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Review

Assessing the Sustainability of Energy-Related Nanomaterial Synthesis: Emphasizing the Need for Energy-Efficient Nanomaterial Preparation Techniques

by
Nazim Hasan
1,2,
Manikandan Muthu
3,
Othman Hakami
1,2 and
Judy Gopal
3,*
1
Department of Physical Sciences, Chemistry Division, College of Science, Jazan University, P.O. Box 114, Jazan 45142, Saudi Arabia
2
Nanotechnology Research Unit, College of Science, Jazan University, P.O. Box. 114, Jazan 45142, Saudi Arabia
3
Department of Research and Innovation, Saveetha School of Engineering, Saveetha Institute of Medical and Technical Sciences (SIMATS), Thandalam, Chennai 602105, Tamil Nadu, India
*
Author to whom correspondence should be addressed.
Energies 2025, 18(3), 523; https://doi.org/10.3390/en18030523
Submission received: 13 December 2024 / Revised: 12 January 2025 / Accepted: 21 January 2025 / Published: 23 January 2025
(This article belongs to the Section A: Sustainable Energy)
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Abstract

:
Sustainable energy has always been the top-priority research discussion, and nanomaterials in energy applications have facilitated the achievement of this goal. For the first time, this review highlights the subtle, overlooked, unaccounted expenditure of energy going into nanomaterial synthesis. In the present article, we give a brief overview of the various nanomaterials used in energy applications and present their general synthesis methods. The lack of data/information on the energy expended on nanomaterial synthesis has been critically pointed out. The alternative, energy-saving, energy-efficient methods, considering sustainability even at the nanomaterial synthesis level, have been put forth as recommendations. This article aims at creating an awareness towards planning of holistic sustainable energy-efficient nanomaterial synthesis processes that will conserve energy. The question projected is: what is the purpose of losing energy during synthesis of energy producing and energy storing nanomaterials?

1. Introduction

Population growth and technology is what drives the 21st-century energy demand [1]. Global power use is 17 trillion watts [2], and energy experts predict that by 2050 it will become 30 trillion watts due to population as well as economic expansion [3]. Energy usage has surged due to population growth, portable electronics, transportation, and domestic and industrial applications. However, energy demand has created a global energy crisis, which has, over recent years, become a compelling cause of concern. Non-renewable sources such as nuclear and fossil (oil, coal, natural gas), as well as renewable energy sources like solar, hydroelectric, ocean, wind energy, geothermal, biomass, and hydrogen energy are the predominant energy sources that are in the game. These supply power to homes and businesses via power lines and other transmission equipment. Affordable and renewable energy is the most pressing demand because sustainable energy drives economic growth and industrialization [4,5]. It is time to save non-renewable energy and find alternative sustainable solutions. Rising ecological concerns and modern society’s needs require the quick development of creative, low-cost, and eco-friendly energy generation and storage solutions [6].
According to the EU Commission, the term “nanomaterial” has been used to described any manufactured or natural material that may be unbound, agglomerated particles having external dimensions between 1 and 100 nm [6]. Recently, the British Standards Institution extended the dimensions to a broader range extending from 1 to 1000 nm size ranges [7]. Nanostructured materials react differently from bulk materials due to size, shape, and distribution [8,9]. Nanomaterials exhibit unique physical, chemical, and biological properties. Increased surface-to-volume ratio, leading to enhanced surface energy and associated biological efficacy has been reported [10,11]. Because of the increasing surface area-to-volume ratio of the nanomaterial, electrode–electrolyte interaction is increased, enabling better ion diffusion kinetics and higher charge/discharge rates. Higher surface area of nanoparticles improves electrochemical performance, energy density, and cycling stability [12]. A nanomaterial-based supercapacitor’s large surface area facilitates greater electrostatic storage capacity, leading to quick energy release and reabsorption. A higher surface area increases the active surface available for photocatalytic activity, leading to effective catalysts in fuel cells, thereby increasing energy conversion. On the whole, inherent properties of nanoparticles, such as their large surface area, size-dependent properties, and catalytic activity, allow for effective energy storage technologies [13].
Physics, chemistry, biology, as well as engineering researchers have benefited from incorporating nanotechnology in their various applications [14]. This has permitted the fabrication of nanoparticles (NPs), nanolayers (NLs), and nanotubes (NTs), which have impacted many human activities [15,16]. Many nanostructured materials have been synthesized in the laboratory for various applications [17]. Nanomaterials have become well established for their application in smart coatings, solar energy systems, and sensing due to their higher efficiency. Solar photovoltaics, hydrogen generation, and fuel cells benefit from their superior properties and cost savings [18,19]. NPs are made by lithography, laser ablation, aerosols, radiolysis, and photochemical reduction. These methods are expensive, energy-intensive, and incur damage on man and surrounding environments [20,21]. Dispersants, surfactants, and chelating agents prevent nanomaterial agglomeration during chemical synthesis. Most of these chemicals become strong environmental pollutants when mass-produced [22]. Thus, green methods minimize cost, energy, product loss, and pollution [11,14,23,24,25]. Microbes, fungi, and plant extracts are well-studied elements of green synthesis processes [26,27]. The three principles of sustainability, such as environmental sustainability, social sustainability, and economic sustainability are interconnected and interdependent. Achieving true sustainability requires a holistic approach that addresses the complex interactions between the environment, society, and the economy. Sustainable energy production as well as sustainable nanomaterial synthesis is very important
The objective of this review is to highlight the wastage of energy involved during the synthesis of energy nanomaterials. For the first time, the overlooked aspects pertaining to energy expenditure during energy nanomaterial synthesis have been highlighted. The need to revisit nanomaterial synthesis methodologies, keeping in mind energy efficiency, has been emphasized. Considering sustainability even at the grass-roots level of nanomaterial synthesis has been recommended. Alternative energy-efficient nanomaterial synthesis approaches have been recommended. This is the first review in this direction that highlights the energy consumption aspect of nanomaterials in energy applications.

2. Energy Application of Nanomaterials—Brief Overview

Energy nanotechnology aims to reduce greenhouse gas emissions and support a low-carbon economy [28]. Nanotechnology benefits all energy chain sectors, and nanomaterials have imprinted their signature on various aspects of energy such as fossil and nuclear fuels, geothermal energy, sun, wind, water, tides, and biomass energy sources. Figure 1 projects an overview of the major energy applications of nanomaterials.

2.1. Nanomaterials in Energy Production

Nano-coated, wear-resistant drill probes extend the oil and gas or geothermal energy development system lifespan and efficiency, saving money. Nanomaterials are used to make lighter, more durable wind and tide power plant rotor blades and wear and corrosion protective coatings for mechanically stressed components. Nanotechnologies are essential for photovoltaic system intensification. Antireflection layers improve plain crystalline silicon solar cell light production and efficiency. Nanotechnologies have improved silicon, dye, and polymer thin-layer solar cells. Polymer solar cells render low cost and flexibility resulting in promising portable devices. Component layer design and organic semiconductor mixture morphology could be optimised by nanotechnologies. Nanostructures like quantum dots and wires may boost solar cell efficiency to above 60% [3,4].
High power plant efficiency requires heat-resistant turbine materials. Power plant and aircraft engine turbine blades can be improved using nano-scale heat and corrosion protection titanium aluminide coatings. Nano-optimized membranes can improve coal-fired power plant carbon dioxide separation and climate-neutral storage. Economic fuel cell applications in vehicles, buildings, and mobile devices are made possible employing nano-structured electrodes, catalysts, and membranes. Thermoelectric energy conversion is yet another promising avenue. Nano-structured semiconductors, owing to their improved boundary layer design, could power textile electronics utilizing body heat [28].
Quantum dots, fullerenes, and CNTs improve solar cell efficiency. Nanotechnology helps construct cheap, efficient solar cells. Quantum dot crystals convert light into electricity, increasing solar cell efficiency by 42% [29]. Researchers have deposited semiconducting thin films of TiO2 (n-type) and NiO (p-type) on an ITO transparent conductive glass plate to build electrodes. A thin TiO2 coating on the upper surface increased efficiency by 18%, that is 50% more than those without coatings [30]. Additionally, polycrystalline Si/doped TiO2 semiconductor thin-film electrodes are reported. Organic polymer PV alternative systems like solar cells (OPV) generate affordable renewable energy from light [31]. Alamri et al. [32] improved solar PV panel energy efficiency with hydrophobic SiO2 nanomaterial. SiO2 enhances PV panel performance. The coated panel’s total efficiency was enhanced by 15% and 5% over the dusty and uncoated panels, making the solar photovoltaic system more efficient. Nanoblack coatings increase solar energy absorption. To improve collector efficiency and conserve energy in solar water heating systems, scientists tested black paint, sol-chrome, black chrome, black nickel, and black anodized aluminium. Since anodic aluminium oxide (AAO) possesses self-organized and well-ordered nanopores, Girginov et al. [33] reported that metal ion electrodeposition in porous alumina enhances the coating [34,35]. Nanotechnology-based nanofluid solar collectors absorb and scatter sunlight [36]. Yousef et al. [37] confirmed the effects of (Al2O3–water) nanofluid on fat-plate solar collectors.
Metal chalcogenide nanomaterials are reported to exhibit excellent optical and electrical properties, as well as to contribute towards batteries, photovoltaics, and display devices [38,39]. The nano-structuring of semiconducting (n-type, p-type, chalcogenide, and nitrides) materials for hydrogen energy generation is also reported. ZnO, CuO, TiO2, and Ag metal nanoparticles have also been reported to be used in dye-based solar cells [40,41,42].
More recently, organic–inorganic metal halide perovskite solar cells (PSCs) have demonstrated the power conversion efficiency (PCE) of 22.7% [43,44]. Perovskite materials are able to function as photovoltaic absorbers because of their extraordinary optoelectronic properties including increased light absorption, ambipolar carrier transport ability, enhanced diffusion length, fabrication ease, and high mobility [45]. Researchers are tying to push the limits to fabricate high-quality perovskite films that can enhance perovskite solar cell efficiency to 25% and beyond. Further, progress has been achieved with the introduction of photoactive semiconductor nanoparticles/quantum dots (QDs) that improves the perovskite layer quality and its optoelectronic attributes by providing controlled nucleation sites that result in crystalline growth along preferred orientations [45].
Sun et al. [46] developed a carbon-free electrocatalyst for proton exchange membrane fuel cells (PEMFC) using a facile surfactant-free wet-chemical approach. This method was used to obtain single-crystalline Pt nanoparticles (NP) on mesoporous Nb-doped TiO2 hollow spheres. A new cost-effective Pt NP support with outstanding corrosion resistance has been found using Nb-TiO2 hollow spheres as a substrate. The Pt-NP/NbTiO2 catalysts outperform the commercial E-TEK Pt/C catalyst in activity and stability. Nanotechnology can produce clean E-85 fuel and petrochemical raw materials, according to Basily et al. [47]. Different nanocatalysts help extract unsaturated hydrocarbons from petroleum wastes. Researchers have examined the yield and product distribution of catalysts like CaO and gold nanocatalyst on CaO. The rod-shaped Au nanocatalyst led to enhanced ethylene production from petroleum wastes, which can be easily converted to ethyl alcohols for clean E-85 fuel.
Matsuda and Moritomo [48] demonstrated the two-electron reaction without structural phase transition in nanoporous cathode material. They examined the valence states as well as charge/discharge properties and structural features of a LixMn[Fe(CN)6] nanoporous cathode. Thin LixMn[Fe(CN)6] films formed by electrochemical synthesis on an ITO transparent electrode yielded 0.83·3.5H2O by substituting Na from Na 1.32Mn[Fe(CN)6], and yielded a high charge capacity and good cyclability. Reforming ethanol over the PtRuMg/ZrO2 catalyst produces hydrogen, according to Chiou et al. [49]. They changed the PtRu/ZrO2 catalyst with Mg and explored OSRE and SRE to produce hydrogen at temperatures below 300 °C with improved ethanol conversion, hydrogen yield, and reduced CO distribution. Compared to the SRE reaction (~250 °C), the OSRE reaction (~390 °C) requires a greater temperature to convert 100% ethanol, rendering the PtRuMg/ZrO2 catalyst more suitable for SRE reaction because of its coke-deposition stability.

2.2. Nanomaterials in Energy Distribution

Energy distribution aims to reduce current transmission losses using inputs from nanomaterials like carbon nanotubes in cables and power lines. Superconductive materials are optimized for lossless current conduction using nanotechnology. Microwaves, lasers, or electromagnetic resonance can give long-term energy benefits. Nanosensory devices and power-electronic components for complex grid control and monitoring could improve power distribution.

2.3. Nanomaterials for Energy Storage

Nanotechnology applications are enhancing energy-related solar, battery, and fuel cell technologies. Nanoscale materials and technologies strengthen solar cells. Nanotechnology has enhanced hydrogen fuel cell durability and efficiency, lithium-ion battery energy density, charge efficiency, and cycle life. Energy-efficient nanocomposites for insulation, window coatings, and other uses have been enabled by nanotechnology. The most promising energy storage is lithium-ion because of its high cell voltage, energy, and power density. New ceramic, heat-resistant, high-performance electrode nanomaterials improve lithium-ion battery capacity and safety. Similar methods have been promoted by Evonik for hybrid and electric vehicles and stationary energy storage. Hydrogen may provide long-term energy benefits. Chemical hydrogen storage cannot meet the auto industry’s 10% hydrogen storage requirement and, hence, nanomaterials, especially nanoporous metal-organic compounds, are sought after to help meet this requirement. Thermal energy storage is benefitted by the use of phase shift materials that help save construction energy. Nanoporous materials like zeolites are used for heating grids or industrial heat storage, which proves beneficial when it comes to energy storage [50].
Graphene is electrically and mechanically strong and biodegradable, super-capacitive, and cheaper than silicon. Si solar cells cannot absorb UV rays, whereas graphene can. Das et al. [51] studied vertical shell-tube thermal energy storage devices, melting carbon-based nanocomposites. PCM nanofillers increased n-alkane thermal conductivity and impacted n-eicosane thermal behaviour. Melting time was measured for SWCNT, nanodiamond, and graphene nanoplatelets. Hussein et al. [52] found that nanographene dispersion improves hydrogen bond breaks and reduces its viscosity, saving 30% energy at 10 g/L during thermal desalination. As graphene increased, energy-saving efficiency decreased. Li-ion batteries are popular energy storage solutions for portable electronics.
Electrodes, catalysts, fuel cell membranes, and polymer electrolyte membranes can use nanomaterials to improve mechanical strength and proton conductivity. Large hydrogen fuel storage is difficult or expensive. Hydrogen has been reported to be stored in CNTs and nanofibers. Carbon nanotube fuel cells store hydrogen, which is the greenest energy source. CNTs store hydrogen by physic-sorption in their cylindrical or interstitial structures [53]. Compared to carbon nanomaterial, Broom et al. [54] have demonstrated more efficient hydrogen storage using nanoporous materials via physisorption. This is owing to the fact that H2 adsorption in nanoporous materials is fast and fully reversible. In materials possessing high BET areas, such as selected MOFs, gravimetric storage capacities exceeding 10 wt% were achieved at cryogenic temperatures of 77 K. Nikitin et al. [55] found that atomic hydrogen in 2.0-nm-diameter single-walled carbon nanotubes (SWCNTs) was about 100% stable at ambient temperature. Hydrogen storage rises 7 wt%. A SWCNT support and platinum catalyst were used to study hydrogen cell power density by Girishkumar et al. [56]. CFE/SWCNT/Pt electrodes have 20% better maximum power density than CFE/CB/Pt. A decreased adsorption temperature or CNT changes boosted SWNTs’ and MWNTs’ hydrogen storage to 4–8% at ambient temperature, according to Orinakova and Orinak (2001) [57]. Amin et al. [58] tested Ni and Pd/Ni nanoparticles on Vulcan XC-72R carbon black electrodes for hydrogen cell methanol oxidation. They discovered that Pd/Ni/C electrocatalysts were more stable than Pd and Ni. Therefore, Pd/Ni/C can be employed in fuel cells as a cheaper methanol oxidation electrocatalyst. It was further observed that bifunctional manganese oxide–silver nanocomposites anchored on graphitic mesoporous carbon reduced oxygen and hindered biofilm formation in long-term microbial fuel cells, according to Khater et al. [59]. Power density is highest in MnOx–Ag/GMC nanocomposites membrane fuel cells (MFCs) compared to Pt/C [59]. Xie et al. [60] developed efficient, cost-effective, and environmentally friendly electrocatalysts for hydrogen and oxygen evolution for water splitting. Adsorption-grown FeS2 nanoparticles were produced on MXene. The FeS2@MXene composite could speed mass/charge transfer and increase water-FeS2 reactive site interaction. Nanostructure active electrodes could improve battery power and recharge time.
Carbon aerogels, nanostructured porous materials with high surface area and electric conductivity, may store electrochemical energy. Pena et al. [61] synthesized carbon aerogels using ferrocene as a source of carbon and catalytic material (Fe). CCVD produced carbon aerogels with a surface area of 780 m2/g (SBET) and pore volumes of 0.55 cm 3/g (VBJH). Their investigation shows that carbon nanostructure aggregation creates mesopores, which increases surface area. These porous Carbon aerogels show promise for gas physisorption, such as molecular hydrogen storage for fuel cells. Titanium coated with organic molecules may be useful for hydrogen storage. Zurani et al. [62] investigated the adsorption of hydrogen molecules on a titanium atom supported by benzene and its catalytic role in H2 dissociation. The authors examined the orbital interactions that bind H2 to Ti and dissociate one H2 molecule.

2.4. Nanomaterials for Energy Conservation

Energy is much needed to improve energy efficiency and reduce waste for sustainable energy supply and energy source development. Nanotechnologies help conserve energy in several ways. Car fuel consumption can be reduced via lightweight nanocomposites-based building materials, wear-resistant engine components, nanoparticular gasoline additives, and low-rolling-resistance tyre mechanical tribology saves energy. Nanoporous thermal insulation for energetic renovation of old structures helps conserve energy. Switchable glasses using nanotechnology components can adjust light and heat flux to reduce energy use [50]. Table 1 gives an overview of the various carbon composite nanomaterials that have been employed for energy-related applications.

3. Techniques That Are Used for the Preparation of Energy Related Nanomaterials

Nanomaterials utilized in the energy sector are synthesized involving various techniques. We review the most common nanomaterial fabrication methods. In top-down nanomaterial synthesis, laser ablation, mechanical milling, etching, electro explosion, and sputtering methods split coarse materials into nanostructured particles. Bottom-up processes include solgel, vapour deposition, and spinning (Figure 2). Cheap mechanical milling, reduces cumbersome materials to nanosize. This method produces copper nanoalloys and aluminium nickel magnesium nanocomposites [89]. Electrospinning is a simple top-down nanomaterial manufacturing method. Nanofiber polymers such as polyurethane nanofibrous membranes have been synthesized in this manner [90]. Nanomaterials are created by sputtering plasma or gas-borne high-energy particles onto solid surfaces. Nanoscale thin film fabrication are possible via sputtering [91]. Laser ablation vaporizes precursors with high-energy lasers to produce nanoparticles, green laser ablation is able to produce noble nanosized metals without the use of chemicals or stabilisers [92].
Chemical vapour deposition (CVD) is reported to produce carbon nanotubes. CVD leads to thin films consisting of vapor-phase chemicals on solidsubstrates [93]. Hydrothermal nanomaterial production is common, it involves heterogeneous nanoparticle synthesis in a liquid media at high temperature and pressure. Hydrothermal synthesis produces nanorods, nanowires, nanosheets, and nanospheres [94]. Wet-chemical sol-gel nanomaterial synthesis is also popular and has resulted in the production of many high-quality metal oxide nanomaterials. Eco-friendly sol-gel approaches have resulted in the production of homogeneous nanomaterials at low temperatures, yielding complex nanostructures and composites [95].

Energy-Saving Alternative Technique: Green Synthesis

Nanostructured materials can be made using green synthesis, which is a simple, low-cost method without the need for expensive equipment. Due to these features, green synthesis stands out amidst other nanomaterial synthesis techniques. Green synthesis methods are replacing physical and chemical approaches due to low energy consumption, harmful chemical release, and sophisticated equipment and synthesis conditions [96]. Aerosols, UV radiation, and thermal disintegration require high pressure and temperatures [97]. Atomized aerosol droplets and nanoparticulate metals are produced at 2400 K flame temperature. Plasma-assisted physical vapour deposition of PdO NPs requires three energy-intensive heat cycles between 250 and 800 °C. Chemical methods always employ expensive, hazardous sodium borohydride, dispersion stabilisers, and organic solvents [98]. Green synthesis uses eco-friendly reducing agents. End-capping agents and dispersants made of green materials save energy and prevent toxic chemical release [99]. Green synthesis employs micro-organisms (fungi, bacteria, algae) [100,101,102] or leaf, flower, root, and peeling extracts [103,104,105,106,107,108,109]. Green materials can lower metal ion valence states via polyphenols and proteins [110]. To manufacture metallic NPs in green synthesis, multifunctional reagents such as polysaccharides act as reducing and capping agents, reducing reactants and boosting atom economy. Green synthesis is non-toxic [103], pollution-free [96], environmentally friendly, economical [111], and more sustainable [112]. Green synthesis of energy nanoparticles has been described by numerous researchers [6,113,114].
Although not many, there have been instances reported where green-synthesized nanomaterials have shown promising results in the energy sector. For example, titanium dioxide-reduced graphene oxide (TiO2-rGO) nanocomposites prepared by green synthesis exhibited increased photocatalytic activity, and, hence, were deployed in lithium-ion batteries [115]. Green-synthesized earth-abundant iron-based nanomaterials have been applied to energy conversion devices for hydrothermal energy applications and were found to show enhanced performance, as well as cost effectiveness [116]. Cellulose has been used to harvest solar energy and utilized in lithium-ion batteries, supercapacitors, electrodes, electrolytes, etc. [117]. Cellulose being a natural polymer, it can yield thin, flexible, biodegradable, biocompatible, and cost-effective components. Green palladium nanoparticles have been used as proficient nano-catalysts [118]. Green-gold nano-catalysts loaded on titanium have been deployed as greener and environmentally friendly nano-catalysts, with tremendous potential for energy conservation [119]. Biomolecules and green solvents have been used for the processing of carbon nanotubes (CNTs) [120]. Life cycle analysis, risk assessment, and utilizing manufacturing processes that are sustainable, as well as greener alternatives, will prove sustainable.
Osman et al. [121] have extensively reviewed the major challenges involved in green synthesis. They have highlighted high energy consumption and long reaction times as the major limitations. They have reported instances where Nasiri et al. [122] reported the use of Ferula persica extracts (600 °C, 3 h) to synthesize silver nanoparticles, while for copper nanoparticles using Guava fruit extract it was 800 °C. The fact being that certain green synthesis methods utilize high temperatures and lengthy processes, consuming significant energy, deems these processes not qualifying the green mandate of environmentally friendly materials. González-Ballesteros et al. [123] synthesized gold nanomaterials at 24 °C using Cystoseira baccata, a brown alga using a process that also required substantial energy. Certain plant extracts require low temperature storage, requiring further energy consumption. Azadirachta indica leaf extract requires storage at 4 °C [97], and dried grass extract requires temperatures below 4 °C [124]. This requires energy-intensive freezers which consume energy.
Producing nanoscale materials at room temperature becomes essential from the reduced energy consumption perspective. Reaction time is crucial in any energy efficient process, however, many green synthesis procedures report long reaction times. For example, mint leaf extract together with iron nitrate requires to be incubated in a shaker for 72 h at 30 °C and 200 rpm in the dark, while iron chloride solution co-incubated with algae requires orbital shaking at 24 °C for 48 h in complete darkness [100]. Similarly, the preparation of nanoparticles of iron from black tea and iron sulfate needs 24 h, followed by 24 h of drying at 250 °C in an oven [125]. On the other hand, copper oxide nanoparticle synthesis from coffee powder extract requires microwave radiation, followed by three hours of boiling and 4–5 h hot air oven drying [97]. What we emphasize here is that, although the process is green, the concomitant energy consumption does not hit a holistic green approach.
Additionally, lengthy extraction procedures are sometimes reported for nanomaterial synthesis, and this is also not ideal [126]. To obtain cherry, mulberry, and oak leaf extracts, drying in an oven for 48 h at 50 °C is needed [127]. Sargassum acinarium and Padina pavonica have to be washed with distilled water and then freeze-dried at 20 °C for 3 days to obtain algal extract [128]. Some procedures are carried out under inert atmosphere, adding to energy consumption. In summary, green synthesis processes, which are otherwise proven to hold an edge over standard nanomaterial synthesis processes, are, on the other hand, branded as an energy-consuming process. This is another area that needs to be worked on so that green methods can be developed to operate on reduced energy consumption. Researchers need to keep in mind these vital criteria when they plan their experiments.

4. Future Perspectives and Recommendations

When renewable, sustainable energy generation is given so much emphasis, the question here is why the energy expenditure aspects involved in the synthesis and fabrication of energy nanomaterials are not being stringently considered and energy saving synthesis methods identified and adopted (Figure 3). Table 2 consolidates the scanty information available on the energy consumption of various nanomaterial synthesis processes, from data available on published reports. Such a clear disclosure of the energy expended in the synthesis process will help in choosing the ideal synthesis process that could make the entire process beneficial from the energy aspect. Every nanomaterial synthesis process should aim to develop such a data sheet on energy consumption during the synthesis process. This review is expected to create an awareness among researchers to pay close attention to this thus-far-ignored aspect. Sustainable energy-using nanomaterials have been trending, and it is now time to look into the use of sustainable nanomaterial synthesis for energy applications; this review seeds this forethought for the first time.
Before planning any synthesis operation, it is vital to collect data on the characteristics of molecules, their effects on the surrounding environment, and how they are transported and end up in the biosphere. This is required in order to achieve sustainability because, with this understanding and background information, scientists will be able to develop nanomaterials that are less harmful to both humans and the environment [140,141]. Solvents are used during green synthesis, however, the recovery of these solvents via the usual distillation method typically requires a lot of energy, and the other option is to make use of auxiliaries and solvents that are less hazardous to human health. Therefore, when it comes to the synthesis of NPs, researchers have concentrated on developing safer solutions, such as systems that do not require the use of solvents or solvents that are not harmful, such as the water/glycerol system [142,143]. Appropriate precursors need to be selected so as to reduce the activation energy of chemical processes, so that conversion can take place at an ambient temperature and the goal of reducing energy consumption is achieved [144]. This can be accomplished by selecting precursors in such a way as to reduce the temperature required for the conversion [145]. The utilization of starch as a reducing agent during the synthesis of Ag–Au bimetallic NPs at room temperature is an excellent illustration of an energy-efficient procedure [146]. The other possibility is to increase the proportion of renewable sources used either for raw materials or for energy. Both of these aspects are quite vital.
Biomass is the most significant category of renewable energy sources [147]. There are numerous examples of the use of renewable material such as cellulose, chitin, starch, and glycerol for NP synthesis [148,149,150,151]. The introduction of new chemicals, increasing energy consumption, and generating more waste should be avoided [145,152]. For instance, the production of metallic NPs with biopolymers such as chitosan can substitute the requirement of capping agents [153,154,155]. In addition, choosing the appropriate catalytic reactions can improve the overall efficiency of the process by reducing the amount of energy required for activation and boosting product selectivity. Because of these benefits, there may be a reduction in the amount of energy and raw materials used, as well as the amount of waste produced [145].
For instance, polyoxometalates, also known as POMs, have the potential to serve as photocatalysts in the synthesis of metallic nanoparticles (NPs), allowing for the reactions to be carried out at room temperature and in a matter of minutes [156]. Another desirable quality in this context is degradability. Chemical products should not have a long half-life in the environment, hence, chemists should design them in such a way that, when they reach the end of their useful lives, they can readily split into molecules that are less complex and less hazardous [157]. Using polymers that are both edible and biodegradable, like gum ghatti, for the purpose of stabilising NPs, for instance, assures that the product will have a short life cycle after being released into the environment [158]. In this manner, it is not necessary to expend any additional energy on the process of decomposing the nanomaterial. This method can reduce energy use while also preventing mishaps and the unintended development of by-products, both of which could result in additional energy expenditure in detoxification or remediation. Figure 4 gives the future propositions we propose towards energy-efficient nanomaterial synthesis.
Sustainable progress requires inputs from various other next-generation carbon nanomaterials. One nanomaterial with exceptional electrical conductivity and mechanical strength is graphene. It is biodegradable, super-capacitive, and cheaper than silicon. Unlike silicon solar cells, graphene can absorb UV light from the sun. CNTs have improved photocatalysis and solar energy conversion. Due to their thermal conductivity, metallic or semiconducting electrical behaviour, and surface area, CNTs are unique. Quantum dots, tiny nanoparticles a few nanometers in size, have been shown to convert solar energy three times more efficiently than the finest solar cell material. An extensive review by Candelaria et al. [159] covers carbon nanomaterial possibilities for energy storage and conversion. Synthetic porous carbon is used in supercapacitors, lithium-ion batteries, porous media for methane gas storage, coherent nanocomposites for hydrogen storage, electrocatalysts for fuel cells, mesoporous carbon (MC) for lithium–sulphur batteries, and porous carbon for lithium–oxygen batteries. It is believed that carbon materials might help build clean, sustainable energy solutions. Fullerene-containing p-type semiconducting polymers accelerate organic photovoltaics [160,161,162,163]. New carbon compounds like carbon nanotubes and graphene are being studied as essential additions for the next generation of optically transparent electronically conductive solar cell films [5,6,164,165,166,167]. Carbon nanotubes and graphenes are researched for batteries, supercapacitors, and fuel cells [168,169,170,171,172,173,174,175]. Diamond, graphite, graphene, fullerenes, carbon nanotubes, and amorphous carbon are carbon structures [176,177].
In addition, new nanofabrication methods must be affordable, environmentally friendly, and of high-quality. The main rationale for using this work’s synthesis approach is to regulate energy and matter in nanoscale material synthesis. Controlling this invention will be necessary. Microwave irradiation uses less energy and is more environmentally friendly than older methods. Microwave irradiation also heats components quickly and uniformly, which is suitable for nanomaterial nucleation and development [178].
Recent developments include plasma discharge nanoscale material synthesis [179,180]. Atmospheric pressure synthesis avoids vacuum equipment, allows quick chemistry due to high species concentrations, and speeds growth. Nanosecond repeated pulsed (NRP) discharges may be ideal for efficient synthesis due to their high mean electron energy. Large amounts of discharge energy go to electron-impact reactions such ionisation, electronic excitation, and dissociation [181]. NRP spark-based nanofabrication uses less energy than prior processes and operates in open air without catalysts or substrate heating. The ability to synthesise on plastic aids hybrid organic–inorganic energy conversion and electronic gadget creation. Plasma discharge fabrication of nanoscale materials takes nanoseconds, making it energy-efficient. NRP synthesised MoO3 nanoscale structures such flakes, spots, walls, and porous networks on polyamide and copper substrates [134]. These authors found that nanosecond discharges can be used for efficient synthesis at atmospheric pressure without catalysts or substrate heating [134]. Other energy-efficient methods include low temperature synthesis or one-pot simple and straightforward synthesis methods.
Other energy-saving nanomaterial-yielding technologies include biodegradable polymer-based nanomaterials and naturally available carbon materials from smoke, soot, and exhausts. Using naturally available nanomaterials will skip the energy-intensive nanomaterial manufacturing process, making energy nanotechnology sustainable. Our group, as well as other researchers, have recovered carbon nanoparticles, nanodots, and nanotubes from soot [182,183,184,185,186,187,188,189]. The energy crisis requires the respect of the current situation and developing nanomaterial synthesis technologies for energy applications.

5. Conclusions

In conclusion, there is no question about the credibility of nanomaterials to impart significant progress in the area of sustainable energy generation and storage. However, this review highlights the importance of identifying energy-efficient processes for the synthesis of energy related nanomaterials. The various nanomaterials and sustainable nanomaterial synthesis processes, including green synthesis and other energy conservative techniques, have been presented. Energy-efficient synthesis methods have been discussed. This review has projected the lapses, and recommendations, more response, and discussions and deliberations are expected as an outcome of this review in near future. The debate raised in this review is expected to revamp nanomaterial synthesis processes, making energy conservation a strict mandate.

Funding

This research received no external funding.

Acknowledgments

The authors extend their appreciation to the Deputyship for Research and Innovation, Ministry of Education in Saudi Arabia, for funding this research work through the project number ISP23-153.

Conflicts of Interest

The authors declare no conflict of interest.

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Energies 18 00523 g001
Figure 1. Overview of the various aspects of the energy sector where nanomaterial applications have been deployed: as an energy source, for energy conversion, energy generation, energy storage and energy saving.
Figure 1. Overview of the various aspects of the energy sector where nanomaterial applications have been deployed: as an energy source, for energy conversion, energy generation, energy storage and energy saving.
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Figure 2. The different synthesis methods that have been reported for nanoparticle preparation using green synthesis techniques. The spheres and rectangles represent micro/nanoparticles.
Figure 2. The different synthesis methods that have been reported for nanoparticle preparation using green synthesis techniques. The spheres and rectangles represent micro/nanoparticles.
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Figure 3. PubMed search results on PubMed using the terms, (A) nanomaterials for energy applications and (B) energy saving nanomaterial synthesis for energy applications. The publication hits in the respective subject area are shown as a function of publication time range.
Figure 3. PubMed search results on PubMed using the terms, (A) nanomaterials for energy applications and (B) energy saving nanomaterial synthesis for energy applications. The publication hits in the respective subject area are shown as a function of publication time range.
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Figure 4. Birds eye perspective of current challenges in energy efficient nanomaterial synthesis and future recommendations.
Figure 4. Birds eye perspective of current challenges in energy efficient nanomaterial synthesis and future recommendations.
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Table 1. Application of advanced carbon nanocomposites for energy generation and storage.
Table 1. Application of advanced carbon nanocomposites for energy generation and storage.
Carbon MaterialCompositeEnergy ApplicationReference
Carbon and its derivatives
Carbon nanotubes (CNT)p-CNT/n-Si structureHybrid solar cells[63]
CNTMoOx coating onto the CNT filmHybrid solar cells[64]
SWCNT and
MWCNT		indium tin oxide (ITO) and poly(3,4-ethylenedioxythiophene), poly(styrenesulfonate) (PEDOT:PSS).Organic Photovoltaic devices[65,66,67]
CNTPEI-SWNT EDLCs
(Polyethyleneimine)		Super capacitors[68]
Aligned carbon nanotubes (A-CNTs)-Super capacitors[69]
CNTPolypyroleSuper capacitors[70]
CNTPEI-SWNT EDLCs
(Polyethyleneimine)		Super capacitors[68]
CNTsMnO2 nanoflakeSuper capacitors[71]
SWCNTTiO2Super capacitors[72]
CNTRuO2Super capacitors[73]
CNTFe2O3Super capacitors[74]
Carbon nano-onionsEtchedSuper capacitors[75]
Carbon nano onionsδ-MnO2 NanosheetsSuper capacitors[76]
Carbon nano-onionsSulfur-dopedSuper capacitors[77]
Carbon nano-onionsCo3O4 doped [78]
CNTsCarbon coatingSuper capacitors[79]
Carbon nano-onionsEtchedSuper capacitors[75]
Carbon nano onionsδ-MnO2 NanosheetsSuper capacitors[76]
Graphene and its derivatives
GrapheneP3HT:PCBMOrganic solar cells[80]
GrapheneCarbon nano-onionsSuper capacitors[81]
Graphene Microribbons-Stretchable micro-super capacitors[82]
Graphene nanosheetspolyaniline (PANI) nanowiresSuper capacitors[83]
GOphenyl isothiocyanate (PITC) Organic solar cells[84]
GOP3HTOrganic solar cells[84,85]
GOpoly(3,4-ethylenedioxythiophene), poly(styrenesulfonate) (PEDOT:PSS) and P(VDF-TrFE) polymersOrganic solar cells[86]
Reduced GOCNTs and a fullerene electron acceptor (PC71BM)Organic solar cells[87]
Reduced GOAgSuper capacitors[88]
Table 2. Energy requirement for the synthesis of nanomaterials from published reports.
Table 2. Energy requirement for the synthesis of nanomaterials from published reports.
NanomaterialProcessing Involved in the Nanomaterial Production/Synthesis MethodTotal Energy RequirementReferences
TitaniumElectric Energy2.2–5 GJ/t[129]
Steam10.4–23.1 GJ/t
Gas9.6–16.1 GJ/t
Coal5–8 GJ/t
-5 (kg CO2 emission/kg Titanium)[130]
TitaniumElectric Energy2.3 GJ/t[131]
Steam9.3 GJ/t
Gas7 GJ/t
Coal-
-4 (kg CO2 emission/kg Titanium)[130]
ZrPlasma40 kg CO2 emission/kg ZrO2[132]
ZrCl4hydrolysis5 kg CO2 emission/kg ZrO2
Zr-octanoate-20 kg CO2 emission/kg ZrO2)
Zr-pentanoate-14 kg CO2 emission/kg ZrO2
Zr-isopropoxide-9 kg CO2 emission/kg ZrO2
ZrO2nano-milling35 kg CO2 emission/kg ZrO2
Zirconium oxideGas26.8 MJ/Kg[133]
Carbon nanotubesThermal furnace1.2 × 108 eV #[134]
Carbon nanotubesMW torch7.2 × 103 eV #[135]
Carbon nanotubesDC arc5 × 104 eV #[23]
GrapheneMW torch1 × 103 eV #[136]
Au nanoparticlesMicrosecond spark1.5 × 103 eV #[136]
Si nanocrystalsMicroplasma6 × 105 eV #[137,138]
WO3 nanoparticlesMicroplasma1.4 × 103 eV #[139]
MoO3 nanosheetsMicroplasma4.2 × 104 eV #[137]
MoO3 nanoflakes/nanowallsNRP spark7.5 × 101 eV #[134]
(GJ/t)—Gigajoules/ton; #—εatom (energy cost of incorporating each atom into the nanostructure); eV—electronvolt.
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Hasan, N.; Muthu, M.; Hakami, O.; Gopal, J. Assessing the Sustainability of Energy-Related Nanomaterial Synthesis: Emphasizing the Need for Energy-Efficient Nanomaterial Preparation Techniques. Energies 2025, 18, 523. https://doi.org/10.3390/en18030523

AMA Style

Hasan N, Muthu M, Hakami O, Gopal J. Assessing the Sustainability of Energy-Related Nanomaterial Synthesis: Emphasizing the Need for Energy-Efficient Nanomaterial Preparation Techniques. Energies. 2025; 18(3):523. https://doi.org/10.3390/en18030523

Chicago/Turabian Style

Hasan, Nazim, Manikandan Muthu, Othman Hakami, and Judy Gopal. 2025. "Assessing the Sustainability of Energy-Related Nanomaterial Synthesis: Emphasizing the Need for Energy-Efficient Nanomaterial Preparation Techniques" Energies 18, no. 3: 523. https://doi.org/10.3390/en18030523

APA Style

Hasan, N., Muthu, M., Hakami, O., & Gopal, J. (2025). Assessing the Sustainability of Energy-Related Nanomaterial Synthesis: Emphasizing the Need for Energy-Efficient Nanomaterial Preparation Techniques. Energies, 18(3), 523. https://doi.org/10.3390/en18030523

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