as easy synthetic methods, defined molecular formula, definite molecular weight, simple purification and structural modification, good batch-to-batch reproducibility, good solubility and film formability. Furthermore, their structural, electrical, and surface properties should be easily tuned by molecular engineering [39], [40]. NDIs also possess good chemical and thermal properties, exceptional charge mobility, simple synthetic procedures, and solution processability among small organic molecules. However, they are rarely investigated for humidity and ammonia sensing applications, and those which are reported are in organic field-effect transistor (OFET) platforms [41], [42]. The OFET based sensors do exhibit good sensitivity, but they have complex fabrication techniques and are prone to device shorting. In contrast, capacitive/amperometric type sensors would provide straightforward, low-cost, and easy to fabricate alternatives, if developed. To this end, we recently studied three NDI derivatives carrying imide side chains of various hydrophilicity for capacitive humidity sensing [24]. The results revealed that different hydrophilic groups on the imide position affect the surface morphology and crystalline structure and indeed interacted differently with water molecules, thereby providing an impact on sensing performance of the humidity sensors.

In this current work, we report on the assembly of nanoporous films of NDI-1 (Fig. 1a) and apply these surfaces to humidity and ammonia sensing applications employing capacitive and amperometric platforms at ambient temperature. There are no examples in the current literature where a simple, NDI-based small molecule is reported to create nanoporous surface and is used to enhance humidity and ammonia gas sensing at room temperature. The idea of generating film porosity to improve gas sensing is an active area of research and to the best of our knowledge, this is the first report of an NDI derivative being used in an amperometric type sensor for ammonia sensing. Several sensing parameters such as sensitivity, response/recovery, hysteresis, non-contact humidity sensing performance, breath analysis, stability, recyclability and selectivity of NDI-1 have been evaluated in detail at room temperature (RT).

Section snippets

Synthesis

The one-step synthetic protocol used for NDI-1 in which both imidation and esterification reactions occur concurrently is given in the Supplementary information (SI).

Thin film sensor fabrication

Silicon (Si) substrates and interdigitated electrodes (IDEs) (10 mm × 6 mm × 0.75 mm dimensions with 10 µm spacing between the fingers) were utilised in the preparation of thin films and sensors (Fig. 1b), respectively. The IDEs were acquired commercially from Micrux technologies and consist of a quartz (SiO2) substrate patterned

Thin film and material characterisations

An image of a thin film of NDI-1 via AFM (Fig. 2a) revealed the effective pattern of nanopores, which were uniformly distributed over the whole surface. The pores vary slightly in their size distribution (200–400 nm), though are remarkably regular. The distribution of these nanopores over the surface were thought to provide a scientific advantage to our previous work [24] by assisting the adsorption, or capture, of target water, for instance, ensuring better sensing performance. The inherent

Conclusions

We were able to successfully develop a prototypical sensor that is highly sensitive to humidity and ammonia at room temperature via employing an NDI-1 active layer. The AFM and SEM analysis revealed a uniform nanoporous structure of a thin film of NDI-1, which we believe attributed to the properties exhibited. The humidity sensing properties of the prepared sensor was studied by exposing it to various humidity levels ranging from 0% to 95% at RT, and the sensor displayed swift response and

CRediT authorship contribution statement

Salman Ali: Conceptualisation, investigation, materials and devices fabrication and characterisations, and writing the original draft. Mohammed A. Jameel: TGA and DSC measurements and assisted in data interpretation. Christopher J. Harrison: Assistance in sensing data collection and writing. Akhil Gupta: Materials synthesis, assisting in data interpretation and writing. Steven J. Lanford and Mahnaz Shafiei: Results analysis, reviewing and finalising the draft, Supervising, Funding and managing

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

This research received funding from the Australian Renewable Energy Agency (ARENA) as part of ARENA's Research and Development Program-Renewable Hydrogen for Export (Contract No. 2018/RND012). The views expressed herein are not necessarily the views of the Australian Government, and the Australian Government does not accept responsibility for any information or advice contained herein. This research was supported by using the Nectar Research Cloud, a collaborative Australian research platform

Salman Ali is an organic electronic devices researcher at the School of Science, Computing and Engineering Technologies, Swinburne University of Technology, Melbourne, Australia. He has completed his MS (Physics) degree from Riphah International University, Islamabad, Pakistan. Presently, he is working on the application of Naphthalene diimides and various other small organic molecules for humidity and gas sensing applications. He is currently doing PhD at the Swinburne University of

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    Salman Ali is an organic electronic devices researcher at the School of Science, Computing and Engineering Technologies, Swinburne University of Technology, Melbourne, Australia. He has completed his MS (Physics) degree from Riphah International University, Islamabad, Pakistan. Presently, he is working on the application of Naphthalene diimides and various other small organic molecules for humidity and gas sensing applications. He is currently doing PhD at the Swinburne University of Technology, Melbourne, Australia, under the supervision of Prof. Steven J. Langford and Dr. Mahnaz Shafiei and awarded Swinburne University Postgraduate Research Award (SUPRA) scholarship for his PhD studies.

    Mohammed A. Jameel is a PhD researcher in the School of Science, department of Chemistry and Biotechnology at Swinburne University of Technology, Hawthorn, Australia. He studies material science. His doctoral project, supported by Swinburne University and CSIRO energy technologies, focus on designing, and Characterising Naphthalene Diimides as organic and perovskite solar cells. He received his master’s degree in science from University of Duhok, Kurdistan, Iraq, with the coordination of the University of Melbourne, Australia.

    Christopher Harrison received his BE and BCompSci from RMIT University in 2013. He then transitioned to a PhD program developing novel digital signal processing techniques for application to acousto-electric sensors. He is currently engaged at Swinburne University of Technology as a research fellow, helping establish a gas sensing laboratory there. His work focuses on the development and application of novel sensing materials targeting the emerging hydrogen energy sector.

    Akhil Gupta is currently working as a Research Fellow at the Faculty of Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, Australia. His research interests lie in the design and development of organic, inorganic and hybrid materials for a number of emerging research areas such as organic solar cells, self-assembly for light-harvesting, molecular gelation and chemical sensing. He completed his PhD at Monash University, Clayton Australia in 2013.

    Mahnaz Shafiei is currently an Associate Professor and former Vice-Chancellor’s Women in STEM Fellow at Swinburne University of Technology and a visiting fellow at Queensland University of Technology (QUT). She received a Bachelor of Science degree in Electrical and Electronics Engineering from AmirKabir University of Technology in Iran, and a PhD degree from RMIT University, Melbourne, Australia in 2011. She followed this with postdoctoral research at QUT and an Australian Endeavour Research Fellowship at Simon Fraser University, Canada. Mahnaz’s research focus is on sensors and nanomaterials and their practical use for health and environmental monitoring. She is investigating new technologies to develop reliable, portable gas sensors with ultra-low power requirements to be embedded in sensor nodes for the Internet-of-Things applications or in mobile systems

    Prof Steven Langford completed his PhD in 1994 from the University of Sydney and was the sole recipient of the Ramsay Memorial Trust Fellowship working with Prof Fraser Stoddart (2016 Chemistry Nobel Prize winner). After an ARC postdoc Fellowship at UNSW, he moved to Monash to begin his independent career in 1998, becoming Professor in 2006 and Head of School in 2010. After a distinguished career at Monash, he joined Swinburne in October 2017 as Dean, Research and Development. His interests involve the application of organic chemistry to new materials research, utilising the principles of supramolecular chemistry to areas as diverse as photosynthetic mimicry, sensors, sustainability, molecular electronics and medicinal chemistry.

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