Editorial: Focus on green nanomaterials for a sustainable internet of things

In the dynamic landscape of the Internet of Things (IoT), where smart devices are reshaping our world, nanomaterials can play a pivotal role in ensuring the IoT’s sustainability. These materials are poised to redefine the development of smart devices, not only enabling cost-effective fabrication but also unlocking novel functionalities. As the IoT is set to encompass an astounding number of interconnected devices, the demand for environmentally friendly nanomaterials takes center stage. This Focus Issue spotlights cutting-edge research that explores the intersection of nanomaterials and sustainability. The collection delves deep into this critical nexus, encompassing a wide range of topics, from fundamental properties to applications in devices (e.g. sensors, optoelectronic synapses, energy harvesters, memory components, energy storage devices, and batteries), aspects concerning circularity and green synthesis, and an array of materials comprising organic semiconductors, perovskites, quantum dots, nanocellulose, graphene, and two-dimensional semiconductors. Authors not only showcase advancements but also delve into the sustainability profile of these materials, fostering a responsible endeavour toward a green IoT future.

In an era defined by the relentless advancement of technology, the Internet of Things (IoT) has emerged as a transformative force, weaving together the physical and digital realms to create a seamlessly connected world [1].As we venture further into this landscape of smart devices, wearables, smart homes, and Industry 5.0, the need for sustainable innovation is more pressing than ever [2].Amidst this pursuit, nanomaterials have risen to a pivotal position [3], offering solutions that not only enhance performance but also align with our aspirations for a greener future.Indeed, nanomaterials are driving progress in a wide spectrum of applications: from sensors [4,5] energy harvesters [6,7], and energy conversion and storage devices [8], to light emitting devices [9,10], and quantum information science [11].Their attributes offer a wide range of opportunities for tailoring their properties to suit specific requirements, making them highly attractive for IoT applications.
The burgeoning expansion of the IoT, however, carries with it a distinct set of challenges, demanding careful consideration of environmental impact and resource utilization.It is, therefore, imperative to champion the development and adoption of eco-friendly nanomaterials, and by extension, devices, which ought to have a high sustainability profile and rely on earth-abundant elements and environmentally friendly fabrication methods [12].The appeal of this approach is two-fold: such materials promise not only to address the pressing issue of resource scarcity, but also to enable the sustainable proliferation of IoT devices.Green nanomaterials point to a future in which sensors that monitor air and water quality * Author to whom any correspondence should be addressed.
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. in smart cities are composed of nanomaterials that themselves have minimal environmental impact; wearable devices powered by energy harvested from the surrounding environment, without producing large volumes of e-waste; and energy storage systems capable of efficiently storing and releasing energy, ensuring a stable supply from renewable sources.
This Focus Issue features papers unveiling a diverse research landscape in green nanomaterials for a sustainable IoT, spanning the domains of circular electronics, materials and devices engineered for smart windows and energy harvesting, energy storage and hydrogen production, and the fundamental properties of emerging optoelectronic nanomaterials (figure 1).These articles illuminate the multifaceted potential of green nanomaterials, demonstrating their adaptability and power across a spectrum of applications.
In a world increasingly driven by technological consumption, the imperative of circular electronics and the ecologically conscious synthesis of electronic materials are key priorities.In this Focus Issue, the papers authored by Cecchi et al and Banerjee et al collectively highlight the transformative potential of circular electronics and sustainable synthesis methodologies.Cecchi et al introduce an environmentally friendly method for recovering non-leaching gold from electronic waste [13].By utilizing aqueous chemistry with hydrogen peroxide and a food chain byproduct, lactic acid, this approach yields gold in its metallic state.The recovered gold not only offers a valuable resource but also finds application in fabricating electrodes for organic field-effect transistors.In the realm of sustainable synthesis methodologies, Banerjee et al's review paper delves into the green synthesis of graphene and its multifaceted IoT applications [14].From sensors to communication devices, this review charts a path toward an interconnected future driven by this versatile nanomaterial.
Advances in device technology are instrumental in shaping the course of the IoT.This Focus Issue comprises an array of studies focusing on the development of cutting-edge devices-ranging from stretchable electronics to advanced computing components and photodetectors-that capitalize on nanomaterials to advance a sustainable IoT ecosystem.Lerond et al's paper introduces an important advance in device design, bringing together stretchable electronics and electrochromic technology [15].By utilizing conductive fibers on elastomeric substrates and a flexible chitosan electrolytic gel, the study demonstrates intrinsically stretchable electrochromic devices.Exploring the crossroads of optoelectronics and neuromorphic computing, Shen et al unveil an optoelectronic synapse based on ZnO nanowires [16].With the ability to emulate synaptic plasticity, this device lays the foundation for efficient, brain-inspired computing systems.Bridging the gap between organic electronic devices and cellulose nanocrystals, Chaulagain et al demonstrate memory devices comprising zinc phthalocyanine covalently conjugated to cellulose nanocrystals [17].The pronounced hysteresis exhibited by these devices in their current-voltage characteristics enables data storage, thereby pointing to the potential of all-organic memory devices.Patel et al's exploration into paper-based photodetectors elucidates the fusion of paper as a flexible substrate with two-dimensional semiconductors as optoelectronic materials.The simple act of rubbing tungsten diselenide (WSe 2 ) crystals onto paper results in cost-effective photodetectors, boasting a spectrally wide photoresponse and excellent mechanical flexibility [18].
The realm of energy harvesting stands as an undisputed frontier in our quest for sustainable IoT.By tapping into ambient energy and optimizing energy harvesting techniques, we pave the way for self-powered devices capable of seamless operation across diverse environments.This Focus Issue features two papers that discuss emerging technologies for harvesting energy from artificial indoor light sources-an approach that could potentially power a myriad of IoT devices in indoor settings.With a focus on organic indoor photovoltaics, Dagar and Brown employ biological/metal-oxide charge transport layers cast from green solvents, achieving enhanced light harvesting under low-intensity indoor lighting [19].In parallel, You et al's endeavor revolves around metal/oxide/metal-based stacks as the back-electrode of indoor organic photovoltaics.By crafting an optical cavity within these stacks, the researchers maximize the photocurrent, thus amplifying the energy-harvesting efficiency of the resultant photovoltaic devices [20].An intriguing implication of this approach lies in the vivid colors generated by these back-electrode stacks, which expand their applicability by blending energy harvesting with aesthetic appeal.
Within their diverse array of potential applications, the domains of efficient energy storage and sustainable hydrogen production emerge as formidable contenders for propelling the IoT toward sustainable trajectory.This Focus Issue showcases advances in energy storage and hydrogen generation that prioritize ecofriendly approaches.Addressing the need for sustainable and abundant materials in energy storage, Brennhagen et al investigate the potential of Bi 2 MoO 6 as an anode material for sodium-ion batteries [21].Through an operando x-ray diffraction investigation, the team unravels the complex mechanisms at play during charge-discharge cycles, laying the groundwork for designing high-performance, eco-friendly energy storage solutions.In the realm of photocatalysis for sustainable hydrogen production, Zhang et al insightfully examine the recent progress in carbon nitride and organic semiconductors as photocatalytic materials [22].These papers collectively underscore the crucial role of nanomaterials in the realm of electrochemistry, enriching energy storage and hydrogen production strategies with direct implications for IoT applications.
At the heart of pioneering device technologies for the IoT resides the need to develop novel optoelectronic materials and attain a fundamental understanding of their properties.Mohanty et al provide an insightful perspective on the potential of layered two-dimensional hybrid perovskites as optoelectronic materials [23].By discussing their structural versatility, synthesis techniques, and potential for stability, this paper showcases a path toward harnessing these materials for sustainable optoelectronics.In the realm of lead-free perovskite derivatives, Schmitz et al explore the impact of halide substitution on tuning the optical properties of 2D monolayer silver-bismuth-bromide double perovskites [24].Through adept manipulation of bandgap characteristics, the researchers showcase the customization potential of these materials to cater to various IoT application requirements, such as indoor photovoltaics to sustainably power IoT sensors [25].In the realm of hybrid organic-inorganic composite nanomaterials, Theurer et al illuminate the unique characteristics of the interface between organic semiconductors and inorganic quantum dots [26].This investigation advances the understanding of the binding mechanisms in such hybrid systems and sheds light on their photophysics, thereby advancing our capacity to exploit these materials for device applications.Notably, these insights might also contribute to the advancement of emerging non-toxic quantum dots, such as InP-based embodiments, which have recently demonstrated promising efficiency and lifetime in light-emitting devices [27].
As we navigate the intricate pathways toward a sustainable IoT ecosystem, this Focus Issue captures important developments in nanomaterials and device technologies striving to adopt sustainable approaches and deliver eco-friendly solutions.As with all nascent technologies, it will be crucial to systematically investigate their sustainability profiles through comprehensive life cycle analyses in the near future, thereby identifying and addressing potential sustainability hotspots [28].The research showcased within this Focus Issue stands as a testament to the collective endeavor thus far, inspiring the continual development of nanomaterial technologies that not only enhance our lives but also safeguard our planet for generations to come.

Figure 1 .
Figure 1.Key themes of the present Focus Issue, covering a wide range of nanomaterials and their applications in energy harvesting, electronics, sensors, and energy conversion and storage.