Chalcogenide Materials for Sustainable Energy and Environmental Applications

Abstract

Chalcogenide materials have garnered significant attention as potential solutions for sustainable energy and environmental applications, attributed to their distinct properties and inherent advantages. This comprehensive review critically examines chalcogenide nanomaterials’ emergence, synthesis, and fabrication methodologies, emphasizing their applicability across various domains, such as energy storage and conversion, photovoltaics, waste management, and water treatment processes. Furthermore, the utilization of chalcogenide nanomaterials in combating air pollution, promoting sustainable agricultural practices, and facilitating biomedical advancements is investigated. The review also encompasses an in-depth analysis of ecotoxicity and environmental safety concerns, regulatory frameworks, and policy implications for adopting chalcogenide nanomaterials. Despite the challenges and future research directions in chalcogenide nanomaterials, their global influence on sustainable development is indisputable. In conclusion, this review stresses the promising prospects of chalcogenide nanomaterials within environmental applications, underlining their potential to reshape the landscape of sustainable energy and environmental technologies.

1. Introduction

The current global energy environment is transitioning toward renewable and environmentally friendly energy sources to decrease the release of greenhouse gases and ease the consequences of climate change. To tackle these difficulties, scientific communities across the globe have been investigating innovative materials that have the potential to facilitate a more environmentally friendly, highly productive, and enduring future [1, 2, 3]. Various materials, including perovskites, skutterudite, metal-organic frameworks, covalent organic frameworks, metal chalcogenides, and intermetallic, have been recognized for their potential to generate alternative energy. Among these materials, metal chalcogenides, which comprise elements from the chalcogen group (oxygen, sulfur, selenium, and tellurium) have garnered attention because of their diverse properties. The origins of chalcogenide nanomaterials can be traced back to the initial investigation of amorphous chalcogenide semiconductors (Figure 1a), throughout the 1960s. Chalcogenide semiconductor nanoparticles exhibit a distinctive amalgamation of optical, electrical, thermal, and catalytic characteristics, rendering them well-suited for diverse applications in sustainable nanotechnology and environmental remediation (Figure 1b) [4, 5].

Figure 1.

Overview of the chalcogenides energy and environmental applications.

These characteristics encompass notable photosensitivity, substantial light absorption, and adjustable band gaps, rendering them well-suited for optoelectronics, solar cells, and photocatalysis implementation. The attractive features of these materials lie in their electronic properties, which include the ability to adjust electrical conductivity and exhibit high carrier mobility. As a result, they have garnered significant interest in their potential applications in electronic devices, such as transistors, sensors, and energy storage systems. Chalcogenide nanostructures possess distinctive thermal properties that render them well-suited for utilization in thermoelectric applications. Transition metal-based catalysts have shown remarkable efficacy in facilitating chemical reactions such as hydrogen evolution and CO₂ reduction, offering a viable and ecologically sound approach to energy production.

Chalcogenide nanomaterials possess promising prospects for environmental remediation, specifically in water purification, air pollution mitigation, and the degradation of organic contaminants [6, 7, 8]. The inherent adaptability of these systems enables the development of tailored approaches for energy conversion, electronics, and environmental remediation. The compatibility of these substances with diverse substrates and materials facilitates the production of sophisticated materials and systems that exploit synergistic features.

The chalcogenide materials listed in Table 1 have garnered attention because of their inherent photosensitivity, rendering them well-suited for applications in data storage and optical devices. Over time, there has been a notable expansion in studies about chalcogenide nanomaterials. This expansion has resulted in substantial progress in developing fabrication techniques and a deeper comprehension of these materials’ inherent features and prospective applications. This chapter aims to briefly analyze chalcogenide nanomaterials, including their distinctive characteristics and significance in sustainable energy conversion and environmental remediation.

S.No	Materials	Applications	Ref.
1	CdX (X = S, Se, Te)	Solar Cells, Photocatalysis	[9]
2	Ag2X (X = S, Se, Te)	Thermoelectric Devices, Superionic Conductors	[9, 10, 11]
3	MoS2, WS2	Lubricants, Catalysts, Energy Storage Devices	[11, 12]
4	Bi2X3, (X = S, Se, Te)	Thermoelectric Devices, Topological Insulators	[13]
5	SnS, SnSe	Solar Cells, Photocatalysis, Batteries	[14]
6	GeX (X = S, Se, Te)	Phase-Change Memory, Infrared Optics	[15]
7	Cu(In,Ga)Se2	Solar Cells	[16]
8	Cu2ZnSnS4	Solar Cells	[17]
9	Ti2S, FeS	Thermoelectric	[18]
10	Sb2S3, Sb2(1 x)Fe2xS3	Thermoelectric	[19]

Table 1.

Chalcogenide nanomaterials and their applications.

2. Metal chalcogenides for energy applications

The distinctive characteristics exhibited by these chalcogenide materials, including their notable electrical conductivity, adjustable bandgaps, and exceptional electrochemical performance, render them very suitable contenders for applications in energy conversion and systems.

2.1 Metal chalcogenides as photovoltaics

2.1.1 Introduction

Chalcogenide photovoltaic materials have emerged as a promising class of compounds for solar cell applications because of their unique properties and tunability. Solar energy has become an increasingly important renewable energy source to address the world’s growing energy demands while mitigating climate change. The development of tools facilitating the efficient use of solar energy is of utmost importance. Photovoltaic (PV) solar cells are semiconductors that convert absorbed solar insolation into electrical energy. Solar cells are crucial in carbonizing energy infrastructure, reducing carbon footprints, and advancing toward a low-carbon society. Solar cells contribute significantly to these objectives besides generating clean, environmentally friendly, and sustainable power.

Materials chemists have identified several materials to observe the solar light function as PV materials. Their capacity for absorption in the visible light spectrum is restricted because of their broadband gap, which exceeds 3.2 eV. The percentage of ultraviolet-visible (UV-Vis) light that reaches the Earth’s surface is 43%, and within this fraction, a mere 5% of solar energy is effectively harnessed. In contrast, around half of near-infrared (NIR) light reaches the Earth’s surface, leading to a notable increase in the efficiency of solar power harvesting. In 2021, worldwide power generated from installed solar modules amounted to a mere 4.5%, while coal-based sources accounted for a far larger share of 37% [20]. Hence, it is imperative to discover novel photo-responsive materials with enhanced efficiency to facilitate more effective solar energy utilization, specifically within the UV-Vis spectrum. Developing a diverse range of new active semiconductors showing sufficient and consistent PV efficiency is a formidable challenge. There are several strategies, such as band gap engineering, enhancing surface area, and improving carrier mobility, to increase the efficiency of solar cell materials. Chalcogenide materials, which encompass a range of compounds containing elements from the chalcogenide group, offer distinct advantages in harnessing solar energy efficiently. This stimulated the scientific community to conduct thorough investigations into various PV materials and structures to attain high power conversion efficiency (PCE) while keeping costs low. Significant progress has been observed globally in recent decades, resulting in remarkable achievements. Notably, quadruple junction solar cells have attained a record-breaking PCE of 44.7% [21]. We expect that solar power will see a decrease in cost in the future years, resulting in an increase in the proportion of PV panels in the global electricity mix annually. The worldwide PV industry, valued at $100 billion, is poised for growth because of new market possibilities in various sectors, such as home, commercial, wearable, and industrial applications [22].

3. Device architecture: metal chalcogenide photovoltaics

The different layers of metal chalcogenide-based photovoltaic devices provide unique purposes that are critical to the device’s overall functionality. Here, we have outlined the different layers of devices, as well as their construction and functions:

Substrate: It is a robust foundation for the device’s construction. It is usually constructed of glass or similar transparent material that lets light pass through and reach the active layers.

Transparent Conductive Oxide (TCO) Layer: The TCO layer is frequently used as the front electrode and is kept on top of the substrate. It allows light into the gadget while providing electrical contact. Indium tin oxide (ITO) and fluorine-doped tin oxide (FTO) are common TCO materials.

The window layer is frequently utilized to increase electrical contact between the TCO and the following layers. It can also function as a diffusion barrier between layers, preventing undesired chemical reactions. It is often composed of a clear substance such as cadmium sulfide (CdS) or zinc oxide (ZnO).

Active layer: It is a device’s heart, usually made of metal chalcogenide like cadmium telluride (CdTe) or copper indium gallium selenide (CIGS). This layer absorbs light energy and produces electron-hole pairs, thrusting off the photovoltaic process.

The Zn (O,S)/CdZnS might be used as a buffer layer and is frequently employed to improve the effectiveness of charge extraction from the absorber layer to the subsequent layers. It aids in reducing energy loss and improving overall device performance.

The Hole Passage Layer (P-type): It aids in passaging positively charged holes created in the absorber layer to the front electrode using materials such as PEDOT:PSS, spiro-OMeTAD, or CuSCN.

Electron Transport Layer (n-type): The ETL allows negatively charged electrons to go from the absorber layer to the back electrode ZnO and TiO₂ are two common ETL materials.

Cathode or Counter Electrode: The back electrode or cathode, catches the electrons carried by the ETL and completes the electrical circuit. The back electrodes can be platinum (Pt) or C-based materials.

These layers collaborate to form an effective photovoltaic device. The different layered transition metal chalcogenide-based PV solar cells are provided in Table 2 and Figure 2. The precise materials and layer layouts might vary depending on the type of metal chalcogenide and the required device performance.

Layers; PV; based on	Electron Transport Layer (ETL)	Hole Transport Layer (HTL)	Active Layer	Counter Electrode	Ref.
CdTe	ZnO,CdS, TiO2	PEDOT:PSS, CuSCN, MoO3, Spiro-OMeTAD	CdTe	Pt, C, Au	[23]
CIGS	ZnO, CdS, TiO2	PEDOT:PSS, MoO3, Spiro-OMeTAD, PTAA	CIGS	Pt, C	[24]
CuInS2	ZnO, TiO2, CdS	Spiro-OMeTAD, PTAA	CuInS2		[24, 25]
MoS2	TiO2,ZnS, Al2O3	MoO3,Spiro -OMeTAD, PEDOT:PSS	MoS2		[26]
Cu2S/CuS	ZnO,TiO2, CdS	PEDOT: PSS, CuSCN	Cu2S/CuS		[27]
Bi2S3	ZnO, CdS		Bi2S3		[28]
In2S3	ZnO,CdS,TiO2		In2S3
PbS/PbSe	TiO2, ZnO, PbSe, CuSCN	PEDOT:PSS, MoO3, Spiro-OMeTAD	PbS/PbSe		[28]
CZTS	ZnO, CdS, TiO2, SnO2	PEDOT:PSS, CuSCN, MoO3, Spiro-OMeTAD	CuZnSnS		[28, 29]
WS2/Ag2S	TiO2, ZnO	MoO3, Spiro-OMeTAD, PEDOT:PSS	WS2/Ag2S		[9]
FeS2/FeSe2NiS2/NiSe2			FeS2/FeSe2; NiS2/NiSe2		[30, 31]
ZnSe/ZnTe	ZnO,TiO2, CdS, MoO3	PEDOT:PSS, CuSCN, Spiro-OMeTAD	ZnSe		[5]
SnS/SnSe			SnS/SnSe		[4]

Table 2.

Presents a list of the various transition metal chalcogenides-based PV solar cells and their successive layers.

Figure 2.

(a) Schematic layout of the several layers of the solar cell, (b) P-N junction, and (c) different possible combinations of the transition metal chalcogenide layers.

Different combinations of transition metal chalcogenides (TMCs), including MoS₂, CrS₂, WS₂, TiS₂, MoSe₂, CrSe₂, WSe₂, TiSe₂, and others, have been seen to exist in metallic, semiconductor, and insulator phases [32]. TMCs encompass a variety of large crystal families that exist in distinct phases, including 1 T, 2H, and 3R. Approximately two-thirds of these materials possess layered structures [33]. Specifically, MoS₂ has a mechanical strength that is 30% higher than steel, and it can undergo rupture after experiencing a deformation of 1%. The substance has been shown to possess exceptional distensibility and strength. Photovoltaic devices have seen a substitution of platinum (Pt) counter electrodes with molybdenum disulfide (MoS₂) in their manufacturing process [34].

4. Performance metrics for metal chalcogenide PVs

The several essential performance parameters of metal chalcogenide PV applications are below [24, 35].

• Efficiency (η): It is a fundamental statistic used to quantify the effectiveness of a PV panel using metal chalcogenides in converting incoming sunlight to electrical energy. Efficiency is commonly quantified as a percentage and can be determined using the following formula: η = (P_in/P_out) × 100%

The variable “P_out” represents the electrical power output of the PV module. P_in represents the incident solar power on the PV module. The efficiency of metal chalcogenides can be influenced by their distinct bandgap properties, prompting endeavors to optimize this parameter.

• Spectral response: The spectral response of a metal chalcogenide PV cell refers to its ability to effectively capture and convert light to varying wavelengths. This aids in evaluating the efficacy of the cell over diverse lighting situations, encompassing the complete solar spectrum.

• Short-Circuit Current (Isc): The Isc is a parameter that denotes the highest current output achievable when the terminals of a PV cell are connected in a short-circuit configuration while adhering to conventional test parameters. The electrical properties of the metal chalcogenide material play a significant role in determining the Isc (short-circuit current) because of its absorption characteristics and electronic properties.

• Open-circuit voltage (Voc): Voc refers to the highest potential difference a PV cell may generate when its terminals are disconnected in a circuit with no current flow. This measurement is conducted under established test settings to ensure consistency and comparability. The determination of the metal chalcogenide’s band-gap energy and other electronic properties is contingent upon several factors.

• Fill factor (FF): FF is a metric that quantifies the efficiency of a PV device in converting the power into electrical output. The academic definition of the term refers to the ratio between the maximum power (Pmax) and the product of the open-circuit voltage (Voc) and the short-circuit current (Isc). A more significant fill factor shows enhanced gadget performance.

• Maximum Power Point (Pmax): The Pmax refers to the highest level of electrical power that may be generated by a photovoltaic (PV) system. This optimal power output is achieved at the specific point when the multiplication of current and voltage is maximized. The aforementioned point is commonly referred to as the maximum power point (MPP).

• Energy Conversion Efficiency: The energy conversion efficiency metric evaluates a PV module’s capacity to convert solar energy into electrical energy, considering solar radiation and temperature fluctuations.

• Temperature coefficient: It characterizes the relationship between their efficiency and changes in temperature. Comprehending these coefficients is crucial for making accurate predictions regarding real-world performance.

• Degradation Rate: Over time, PV devices may see a decline in performance. The degradation rate is a metric that quantifies the yearly decline in performance, offering valuable information regarding the enduring dependability of photovoltaic systems based on metal chalcogenides.

Stability and reliability measures evaluate the capacity of photovoltaic (PV) devices to sustain their performance over an extended period, encompassing their resistance to external elements, such as humidity, temperature, and ultraviolet (UV) radiation. In recent years, technical advancements in solar energy have significantly progressed, surpassing the first stages witnessed in the 19th century. However, the average efficiency of solar panels remains at approximately 15–20%. This implies that about 80–85% of the incident raw energy originating from our primary celestial body is dissipated. In addition, it is noteworthy that silicon solar cells, which represent the predominant photovoltaic technology in use, possess a theoretical efficiency limit of approximately 35% (Table 3).

Single-junction				Multiple-junction
S.No	Materials	Efficiency %	Ref.	Materials	Efficiency %	Ref.
1	Si	26.8	[36]	InGaP/GaAs/InGaAs	37.9	[37]
2	GaAs	29.1	[38]	GaInP/GaAs	32.8	[39, 40]
3	InP	24.2	[41]	GaInP/GaInAsP/S	35.9	[42]
4	CIGS	23.3	[43]	GaInP/GaAs/Si	35.9	[44]
5	CIGSSe	20.3	[45]	GaAsP/Si	23.4	[46]
6	CdTe	21.0	[47]	GaInP/GaInAs/Ge; Si	34.5	[48]
7	CZTSSe	12.1	[47]	Perovskite/Si	33.7	[48]
8	Perovskite	24.3	[49]	Perovskite/perovskite	28.2	[50]
9	GaInP	22.0	[51]	GaInP/GaAs/GaInAs	37.8	[52]
10	Organic	15.2	[53]	4 J Minimodule	41.4	[40]

Table 3.

A list of transition metal chalcogenide PVs and their efficiencies.

5. Challenges in chalcogenide photovoltaics

The materials mentioned above have exhibited exceptional performance across a range of applications. Current research and development endeavors are concentrated on several aspects to augment their effectiveness and explore untapped potential. The following are a few prominent domains in which research is currently advancing.

• Materials Toxicity: Certain metal chalcogenides possess hazardous elements such as cadmium and lead. Researchers are now investigating alternate materials or procedures that are less harmful to address the potential concerns of both the environment and human health. For example, the use of non-toxic substances such as copper zinc tin sulfide (CZTS) or copper zinc tin selenide (CZTSe)

• Efficiency and Performance: The goal of power conversion efficiencies (PCE) on par with popular technologies, such as silicon-based solar cells, has been a significant obstacle. One potential solution for achieving better efficiencies is to focus on improving the quality of crystals, optimizing processes for depositing films, and boosting the mobility of charge carriers. The investigation of band-gap engineering, tandem cell architectures, and sophisticated light trapping methods is also underway.

• Stability and Reliability: Certain metal chalcogenides exhibit susceptibility to environmental factors such as moisture, temperature, and light exposure, resulting in deterioration and reduced longevity. Encapsulation methods, protective coatings, and a deeper understanding of degradation processes may improve these issues.

• Material Homogeneity and Defects: Material defects, compositional non-uniformities, and grain boundaries may impact the transit and recombination of charges inside a material.

• Advanced Material Engineering: The enhancement of electrical and optical properties in metal chalcogenides may be achieved by modifying their composition and structure. This strategy holds promise for enhancing solar absorption by adjusting the band gaps by the solar spectrum.

• Interface Engineering: To maximize the capture of charge carriers and reduce recombination, interface engineering includes carefully designing interfaces between various materials. This promotes effective charge transport. Incorporating charge-selective layers, such as buffer layers or electron/hole transport layers, is often used in engineering to enhance the overall efficacy of devices.

• Scalable Fabrication Methods: The challenge is in developing economically viable and scalable methodologies for mass manufacturing metal chalcogenide solar cells on a big scale. The proposed approach involves investigating printing techniques and roll-to-roll manufacturing methods to achieve efficient and cost-effective fabrication processes.

• Environmental Impact Reduction: Reducing the environmental footprint of production processes by recycling and reusing materials to reduce waste. Alternative chalcogen sources and sustainable synthesis routes are investigated as a solution.

6. Emerging progress and current trends in chalcogenide photovoltaics research

Recent developments have influenced the field of solar energy conversion and changing research trends in metal chalcogenide photovoltaics. An area of significant research is the investigation of tandem and multifunction solar cells, which integrate metal chalcogenide solar cells with other photovoltaic technologies to enhance overall efficiency. Combining different elements or components enables efficient absorption of light over a broader range of solar wavelengths, resulting in improved overall performance of the device. Another area of research that is as intriguing is the exploration of hybrid architectures, which include the seamless integration of metal chalcogenides with perovskite materials. This study’s primary aim is to enhance light absorption and optimize the separation of charges inside perovskite layers, with the potential to increase efficiency [54, 55, 56].

The process of bandgap engineering through anion alloying, such as in the case of BaZrO₃, results in the material exhibiting insulating properties with a bandgap of approximately 5 eV. In contrast, the bandgap of BaZrS₃ is approximately 1.8 eV. Similarly, the bandgaps of these materials can be controlled by including different anions and cations during the synthesis of chalcogenide perovskites. The substitution of titanium (Ti) on the zirconium (Zr) site and the incorporation of selenium (Se) with sulfur (S) have yielded promising outcomes. For instance, the chalcogenide perovskite BaZr_0.75Ti_0.25S₃ resulted in a reduced bandgap of 1.43 eV. This value is in close proximity to the optimal value, known as the Shockley-Queisser limit. In contrast, the BaZr(S_0.6Se_0.4)₃ showed a decrease of 180 meV in the bandgap of BaZrS₃ [57, 58].

Selectively removing photo-generated electrons through the electron transport layer is frequent in photovoltaic devices. Through a large Schottky barrier at the perovskite material-ETL border, the electron transport layer (ETL) prevents electron vacancies from moving. The heterojunction tunneling mechanism (HTM) inhibits electron mobility, making whole extraction easier. Thus, inadequate charge transfer and charge flow obstruction will cause charge recombination at the contact, resulting in energy loss. Interface recombination involves non-radiative charge carrier recombination. Contrasting energy levels, surface imperfections, and carrier migration toward the interface cause this process.

The enhancement of understanding about defects and their subsequent impact on the functioning of devices has facilitated the development of more efficient solutions for defect passivation. By strategically implementing surface treatments and interface engineering approaches, scholars have successfully reduced recombination rates and prolonged the lifespans of charge carriers [5, 6, 8, 59, 60, 61]. Innovative techniques for characterizing materials and electronics, such as high-resolution imaging and spectroscopy, have provided significant revelations about their fundamental properties and complex operational mechanisms. Methods like Kelvin probe microscopy, terahertz spectroscopy, and time-resolved photoluminescence. Flexible and transparent substrates have facilitated the emergence of metal chalcogenide solar cells that possess flexibility and partial transparency. This technological advancement allows for smoothly incorporating these devices onto various surfaces and objects, broadening their uses.

This work investigates novel thin film device light absorption methods. These include studying plasmonic nanoparticles, nanostructured surfaces, and photonic crystals to improve light absorption. Machine learning and computer modeling speed up material discovery and device optimization. This partnership predicts material properties and determines the best experimental methods. Academics are exploring scalable fabrication methods to increase metal chalcogenide solar cell production. These methods include solution-based and roll-to-roll manufacturing.

Implementing environmentally sound synthesis processes and recycling procedures has been driven by a strong dedication to sustainability. A significant focus on replacing uncommon and dangerous elements with readily available earth-abundant resources characterized this dedication. Using metal chalcogenide solar cells in architectural components such as glass, windows, and facades produces electricity while enhancing visual appeal and promoting novel architectural concepts. The successful exceeding of the 30% criterion instills assurance that cost-effective photovoltaic systems with superior performance can be effectively introduced into the market.

Surpassing this threshold instills a sense of assurance that high-performance, cost-effective photovoltaic systems can be successfully introduced to the market. There is increasing competition among materials scientists, as seen by recent developments reported by LONGI, a prominent Chinese corporation that holds a significant market share in global solar panel production. Recently unveiled its latest innovation as a tandem solar panel with an impressive efficiency rating of 33.5% [62]. Although this situation is indeed thrilling, it is important to note that we are merely at the initial stages. Researchers have been endeavoring to address this issue for an extended period. A team at the National Renewable Energy Laboratory (NREL developed a panel with an efficiency of 47%). However, regrettably, the cost of this model renders it is impractical for widespread adoption [58]. A recent study conducted by researchers at the Fraunhofer Institute for Solar Energy Systems ISE has shown a notable enhancement in the efficiency of the most advanced four-junction solar cell to date. Implementing a novel antireflection coating achieved this improvement. Specifically, the efficiency of the solar cell was raised from 46.1 to 47.6%. This achievement represents a significant global milestone, as there is currently no solar cell that surpasses its efficiency on a global scale [63].

6.1 Thermoelectric chalcogenides

1. Thermoelectricity and its Importance: The reports from the global footprint network provide an essential analysis showing a potential decline in fossil fuel reserves during the next 50 years. In recent years, the combustion of fossil fuels has had a significant impact on the ecosystem and climatic conditions. Carbon dioxide output has reached a historically high level in the last decade, a concerning indication for the future. Several data show that the energy used in the whole consumption amounts to just 40%, while the remaining energy is released into the environment as heat (Figure 3a). Scientific findings have bolstered the case for the immediate implementation of coordinated measures to mitigate environmental catastrophes. The increasing global energy requirements, the diminishing reserves of fossil fuels, and the growing apprehensions over climate change have prompted an extensive study of alternative energy sources and energy conversion methods. There are a few alternate techniques for mitigating the energy problem, one of which involves the recovery of spent energy wasted heat. We used thermoelectric modules as a kind of energy conversion technology [64].

   The thermoelectric generator is a solid-state configuration that directly converts thermal energy into electrical energy. It can function as a solid-state refrigerator, as seen in Figure 3(b). The processes mentioned above are founded upon the Seebeck and Peltier effects principles, which just need a temperature difference inside a material to generate electricity. Thermoelectric materials have several benefits, including noise reduction, compact dimensions, expandability, long-lasting performance, absence of mechanical forces, and lack of chemical reactions. The material chemist’s primary goal is to find thermoelectric materials that are both efficient and environmentally beneficial while also being economically viable. The thermoelectric notion has been acknowledged for over 150 years, but its comprehension of electron and phonon transport events has solidified its foundation.

2. Thermoelectric properties: Thermoelectric figure of merit

   The efficiency of converting heat to electricity in thermoelectric materials is evaluated using the dimensionless figure-of-merit, zT. This figure-of-merit is calculated using the equation zT = S²σT/κ, where S represents the Seebeck coefficient, σ represents the electrical conductivity, T represents the temperature, and κ represents the thermal conductivity. To get an adequate power output, a substantial temperature difference is imperative, denoted as ΔT = Th − Tc. The good thermoelectric material must exhibit good efficiency η seen across a limited temperature difference ΔT. The highest efficiency η across a finite temperature difference ΔT. We can readily estimate this value using S, ρ, and κ.

   The effectiveness of thermoelectric systems is contingent upon many parameters, including the Carnot efficiency and the thermoelectric characteristics, including the Seebeck coefficient, thermal conductivity, and electrical resistivity. The average energy conversion efficiency of about 1 ZT is 10%. If the ZT value is around 3, it results in a 20% increase. Once the thermoelectric figure of merit (ZT) reaches around 4, the conversion efficiency may increase to as high as 30%. At present, we have successfully attained a ZT value greater than 2, which has enabled its practical use. In order to attain thermoelectric materials with high efficiency, it is essential that the materials exhibit a substantial Seebeck coefficient and electrical conductivity while simultaneously maintaining a high coefficient of thermal conductivity. Figure 4 illustrates a graphical representation of the thermoelectric module.

   In contemporary times, there has been a significant focus on chalcogenides, half-Heusler compounds, clathrates, Zintl phases, and skutterudite owing to their notable high thermoelectric figure of merit (ZT). Most of the materials mentioned above are mostly composed of heavy metals and are characterized by a limited abundance. Therefore, the development of thermoelectric materials encompasses the attainment of a high figure of merit (ZT) as well as the need to operate across a broad temperature range using materials that are both non-toxic and economically viable. Most thermoelectric materials currently in the market are based on telluride compounds, with notable examples, including PbTe, Bi₂Te₃, NaPb_xSbTe_x + 2, and AgSbTe₂. These materials offer several advantages, such as low thermal conductivity (2.3 W/m.K for PbTe and 1.7 W/m.K for Bi2Te3), a high Seebeck coefficient of 500 μV/K, and the ability to be easily doped with either p-type or n-type atoms. The exorbitant cost of tellurium-based thermoelectric materials has prompted researchers to explore alternate options. Among these alternatives, sulfur (S; Bi2S3) and selenium (Se; PbSe) have been promising candidates because of their low thermal conductivity and high Seebeck coefficients.

3. Approaches to improve the thermoelectric efficiency

   1. Doping to Modify Charge Carrier Concentration: The doping process entails the introduction of dopants, which are foreign elements, into the lattice structure of chalcogenide materials to alter their electrical characteristics. The manipulation of dopants enables the regulation of charge carrier concentration, exerting an impact on electrical conductivity and the Seebeck coefficient. In the process of n-type doping, introducing extra electrons augmented the electron concentration. Conversely, p-type doping involves the introduction of holes to elevate the engagement of holes. The selection of a dopant, together with its concentration and placement within the crystal lattice, may substantially influence the electrical characteristics of a material, affecting its thermoelectric efficiency.

   2. Nanostructuring and Grain Boundary Engineering: The process of nanostructuring entails the deliberate manipulation of a material’s structural composition at the nanoscale dimension, resulting in the formation of minute features such as nanoparticles, nanowires, or nanograins. These characteristics can disperse phonons, which are vibrations that transfer heat and increase electron scattering. As a result, the material’s thermal conductivity is reduced, and its thermoelectric efficiency is improved. The primary aim of grain boundary engineering is to optimize the grain size and distribution inside a material. This is because grain borders have the potential to function as scattering foci for phonons, resulting in a decrease in thermal conductivity.

   3. Band-gap engineering: The term “band engineering” pertains to the manipulation of the electronic band structure of a material to enhance its electronic characteristics to optimize thermoelectric performance. This process entails changing the location and breadth of the energy bands to optimize the Seebeck coefficient and electrical conductivity. The aim of energy filtering is to selectively permit the transmission of specific energy levels of charge carriers while impeding others, enhancing the material’s thermoelectric characteristics. This technique shows significant use in materials characterized by intricate band topologies.

   4. Phonon Engineering and Reduction of Thermal Conductivity: Phonons transmit thermal energy inside materials. The enhancement of efficiency in thermoelectric chalcogenides reduces thermal conductivity. Phonon engineering encompasses the deliberate introduction of scattering centers, such as nanoparticles or point defects, to impede phonons’ efficient heat transfer capabilities by disrupting their propagation. In addition, using materials characterized by low phonon velocities or pronounced anharmonic might contribute to mitigating thermal conductivity.

   5. Transport Phenomena: The efficiency of chalcogenides hinges on their ability to control thermal conductivity while maintaining high electrical conductivity. Phonon scattering plays a pivotal role in reducing thermal conductivity, involving interactions with grain boundaries, nanostructures, and point defects is equally vital, governed by electrical conductivity and the Seebeck coefficient. The engineering mentioned above exhibits interconnectivity and provides the potential for synergistic outcomes when combined.

Figure 3.

Schematic representation of the Seebeck and Peltier effect.

Figure 4.

Graphic representation of the thermoelectric module.

Thermoelectric chalcogenide materials often possess complex crystal structures, ranging from layered to more intricate frameworks. Heavy elements and intrinsic anisotropy in their crystal structures contribute to their enhanced thermoelectric performance. The field of thermoelectric chalcogenides boasts several noteworthy examples that highlight their potential. One example is lead chalcogenides, specifically PbTe, known for its impressive ZT values at elevated temperatures. Additionally, copper chalcogenides (Cu₂Se and Cu₂Te) exhibit promising thermoelectric properties because of their complex crystal structures and inherent anisotropy (Table 4).

Material	Maximum ZT	Temperature Range	Reference
PbTe	∼2.2	600–900 K	[65]
Cu2Se	∼1.5	RT	[66]
Cu2Te	∼1.3		[66]
Sb2X3	1.1–1.8 eV		[67]
BaZrS3	1.75–1.94		[68]

Table 4.

A list of transition metal chalcogenide PVs and their efficiencies.

Challenges and Future Directions: Despite significant progress, challenges persist in enhancing the efficiency and viability of thermoelectric chalcogenides. Balancing trade-offs between electrical and thermal properties remains intricate, demanding novel material design strategies. Sustainability considerations of material sourcing and processing are vital for the widespread adoption of these materials.

The future of thermoelectric chalcogenides lies in the convergence of advanced computational modeling, materials synthesis techniques, and device engineering. Researchers can pave the way for transformative applications in energy harvesting and utilization by delving into the fundamental interactions governing their thermoelectric behavior. The versatility of thermoelectric chalcogenides finds applications in various domains. Solid-state thermoelectric generators, capable of harvesting waste heat from industrial processes or vehicle exhaust, hold promise in sustainable energy production. Their compatibility with flexible substrates paves the way for wearable energy devices, powering electronics from body heat. Innovative approaches involve integrating thermoelectric chalcogenides into hybrid systems, such as coupling them with photovoltaics for efficient solar energy conversion. Their potential in space exploration, where temperature gradients abound, showcases their utility in extreme environments.

6.2 Chalcogenide nanomaterials for energy conversion

6.2.1 Photocatalysis

The escalating environmental concerns have been driven by the expansion of the human population and the industrial revolution. The industrial sector discharged approximately 400 million metric tons of chemicals, including harmful solvents and metal ions, into the water. Besides industrial waste, many human activities, such as using pesticides, fertilizers, and the disposal of home trash, also contribute to the contamination of water bodies (see Figure 1). The abovementioned businesses emitted various detrimental organic substances, including formaldehyde, azo dyes, dioxins, pesticides, and heavy metals. The potential consequences of such actions threaten both human well-being and the ecological balance inside our natural water reservoirs. Each year, around 15% (equivalent to one thousand tons) of non-biodegradable textile dyes are discharged into streams and aquatic bodies through the wastewater generated by the textile industry. Typically, in the dyeing and finishing industry, an approximate range of 120–280 liters of water is utilized per kilogram of garment throughout the processing stage [69, 70, 71].

Based on an estimate provided by the World Bank, it has been determined that around 17–20% of water pollution can be attributed to the activities of textile businesses. Most organic dyes have been found to possess carcinogenic properties, posing significant health risks to both humans and marine species. These toxins might induce many health issues, including but not limited to cancer, neurological impairments, cardiovascular disorders, and digestive ailments, even when present in minimal quantities. Many actions have been identified as factors that lead to the gradual disruption of water flow in the foreseeable future (Figure 5) [69, 72].

Figure 5.

Source of water pollution chemical contamination.

Therefore, there is an urgent need to detoxify or remove deleterious chemical contaminants from water to ensure long-term sustainability. Various standard water treatment methods were accessible, such as membranes, ion exchange, filtration, remediation, coagulation, and sedimentation. Several advanced oxidation techniques, including photocatalysis, wet oxidation, and sonolysis, are recognized for their ability to mineralize resistant substances. In recent decades, there has been a growing interest in utilizing advanced oxidation processes (AOPs) to eliminate harmful organic chemicals from water [73, 74, 75, 76]. This approach is one of several detoxification methods that has garnered significant attention. The integration of many study fields within the realm of Photocatalysis has been significantly driven by the imperative for environmental reform, clean hydrogen (H₂) fuel generation, and the conversion of CO₂ (Figure 6). Photocatalysis has several advantages compared to traditional catalytic processes, such as eliminating hazardous stages and operating at elevated temperatures and pressures. Studies have shown that using semiconductor photocatalysis in photo-oxidation processes yields water treatment efficiencies of over 95%.

Figure 6.

Mechanism and uses of photocatalysis.

In the water treatment process, four distinct tactics have facilitated the degradation of pollutants by Photocatalysis (Figure 6a): (1) Strategies involve the creation of electron-hole pairs, known as photoexcitation, which occurs when light is absorbed, (2) The process of ionization of water (H₂O), (3) the phenomenon of oxygen ion-sorption, (4) the protonation of superoxide.

$$\text{Photocatalyst}+\mathrm{h\nu }\left(\mathrm{UV}\right)\to \text{Photocatalyst}\left({\mathrm{e}}^{-}\left(\mathrm{CB}\right)+{\mathrm{h}}^{+}\left(\mathrm{VB}\right)\right)$$E1

$${\mathrm{H}}_2\mathrm{O}\left(\mathrm{ads}\right)+{\mathrm{h\nu }}^{+}\left(\mathrm{VB}\right)\to {\mathrm{OH}}^{●}\left(\mathrm{ads}\right)+{\mathrm{H}}^{+}\left(\mathrm{ads}\right)$$E2

$${\mathrm{O}}_2+{\mathrm{e}}^{-}\left(\mathrm{CB}\right)\to {{\mathrm{O}}_2}^{-●}\left(\mathrm{ads}\right)$$E3

$$\mathrm{Dye}+{\mathrm{OH}}^{●}\to {\mathrm{Co}}_2+\mathrm{H}2\mathrm{O}\left(\mathrm{dye}\text{intermediates}\right)$$E4

$$\mathrm{Dye}+{\mathrm{h}}^{+}\left(\mathrm{VB}\right))\to \text{Oxidation products}$$E5

$$\mathrm{Dye}+{\mathrm{e}}^{-}\left(\mathrm{CB}\right)\to \text{Reduction products}$$E6

The dynamic nature of this field causes the use of materials that are economically viable, devoid of toxicity, environmentally sustainable, competitive, and easily synthesized. Using a wide range of materials in the field has gained significant appeal. Many materials, including graphene, titanium dioxide (TiO₂), [2] zinc oxide (ZnO), copper, tin sulfide (Cu₂SnS₃), zirconium dioxide (ZrO₂), cerium dioxide (CeO₂), iron oxide (Fe₂O₃), tin dioxide (SnO₂), strontium titanate (SrTiO₃), tungsten trioxide (WO₃), among others, have been investigated for their potential multifunctional applications. Titanium dioxide (TiO₂) is widely acknowledged as the predominant material among semiconductors, renowned for its potential as a photocatalyst. The titanium dioxide (TiO₂) material, possessing a band-gap energy of 3.2 eV, predominantly exhibits absorption characteristics toward ultraviolet (UV) light with a wavelength of approximately 386 nanometers (nm). This specific UV wavelength accounts for a mere 5% of the total solar energy that reaches the Earth’s surface. Therefore, there is a significant interest in developing a photocatalyst with a band gap lower than 3.2 eV to improve light absorption within the visible spectrum. A few photocatalytic items have been designed and commercially introduced.

In recent times, there has been a notable surge in the attention received by metal-chalcogenides among researchers. This heightened interest can be attributed to these materials’ captivating electronic structure and optical capabilities, particularly in their application in photocatalysis. Tin Selenide SnS, MoSe₂ metal chalcogenide show a versatile material with a two-dimensional layered structure that finds utility in several fields, such as thermoelectricity, photodetection, solar energy conversion, photocatalysis, gas sensing, battery technology, and topological insulation (refer to Figure 6b). SnSe possesses a direct and indirect band gap ranging from 0.9 to 1.8 eV. Its high absorption coefficient, exceeding 10⁴ cm⁻¹, contributes significantly to its exceptional optical absorption capabilities. Despite demonstrating significant potential as a photocatalyst, further research and development are necessary because of limitations in certain aspects within a singular material composition.

One potentially helpful photocatalyst relies on three fundamental properties of semiconductor materials: 1. Band-gap, 2. Rate of separation of electron-hole pairs, and 3. Optical absorption efficiency. Within the context of a wide band-gap semiconductor, electron excitation can be observed when subjected to ultraviolet radiation, as this type of radiation possesses the energy to stimulate the electrons into an activated state. In contrast, small band-gap semiconductors can excite electrons within the visible energy range. The early excitement around this phenomenon has primarily stemmed from the properties of individual materials. However, the rapid recombination of electron-hole pairs and the limited efficiency of surface catalytic reactions pose significant challenges to the overall performance of many photocatalytic materials. Many preliminary endeavors in this domain have shown that a semiconductor photocatalyst comprising a single component cannot meet all the conditions.

We have devised various solutions to address these limitations, including using band-gap engineering via the fabrication of heterostructures. An extensive investigation in this field has shown that optimizing the energy band alignments at the interfaces of two semiconductors can significantly enhance the separation of charges by suppressing the recombination of photo-generated electron-hole pairs. Numerous endeavors have been undertaken to improve efficiency by mitigating recombination rates and broadening light absorption using surface modification, metal and non-metal ion doping, and composite formation. One promising method involves the fabrication of heterostructure nanocomposites, wherein incorporating additional semiconductors or metals has been identified as a viable strategy for augmenting their photocatalytic capabilities (see Figure 7) [5, 6, 7, 8].

Figure 7.

Types of electron-hole migration in heterojunction photocatalyst.

Multiple studies have shown that heterostructure nanosheets, interconnected through van der Waals (vdW) forces, have superior properties to individual 2D nanosheets [76, 77]. A significant number of heterostructure composites have been investigated in the field, including but not limited to MnFe₂O₄/rGO, CdMoO₄/g-C₃N₄, BiVO4/g-C₃N₄, Bi₂MoO₆/g-C₃N₄, CdxZnyS/g-C₃N₄, and Ti₃C₂/g-C₃N₄, WO₃/ZnIn₂S₄, CuFe2O₄/Ti₃C₂, MoS₂/TiO₂, SnO₂/ZnSe(N2H4)_0.5, MOSe₂/GaN, MoSe₂/InN, and CuO/TiO₂. Besides the oxide mentioned above heterostructures, there have been reports of metal chalcogenide (MoSe₂, SnX, and ZnSe) heterostructures, including SnSe₂/Se, SnSe/SnO₂, SnSe₂/SnSe, SnSe/g-C₃N₄, SnO₂/ZnS, SnSe/SnO2, MoS₂/ZnSe, ZnTe/ZnSe, MoSe₂/GaN, MoSe2/InN, GaN/MoSe2/GaN, and ZnO–ZnSe. However, the number of reported metal chalcogenide heterostructures remains limited, and significant efforts are currently being made to improve their efficiency [4, 6, 7, 8, 61].

7. The future prospects of metal chalcogenides in photovoltaics, Thermoelectrics, and catalytic applications

A review of this study shows that metal chalcogenides exhibit distinct characteristics that render them promising contenders for sustainable energy generation and conversion technologies. In this concluding section, we will engage in a reflective analysis of the future prospects of these materials within the areas mentioned above while delving into their applications in these contexts.

The pursuit of highly efficient and economically viable solar energy conversion technology continues to be a primary focus in our endeavors to tackle worldwide energy-related issues. Metal chalcogenides provide many characteristics within the field of photovoltaics that present encouraging potential for the future. Metal chalcogenides have the potential to be incorporated into tandem solar cell configurations to improve the efficiency of converting solar energy. Combining materials with complementary band gaps can efficiently use the complete solar spectrum. The production of thin-film solar cells holds promise for cost reduction in manufacturing processes and the exploration of novel applications in the realm of flexible and lightweight solar panels.

Further investigation into metal chalcogenide quantum dots and nanowires might contribute to advancing innovative solar systems with high efficiency and adjustable electrical characteristics. Sustainable energy challenges include efficiently capturing and turning waste heat into electricity. Metal chalcogenides can enhance energy efficiency by harnessing waste heat in industrial operations and exhaust systems. Developing efficient, flexible, lightweight thermoelectric materials can enable portable and wearable energy harvesting systems. The non-toxic properties of many metal chalcogenides support the increasing focus on environmentally friendly materials in the energy industry. Metal chalcogenides with efficient catalytic characteristics are ideal for green and sustainable chemical manufacture and environmental remediation.

Future studies may involve applications in next-generation batteries and supercapacitors. It may also aid quantum computing, communication, and dot-based gadgets. These materials are essential to our sustainable and energy-efficient future because of their adaptability and breakthroughs in materials science and engineering. As we realize their potential, metal chalcogenides will become more relevant in twenty first-century energy and environmental challenges.

8. Conclusion

In summary, this extensive analysis shows the potential of chalcogenide materials in addressing global challenges related to sustainable energy and environmental issues. The distinctive characteristics and utilizations of these technologies encompass photovoltaics, thermoelectricity, and water treatment. It is necessary to recognize the inherent difficulties and the requirement for further investigation in the realm of chalcogenide nanomaterials. This covers the evaluation of ecotoxicity, assessment of environmental safety, and implementation of stringent regulatory measures. Chalcogenide nanomaterials are poised to have a substantial influence on global sustainable development, notwithstanding the obstacles that may arise. The utilization of chalcogenide nanoparticles has the potential to significantly transform sustainable energy and environmental solutions within a global context that prioritizes sustainability. Ongoing research and development efforts hold the potential to facilitate the emergence of a cleaner and more sustainable future by introducing promising advancements.