Translational potential of CuSe nanostructures as advanced energy materials: fundamental insights and emerging multifunctional solar energy conversion applications

Abstract

Extensive research efforts in past few decades have identified thousands of nanoparticles for various potential applications. However, only a few have found relevance in real-world industrial applications. A key question that continues to challenge material researchers is “what truly defines the foundation for designing nanomaterials that can meet all the critical criteria for industrial applications”? This tutorial review begins by highlighting the strategic significance of both metallic and non-metallic components in semiconductor nanomaterial systems. It emphasizes that the intelligent integration of these components can markedly develop the functional properties of semiconductor nanoparticles (NPs). Such synergistic development makes these materials highly attractive for a wide range of industrial applications. Cu–Se synergy mitigates Cu toxicity and yields a low band gap semiconductor with complementary electronic properties of Cu and Se, positioning CuSe as a promising candidate for next-generation solar energy conversion and healthcare technologies. This review emphasizes the crucial role of nanostructures (NSs) design in influencing the photogenerated electron–hole pair's dynamics, detailing various strategies employed to fabricate diverse 0D–3D CuSe NSs. CuSe NSs are also reviewed for their multifunctional solar energy conversion applications, including photocatalysis and photovoltaic cells. Extending beyond solar energy, the promising potential of CuSe NSs in energy storage systems and biomedical applications showcases their versatility and wide-ranging applicability. With a consolidated overview of the findings, the current challenges and future perspectives for harnessing the full potential of CuSe NSs, as advanced multifunctional energy materials, are discussed. Eventually, potential future industrial applications are discussed followed by a summary and outlook.

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Vikas Kumar

Vikas Kumar is a Senior Research Fellow in the Department of Physics at the National Institute of Technology (NIT) Kurukshetra, India. He completed his Bachelor of Science (BSc, 2015–2018) from Government College, Hisar, India. After graduation, he joined the Department of Physics and Photonics at the National Institute of Technology Hamirpur, India, for his Master's in Physics (MSc, 2018–2020). Following his post-graduation, he joined NIT Kurukshetra to pursue his doctoral studies. His research interests focus on the design and development of inorganic semiconductor materials for solar energy conversion and sustainable fuel production. His research has already made significant contributions to the scientific community, with publications in renowned journals such as RSC Nanoscale advances and Material letters.

Sahil Thakur

Sahil Thakur is currently pursuing his PhD degree at the National Institute of Technology, Hamirpur, India. His research primarily focuses on creating and developing semiconducting functional nanomaterials, i.e. metal oxides and 2D nanostructure-based photocatalysts, with particular emphasis on their applications in next-generation energy and environmental technologies. He has been awarded with prestigious CSIR Junior Research Fellowship. His research has already made significant contributions to the scientific community, with publications in renowned journals such as ACS Langmuir, Chemosphere, ChemComm, and Advanced Powder Materials. His work promises to push the boundaries of materials science and contributes to address the critical challenges in sustainable energy and environmental solutions.

Jai Prakash

Dr Jai Prakash is working as an Assistant Professor in the Department of Chemistry, NIT Hamirpur. He completed his PhD (2007–2012) in India, and afterwards, he worked as a Postdoctoral Researcher at INRS-EMT, Quebec (Canada), Aix-Marseille University (France), Universite Libre de Bruselles (Belgium) and University of the Free State, (South Africa). He has received several national and international scientific awards including DAAD Academic Award (2018), Merit Scholarship Award-2017 (Canada), Promising Young Researcher Award-2016 (South Africa), Prestigious INSPIRE Faculty Award-2015 (DST, India), Guest Scientist-2014 (NIMS, Japan), SSHN-2024 (France), and World's Top 2% Scientists (2021–2025). He has published more than 150 articles including reviews and book chapters. His major research fields are functional nanomaterials for energy and environmental applications. He has citations of more than 5500 with an h-index of 45 (November 2025).

Sushil Kumar Kansal

Dr Sushil Kumar Kansal is a Professor of Chemical Eng. at Panjab University, Chandigarh. He has many fellowships and awards in his credit including: DAAD Fellowship, Germany; Visiting Scientist to South China University of Technology, Guangzhou, China; Visiting Faculty to University of Waterloo, Canada under Indo-Shastri Canadian Fellowship; Mentor, Newton-Bhabha India-UK Advanced Training Course in Waste Water Treatment at University of Hull, UK. Professor Kansal’s total number of publications is 190, out of which 165 are in international journals, and 25 as book chapters, having an h-index of 61, with total citations ≥16000 (as of November 2025).

Hendrik Christoffel Swart

Prof. Hendrik C. Swart is an internationally acclaimed Researcher and is currently a Senior Professor in the Department of Physics at the University of the Free State, South Africa. He started research on luminescence materials in South Africa at the beginning of 1996. He has more than 975 publications in international peer reviewed journals and 120 peer-reviewed conference proceedings, and he is editor/author or co-editor/author of 100 book chapters and/or books with more than 31 000 cited author references, an h-index of 78, and more than 800 national and international conference contributions (authored and co-authored).

Mikhael Bechelany

Dr Mikhael Bechelany, born in 1979, earned his PhD in Materials Chemistry from the University of Lyon in 2006, focusing on 1D nanostructures. After completing a postdoctoral degree at EMPA (Switzerland), he joined the European Institute of Membranes (CNRS) in Montpellier in 2010 as a Researcher. His expertise lies in nanomaterial synthesis using ALD, electrospinning, and 3D printing for applications in energy, health, and the environment. Actively collaborating with academic and industrial partners, he participates in European and national projects. As of early 2025, he has authored 405 publications, holds 18 patents, and is involved in 6 start-ups while serving on the editorial boards of multiple scientific journals.

Awnish Kumar Tripathi

Dr Awnish Kumar Tripathi is an Assistant Professor in the Department of Physics at National Institute of Technology Kurukshetra, India. He has been actively contributing for the development of phenomenological models that could explain the anomalous optoelectronic features observed in semiconductor materials and devices. His expertise has been pivotal for the understanding and controlling the defect and disorder-mediated charge carrier dynamics that are key requirements for designing semiconductor nanostructures with desired optoelectronic properties. Dr Tripathi has authored more than 25 research articles in leading journals such as Physical Review B, Applied Physics Letters, Nanoscale Advances, Organic Electronics, Superlattices and Microstructures.

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1 Introduction

Developing sustainable energy resources has become one of the most urgent tasks for human beings for well-known reasons. Solar energy is promising due to its unparallel advantages like immense capacity, universal accessibility, inexhaustible nature, and environmental friendliness.^1–4 The Earth receives approximately one hundred and twenty thousand terawatts of solar radiation,⁵ which largely exceeds the global human energy consumption of merely 18–20 terawatts.⁶ This enormous disparity highlights the untapped potential of solar energy as an energy source. However, transforming this abundant incident energy into a practical and usable form poses a critical challenge.^7–11 It can be utilized through photovoltaic (PV),^12,13 photothermal (PT),^14,15 and photocatalytic (PCs)^16,17 processes, as shown schematically in Fig. 1(a). In PVs, a system harnesses solar electromagnetic radiation to generate electricity, offering a promising avenue for sustainable energy production.^18–23 Both technical experts and end-users now recognize solar PV as one of the most important renewable energy sources in terms of its globally installed capacity.²⁴ On the other hand, in PT, the light energy is converted into heat energy,²⁵ where semiconductor materials act as light absorber and convert into heat energy. In addition to solar energy conversion for a sustainable environment,^26,27 this technique has a great deal of utility in medical sciences.^28–34 Photocatalysis (PCs) represents the third major approach, wherein photons with energy exceeding the bandgap energy of the semiconductor are absorbed, and lead to the generation of an electron–hole pair (exciton).^35–37 Based on their chemical potential, these photogenerated charge carriers participate in chemical reactions, playing critical roles in various photocatalytic activities, i.e., water splitting, degradation of environmental pollutants, etc.^38–40 The semiconductor materials driving this process are designated as ‘photocatalysts’.

Fig. 1 (a) Multidimensional NSs of CuSe and their applications across four major domains. (b) Number of publications on CuSe for different applications published during 2021–2025 (data: Web of Science database) containing keywords “CuSe, photocatalytic”; “CuSe, photovoltaic”; “CuSe, energy storage”; or “CuSe, biomedical” in their title or keywords.

While these processes offer a promising pathway toward transitioning to renewable energy, they also present a critical challenge. One of the major challenges is designing and developing materials with precisely tailored properties that can efficiently capture solar energy and convert it into desired forms.^41,42 An ideal material for these applications should possess several key characteristics.^43,44 It includes an appropriate band gap for efficient photon absorption, high thermal stability to withstand operational temperatures, minimal toxicity to ensure environmental and human safety, cost-effectiveness to enable large-scale production, and manufacturing simplicity to facilitate widespread implementation. Materials satisfying these criteria can function effectively across the above-mentioned applications. In the last few decades (1970–2010), transition metal oxides (TMOs) such as TiO₂,^45–48 ZnO,^49–51 WO₃,^52,53 and Bi₂O₃ (ref. 54 and 55) have been widely employed, considering their low toxicity, simple synthesis process, and cost-effectiveness. However, when utilizing these for solar energy conversion, their significant band gaps (≥3 eV) limit their absorption capabilities to the ultraviolet (UV) region, constituting less than 4% of the total solar spectrum.^56,57 Researchers have extensively investigated strategies such as doping and composite formation to improve the Vis-NIR range absorption of existing materials.^58–60 However, these approaches offer limited improvement in capturing a broader portion of the solar spectrum. This underscores the need to move beyond traditional materials and develop novel compounds that can effectively utilize the full range of solar radiation.

Thanks to rapid advancements in materials science, it is well understood that both their metallic and non-metallic components strongly influence the electronic structure of compound semiconductors.⁶¹ In particular, the non-metallic counterpart plays a critical role in determining the electronic band structure of the complex. Fundamentally, the high electronegativity of the non-metal leads to deeper the valance band edge. Therefore, TMOs often exhibit high band gap values due to the high electronegativity of oxygen. It stems from its small ionic radius and position at the top of the chalcogen group (group 16). In contrast, as we move down in this group, elements such as sulfur (S), selenium (Se), and tellurium (Te) possess larger atomic sizes and greater polarizability.^62–64 When these chalcogen ions combine with transition metal ions, it improves electrical conductivity and results in narrower band gaps in the corresponding transition metal chalcogenides (TMCs). The same have been experimentally investigated and reported by various research groups.^62–71 Notably, TMCs such as CdS and ZnS were most studied for many solar energy conversion applications. The toxicity of cadmium (Cd) in CdS and the wide bandgap energy (3.6 eV) of ZnS are proving a significant constraint to their commercialization.^72–79 Other well-known TMCs including CdTe, CdSe, and GaAs present additional concerns related to low natural abundance, high production costs, and environmental hazards associated with their trace metal concentration. For instance, the practical implementation of CdTe solar cells suffers from high carcinogenicity of Cd and low abundance of Te. The production just 10% of global electricity using CdTe solar cells at current efficiency levels would require approximately 1900 tons of Te, while annual global production is merely about 60 tons.⁸⁰ The situation is even more challenging for In and Ga, which have even lower yearly production volumes and higher prices.^80–82

The larger ionic radius and lower electronegativity of Se than those of oxygen (O) and sulphur (S) result in significantly higher electrical conductivity of Se-ions (∼1.2 × 10⁻³ Sm⁻¹), surpassing that of S-ions (∼1 × 10⁻²⁸ Sm⁻¹). This feature influences metal selenides' charge transport capabilities and bandgap properties compared to oxide and sulphide NPs. In recent years, CuSe has emerged as a promising p-type TMC, attracting considerable research attention.^83–85 The electronic structure of Se 4p ([Ar] 3d¹⁰ 4s² 4p⁴) states facilitates strong chemical hybridization with the localized Cu 4s ([Ar] 3d¹⁰ 4s¹) states. This enables effective charge transfer, a key determinant of CuSe's performance in electrochemical and catalytic applications.⁸⁶ In addition, the ease of inducing a positive surface charge makes this Cu-based nanomaterial a standout in the spotlight of biomedical research, including antibacterial action, photothermal therapy, and targeted drug delivery.^87–89 A detailed discussion on the significance and additional factors contributing to its potential promise of CuSe is provided in Section 2 of the article. Fig. 1(a) illustrates the multidimensional structures of CuSe and highlights its application in major domains such as photovoltaics, photocatalysis, energy storage, and biomedical applications.

Despite its immense potential, CuSe remains an underrated material in the scientific literature. Fig. 1(b) shows the bar graph of research trends in CuSe applications during 2021–2025, highlighting the research opportunities in these less-explored domains. Although various research groups have demonstrated the effectiveness of CuSe in PCs, a comprehensive review that consolidates these findings and highlights CuSe as a promising semiconductor for future multifunctional applications is crucial. Very recently, Chawla et al.⁹⁰ have reviewed the synthesis of CuSe NSs. Their work offers valuable insights into CuSe synthesis and the impact of chemical composition on structural and PCs properties. Still, a detailed exploration of solar energy conversion and biomedical applications remains limited. The present review highlights the significance of CuSe NPs, with particular emphasis on various nanoarchitecture designs that influence their electronic and optical behavior. Beyond its role in solar energy conversion, the present article explores the potential of CuSe across multiple scientific domains, including energy storage and its emerging applications as antibacterial and photothermal therapy for cancer treatment, highlighting its significant potential in medical science. Finally, it concludes with a forward-looking perspective on the future advancements in CuSe research. By presenting this comprehensive analysis, the review aims to inspire innovation and interdisciplinary research using semiconductor materials, ultimately contributing to advancing next-generation, high-efficiency, and sustainable energy technologies.

2 Historical evolution of photovoltaic and photocatalytic systems: rise of CuSe-based NSs, challenges, and strategy advancement

The roots of PV and PC technologies can be traced back to 1839, when Edmond Becquerel discovered the PV effect. He observed that an illuminating electrode immersed in an acidic solution generated a voltage and current, a foundational observation in light-induced electrical phenomena.^91,92 Although early observations laid the groundwork, it was not until 1921 that Sir Albert Einstein received the Nobel Prize for explaining the photoelectric effect. This milestone sparked a wave of breakthroughs, including developing the first silicon (Si)-based solar cell in 1954 and then photocatalytic water splitting on a titanium dioxide (TiO₂) electrode by Fujishima and Honda in 1973. These landmark discoveries marked critical turning points in the evolution of PV and PC technologies to convert solar energy into usable form. In the wake of these breakthroughs, the materials science community has intensified its pursuit of advanced nanomaterials capable of efficient solar energy conversion. For effective utilization in solar energy conversion technologies, nanomaterials must exhibit solar-spectrum-like absorption and possess precisely aligned band edges. The critical role of band edge alignment, particularly in PCs, has been extensively discussed elsewhere in the literature.^93–96 A graphical representation of the band gap (E_g) and the corresponding band edge positions of designed TMOs and TMCs is provided in Fig. 2(a) and (b) to illustrate this concept.

Fig. 2 Band edge position of various (a) TMOs and (b) TMCs.

The promise of CuSe NPs in solar energy harvesting is truly remarkable.^90,97,98 Owing to their narrow band gap, excellent thermal stability, and low cost, CuSe-based materials have attracted growing attention as sustainable candidates.⁹⁰ Beyond these intrinsic advantages, CuSe NPs exhibit a localized surface plasmon resonance (LSPR) effect, typically uncommon in traditional semiconductors.⁹⁷ In LSPR, the conduction electrons within the NPs oscillate collectively in resonance with incident light, resulting in the generation of a strong localized electromagnetic field at the particle surface. This plasmonic excitation facilitates the production of hot electrons, which can effectively transfer to the semiconductor's conduction band, thereby increasing the population of photoinduced charge carriers. The enhancement in carrier density and mobility plays a decisive role in reducing the electron–hole recombination rate, a critical factor governing the efficiency of solar energy conversion.⁹⁹ In a recent study, Gan et al.¹⁰⁰ reported the establishment of the LSPR behavior in water-dispersed CuSe NPs. This LSPR effect in water-dispersed CuSe NPs showcases their effectiveness to harvest solar energy efficiently for photocatalytic energy conversion. As discussed in the previous section, the metallic and non-metallic constituents of the compound play a pivotal role in determining the electronic band structure of a semiconductor. Considering this, the existence of CuSe in both stoichiometric and non-stoichiometric forms provide a versatile platform to influence its optical and electronic properties.¹⁰¹ By simply varying the chemical composition, these properties can be tailored to meet the specific requirements of targeted optoelectronic applications.¹⁰² For example, Cu-rich (metallic-rich) compositions, e.g., Cu₂Se and Cu_1.8Se, tend to exhibit a narrower band gap. This is attributed to the increased density of free carriers (free electrons), introduced by the excess Cu ions. These additional carriers increase the Fermi level and elevate the energy of the highest occupied molecular orbitals (HOMO), resulting in a reduction in E_g. Conversely, Se-rich (non-metallic-rich) compositions such as CuSe₂ and Cu₃Se₂ typically show a wider E_g value. Therefore, by varying the relative concentrations of Cu and Se, the band structure of CuSe can be engineered accordingly. In addition to affecting the E_g, changes in chemical composition also influence the positioning of the constituent atoms. Such compositional alterations lead to changes in the crystal structure of the resulting chemical composition, which, in turn, significantly affect their thermal stability. A summary of investigations by various research groups exploring the effects of different chemical compositions on crystal structure, E_g, and thermal stability is presented in Table 1. In addition to showcasing the star potential as base material for PV and PCs, this composition tuning affects expanding its role in other components to advance these technologies.^103–106 Over time, CuSe has been increasingly explored as an efficient counter electrode (CE) material in PV, a cathode or anode material in energy storage devices,^107–112 a sensitive platform for chemical sensing,^113–116 and an NIR-II light-responsive photothermal agent for cancer therapy in biomedical applications.^117–121 It underscores its remarkable versatility, highlighting its growing importance in the development of sustainable energy solutions and advanced healthcare technologies.

Table 1 Fundamental properties of CuSe nanoparticles in different stoichiometric and non-stoichiometric chemical compositions

Chemical composition	Crystal phase	E g (eV)	Thermal stability	Ref.
α-Cu2Se	Monoclinic	0.8–1.0 eV	α-stable at <400 K	122 and 123
β-Cu2Se	Cubic		β-stable at >400 K
Cu1.8Se	Cubic	1.10	Stable	124 and 125
Cu3Se2	Tetragonal	1.77	Stable	126 and 127
CuSe	Hexagonal	2.0–2.5	Stable	128 and 129
CuSe2	Cubic	2.0	Stable	130 and 131
Cu2−xSe	Cubic	1.9–2.7	Stable	132 and 133

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In addition to enabling broad solar spectrum absorption, the low E_g value of CuSe presents a dual-edged advantage. While it enhances the light-harvesting efficiency, it also increases the risk of rapid recombination of photogenerated electron–hole pairs. This recombination significantly limits the effective utilization of charge carriers, reducing the efficiency of both PV and PC processes. The fundamental trade-off between broad light absorption and efficient charge separation emerges as a central obstacle in advancing CuSe-based solar technologies.¹³⁴ Structural dimensionality is one important element that can have a big impact on the optical and electronic characteristics of semiconductor NPs.^135,136 For example, a sphere with a nanometer dimension has a high specific surface area, resulting in a lower recombination rate of photogenerated electron–hole pairs.¹³⁷ One-dimensional fibers or tubes have advantages about less recombination because of the short distance for charge carrier diffusion, light-scattering properties, and fabrication of self-standing nonwoven mats.^135,138 Two-dimensional (2D) nanosheets have smooth surfaces and high adhesion, whereas three-dimensional (3D) monoliths may have high carrier mobility due to their interconnecting structures.^139–142 In his intriguing report, Tsakalakos¹⁴³ comprehensively examines various NSs for PV applications, delineating their advantages and disadvantages. The authors reported that 2D NSs (e.g., Quantum wells) offer benefits such as charge carrier confinement. It creates a structured carrier layer featuring sharp energy levels and excellent transport properties to improving absorption due to a denser band edge state. Complementing to this, Yu et al.⁴ offered a thorough overview of the influence of different NSs on the optical properties of semiconductor NPs. Covering a range of materials including Si, CdTe, CdSe, CdS, ZnO, and CuInSe₂ (copper indium selenide), they summarize the recent advancements in the study of the optical properties of different NSs. Fujishima, who discovered this phenomenon in PCs,¹⁴⁴ also emphasizes the importance of NS design for improved PC performance of a photocatalyst.¹⁴⁵ The authors discuss the impact of these NS designs on the generation and recombination of photogenerated electron–hole pairs, thereby increasing the efficiency of the PC process. In addition to these comprehensive reviews, numerous reports provided detailed insights into the significance of NS design.^{143,146–163} These studies demonstrate that structural variations at the nanoscale can significantly influence light absorption, photo-induced carrier dynamics, and thus, the overall efficiency. It underscores the importance of NS engineering in advancing solar cell technologies. Concluding these, optimizing NSs is a highly effective strategy for designing semiconductor NPs with improved solar energy conversion performance.

3 CuSe NSs: brief synthesis techniques, growth mechanism, and tailoring of various properties

In the context of material synthesis, the synthesis technique plays a crucial role in determining the properties of the resulting NPs.¹⁶⁴ Each synthesis route offers unique advantages and is often selected based on the desired structural and functional characteristics of the target material. For instance, the hydrothermal method is widely preferred for producing highly pure and well-crystallized NPs. The closed reaction environment and the elevated pressure developed inside the autoclave facilitate uniform nucleation and growth processes, thereby improving the crystallinity and structural integrity of the obtained NPs.¹⁶⁵ In contrast, for introducing the crystal defects or surface disorder, chemical routes such as the sol–gel, co-precipitation, chemical bath deposition methods, and combustion methods are often employed.^166–170 Hence, to incorporate the desired features in CuSe NPs, the detailed investigation and optimization of the synthesis technique is crucial. Gurin et al.¹⁷¹ elaborated a detailed procedure to study the chemical composition of CuSe NPs embedded in a sol–gel-derived silica matrix. The authors proposed an interesting strategy for the preparation of CuSe NPs using this approach. In the first step, CuO-doped xerogels were synthesized, which were subsequently subjected to high-temperature treatment in a hydrogen atmosphere while introducing Se vapor in a closed vessel. This method allowed systematic investigation of the influence of Se vapor pressure and the concentration of Cu-ions, determined by the initial CuO doping, on the chemical composition and optical properties of the resulting NPs. Hankare et al.¹⁷² reported the fabrication of CuSe thin films on glass substrates at low temperatures using the chemical wash deposition method. In this process, a glass substrate was positioned perpendicular to a Se-containing solution, and Cu-ions were introduced dropwise while carefully controlling the pH of the system. The temperature of the solution was maintained at ∼278 K during the deposition process, and after 120 minutes, the system was allowed to gradually cool down to ∼300 K. The resulting CuSe thin films were found to exhibit a hexagonal crystal phase. Furthermore, the study revealed that thickness of the film was strongly dependent on the deposition temperature and deposition time, highlighting the critical role of process parameters in tuning the structural and physical properties of the CuSe thin films. Xiao et al.¹⁷³ reported the synthesis of CuSe NPs using a simple chemical process. In a typical experiment, Cu and Se precursors were directly mixed and subsequently heated at 200 °C under an inert atmosphere. The authors observed that the resulting product CuSe NPs correspond to a non-stoichiometric phase of Cu_2−xSe with a cubic crystal structure. In a more recent study, Kumar et al.^{98,134,174,175} demonstrated the synthesis of CuSe nanosheets using a hydrothermal approach, employing less toxic and more economical precursors. This environmentally friendly route not only simplifies the synthesis process but also provides superior control over the chemical composition and crystal size of the resulting NSs. In addition to these studies, several other research groups have reported the successful fabrication of CuSe NSs using diverse synthesis techniques, including solvothermal, microwave-assisted, chemical bath deposition, and electrodeposition methods. A comprehensive summary of the reported synthesis methods, corresponding experimental conditions, and observed material characteristics is presented in Table 2 for the ease of comparison and better understanding of the synthesis–structure–property relationship in CuSe systems.

Table 2 Synthesis route and structural, optical, and photocatalytic performance of CuSe NS-based photocatalysts

Chemical composition	Synthesis route	Nanostructure design	E g (eV)	Target pollutant	Degradation %	Time taken (minutes)	References
Cu1.8Se	Co-precipitation	Nanoplate	2.0	MB	90	60	244
CuSe	Chemical solution deposition	Thin film	2.2	Congo red	91	150	236
CuSe	Reflux condensation	Nanospheres	2.3	RhB and MB	87 & 76	90	193
CuSe	Hydrothermal	Quantum dot	2.0	RhB	100	6 h	242
CuSe	Rapid injection method	Nanosheet	1.4	—	—	—	245
CuSe	Microwave assisted-reaction	Nanosheet	1.8	—	—	—	246
Cu2−xSe	Hydrothermal synthesis	Porous micro-networks	2.6	R6G	99	15	247
CuSe	Selenization process	Monolithic	2.2	MB	97	120	248
CuSe	Chemical process	Cube	2.0	Congo red	99	150	236
CuSe	Chemical precipitation	Nanoplate	3.5	Imidacloprid	99	70	249
CuSe	Hydrothermal	Nanoplate	0.8	RhB	91	60	250
Cu1.8Se	Co-precipitation	Nanoplate	2.0	MB	99	75	233
CuSe	Hydrothermal	Nanoplate	2.1	MB	60	180	251
CuSe	Hydrothermal	Fernwort-like	1.7	MB	100	12	241
CuSe	Hydrothermal	Nanosheet	0.8	RhB	99	120	234
CuSe	Hydrothermal	Nanosheet	1.8	MB	99	45	134
CuSe	Electrospinning	Nanofiber	—	—	—	—	221
Cu2Se	One-pot solution	Nanoparticle	1.6	MB	42	18	177
Cu2Se	Hydrothermal	Nanoparticle	2.0	RhB	70	35	252
Cu2Se	Hydrothermal	Nanosheet	2.3	MB	99	60	253
Cu1.8Se	Co-precipitation	Nanoparticles	2.0	MB	45	60	254
Cu2−xSe	CVD	Nanoplate	2.5	Tetracycline hydrochloride	98	90	255
Cu2−xSe	Co-precipitation	Nanoplate	2.1	MB	90	120	256
Cu2−xSe	Biosynthesis	Nanosphere	2.1	MB	99	300	257
CuSe1−xS	Hydrothermal	Nanoflakes	1.8	MB	100	15	258
CuSe@GO	Hydrothermal	Nanoparticle	2.4	MG	90	45	259
CuSe@ZnO	Sonication	Nanoparticle	2.5	MB	90	15	260
CuSe@ZnSe	Ion-exchange synthesis	Nanoflower	2.3	MO & MB	99	60	261
CuSe@TiO2@GO	Hydrothermal	Nanoparticle	2.5	MB	99	80	109
Cu2−xSe@TiO2	Direct chemical process	Nanoparticle	3.0	Formic acid	99	45	262
Cu3Se2@rGO	Co-precipitation	Nanoplate	1.9	MB	96	60	263
Cu2−xSe@FeSe2	Hydrothermal	Nanoparticle	1.65	Tetracycline hydrochloride	90	—	264

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In a similar vein, to investigate the growth mechanism, Zhang et al.¹⁷⁶ conducted a comprehensive investigation into the synthesis of CuSe using various methods. They demonstrated that by carefully adjusting the reaction parameters, particularly the effective Cu/Se ratio, it is possible to achieve CuSe with different chemical compositions, primarily by tuning the solution's reaction temperature and heating time. The proposed growth mechanism for different NSs for CuSe is provided in Fig. 3(a), and the obtained XRD pattern confirming the chemical composition is provided in Fig. 3(b). Building on this, Wang et al.¹⁷⁷ and Liu et al.¹⁷⁸ discussed in detail the impact of various reaction parameters on the morphology and structural properties of CuSe NSs. Fig. 3(c) and (d) present the schematic diagram of the synthesis route and morphological analyses. Gu et al.¹⁷⁹ reported a successful synthesis of concentrated hexagonal CuSe nanoflakes using an alkaline hydrothermal method with a lateral size of 200–800 nm and a thickness of 15–40 nm. Interestingly, it was observed that the amount of PVP, used as a capping agent, significantly affects the optical absorption properties of the prepared CuSe nanosheets. The reported absorption properties for prepared CuSe NSs are shown in Fig. 3(e). A well-complemented to this, Chawla et al.⁹⁰ investigated the impact of capping agent Ethylene diamine tetra acetic acid (EDTA) concentrations on phase purity and substantial effect on the size resulting CuSe nanoplates. Recently, Patel et al.¹⁸⁰ have reported a unique synthesis method for producing highly crystalline CuSe NSs, as illustrated in Fig. 3(f). In their study, the authors emphasized that the stoichiometry and phase of CuSe play a critical role in determining its thermal stability and band gap. Notably, they observed a reversible phase transition, where CuSe transforms from a hexagonal lattice to an orthorhombic structure at 325 K, and reverts to the hexagonal phase at 410 K. Furthermore, photocurrent measurements, as shown in Fig. 3(g), demonstrate an induced current under solar-like illumination, highlighting the potential of CuSe NPs for solar energy conversion applications. In addition to these discussions, many more studies are being conducted about the synthesis pathway of CuSe NPs.^181–198 A description of the adopted synthesis routes, corresponding NS design, and the optical absorption edges reported by various research groups is provided in Table 2.

Fig. 3 (a) Proposed growth mechanism of CuSe nanosheets. (b) XRD patterns of Cu_2−δSe and CuSe nanosheets.¹⁷⁶ (c) One-pot solution-based fabrication of CuSe with controllable morphologies based on Cu : Se ratios.¹⁷⁷ (d) FESEM image showing hexagonal CuSe nanosheets.¹⁷⁸ (e) UV-visible absorption spectrum of CuSe nanosheets prepared by alkaline hydrothermal synthesis.¹⁷⁹ (f) Illustration of ultrasonic exfoliation methodology for CuSe nanoparticle preparation. (g) Photocurrent dynamics, demonstrating photoelectrochemical activity under a 395 nm monochromatic light exposure.¹⁸⁰ Reprinted with permission from (a and b) ref. 176 (Elsevier), copyright 2010; (c) ref. 177 (RSC), copyright 2017; (d) ref. 178 (Nature), copyright 2014; (e) ref. 179 (RSC), copyright 2014; and (f and g) ref. 180 (RSC), copyright 2021.

3.1 Zero-dimensional CuSe nanospheres and quantum dots

Quantum dots (QDs) represent an appealing subject for scientific exploration due to their unique properties as three-dimensional (3D) confined systems.¹⁹⁹ They have gained significant scientific interest as they offer a way to create ‘artificial atoms’-zero-dimensional structures where charge carriers are confined in all three spatial dimensions.²⁰⁰ Moreover, these spheres possess a high specific surface area, high pore volume, and pore size, which finally increases the size of the accessible surface area of the nanomaterials.^199,201 Overall, these increases result in better performance in chemical reactions that occur on the material's surface, e.g., PCs. Furthermore, these structural characteristics improve the light-harvesting efficiency of the materials by maximizing light penetration into their interior. This improved light absorption makes them promising candidates for various optoelectronic applications including PV and PCs.²⁰² Considering these, the beneficial properties derived from their unique structures have been reported in publications for many TMO NPs such as TiO₂ (ref. 116) and ZnO.¹⁹⁶ Similarly, in the context of CuSe, Sonia et al.¹⁹³ reported the preparation of different CuSe nanospheres using a conventional hydrothermal method by varying the reaction parameters such as reaction temperature and reaction time. It was observed that CuSe samples prepared at a lower reaction temperature (50 °C) show the formation of irregular structures up to 2 µm (Fig. 4(a)), whereas the sample refluxed at a higher temperature of 70 °C shows porous CuSe NPs, as shown in the inset of Fig. 4(b). It was observed that the average crystallite size decreases by increasing the reaction time from ∼44.73 (for 4 h) to ∼23.73 nm (for 12 h), resulting in a blue shift in the absorption properties of CuSe nanospheres. Additionally, no appreciable change in crystallite size was noticed in the reaction time of 12 h and at the reaction temperatures of 50, 70, and 90 °C, indicating that the reaction time plays a vital role in determining the surface structure of CuSe NPs. Recently, Das et al.²⁰³ have reported on producing CuSe_1−xTe_1+x nanospheres using microwave, with particle sizes ranging from 50 to 80 nm. It was observed that when the concentrations of Se and Te were varied, the morphology of the Cu–Se–Te NSs shifted from nanosheets to nanospheres. Notably, a significant change in bandgap value was reported with different Se/Te ratios, as shown in Fig. 4(c). The proposed growth mechanism responsible for the observed morphology is illustrated in Fig. 4(d). Based on the FESEM outcomes, it has been concluded that the resulting sample exhibited totally deformed edges and clean nanospheres at lower Se/Te ratios. The researcher reached an interesting outcome that there was a relationship between the increase in dopant atomic radius and the size of nanospheres. Similarly, Demazeau et al.²⁰⁴ reported a one-step solvothermal method for the synthesis of CuSe nanospheres using ethylene diamine as the solvent. Interestingly, the author observed that replacing the solvent from ethylene diamine by ethylamine affected the nucleation process and resulted in a change in the morphology from nano-whiskers to nanospheres. The work is well complemented by Kaviyarasu et al.;²⁰⁵ the authors reported that by the incorporation of amines into the reaction mixture, different NSs can be obtained easily at moderate temperatures as a result of the high reactivity of the amine–chalcogen complex. In a typical work, a two-step hydrothermal method was used for the synthesis of CuSe QDs in which CuCl₂ (metal halogen), Se powder (elemental chalcogen) and ethylene glycol (amine) were used as starting materials. Fig. 4(e) shows the reported HR-TEM image of the prepared CuSe sample. Additionally, they have investigated the change in the optical behavior of the prepared CuSe NSs with the increase in reaction time. It has been noted that the band gap decreases from 2.2 eV to 1.38 eV with a reaction time increase from 4 to 12 h because of the quantum confinement effect.

Fig. 4 (a and b) FESEM images of CuSe nanospheres.¹⁹³ (c) Band gap variation with feeding ratio variation and (d) proposed microwave synthesis process.²⁰³ (e) HRTEM image of CuSe nanospheres.²⁰⁵ (f) Obtained CuSe NSs for different concentrations of orange juice.²¹⁰. (g) Chemical and optical characteristics of the CuSe nanospheres.²¹¹ Reprinted with permission from (a and b) ref. 193 (Elsevier), copyright 2013; (c and d) ref. 203 (ACS), copyright 2023; (e) ref. 205 (Elsevier), copyright 2016; (f) ref. 210 (Elsevier), copyright 2021; and (g) ref. 211 (ACS), copyright 2017.

Very recently, some natural products such as corncob and egg white have been well used to synthesize various functional materials with unique NSs.^206,207 The complex chemical composition of fruit juice provided potential to the synthesis of nanomaterials with special morphology.^208,209 Shu et al.²¹⁰ developed a facile one-pot green hydrothermal route to prepare CuSe at the gram level by using different natural juices (pitaya, grape, litchi and watermelon juices) to provide potential for the synthesis of nanomaterials with special morphology. By varying the volume of orange juice, three kinds of CuSe with different morphologies, namely, dendritic, nanoflower and nanospheres were synthesized, as shown in Fig. 4(f). Furthermore, to investigate the role of reaction time and temperature in the synthesis of CuSe, the synthesis process was monitored from 1 to 7 h, whereas the reaction temperature was adjusted from 110 to 190 °C. The authors reported that the growth of CuSe NPs follows a typical time-dependent process. The author ends up with concluding that vitamin C and carbohydrates present in orange juice work as reducing and capping agents. The work was well complemented by other researchers in which they reported the effect of kiwi juice on other non-stoichiometry composition of CuSe.²¹¹ The reported outcomes providing insights into the structural and optical properties of CuSe NSs are presented in Fig. 4(g).

3.2 One-dimensional CuSe nanofibers and nanotubes

Since the discovery of carbon nanotubes (CNTs) by Iijima in 1990, nanotubes (NTs) have been recognized for their exceptional electronic and mechanical properties, with a high surface area and large pore volume. Numerous studies have demonstrated that the NT architecture significantly improves the light utilization efficiency.²¹² The reduced recombination rate of photogenerated electron–hole pairs, facilitated by the high surface-to-volume ratio in tubular and fibrous structures, further supports their suitability for various advanced applications.²¹³ Additionally, the ability to assemble these 1D materials into self-standing nonwoven mats opens up further possibilities.^214,215 As a result, the creation of CuSe NT structures for energy and environmental applications has been the focus of several research efforts. Zhang et al.²¹⁶ presented a chemical reaction pathway for the preparation of CuSe crystalline NTs, wherein trigonal Se (t-Se) was utilized as a template-directed reagent. By reacting with Cu NPs, the t-Se NTs, which were serving as reactants as well as templates in synthesis procedure, were transformed into crystalline CuSe NTs. The FESEM images of the Se-NTs used as templates and the produced CuSe NTs are shown in Fig. 5(a) and (b), respectively. Besides CuSe NTs, the authors reported that other CuSe with 1D NSs, such as Cu₃Se₂, Cu_2−xSe_x, and Cu₂Se, were generated by varying the ratio of Cu to Se in the precursors. Similarly, Wei et al.²¹⁷ reported the electrochemical production of CuSe NTs with an Se-template-type structure. The obtained FESEM image, shown in Fig. 5(c), for prepared CuSe NTs gives excellent correlation with Zhang et al.'s²¹⁶ outcomes. The authors thoroughly investigated the effects of various factors, including scan rate, scan potential scope, and the acidity of the electrolyte solution, on the electrochemical and electro-generated chemi-luminescence properties of the hexagonal crystal-phase CuSe NTs. In one such study, the fabrication of a Se-nanotube, a crucial template to synthesise CuSe nanotubes, was reported by Chen et al.²¹⁸ The authors reported a preparation strategy for Se NTs utilizing a chemical method without using of any template, demonstrating that the preparation of Se NTs without using any template in any step of the process. The proposed growth mechanism for these Se NTs, as depicted in Fig. 5(d), with the FESEM images provided in Fig. 5(e), clearly illustrate the successful formation of nanotube-like structures in the produced Se NSs, confirming the effectiveness of the synthesis approach. As described by Xu et al.,²¹⁹ cubic Cu_2−xSe nanowire bundles were reacted with a sodium citrate solution at ambient temperature to create a CuSe nanowire with lengths of hundreds of micrometers. The nanowire was composed of several aligned nanowires with diameters of roughly 10–15 nm. Jagminas et al.²²⁰ reported the manufacture of CuSe nanorods using an electrochemical technique in the presence of an alumina pore employed as a template. It was observed that the composition and NSs of the prepared sample grown within the alumina pore depend greatly on the concentration ratio of Cu/Se and the pH of the reaction electrolyte. It has been observed that the length of the nanowire relies on the state of the AC electrolyte and expands by up to 10 µm over the course of the deposition process.

Fig. 5 (a) FESEM images of Se. (b) FESEM images of CuSe nanowire.²¹⁶ (c) FESEM images of CuSe nanowire.²¹⁷ (d) Proposed growth mechanism of Se-nanotube. (e) FESEM images of Se-nanotube.²¹⁸. (f and g) Proposed growth mechanism and optical absorption spectra of CuSe-nanotube, respectively.²²¹ Reprinted with permission from (a and b) ref. 216 (ACS), copyright 2006; (c) ref. 217 (Elsevier), Open access 2008; (d and e) ref. 218 (Elsevier), copyright 2010; and (f and g) ref. 221 (AJC) Open access 2018.

Sibokozar et al. are a step forward in this²²¹ by demonstrating the synthesis of CuSe nanofibers through a modified electro-spinning method, called the foaming-assisted electro-spinning method. It was observed that the production of large-pore mesoporous CuSe nanofibers depends critically on the amount of PMMA used as the foaming agent. The proposed growth mechanism and optical absorption characteristics of prepared CuSe nanofibers are shown in Fig. 5(f) and (g). The authors also investigate the impact of PMMA concentration on the average diameter of CuSe nanofibers. Interestingly, as they increase the CuSe NP concentration to 0.2% to 1.8% in a PMMA solution, the average diameter of the obtained CuSe nanofibers increases from 2.35 ± 0.80 µm to 10.25 ± 4.40 µm. It has been noted that the produced sample maintains uniform large-pore-sized mesoporous CuSe nanofibers and uniform 1D mesoporous NS with good purity.

3.3 Two-dimensional CuSe nanosheets

A nanosheet is a material with a high aspect ratio, flat surface, and nanoscale flake shape having a lateral size range of several tens of micrometers to the submicrometer level and a thickness of 1–10 nm.¹³⁵ Having low turbidity, excellent adhesion to substrates, and high smoothness,^222,223 the planar structure of 2D materials endows them with abundant surface atoms and certainly provides sufficient space for interface-related processes.^224,225 Complementing these, the significantly low thickness of 2D materials is another important structural feature. Upon reaching nanoscale and even atomic levels of thickness, photo-induced charge carriers can easily move from bulk to surface over a nanoscale distance.^226,227 This decrease in traveling length dramatically suppresses the electron–hole recombination during charge transport and preserves more charge carriers for surface reaction, resulting in improved efficiency for optoelectronics applications.^228,229 Considering that 2D NSs play a crucial role in altering the photogenerated carrier dynamics to improve the optical conversion efficiency of CuSe, particularly in energy and environmental applications, Zhang et al.¹⁷⁶ reported the fabrication of hexagonal-shaped nanosheets of CuSe and Cu_2−xSe with different phases by simply varying the reaction temperature and feeding ratio of Cu/Se through the green-acetate paraffin method without using any poisonous solvents. It was observed that the feeding ratio has a significant impact on the morphology of the prepared CuSe NSs. Specifically, lower effective concentrations decrease the nucleation rate of CuSe clusters, which, in turn, degrades the morphology homogeneity and forms two different types of clusters as a result: large plates and small plates. However, smooth nanoplates were preferred at greater feeding ratios and temperatures because of improved nucleation processes, as shown in Fig. 6(a). Lingzi Li et al.²³⁰ reported the creation of 2-D CuSe nanosheet arrays on Au-coated polyethylene terephthalate (PET) substrates by a facial electro-deposition method. Copper chloride (CuCl₂) and selenium dioxide (SeO₂) were utilized as CuSe source materials, resulting in thin, homogenous, distorted hexagonal-shaped CuSe nanosheet nanodiscs. In this study, the authors systematically explored the optimal conditions such as deposition voltage and temperature, for synthesizing highly crystalline CuSe nanosheets, as shown in Fig. 6(b). Their findings revealed a clear distinction in the role of these parameters: while the deposition voltage primarily influences the thickness of the CuSe layer, the temperature played a crucial role in determining the crystallinity of the final product. The interplay between process parameters and material characteristics is illustrated in Fig. 6(c), which shows the variation in deposition thickness as a function of deposition voltage. This work was well complemented by Jadhav et al.,²³¹ who reported on the use of chrono-amperometry technology in the fabrication of 2D CuSe nanosheets. Fig. 6(d) shows the schematic diagram of the reported synthesis process. The authors used stainless steel, platinum, and calomel electrodes as working, counter, and reference electrodes, respectively. The reported FESEM images, as shown in Fig. 6(e), revealed the 2D NSs with an average thickness of 34 nm and an estimated lateral size of 700 nm. Xue-Jun Wu et al.²³² reported the preparation of CuSe nanosheets, which were synthesized by a hot-injection method using copper(II) sulfate (CuSO₄), Se powder, and dichloride-diphenyl-trichloroethane (DDT) as precursor and capping agents, respectively. As shown in Fig. 6(f), the FESEM images show that the as-synthesized 2D CuSe nanocrystals were hexagonal or triangular in shape with an average lateral size of 0.6–1.6 µm. The different structural outcomes reveal that the addition of DDT was essential to preserving the CuSe nanosheet's structural integrity. The reported optical investigation, as shown in Fig. 6(g), revealed that the obtained NSs exhibited excellent absorption behavior within the visible range of the solar spectrum. In a report, the synthesis of 2D CuSe nanoplates using the co-precipitation method by simply varying the Cu/Se ratio was reported by Morteza Nouri et al.²³³ It was observed that the sample with a Se/Cu ratio of 0.6 has NPs as its morphology, but the sample with a ratio of 1.4 has hexagon-shaped nanodiscs. The reported FESEM images for these hexagonal-shaped nanodiscs are shown in Fig. 6(h). Additionally, the optical absorption analysis gives the estimated band gap value, ranging from 1.6 to 2.2 eV, revealing the strong potential of these NSs for solar energy conversion applications. Similarly, Poola et al.,⁷⁷ Chen et al.,²³⁴ and Xin Hou et al.²³⁵ reported employing a one-step hydrothermal process to synthesize 2D CuSe NSs at a comparatively low temperature by a one-step hydrothermal method. According to the SEM examination, the as-synthesized products had a hexagonal NS morphology and a 100–200 nm thickness for CuSe NSs with varying lateral dimensions. Recently, Nadhar Globoid²³⁶ and colleagues have reported on a method for creating 2D CuSe NSs by chemical deposition. The authors examined the effect of reaction media, such as pH, on the resulting CuSe nanocrystals' size and form. Interestingly, complexing agents were observed to affect the stoichiometry of products. For example, CuSe exhibits an optimal hexagonal crystal structure with a nanodisc morphology at pH = 10. Recently, Kumar et al.¹³⁴ have reported the fabrication of 2D CuSe nanosheets, demonstrating a simple yet effective strategy to tailor their longitudinal thickness by adjusting the pH of the reaction medium. This approach leverages H⁺-modulated nucleation and growth mechanisms, enabling control over the nanosheet morphology. The study identified an optimal thickness that facilitates efficient transfer of photogenerated electron–hole pairs to the surface, significantly reducing their recombination rate. This reduction is reflected in the photoluminescence (PL) spectra of the samples synthesized under varying reaction conditions, as shown in Fig. 6(j).

Fig. 6 (a) FESEM images of CuSe NSs.¹⁷⁶. (b and c) FESEM images and deposition characteristics of CuSe NSs, respectively.²³⁰. (d and e) Schematic of the fabrication techniques and formed CuSe NSs, respectively.²³¹. (f and g) FESEM images and optical absorption characteristics of CuSe NSs, respectively.²³² (h) FESEM images of CuSe NSs.²³³. (i and j) HRTEM images and PL spectra of the prepared CuSe NSs, respectively.¹³⁴ Reprinted with permission from (a) ref. 176 (Elsevier), copyright 2010; (b and c) ref. 230 (ACS), Open source 2017; (d and e) ref. 231 (Elsevier), Open access 2021; (f and g) ref. 232 (Wiley-AC), Open source 2014; (h) ref. 233 (Elsevier), copyright 2020; and (i and j) ref. 134 (Elsevier), copyright 2024.

3.4 Three-dimensional flower-like and other CuSe morphologies

Nonetheless, the application of semiconductor NSs is not limited to the above-mentioned morphologies. Due to the immense potential for various applications of nanostructure materials, researchers are trying to achieve more suitable NSs to assist these purposes. Various advanced structures have been designed in congruence with those of the well-accepted NSs reported so far in terms of their applications. A thermal evaporation method for depositing a skinny layer of CuSe on a glass substrate was described by Liew et al.²³⁷ The reported XRD pattern, shown in Fig. 7(a), reveals the successful fabrication of the crystal structure of CuSe. The AFM profile fit diagram shows that the deposited film has a smooth surface with a mean square roughness of 3.5 nm. Moreover, the authors investigated the effect of layer-by-layer deposition on the electric properties of the CuSe thin film. Recently, Keyan et al.²³⁸ have reported the synthesis of 3D NSs of Ag-doped CuSe nanocomposites via a simple one-step hydrothermal process. The reported FESEM images (Fig. 7(b) and (c)) show that the prepared samples have cauliflower-like NSs, where a thorough explanation of the produced CuSe NSs' electrochemical activity was offered. The results of the HRTEM analysis showed that the produced NSs have three-dimensional structures with an estimated 350 nm average dimension. Li et al.²³⁹ reported synthesizing a series of CuSe NSs by a chemical method using CuSO₄ and SeO₂ as their starting materials. The effectiveness of this synthesis route was revealed through SEM and TEM analyses, which showed that the CuSe sample exhibited two types of morphologies, as shown in Fig. 7(d) and (e), where larger-sized sheets and smaller-sized spherical bulks were visible. Interestingly, while investigating the effect of calcination on the morphology of prepared samples, it was observed that all the NPs of Cu_2−xSe disappeared and fused into a large bulk structure. Liu et al.²¹¹ utilized a one-pot rapid, facile, and green hydrothermal route for the synthesis of 3D CuSe NSs in which kiwi juice is employed as the coating and reducing agent. As shown in Fig. 7(f), the reported FESEM images reveal the dendritic with hierarchical side branch's structure. It was observed that the growth of the dendritic NSs is a time-dependent process. As the reaction proceeded, the initial CuSe experienced morphological evolution and got converted into perfect dendrites with a high density of secondary branches, and then got destroyed with a longer reaction time. Moreover, the changes in the dendritic CuSe patterns were observed with the increase in reaction temperature and branch arrays were developed better at 160 °C than at 130 °C. Recently, Sajjad et al.²⁴⁰ have reported a 3D CuSe NS synthesized via a facial wet chemical process using Se powder and CuCl₂ as starting materials. Fig. 7(g) and (h) show the reported EDS spectra and morphological SEM investigation shows that prepared samples exhibit micro-flower like morphology with no prominent residues and impurity structures. In particular, the author investigated the electrochemical behavior of prepared CuSe NS and reported the improved capacitance, excellent durability, and superb stability of the electrode coated with CuSe-nanoflowers (NFs). Mao et al.²⁴¹ reported a green hydrothermal template-free method to prepare single-crystalline superstructures of fern-like CuSe, as shown in Fig. 7(i), by using polyvinylpyrrolidone (PVP) as both the reductant and the surfactant. In a particular work, the authors investigated the effect of different synthesis parameters such as precursor molar ratios, precursor combinations, and the molecular weight of PVP on morphologies, phases and structural forms of CuSe. The authors provided a detailed explanation of the effect of PVP concentration on the morphology, concluding that PVP acts primarily as the reducing agent. Notably, CuSe exhibits a fernwort-like morphology even at low concentrations of PVP. The reported optical absorption spectrum (Fig. 7(j)) reveals strong absorption in the visible range. Shu et al.²¹⁰ used orange juice as the reducing agent in their study. The FESEM investigation revealed that by varying the volume of orange juice, three types of CuSe with different morphologies, namely dendrites, nanoflowers and nanospheres, were synthesized, as shown in Fig. 7(j) and (k). The authors came up with concluding that synthesis parameters such as reaction time, reaction temperature, precursor ratio, vitamin C, and carbohydrates present in fruit juices can be used as reducing and capping agents to achieve some special types of microstructure architecture of CuSe nanoparticles.

Fig. 7 (a) XRD pattern of the prepared CuSe NSs.²³⁷. (b and c) FESEM images for CuSe nano-cauliflowers.²³⁸ (d) FESEM image of CuSe NSs. (e) HRTEM images of CuSe NSs.²³⁹ (f) FESEM image of CuSe NSs.²¹¹ (g) FESEM images of CuSe NSs. (h) EDS spectra of the prepared CuSe NSs.²⁴⁰ (i) FESEM images of CuSe NSs. (j) Optical absorption spectra of CuSe NSs.²⁴¹. (k and l) FESEM images of CuSe NSs.²¹⁰ Reprinted with Permission from (a) ref. 237 (Springer), Open access 2009; (b and c) ref. 238 (Springer), Copyright 2023; (d and e) ref. 239 (RSC), Open access 2021; (f) ref. 211 (ACS), Copyright 2017; (g and h) ref. 240 (Elsevier), Copyright 2022; (i and j) ref. 241 (Springer), Copyright 2016; (k and l) ref. 210 (Elsevier), Copyright 2022.

4 Multifunctional applications of CuSe semiconductor NSs in energy, environment, and biomedical research through solar energy conversion

4.1 Photocatalytic performance

The schematic diagram depicting the photocatalytic process along with the required band edge potentials is shown in Fig. 8(a). The last section depicted above shows the basic properties of various CuSe NSs, having a significant impact on the optical and structural properties. Therefore, many researchers have investigated the role of CuSe NSs as photocatalysts for the photodegradation of probe pollutants under different irradiation sources. For example, Kaviyarasu et al.²⁴² reported the PC performance of CuSe QDs prepared via the reflux condensation method for the photodegradation of the well-established organic pollutant Rhodamine Blue (RhB) dye under UV and visible light irradiations. The photocatalytic performance of the samples revealed that the as-synthesized CuSe nanosphere exhibited better photocatalytic degradation of RhB dye by almost 99.1% in 6 hours at a degradation rate of 0.27 min⁻¹. Liu et al.²¹¹ reported that a low E_g value of 1.57 eV and high surface area of dendritic CuSe with hierarchical side-branch NSs were highly favorable for the degradation of MG dye in aqueous solutions under natural sunlight irradiation. The authors present an investigation on the production and involvement of different radicals in the MG photodegradation process using ESR analysis utilizing TEMPO and DEMPO as solvents. However, Li et al.²²⁵ successfully synthesized CuSe hierarchical NSs on the surface of TiO₂ NTs by a facile and green method. The FESEM image displayed in Fig. 8(b) shows rose-like CuSe hierarchical NSs. Furthermore, the photocatalytic activity of the prepared CuSe-sensitized TiO₂ NSs was examined using a persistent organic pollutant (POP), anthracene-9-carboxylic acid (ACA), under simulated solar light irradiations, indicating an improved photocatalytic degradation performance with an excellent recyclability up to 10 cycles. The obtained absorption spectra collected at different intervals of light irradiation, as shown in Fig. 8(c), demonstrate the successful degradation of the pollutant. The improved degradation process was attributed to efficiently inhibiting the recombination of photogenerated electron–hole pairs and prolonging the life of the photogenerated carriers. Farooq et al.²⁴³ reported the fabrication of hexagonal shape 2D NSs of CuSe for the degradation of Estron (E1), a major endocrine-disrupting chemical (EDS). In particular, the authors investigate the adsorption and photocatalytic degradation behavior of the prepared CuSe nanosheets, as shown in Fig. 8(d). It was observed that the prepared samples show an excellent adsorption property with an adsorption capacity of 1.44 mg g⁻¹, with an adsorption rate constant of 0.037 min⁻¹, showing complete adsorption in 30 minutes. For photocatalytic degradation under visible light irradiation, the observed degradation rate constant is 0.013 min⁻¹, and the samples show more than 90% degradation in 60 minutes. Moreover, it has been observed that the prepared CuSe samples have excellent recyclability. To compare the effectiveness of traditional photocatalysts, the authors proposed a photodegradation comparison of the well-established photocatalyst TiO₂. They observed an excellent photocatalytic degradation efficiency of the CuSe photocatalyst. Recently, aiming to improve the NIR range absorption of CuSe nanosheets, Kumar et al.¹⁷⁵ reported the fabrication of cobalt (Co)-doped CuSe nanosheets and investigated the effect of Co-doping concentration on their optical and photocatalytic properties. Photocatalytic performance improved to a certain Co-doping level. Still, it declined beyond that, attributed to Co-induced crystal defects acting as recombination centers for photogenerated charge carriers after a certain doping level. The estimated variation in band gap and rate constant with different doping concentrations for the prepared CuSe nanosheets is shown in Fig. 8(f). At a higher Co content, the emergence of a cubic phase further reduced the photocatalytic efficiency, highlighting the superior performance of the hexagonal crystal phase of the CuSe photocatalyst. The same research group also investigated the impact of iron doping, particularly in the Fe³⁺ oxidation state, on modifying lattice parameters to enhance light absorption and improve photocatalytic performance.¹⁷⁴ In this study, the authors established the fundamental relationship between lattice parameter modifications and their effect on the E-k diagram, demonstrating how such changes improve NIR absorption in CuSe photocatalysts. They further examined the CuSe nanosheets' photocatalytic activity and demonstrated effective degradation of various organic pollutants, with improved rate constants illustrated in Fig. 8(e). Beyond the studies discussed above, numerous research groups have investigated and reported the photocatalytic properties of CuSe NSs, with their crucial reported parameters summarized in Table 2. These findings highlight the potential of CuSe NSs as a next-generation, cost-effective, and highly efficient photocatalyst, offering promising avenues toward sustainable energy solutions.

Fig. 8 (a) Schematic of the photocatalytic process. (b and c) FESEM images and absorption spectra of the collective sample at different intervals of light irradiation for anthracene-9-carboxylic acid (ACA) using a CuSe@TiO₂ nanoflower.²⁶⁵ (d) FESEM images and the observed adsorption capability of CuSe nanosheets.²⁴³ (e) Variation in rate constant for the photodegradation of various organic pollutants with Fe³⁺-doping in CuSe nanosheets.¹⁷⁴ (f) Band gap variation and rate constant variation with Co-doping concentrations for CuSe nanosheets.¹⁷⁵ Reprinted with permission from (b and c) ref. 265 (Elsevier), Copyright 2022; (d) ref. 243 (Elsevier), Copyright 2023; (e) ref. 174 (Elsevier), Copyright 2024; (f) ref. 175 (Elsevier), Copyright 2025.

4.2 Photovoltaic (PV) applications

As discussed in the previous sections, the stoichiometric compositions CuSe with a hexagonal crystal phase exhibit particularly favorable conditions for natural solar-driven photocatalysis. Besides its role in photocatalysis, the naturally low band gap of CuSe makes it a promising material for converting solar energy into electricity through PV cells. The schematic representation of the solar energy conversion process using PV technology is presented in Fig. 9(a). By carefully modifying the electronic properties of CuSe, particularly through compositional tuning, it can achieve the necessary conductivity, making it a strong candidate for effective CE performance in quantum dot-sensitized solar cells (QDSCs). To serve as an ideal CE, a material must possess high electrical conductivity.^266,267 Thus, in the realm of photovoltaic application, the focus shifts toward non-stoichiometric compositions of CuSe, which have been found to exhibit significantly improved conductivity. For example, Bo et al.²⁶⁸ conducted a systematic study to compare the power conversion efficiency (PCE) of CuSe-based CE with that of the conventional CE commonly used in QDSCs, such as platinum (Pt) and copper sulfide (Cu₂S). In this work, the researchers introduced a novel, cost-effective method for fabricating CuSe films directly on a red Se substrate. A schematic representation of the fabrication process is presented in Fig. 9(b). The performance evaluation revealed that the QDSC assembled with a Cu_2−xSe CE achieved a notable average PCE of 3.80%, slightly higher than the 3.72% PCE obtained from QDSCs utilizing optimized Cu₂S-brass CEs. In stark contrast, the conventional Pt-based CE, used as a reference under identical conditions, demonstrated a significantly lower average PCE of only 1.06%. The obtained Tafel polarization curve for the fabricated electrode is shown in Fig. 9(c). This improvement in conversion efficiency highlights the potential of Cu_2−xSe as a superior and economically viable alternative to traditional noble metal-based CEs in QDSCs. Similarly, Xu et al.²⁶⁹ reported the fabrication of Cu_2−xSe nanoflakes with a cubic crystal structure and explored their application as the CE in ZnSe/CdSe-QDSCs. The FESEM images, along with the electrochemical characterization, shown in Fig. 9(e), demonstrated excellent conductivity of the Cu_2−xSe CE. This high conductivity highlights the suitability of these NSs as CE for PV applications. Furthermore, performance evaluation revealed a significant enhancement in device metrics, including fill factor (FF) and short-circuit current density (J_SC), ultimately leading to a remarkable improvement of 128% in the PCE compared to Pt CEs. Building upon these findings, Zhang et al.²⁷⁰ extended the investigation by exploring the effect of different non-stoichiometric compositions of CuSe on the optoelectronic performance of QDSCs CEs. In their investigation, the authors specifically examined the impact of different Cu : Se feeding ratios, crucial to prepare different non-stoichiometric compositions of CuSe (detailed discussion is provided in Sections 1 and 2 of this article), on the PCE using Cu_2−xSe CEs in CdSe/CdSeTe-based QDSCs. Among the tested compositions, the most significant performance improvement was achieved using a Cu : Se feeding ratio of 1 : 4. The QDSCs fabricated with these Cu_2−xSe/FTO electrodes exhibited a remarkable PCE of ∼7%, demonstrating the superior functionality of this non-stoichiometric variant. The detailed PV parameters for the optimized 1 : 4 Cu : Se ratio: a J_sc value of 16.08 mA cm⁻², an open-circuit voltage (V_oc) of 0.643 V, an FF value of 62.76%, and an overall PCE of 6.49% in some configurations. To further interpret the PV characteristics of the different CEs, the results are shown in Fig. 9(f). The order of 1 : 4 > 1 : 6 > 1 : 2 could be determined from the slopes in the anodic and cathode branches, indicating that the exchange current densities (J₀) are in the same order, and thus revealing the most minor charge transfer resistance for 1 : 4 and the largest one for 1 : 2. These obtained outcomes reveal the highest conductivity of the CuSe sample prepared with a Cu : Se feeding ratio of 1 : 4. Additionally, the superior performance of the 1 : 4 ratio was further validated by incident photon-to-current efficiency (IPCE) measurements. The Cu_2−xSe CEs prepared with this ratio achieved an IPCE of ∼80% within the 400–650 nm wavelength range, highlighting its excellent light-harvesting and electron transfer capabilities. Moreover, several research groups have explored and reported various chemical compositions including both stoichiometric and non-stoichiometric forms, as counter electrodes for PV. Key parameters such as the type of PV system, PCE, FF, and their corresponding advantages are summarized in Table 3.

Fig. 9 (a) Schematic showing the PV mechanism. (b) Synthesis scheme and photographs of Cu_xSe CEs on FTO substrates. (c) Tafel curves of different CEs in a 2 M Na₂S/S electrolyte (−0.6 to +0.6 V).²⁶⁸ (d) Transverse current–voltage characteristics of CuSe thin films.²⁷² (e) FESEM images and CV curve for CuSe nanosheets.²⁶⁹ (f) IPCE spectra and J–V characteristics of CdSe QDSCs with Cu_xSe/FTO CEs.²⁷⁰ Reprinted with permission from (b and c) ref. 268 (RSC), Copyright 2014; (d) ref. 272 (Springer), Copyright 2023; (e) ref. 269 (Wiley), Copyright 2015; and (f) ref. 270 (Elsevier), Copyright 2016.

Table 3 Photovoltaic performance of CuSe NS-based devices

Electrode material	QCDS (sensitizer@base material)	Utilized as	Merits	PCE (%)	FF	Ref.
CuSe	CdSe@TiO2	CE	Having a net-like morphology results in low resistance with improved catalytic activity	5.77	60	285
CuSe	CdS@TiO2	CE	Showing ∼2 times PCE compared to a conventional Pt electrode	1.68	58	189
Cu2Se	CdS/CdSe@ZnO	CE	J sc observe to increase from 6.02 mA cm^−2 to 11.26 mA cm^−2 (compare to Cu2S)	2.68	—	286
Cu3Se2	CdSeTe	CE	Good conductivity, stability, and high catalytic activity provided by EIS and Tafel plot	8.72	60.73	270
CuSe/Cu0.78S	CdS/CdSe@TiO2	CE	With varying Cu/S ratio-Jsc, Voc, FF, and PCF changes	4.82	40	287
Cu2S/Cu2Se	CdSe@ZnO	CE	J sc observed to increase ∼4 times compared to the individual	4.60	40	288
Graphene/Cu2−xSe	ZnCulnSe@TiO2	CE	Low charge transfer resistance	6.67	45.10	289

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In addition to these studies on the utilization of CE, numerous research groups have detailed the investigation of the utilization from a solar cell application point of view. Ambade et al.²⁷¹ used the chemical deposition approach to manufacture nanocrystalline thin films of CuSe on glass and tin-doped indium oxide substrates. The absorption characteristics and photocurrent measurements indicate that it is highly suitable for a broad solar spectrum. This work is well complemented by Arora et al.²⁷² The CuSe layers with a thickness of 150 nm have been deposited onto cleaned glass and ITO substrates using the resistive heating thermal evaporation technique. The conductivity outcomes, shown in Fig. 9(d), used a two-probe method, revealing an increase in conductivity with the increase in annealing temperature, giving a positive sign to utilize CuSe thin films for solar PV application. However, to examine the impact of thickness and Y-doping on the optical and electric properties of CuSe samples, Akpu et al.²⁷³ fabricated the pure and Y-doped CuSe thin films of varying thickness on glass slides by spray pyrolysis. The E_g value of the prepared sample was observed to drop from 3.0 eV to 2.7 eV when the doping concentration increases. It has been observed that the electrical parameters were significantly influenced by the Y-doping and thickness of CuSe films. As the Y concentration in CuSe increases from 0 to 0.04 mol%, the resistivity increases from 3.450 × 10⁻⁶ Ω m to a maximum value of 7.704 × 10⁻⁶ Ω m with the increase in thickness from 152 nm to 198 nm and decreasing conductivity values of 2.898 × 10⁵ (Ω m)⁻¹ to 1.298 × 10⁵ (Ω m)⁻¹.

Recently, chalcopyrite solar cells, named so because of the thin absorber layer of Cu-based chalcopyrite materials used in these cells, are one of the frontrunners in thin-film PV technology owing to their tunable direct bandgap, significant absorption coefficient, and long-term stability.^274,275 Among all Cu-chalcopyrite materials, copper indium selenide (CISe) and copper indium gallium selenide (CIGSe) are most suitable for use as a light-absorbing layer.^276,277 The role of secondary CuSe phase in enhancing the performance of multiphase CuInSe₂ for PV applications is of significant importance in the quest for efficient and sustainable solar energy technologies.²⁷⁸ CuInSe₂ is a well-known semiconductor material with favorable optoelectronic properties and has received considerable attention in the last few years in solar cell applications.²⁷⁹ However, the introduction of a secondary CuSe phase offers several advantages. First, it serves as a potential carrier of excess Cu atoms, thereby reducing the formation of detrimental defects in the CuInSe₂ lattice.²⁸⁰ This results in improved charge carrier mobility and reduced recombination losses, leading to enhanced device performance. Moreover, the presence of CuSe can modify the band structure of CuInSe₂, tailoring its optical and electronic properties to better match the solar spectrum.²⁶⁰ Additionally, the formation of heterojunctions between CuInSe₂ and CuSe phases can facilitate efficient charge separation and transport, further boosting the device efficiency.²⁸¹ Furthermore, the incorporation of CuSe phase can enhance the stability and reliability of CuInSe₂-based solar cells by mitigating degradation mechanisms such as Cu diffusion and grain boundary reactions.²⁸² Overall, the synergistic interaction between CuInSe₂ and CuSe phases holds great promise for advancing the efficiency, stability, and cost-effectiveness of PV devices, contributing to the widespread adoption of solar energy as a clean and renewable energy source.^80,283,284

4.3 Energy storage

As observed in the previous section, electrochemical investigations using cyclic voltammetry (CV) have demonstrated these nanomaterials' excellent conductivity and stability. These properties support their application in an important component (counter electrode) for PV cells and open new avenues for their use in energy storage systems.²⁹⁰ Therefore, CuSe NPs have attracted considerable attention as efficient materials for next-generation energy storage technologies. Pazhamalai et al.²⁹¹ reported the fabrication of a CuSe₂/Cu electrode directly on a Cu-foil substrate using a Se precursor and Cu source by a simple hydrothermal method. Fig. 10(a), (inset) presents the images of the Cu foil before and after the hydrothermal process, illustrating the successful formation of a binder-free, hierarchical nano-needle-like structure. Electrochemical characterization using CV, as shown in Fig. 10(b), exhibited distinct redox peaks, confirming the pseudocapacitive behavior of the electrode. Galvanostatic charge–discharge (GCD) curves further validated this. Together, these analyses reveal that the specific capacitance of the resulting structure is approximately 1040 F g⁻¹. The obtained results demonstrate the excellent potential of CuSe NSs for electrochemical applications. However, despite the promising outlook of Cu-based selenide catalysts, several inherent challenges remain. Primarily, the limited number of active sites restricts effective contact with the electrolyte, thereby reducing the electrochemical activity.^292–295 In addition, the intrinsically low electrical conductivity of pristine CuSe, arising from its semiconducting nature, hampers rapid electron transport and consequently limits the overall electrochemical performance.^183,296,297 To overcome these limitations, strategic coupling of CuSe with conductive or catalytically active materials can significantly enhance the density of active sites and facilitate faster charge transfer, ultimately improving the overall electrochemical efficiency.^298–301 Tan et al.³⁰² reported the fabrication of CuSe-decorated CNTs as a cathode in microbial fuel cells (MFCs), showing significantly improved oxygen reduction reaction (ORR) activity compared to individual CNTs and CuSe in the natural electrolyte. Various mass ratios of CuSe@CNT composites were synthesized and fabricated as electrodes to optimize their performance for the enhanced ORR activity and as electrode materials in microbial fuel cells (MFCs). The authors observed that the reduced particle size of CuSe within the hybrid structure facilitates greater exposure of active sites. Furthermore, electrochemical impedance spectroscopy (EIS) analysis revealed that the specific hybrid composition exhibits a significantly improved charge transfer process compared to pristine CuSe. As shown in Fig. 10(c), the sample with a CuSe@CNT ratio of 1 : 1 achieved a higher power density of 425.9 ± 5 mW m⁻², which is approximately 1.90 and 1.65 times higher than that of CNTs (244.0 ± 4 mW m⁻²) and CuSe (258.8 ± 6 mW m⁻²), respectively. The combined enhancement in active surface area and charge transport efficiency contributes to the superior electrochemical activity of the CuSe nanostructure. It suggested that CuSe@CNT is a promising ORR catalyst for MFC applications. The schematic representation of the proposed composite architecture and the mechanism responsible for the improved activity is illustrated in Fig. 10(d). Recently, to introduce the aforementioned properties (enhanced charge transfer and increased active surface area), Karuppasamy et al.¹¹² employed a highly conductive carbon network of graphene to synthesize a CuSe–CoSe₂ nanocomposite decorated on graphene oxide (CCS@GO). The observed FESEM image for the formed NSs is shown in Fig. 10(d). The CV and GCD profiles of the prepared NSs, as shown in Fig. 10(d), highlight the high performance of the hybrid supercapacitor, demonstrating a specific capacitance (C_sp), energy density, and power density of 192.8 F g⁻¹, 54.6 W kg⁻¹, and 700 Wh kg⁻¹, respectively. Furthermore, to evaluate the practical applicability of the CCS@GO composite, a hybrid asymmetric supercapacitor was fabricated and demonstrated for its electrochemical performance. As illustrated in Fig. 10(d), the CCS@GO material served as the positive electrode, while commercially available high-porosity activated carbon (AC) with a large surface area was employed as the negative electrode. The assembled device exhibited remarkable cycling stability, retaining 83% of its specific capacitance even after 10 000 charge–discharge cycles, highlighting its excellent potential for practical energy storage applications. The authors concluded that the enhanced electrochemical performance of the hybrid material can be attributed to its improved porosity and higher electrical conductivity compared to the individual counterparts. The CCS@GO composite exhibited a notably low charge-transfer resistance (0.52 Ω), which is ascribed to the synergistic interaction between CuSe and CoSe₂ uniformly anchored on the conductive GO sheets, thereby facilitating efficient charge transport and superior electrochemical activity. Very recently, Manikandan et al.³⁰³ have developed a CuSe@MXene (CMxene) hybrid, utilizing the high surface area, excellent conductivity, and abundant active sites of MXenes to enhance the electrochemical performance. The composite exhibited larger CV peak currents and broader redox areas than those of pristine CuSe, indicating superior charge storage capability. GCD analysis at 1 A g⁻¹ confirmed its pseudocapacitive behavior with excellent coulombic efficiency. A slight shift in redox peaks upon MXene integration revealed strong interfacial interaction, while the synergistic effect between conductive Ti₃C₂T_x sheets and CuSe active sites significantly improved ion diffusion and charge transfer, resulting in enhanced specific capacitance. For the utilization of CuSe characteristics for developing high-performance battery materials, Yang et al.¹⁸⁴ reported the successful utilization of the mesoporous structure developed in CuSe for the efficient adsorption of magnesium (Mg)-ions, enabling electric double-layer capacitor (EDLC)-type behavior. This approach facilitated the design of an effective, low-cost electrode for rechargeable Mg-batteries. The authors presented a compelling study by constructing a model Mg-battery system and systematically investigating its performance, offering valuable insights into its potential for real-world energy storage applications. The effect of different current densities (10, 20, 5,0, 100 mA g⁻¹) on different performance parameters such as reversible current density, stable capacity, long-term stability, and reversible capacity of CuSe NPs as cathode materials for reversible rechargeable Mg-battery was investigated. The CV curves, shown in Fig. 10(e), exhibit overlapping reversible oxidation and reduction peaks across multiple cycles, highlighting the excellent electrochemical reversibility and stability of the CuSe electrode. The CuSe NPs, as cathode materials for rechargeable Mg-batteries at a current density of 20 mA g⁻¹, can display a remarkable reversible specific capacity of 241.2 mAh g⁻¹ at room temperature. Furthermore, the stable capacity of CuS reveals 200 mAh g⁻¹ at 50 mA g⁻¹ (60 cycles). The CuSe NPs provide long-term cycling stability with reversible capacity maintained at 107.7 mAh g⁻¹ at a current density of 100 mA g⁻¹ over 150 cycles. In addition to the above-discussed reports, extensive research underscores the potential of CuSe-based materials for energy storage applications in batteries and supercapacitors. The reported key performance parameters are summarized in Table 4, demonstrating their suitability for both applications. By utilizing various synthesis strategies and combining CuSe with other materials, these researchers have demonstrated significant improvements in energy density, power density, cycling stability, and overall device efficiency, further solidifying CuSe NPs' role as a promising candidate for next-generation energy storage technologies.

Fig. 10 (a) High-magnification FESEM images of the CuSe₂/Cu foil. (b) Cyclic voltammetry profiles of the CuSe₂/Cu electrode at different scan rates.²⁹¹ (c) Power density comparison of different cathode catalysts.³⁰². (d) Schematic of the hybrid asymmetric supercapacitor using a CCS@GO positive electrode and activated carbon negative electrode, with galvanostatic charge–discharge profiles of CCS@GO at different current densities.¹¹² (e) CV curves and charge–discharge voltage profiles of the CuSe nanoparticle electrode at different cycles (50 mA g⁻¹).¹⁸⁴ Reprinted with permission from (a and b) ref. 291 (Elsevier), Copyright 2016; © ref. 302 (Elsevier), Copyright 2016; (d) ref. 112 (Elsevier), Copyright 2022; and (e) ref. 184 (Elsevier), Copyright 2019.

Table 4 Energy storage performance of CuSe-based NSs

Supercapacitor characteristics
Electrode material	Specific capacitance (F g^−2)	Solution resistance (Rs Ω cm^−2)	Charge transfer resistance (Rct Ω cm^−2)	Energy density/Power density (Wh Kg^−1/W Kg^−1)	Stability (capacitance retention/cycle)	Ref.
CuSe	490	3.24	6.46	—	88/11 090	240
CuSe/Mxene	570	1.52	1.37	59.38/800	98/1000	303
Cu2Se/GO	219	0.56	1.45	21.1/400	91/10 000	304
β-Cu2Se	117	0.7	1.8	48/5500	85/10 000	305
Cu3Se2@Cu	928	0.58	1.58	30.8/461	90/5000	306
CuSe2@rGO	104	0.83	3.50	28.3/1538	88/5000	307
CuSe/CoSe2@GO	192	0.96	0.52	54.6/700	87.5/10 000	112

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Battery type characterizations
Electrode material	Electrode	Battery type	Capacity (mAh g^−1)	Remarks	Ref.
CuSe	—	Aqueous battery	285	A CuSe//Zn hybrid pouch cell was designed as a proof-of-concept	308
Cu1.8Se	Cathode	Li–S ions	6.07 mAh cm^−1	Optimization in the crystal phase and Cu-vacancy promotes effective LiPS trapping and Li–S reaction kinematics	309
β-Cu2Se	Cathode	Mg-ions	240	A reduction in crystallinity of β-Cu2Se improves capacity from 120 to 240 mAh g^−1	310
CuxSe	Anode	Li-ions	645	Having stable reversible capacity at a high current density of 30 A g^−1	311
Cu2−xSe	Anode	Zn-ions	114.8	Cubic crystal phase providing excellent conductivity and negligible capacity loss (0.0002/cycle)	312
Cu2−xSe	Anode	Zn-ions	140	Highly flexible with 91% capacity retention after 20 000 cycles	313
Cu2Se/CoSe	Cathode	—	108.6	Show excellent initial capacity retention of 113% after 10 000 cycles	314

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4.4 Biomedical sciences

Copper and its compounds such as Cu₂O and CuO have gained significant attention in biomedical applications due to their excellent antibacterial, antifungal, antiviral, and anti-inflammatory properties. The redox activity of copper ions (Cu⁺/Cu²⁺) enables the generation of reactive oxygen species (ROS), which play a vital role in microbial killing and cancer cell apoptosis.^87,315,316 Despite these advantages, the primary concern is their potential cytotoxicity, especially at high concentrations or prolonged exposure, which can damage healthy cells and tissues.^317–319 The uncontrolled release of copper ions can lead to oxidative stress, inflammation, and disruption of normal cellular functions. However, cytotoxicity concerns are significantly reduced in Cu-based selenides, particularly due to the relatively low toxicity of the CuSe complex, making it a favorable candidate for various biomedical applications.^320,321 Their tunable optical properties make them suitable candidates for imaging modalities like photoacoustic and fluorescence imaging, enabling non-invasive visualization of biological tissues with high sensitivity and resolution.^322,323 Additionally, CuSe NPs can serve as effective drug delivery carriers due to their large surface area, high drug loading capacity, and controlled release capabilities, facilitating targeted and sustained drug delivery to specific sites within the body.^324,325 Very importantly, under controlled reaction conditions, a p-type surface charge can be induced on CuSe, which promotes strong electrostatic interactions with the naturally n-type surface charged bacterial membranes.¹¹⁹ This enhanced attraction promotes effective bacterial adsorption, ultimately leading to their destruction and underscoring CuSe's strong antibacterial potential. Owing to these promising intrinsic properties, CuSe has gained considerable interest in biomedical research, where several groups have explored and reported its use-along with its modified derivatives-as a potential material for diverse biomedical applications.

4.4.1 In vitro studies. Shi et al.¹¹⁹ report the electrochemical synthesis of electro-stimulated CuSe (E-CuSe), which possesses a positively charged surface specifically engineered to improve the antibacterial efficacy. The schematic diagram showing the synthesis and produced surface positive charge is shown in Fig. 11(a). The zeta potential measurements (Fig. 11(b)) reveal a distinct contrast between the negatively charged surface of pristine CuSe and the positively charged surface of E-CuSe. The electric field, induced due to the positive surface of E-CuSe, facilitates targeted negative bacterial capture, triggering a localized and controlled release of Cu-ions. This targeted interaction significantly improved bactericidal activity making E-CuSe highly effective against pathogenic bacteria. The authors also investigated the comparative cytotoxicity of CuSe with conventional Cu₂O, as illustrated in Fig. 11(c). The results revealed that the synthesized CuSe NPs exhibited significantly lower cytotoxicity than the conventional Cu₂O, highlighting its superior biocompatibility and potential suitability for biomedical applications. Recently, Patel et al.¹²⁰ have reported the antimicrobial and antioxidant properties of pristine, Ni-and Zn-doped CuSe NPs for antimicrobial effects against S. aureus, Pseudomonas aeruginosa, Proteus vulgaris, Enterobacter aerogenes, and E. coli bacteria. It was observed that the Zn-doped CuSe NSs demonstrated the highest sensitivity against all bacteria. In a recent study, Padilla et al.³²⁶ have reported the synthesis of Cu_2−xSe NPs and carried out in vitro antimicrobial investigations against S. aureus, E. coli, and C. albicans using the agar well diffusion method. The obtained CuSe NSs are shown in Fig. 11(d). Remarkably, they found that even a minimal concentration, just 1 wt% of CuSe NPs relative to the bacterial content, was sufficient to completely eliminate the microbial colonies, demonstrating the exceptional antibacterial efficacy of Cu_2−xSe NPs. The in vitro test results obtained at different concentrations of CuSe are presented in Fig. 11(e). Complementing these findings, Li et al.¹¹⁸ explored the antimicrobial and photothermal antibacterial activity of CuSe NSs. Their study revealed that a concentration of 40 µg mL⁻¹ of CuSe could completely eradicate the bacteria (1 × 10⁵ CFU mL⁻¹) under normal conditions. More interestingly, when the bacterial culture was exposed to light irradiation, the photothermal effect of CuSe significantly enhanced its antibacterial performance, enabling complete bacterial destruction at a reduced concentration of only 20 µg mL⁻¹. These studies collectively highlighted the synergistic effects of CuSe's intrinsic antimicrobial properties and its strong photothermal response. Various research groups, listed in Table 5, have demonstrated and reported the antimicrobial activities of CuSe NSs showing great promise as multifunctional materials for next-generation biomedical applications.

Fig. 11 (a) Schematic of the preparation of electrolytic CuSe (E-CuSe). E-CuSe was collected on the platinum sheet at the cathode. The possible crystal structures of E-CuSe are shown in the dashed box. (b) Zeta potential information of CuSe (blue) and E-CuSe (purple) in the physiological pH range. (c) Survival rate of L929 cells treated with different concentrations of Cu₂O and E-CuSe.¹¹⁹ (d) HRTEM image of the obtained CuSe nanostructure. (e) Antibacterial activity of the prepared CuSe nanostructure at different weight percentages against S. aureus bacteria.³²⁶ Reprinted with permission from (a–c) ref. 119 (Elsevier), Copyright 2023; and (d and e) ref. 326 (MDPI), Open access 2024.

Table 5 Applications of CuSe NSs in biomedical applications

Nano conjugate	Photothermal conversion efficiency	Bacteria & other	Mechanism	NIR laser (nm)	Effectiveness (bacteria kill %)	Ref.
CuSe	45	MRSA	PTT, PDT	1064	>99	118
		E. coli
Bio-CuSe	76.9	Tumor ablation	PTT	1064	>87	327
Bio-CuSe/PVDF membrane	30	E. coli	PTT	1064	>97.5	117
		B. subtilis
Cu2Se	61.16	S. aureus	PTT, a bacteriostatic agent	1064	>72	329
		E. coli
Cu2Se	50.89	Modal 4T1 tumors	PTT, CDT	1064	Highly effective	330
Cu2Se	51	HeLa	PTT	808	Highly effective	331
		Cancer cell
Cu2−xSe	22	Colorectal cancer cell (HCT-116)	PTT	800	Highly effective	83
Cu2−xSe	42	Bladder cancer	PTT	1064	Highly effective	332
Cu2−xSe	—	—	Cu^2+-ions release	Recover Cu^2+ deficiency		333
				Treatment of fatty liver

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4.4.2 In vivo studies. Building upon the strong in vitro antibacterial outcomes, several research groups have extended their investigations to in vivo studies using living organisms. These studies aimed to evaluate the biocompatibility, therapeutic efficiency, and potential biomedical applications of CuSe-based materials under physiological conditions, thereby providing deeper insights into their practical applicability in real biological environments. Li et al.¹¹⁸ investigated and reported the photothermal and photodynamic therapeutic properties of CuSe nanosheets. The photothermal analysis results reveal that even at a low concentration of 60 µg mL⁻¹, the CuSe dispersion exhibits a rapid temperature rise and reaches up to 65 °C within just 5 minutes when irradiated with a laser source (1064 nm, 2 W cm⁻²). The in vivo studies demonstrate the excellent photothermal conversion efficiency of CuSe NSs, highlighting their strong potential for photothermal therapeutic applications. The schematic representation of their in vivo experimental setup is illustrated in Fig. 12(a). The study demonstrated that CuSe NSs exhibited remarkable wound-healing efficacy against MRSA-infected wounds (1 cm² area, 50 L bacterial suspension containing 1 × 10⁹ CFU) in rats. Under near-infrared (NIR) light irradiation, the CuSe-treated wounds showed complete healing within just 8 days, confirming the material's strong photothermal-assisted antibacterial and regenerative capabilities. The author claimed that CuSe induced photodynamic conversion of O₂ into highly toxic reactive oxygen species (ROS), which can cause significant oxidative stress and damage to bacteria. In addition, CuSe NSs can efficiently consume glutathione (GSH) at bacterial infection sites, thus further enhancing their sterilization efficacy. Furthermore, cytotoxicity assessments performed on the treated rats revealed no observable adverse effects on major organs or blood cells, emphasizing the excellent biocompatibility and biosafety of CuSe nanosheets for potential biomedical and therapeutic applications. Complementing this, Shi et al.¹¹⁹ conducted in vivo experiments on rats infected with ESBL E. coli. Substantial healing was observed within 7 days of post-treatment. Cytotoxicity assessments were carried out to ensure biosafety by analyzing blood samples from the treated rats. The results indicated no adverse effects on major organs such as the liver, kidneys, or overall blood cell health, suggesting excellent biocompatibility. Furthermore, transcriptome analysis was performed to examine the potential for genetic disruption. This analysis revealed no detectable alterations in DNA structure, reinforcing the genetic safety profile of the CuSe nanosheets. In a related study, Patel et al.¹²⁰ investigated the effect of Zn and Ni doping on the biocompatibility and toxicity of CuSe NPs through in vivo experiments conducted on Artemia salina (brine shrimp). The detailed experimental procedures and analytical methods are described in their original report.¹²⁰ The in vivo lethality assay (LC), which determines the LC₅₀ value (the concentration required to kill 50% of the organisms), revealed that the undoped CuSe NPs exhibited an LC₅₀ value of approximately 25 µg mL⁻¹. The mortality rate represents the percentage of organisms (e.g., brine shrimp in this study) that succumb at a given NP concentration, thereby indicating the toxicity level and the amount of CuSe NPs required to cause lethality in the tested organisms. The relationship between the mortality rate and the LC₅₀ value is observed to decrease progressively with Zn and Ni doping, indicating that doping enhanced the intrinsic toxicity of CuSe NPs. This increase in toxicity suggests that metal ion substitution within the CuSe lattice alters the physicochemical and surface properties of the NPs, thereby influencing their biological interactions and cellular responses. Wang et al.³²⁷ synthesized biogenically derived Cu_2−xSe NPs and systematically optimized the Cu : Se precursor ratio to enhance their photothermal efficiency. The HRTEM image of the prepared samples shows the formation of uniform, ultra-small NPs, a size regime particularly favorable for biomedical applications. The photothermal evaluation of these NPs, also presented in Fig. 12(b), reveals the strong dependence of photothermal performance on the precursor feeding ratio. The optimization of this ratio notably enhanced the light-to-heat conversion efficiency of CuSe, where even a low concentration of 50 µg mL⁻¹ resulted in a rapid temperature increase up to 46 °C under laser irradiation. Moreover, the elevated temperature was maintained for a longer duration, as illustrated in Fig. 12(c), demonstrating the excellent photothermal stability and energy retention capability of the optimized CuSe NPs. Furthermore, when assessed for cancer cell treatment under NIR irradiation (1064 nm), the optimized CuSe NPs exhibited excellent therapeutic efficacy, effectively destroying cancer cells while showing no noticeable cytotoxic effects on vital organs such as the kidney, lungs, and brain. These findings clearly emphasize that the precursor ratio optimization plays a crucial role in tailoring the biomedical performance and photothermal therapeutic potential of CuSe NPs. In a recent study, Su et al.³²⁸ have developed CuSe NSs modified with n-octadecanethiol (ODT) end-functionalized poly(methacrylic acid) (PMAA) ligands. These functionalized NPs were uniformly incorporated into a decellularized periosteum/sodium alginate (ECM-SA) matrix to fabricate an ODT-PMAA@Cu₂Se NP-doped ECM-SA functional wound dressing for treating infected diabetic wounds. When evaluated in a mouse model, the OP@Cu₂Se/ECM-SA composite exhibited remarkable antibacterial efficacy, with revealing a reduction in bacterial activity to 1.97%, confirming its strong therapeutic potential. The schematic illustration depicting the synthesis process and in vivo experimental outcomes is presented in Fig. 12(d). Moreover, several other research groups have synthesized and systematically evaluated the biocompatibility, antibacterial efficiency, and photothermal properties of CuSe-based NPs and their doped derivatives. A comparative summary of their key findings, including photothermal conversion efficiency, target bacterial strains, killing mechanisms, and overall antibacterial performance, are presented in Table 5, providing a comprehensive overview of CuSe's emerging role in biomedical and antibacterial research.

Fig. 12 (a) Synthesis of CuSe NSs and promotion of the healing of bacterium-infected wounds.¹¹⁸. (b and c) Photothermal heating curve of CuSe NS suspensions (50 µg mL⁻¹) under laser irradiation, and cooling curve after the laser was shut off.³²⁷ (d) Schematic of the design strategy and application of OP@Cu₂Se/ECM-S, and its antibacterial activity in the presence of near-infrared light.³²⁸ (e) Mn : CuSe@BSA-FA-Ce6 dispersion in different media and its MRI properties: T₁-and T₂-weighted images, inverse relaxation time (1/T) versus Mn concentration plot, and calculated r₁ and r₂.³³⁵ (f) COF–CuSe characterization: SEM images, DPBF degradation under a 650 nm laser, and temperature response under an 808 nm irradiation (1.5 W cm⁻²).³³⁴ Reprinted with permission from (a) ref. 118 (Elsevier), Copyright 2024; (b and c) ref. 118 (ACS), Copyright 2023; (d) ref. 328 (Elsevier), Copyright 2024; (e) ref. 335 (ACS), Copyright 2019; (e) ref. 334 (ACS), Copyright 2019.

4.4.3 Multifunctional biomedical potential of CuSe via composite engineering. Considering recent advancements in the design of multifunctional nanomaterials capable of performing multiple therapeutic and diagnostic tasks with a single dose in biomedical applications, Hu et al.³³⁴ reported the fabrication of monodisperse CuSe-sensitized covalent organic framework (COF) photosensitizers. They demonstrated significantly enhanced photodynamic (PDT) and photothermal (PTT) effects for cancer therapy. In this study, the COF material was an effective PDT agent, exhibiting excellent PDT effect under 650 nm or 808 nm laser irradiation. To evaluate the COF's ability to generate singlet oxygen (¹O₂), a crucial reactive species in photodynamic therapy, the authors employed 1,3-diphenylisobenzofuran (DPBF) as a chemical probe. DPBF readily reacts with ¹O₂, resulting in a measurable decrease in its characteristic absorption peak, as shown in Fig. 12(f). The observed reduction in the DPBF absorption intensity upon light irradiation confirmed the successful generation of ¹O₂. Notably, when the COF was decorated with CuSe, an improvement in ¹O₂ generation was observed, highlighting the synergistic enhancement and excellent potential of CuSe–COF hybrids as powerful PDT agents. The PTT properties of the prepared CuSe–COF combination were evaluated by monitoring the system's temperature rise under laser irradiation. As shown in Fig. 12(f), the temperature increased to approximately 55 °C within 5 minutes of exposure, demonstrating efficient photothermal conversion. The in vitro and in vivo experiments indicated a therapeutic effect on killing cancer cells and inhibiting tumor growth. This significant thermal response under controlled irradiation highlights the promising potential of the proposed nanohybrid for combined PDT and PTT cancer therapy using a single NP platform. Extending this, Wu et al.³²¹ reported the fabrication of CuSe with zeolite imidazolate framework-67 (ZIF-67) to build a multifunctional platform (ZIF-67@CuSe@PVP) with efficient chemo-photothermal therapy for cancer treatment. In vivo biodistribution evaluation revealed the passive tumor-targeting ability of ZIF-67@CuSe@PVP@DOX via the enhanced permeability and retention (EPR) effect. Both in vitro and in vivo data demonstrated the excellent anticancer efficacy of ZIF-67@CuSe@PVP in tumor-bearing mice. In a step forward in designing a theranostic agent as a material having properties of diagnosing and treating, Deharvi et al.³³⁵ reported the design of manganese (Mn)-doped CuSe NSs (Mn : CuSe@BSA) using a biomimetic mineralization approach. In this strategy, bovine serum albumin (BSA) was employed to regulate the nucleation and growth of CuSe NPs, yielding nanoscale-sized particles, making them soluble in different solvents, favorable for efficient bio-distribution and targeted delivery within the body. Mn-ions were doped into the CuSe lattice to endow the magnetic resonance imaging (MRI) contrast capabilities, while CuSe served as the PT agent. Folic acid (FA) and chlorin e6 (Ce6) were conjugated onto the BSA corona, forming the Mn : CuSe@BSA-FA-Ce6 nanohybrid to further target specificity. FA facilitates active targeting of folate receptor-overexpressing cancer cells, while Ce6 provides photosensitizing capabilities for synergistic PDT therapy. To experimentally evaluate the diagnostic capabilities of the prepared NSs, the MRI performance of the Mn : CuSe@BSA-FA-Ce6 agents was assessed by measuring the relaxivity values of water protons and capturing both T1-and T2-weighted phantom images. In addition to showing excellent solubility of prepared NSs in different solvents, Fig. 12(e) shows that the presence of Mn in the theragnostic agent significantly shortens the T₁ and T₂ relaxation times, resulting in enhanced signal intensity in T₁-weighted images and decreased signal intensity in T₂-weighted images. These results demonstrate the excellent potential of the developed CuSe NPs for diagnostic imaging of target organisms.

5 Major challenges, possible solution, and future prospects

5.1 Optimization of band edges

The solar energy conversion efficiency of CuSe NSs is highly influenced by their sensitivity to the solar spectrum, which spans ultraviolet (UV, ∼4%), visible (∼46%), and near-infrared (NIR, ∼50%) regions. Although the intrinsic bandgap of CuSe is well suited for efficient absorption of visible and NIR light, a key challenge in its application as a photocatalyst lies in precisely tuning the compound's valence band (VB) and conduction band (CB) edge positions. The reported VB and CB positions of CuSe are approximately 1.32 eV and −0.32 eV, respectively. While the CB edge of CuSe offers a favorable potential for the generation of reactive oxygen species (e.g., superoxide radicals) through photogenerated electrons, its valence band (VB) edge is not sufficiently positive to oxidize water into hydroxyl radicals or to effectively drive water splitting for hydrogen evolution. A promising strategy to overcome this challenge involves engineering the chemical composition of CuSe, particularly by incorporating different stoichiometric and non-stoichiometric phases. By selectively varying the metallic (Cu) and non-metallic (Se) contents, modulating the band edge positions may be possible to achieve a more favorable redox potential alignment. However, such optimization requires robust theoretical modeling to predict and guide compositional modifications, a domain currently underexplored in the existing literature. The lack of comprehensive theoretical studies and systematic experimental validation presents a significant gap, especially concerning the photocatalytic applicability of compositionally tuned CuSe materials. Achieving this optimization could shift the band edge's positions and improve the potential of the photogenerated electron–hole pair in CuSe to participate in water splitting.

5.2 Flexible device integration

The development of flexible CuSe-based energy devices opens up new possibilities for next-generation solar and optoelectronic systems. As discussed in the present article (Section 4), the excellent electrical conductivity, strong optical absorption, and mechanical adaptability of CuSe NSs can be effectively incorporated into bendable and stretchable platforms. Future progress in this direction will rely on solution-processed and low-temperature fabrication methods that allow direct integration of CuSe thin films or hybrid layers onto flexible substrates such as polyethylene terephthalate (PET), polyimide (PI), or conductive textiles. Embedding CuSe within polymer or elastomer matrices can further improve structural stability and flexibility while maintaining efficient charge transport. These advancements will enable the creation of lightweight, wearable, and portable energy-harvesting systems, including flexible solar modules, photothermal patches, and self-powered biosensors. Overall, enhancing the mechanical compliance and structural resilience of CuSe-based materials will be vital for their successful application in next-generation flexible and multifunctional energy devices.

5.3 Hybrid material durability

Ensuring the durability and long-term stability of base materials is essential for their reliable performance in solar and energy conversion devices. Although CuSe NSs exhibit excellent optical and electronic properties, their structural integrity and interfacial stability can degrade under prolonged illumination, thermal cycling, or environmental exposure. To overcome these challenges, future work should focus on interface engineering, defect control, and surface passivation to minimize oxidation and ion migration within the hybrid framework. Incorporating CuSe into robust host matrices, such as conductive polymers, metal oxides, or carbon-based supports, can significantly enhance mechanical and chemical stability while preserving charge transport pathways. Additionally, the design of encapsulation layers and protective coatings can prevent moisture or oxygen ingress, extending device lifetime under operational conditions. By optimizing the chemical bonding, lattice compatibility, and interface coherence between CuSe and the host matrix, researchers can achieve hybrids that combine high efficiency with superior operational stability, paving the way for durable and resilient CuSe-based energy systems suitable for practical applications.

5.4 Scalability and advanced recycle technique

Achieving scalable and sustainable fabrication of required materials is a crucial step toward their real-world implementation in any technology. Although laboratory-scale syntheses of CuSe have demonstrated excellent control over particle size and composition, most existing methods remain limited in throughput and reproducibility. Future efforts should emphasize the development of cost-effective, environmentally benign, and high-yield synthesis routes, such as hydrothermal, solvothermal, sol–gel, or biogenic processes, which can be easily adapted for large-scale production. Furthermore, integrating CuSe NSs into printable inks and coating formulations will enable compatibility with scalable manufacturing platforms, including roll-to-roll processing, inkjet printing, and spray coating. Such approaches will not only reduce production costs but also facilitate the fabrication of large-area, flexible, and lightweight devices. Equally important is the optimization of resource-efficient and recyclable device architectures, ensuring minimal environmental impact during fabrication and disposal. Advancing these scalable and sustainable strategies will be key to transforming CuSe nanomaterials from promising laboratory systems into commercially viable components for next-generation energy technologies. In addition to provide large-scale production, researchers should focus on designing advanced recyclable technologies to efficiently collect and reuse produced CuSe nanoparticles. In our view, incorporating additional properties, such as magnetic characteristics through doping or forming composites, could be a promising strategy. These properties would enable the successful recovery of CuSe nanoparticles after their use in specific applications such as photocatalytic wastewater treatment. By enhancing the recyclability of CuSe NSs, this approach would not only make large-scale applications more feasible but also contribute to reducing environmental impact. Long-term chemical and photothermal stability in real-world conditions must be rigorously evaluated. In addition, the recyclability, lifecycle assessment, and eco-toxicity of CuSe materials should be thoroughly studied to align with sustainability goals in solar energy technologies.

6 Potential future applications: from lab to industries

Building on the above-mentioned fundamental advantages and complemented by possible low-cost and environmental risks, the unique semiconductor chemistry and reported multifunctional performance of CuSe are driving its recognition as a promising material for emerging industrial technologies. In the solar energy sector, its narrow band gap, high thermal stability, and non-toxic nature make it an attractive candidate for photovoltaic devices, especially as an absorber layer in thin-film solar cells. Considering these advantages, several globally recognized companies with established expertise in solar panel technology development and their installation actively adopted copper indium gallium selenide (CIGS) for the fabrication of thin-film solar panels. These CIGS-based devices have demonstrated significantly higher power conversion efficiency (PCE) compared to the conventional Si-based solar cells, while also offering advantages in terms of cost-effectiveness and flexibility. A detailed fundamental discussion of CIGS and its structural, chemical, and functional relevance are provided in Section 4.1 of the article. This industrial shift highlights the increasing demand for advanced materials such as Cu-based selenides, where CuSe emerges as a simpler and potentially more economical alternative. In the automotive industry, particularly within the rapidly expanding electric vehicle (EV) segment, CuSe shows great potential in thermoelectric generators for waste heat recovery systems. This is largely attributed to its inherently low thermal conductivity and high electrical conductivity properties that can be fine-tuned by adjusting the copper-to-selenium atomic ratio, owing to its non-stoichiometric crystal structure. The ability to engineer its electronic and thermal properties via simple compositional modifications offers a scalable route for optimizing the device performance. In addition to its established properties, the low thermal conductivity and near-infrared (NIR) light responsiveness of CuSe make it a promising candidate for photothermal and photodynamic cancer therapy. Beyond healthcare applications, CuSe nanostructures have shown remarkable potential in solar-light-driven photocatalysis hydrogen production, making them attractive for clean and low-cost hydrogen energy industries. Additionally, their ability to harness the natural solar spectrum for photocatalytic degradation of water-soluble organic pollutants offers a sustainable solution for industrial wastewater treatment cell. This is particularly relevant in textile and dye-based industries, where large volumes of colored effluents are discharged into water streams. Furthermore, its electrochemical properties position it as an effective electrode material for energy storage applications. A comprehensive overview of these applications, along with a schematic, is illustrated in Fig. 13.

Fig. 13 Present and possible future industrial utilization of CuSe-based NSs.

7 Summary and general outlook

To convert the light energy into electricity using a photovoltaic device, or to energy-rich molecules (solar fuels) via the photocatalysis process, invariants start with the design of new, or the modification of existing, semiconductor nanomaterials. Therefore, the present review gives a comprehensive overview of CuSe semiconductors, p-type TMCs, highlighting their promising potential across energy conversion, energy storage, and biomedical applications. The analysis of CuSe's fundamental properties revealed that its chemical composition and crystal phase play a critical role in shaping its optical and electrical behavior. For instance, the hexagonal crystal phase exhibits semiconducting characteristics with a band gap ranging from 1.8 to 2.5 eV. While the cubic phase, commonly associated with non-stoichiometric compositions, tends to show a lower band gap and a higher electrical conductivity. Emphasizing the NS design, different CuSe NSs are systematically overviewed for their functional properties. CuSe NSs analysis shows that CuSe nanosheets, which offer a high active surface area, can be synthesized by simply varying the reaction time and temperature, commonly using electrodeposition or hydrothermal techniques. Nanospheres and other 3D CuSe architectures are typically obtained through hydrothermal synthesis with reducing and capping agents. CuSe nanotube (NT) structures require template-assisted chemical methods, while nanofibers can be produced via a modified electrospinning technique. In evaluating the photocatalytic performance of CuSe NSs, the hexagonal structure with stoichiometric CuSe form has consistently exhibited superior photocatalytic activity among the various crystal phases of CuSe. Furthermore, NSs such as CuSe nanosheets and dendritic architectures with branched morphologies have demonstrated significantly enhanced photocatalytic behavior. While nanosheets offer advantages in terms of ease of synthesis, dendritic structures provide significantly higher photodegradation efficiency, albeit with more complex fabrication requirements. In PVs, by tailoring its chemical composition and phase, non-stoichiometric variants such as Cu_2−xSe have outperformed conventional platinum (Pt) electrodes, offering a more cost-effective and efficient alternative for high-performance PV devices. Besides energy conversion, CuSe-based nanocomposites have shown promise in energy storage applications, exhibiting high energy density and improved stability. In biomedical applications, the high surface area achieved through nano-structuring facilitates efficient drug loading and targeted delivery. Moreover, non-stoichiometric CuSe compositions with enhanced photothermal properties have demonstrated potential for cancer therapy. In addition, the high surface area of CuSe NSs further enhances their capacity as a drug delivery system, allowing for efficient loading and controlled release of therapeutic agents. This combination of antibacterial efficacy, NIR-driven therapeutic potential, and drug delivery capability positions CuSe NSs as a versatile and auspicious material for future biomedical applications, particularly in combating infections and improving targeted treatment outcomes.

Conflicts of interest

There is no conflict of interest to declare.

Data availability

No primary research results, software or code have been included, and no new data were generated or analysed as part of this review.

Acknowledgements

The first author (VK) gratefully acknowledges the Department of Physics, National Institute of Technology Kurukshetra, for providing essential facilities and Central Library, National Institute of Technology, for granting full access to published articles through journal subscriptions. The author further thanks Mr Vipin Kumar, Mr Vinod Kumar, and Dr Bajrang Lal from the Department of Chemistry, National Institute of Technology Kurukshetra, for their conceptual discussions. The authors acknowledge the Ministry of Education, India, for providing the fellowship. This work is supported by the National Research Foundation of South Africa (No. 84415) and the South African Research Chairs initiative of the Department of Science and Technology. Author (JP) also acknowledges NRF South Africa for the NRF rating. The financial assistance from the University of the Free State is highly recognized.

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